Silicon α-effect: A systematic experimental and computational study of the hydrolysis of Cα- and Cγ-functionalized alkoxytriorganylsilanes of the formula type ROSiMe2(CH 2)nX (R = Me, Et; N = 1, 3; X = functional group)

Article

pubs.acs.org/Organometallics

Silicon α‑Eﬀect: A Systematic Experimental and Computational Study

of the Hydrolysis of C_α- and C_γ‑Functionalized

Alkoxytriorganylsilanes of the Formula Type ROSiMe₂(CH₂)_nX (R =

Me, Et; n = 1, 3; X = Functional Group)

Andre Berkefeld,^†Celia Fonseca Guerra,^‡Rudiger Bertermann,^†Dennis Troegel,^§Jurgen O. Daiß,^∥

́

̈

Jurgen Stohrer,^§F. Matthias Bickelhaupt,*^,‡,⊥and Reinhold Tacke*^,†

̈

^†Institut fur Anorganische Chemie, Universitat Wurzburg, Am Hubland, D-97074 Wurzburg, Germany

̈

^‡Department of Theoretical Chemistry, Amsterdam Center for Multiscale Modeling (ACMM), VU University Amsterdam, De

Boelelaan 1083, NL-1081 HV Amsterdam, The Netherlands

^§Wacker Chemie AG, Consortium fur elektrochemische Industrie, Zielstattstrasse 20, D-81379 Munchen, Germany

̈

^∥Wacker Chemie AG, Johannes-Hess-Strasse 24, D-84489 Burghausen, Germany

^⊥Institute for Molecules and Materials, Radboud University Nijmegen, Heyendaalseweg 135, NL-6525 AJ Nijmegen, The Netherlands

S

* Supporting Information

ABSTRACT: To understand the silicon α-eﬀect in terms of an enhanced

reactivity of the Si−OC bond of α-silanes of the formula type

ROSiMe₂CH₂X compared to analogous γ-silanes ROSiMe₂(CH₂)₃X (R

= Me, Et; X = functional group), a systematic experimental and

computational study of the kinetics and mechanisms of hydrolysis of

such compounds was performed. For this purpose, a series of suitable

model compounds was synthesized and studied for their hydrolysis kinetics

in CD₃CN/D₂O under basic and acidic conditions, using ¹H NMR

spectroscopy as the analytical tool. To get more information about the

reaction mechanisms, the experimental investigations were complemented by computational studies. These investigations

demonstrated that the silicon α-eﬀect cannot be rationalized in terms of a special single eﬀect. The reactivities observed rather

result from a summation of diﬀerent components, such as electronic and steric eﬀects, pD dependence, and hydrogen bonds

between the functional group (or even protonated functional group) and the alkoxy leaving group. Therefore, the term “silicon

α-eﬀect” should not be used furthermore to explain the hydrolysis reactivity at the silicon atom of alkoxyorganylsilanes with

functional groups in α- or γ-position of the organyl groups (so-called α- or γ-silanes).

polymers.¹⁻³In particular, these α-silanes can be used to

modify organic polymers (e.g., polyurethanes or polyethers) by

C_α-Functionalized trialkoxymethylsilanes and dialkoxydimethyl-

introducing them via their functional group X into α-silane-

INTRODUCTION

■

silanes of the formula types Ia and Ib (known as α-silanes,

Scheme 1) constitute a very interesting class of compounds

with unique properties that can be exploited in a wide range of

cross-linking technologies for the preparation of a variety of

terminated prepolymers that are highly reactive toward water.

Hydrolysis of the Si−OMe or Si−OEt groups of these

silylmethyl-end-capped prepolymers leads to the corresponding

silanols, which then undergo condensation reactions to give a

two- or three-dimensional polysiloxane network constructed

from Si−O−Si moieties. As the reactivity of α-silane-terminated

prepolymers against humidity and/or moisture is very high and

tunable, such materials are predestinated for applications in

sealants, adhesives, coatings, and spray foams.¹⁻³In contact

with moisture, certain α-silane-terminated prepolymers cross-

link 10−1000 times faster than the corresponding prepolymers

produced from conventional C_γ-functionalized trialkoxypropyl-

silanes and dialkoxymethylpropylsilanes of the formula types

IIa and IIb (known as γ-silanes, Scheme 1). This reactivity

a

Scheme 1. α-Silanes Ia and Ib and γ-Silanes IIa and IIb

a

Received: January 29, 2014

R = Me, Et; X = functional group.

A

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phenomenon has been termed “α-eﬀect”. Although α-silanes of

the formula types Ia and Ib constitute the basis of a very

important and economically successful technology platform in

the silicone industry, the α-eﬀect is not yet fully understood.

From an industrial point of view, the α-eﬀect was ﬁrst

observed in the late 1960s by researchers at Bayer AG as an

increase in reactivity at the silicon atom and was already used at

that time for cross-linking of α-silane-terminated prepolymers.⁴

Surprisingly, this initial work was interrupted after 1975 for

almost 25 years, until this chemistry was reactivated by

scientists at Wacker Chemie AG and then developed into a

practical and commercial enterprise.⁵

Scheme 3. Model Compounds Studied for Their Hydrolysis

Reactivity

The term α-eﬀect was introduced to describe the unique

reactivity of α-silanes at both the silicon atom and the

functional group X (mainly NR₂, OC(O)R, N(H)COOMe)³⁻⁵

and to address some unusual physicochemical and spectro-

scopic properties of these compounds; that is, the α-eﬀect is

not sharply deﬁned and its nature has been a matter of debate

for many decades.^6,7As shown exemplarily for the α-silane

(MeO)₃SiCH₂NMe₂in Scheme 2, for a long time the α-eﬀect

Scheme 2. Hypothetical Intramolecular Si···N Interaction in

(MeO)₃SiCH₂NMe₂as a Model to Explain the α-Eﬀect

variation of (i) the proton aﬃnities of the functional groups X

and the alkoxy groups (→ impact on the degree of

protonation) and (ii) the electronegativities of the functional

group X (→ impact on the electrophilicity of the silicon

center). Furthermore, this selection also implies some variation

of steric eﬀects. This collection of model compounds was

designed to allow the investigation of both the α/γ-eﬀect

(comparison of α- and γ-silanes with identical functional groups

X) and the α/α′-eﬀect (comparison of α-silanes with diﬀerent

functional groups X).

has been rationalized in terms of an intramolecular donor−

acceptor interaction between the lone pair of the organofunc-

tional group (in this case NMe₂) and the silicon atom.

However, in a series of more recent experimental and

computational studies (structure determination of some α-

silanes and analysis of the binding situation in these

compounds), Mitzel et al. have clearly demonstrated that this

picture is incorrect and cannot be used to explain the α-eﬀect.⁸

Although the kinetic cross-linking behavior of α-silanes is

determined by both the hydrolysis and the condensation

reactivity, the term “α-eﬀect” is mainly used in relation to the

hydrolysis reaction because this step is assumed to be much

faster than the condensation. To understand the α-eﬀect in

terms of the enhanced hydrolysis reactivity of the Si−OC bond

of α-silanes (Ia, Ib) compared to the analogous γ-silanes (IIa,

IIb), we have performed a systematic experimental and

computational study of the kinetics and the mechanisms of

the hydrolysis of the model compounds 1a−13a, 1b−6b, 8b−

10b, and 13b (Scheme 3). To simplify matters, only

monoalkoxysilanes with one hydrolyzable Si−OR group were

chosen for these studies.

RESULTS AND DISCUSSION

■

Syntheses. Compounds 1b and 13b were synthesized by

treatment of dimethoxydimethylsilane (14) with n-propyl- or n-

pentylmagnesium bromide in diethyl ether (yields: 1b, 85%;

13b, 72%; Scheme 4).

Scheme 4. Syntheses of Compounds 1b and 13b

Depending on the pH value of the aqueous reaction medium,

diﬀerent reaction mechanisms for the hydrolysis of the model

silanes 1a−13a, 1b−6b, 8b−10b, and 13b have to be

considered. Generally, the hydrolysis of the alkoxysilanes

studied can be described by eq 1:

Alkoxysilane + Nucleophile → Silanol + Alkanol

(1)

Compounds 3a and 13a were prepared according to Scheme

5, starting from trichlorosilane (15). Thus, treatment of 15 with

the corresponding alkyl chloromethyl ether (MeOCH₂Cl or

nPrOCH₂Cl) and triethylamine, in the presence of copper(I)

chloride, furnished the (alkoxymethyl)trichlorosilanes 16 and

17.^8b,9Reaction of 16 with one molar equivalent of

ethylmethylamine in tetrahydofuran (THF), in the presence

of ethyldimethylamine as a hydrogen chloride acceptor, and

subsequent treatment with an excess of methylmagnesium

The nucleophile in this reaction can be H₂O or OH⁻, and

the alkoxysilane (electrophile) is protonated or nonprotonated.

Protonation can occur at the alkoxy group (OMe, OEt) and/or

the functional group X (OMe, OAc, NMe₂, N(H)COOMe,

N(Me)COOMe, N(H)Ph, OPr), depending on the diﬀerent

proton aﬃnities of these groups and the pH value of the

reaction medium. For the selection of the model compounds

studied, we aimed to use compounds that demonstrate a broad

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methanol in N,N-dimethylformamide furnished

MeOSiMe₂CH₂N(H)C(O)OMe¹²(6a). Compound 10a was

synthesized analogously to 5a.

Scheme 5. Syntheses of Compounds 3a and 13a

Compounds 3b−6b and 10b were synthesized according to

Scheme 7, starting from (3-chloropropyl)methoxydimethyl-

Scheme 7. Syntheses of Compounds 3b−6b and 10b

bromide in diethyl ether aﬀorded a mixture of MeEtNSiMe₂-

CH₂OMe (18), (MeEtN)₂SiMeCH₂OMe (19), and ClSiMe₂-

CH₂OMe (20). Compounds 18 and 20 were isolated by

fractional distillation and then transformed into 3a by

treatment with methanol and catalytical amounts of hydrogen

chloride (18) or by reaction with methanol and ethyldimethyl-

amine (20). Both syntheses were performed in n-pentane, and

3a was obtained with a total yield of 2% (relative to 15).

Compound 13 was synthesized analogously (17 → 21/22/23

→ 13a) and isolated with a total yield of 8% (relative to 15).

Compounds 4a−6a and 10a were synthesized according to

Scheme 6, starting from (chloromethyl)methoxydimethylsilane

(2a) or (chloromethyl)ethoxydimethylsilane (24). Thus, treat-

ment of 2a with sodium acetate in THF furnished

MeOSiMe₂CH₂OAc¹⁰(4a). Reaction of 2a with an excess of

dimethylamine in an autoclave aﬀorded MeOSiMe₂CH₂NMe₂

(5a),¹¹and treatment of 2a with potassium cyanate and

silane (2b) or (3-chloropropyl)ethoxydimethylsilane (25).

Thus, treatment of 2b with sodium methoxide in methanol

aﬀorded MeOSiMe₂(CH₂)₃OMe^5a(3b), and reaction of 2b

with sodium acetate in THF, in the presence of the phase

transfer catalyst tetra-n-butylphosphonium chloride, furnished

MeOSiMe₂(CH₂)₃OAc¹⁰(4b). Treatment of 2b with an excess

of dimethylamine in an autoclave aﬀorded MeOSi-

Me₂(CH₂)₃NMe₂(5b), and reaction of 2b with potassium

cyanate and methanol in N,N-dimethylformamide furnished

MeOSiMe₂(CH₂)₃N(H)C(O)OMe¹²(6b). Compound 10b

was synthesized analogously to 5b.

Puriﬁcation of 6b by distillation yielded a small amount (ca.

14 mol %) of the ﬁve-membered ring compound 26 (formed

by intramolecular condensation of 6b), which could be

transformed back to 6b by treatment with a stoichiometric

amount of methanol at room temperature (Scheme 8).

Compound 26 was enriched by distillation to validate its

identity (for details, see the Supporting Information).

Scheme 6. Syntheses of Compounds 4a−6a and 10a

Scheme 8. Formation of Compound 26 and Its

Transformation to 6b

Compound 7a was synthesized according to Scheme 9,

staring from MeOSiMe₂CH₂Cl (2a). Thus, treatment of 2a

with an excess of methylamine in an autoclave aﬀorded

MeOSiMe₂CH₂N(H)Me¹³(27), which upon reaction with

methyl chloroformate and triethylamine in n-pentane furnished

7a.

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the preparation of analytically pure products (NMR and GC

analysis), if necessary at the expense of the yield. The identities

of all the compounds synthesized were established by elemental

analyses (C, H, N) and NMR spectroscopic studies (¹H, ¹³C,

²⁹Si).

Scheme 9. Synthesis of Compound 7a

Kinetics of Hydrolysis. General Considerations. The

kinetics of the hydrolysis of nonfunctional alkoxytriorganylsi-

lanes have already been studied extensively, and it has been

demonstrated that the reaction rates are inﬂuenced by acids,

bases, solvents, temperature, and steric and electronic eﬀects.¹⁶

The pH-dependence (here pD-dependence; see below) of the

hydrolysis kinetics shows two clearly separated diﬀerent

regimes for all silanes investigated in the present study: rate

constants generally increase with increasing pD values above ca.

pD 8.5, while at lower pD values (up to ca. pD 7) the rate

constants decrease with increasing pD values. These ﬁndings

clearly indicate the presence of diﬀerent reaction mechanisms

under basic (pD > 8.5) and acidic conditions (pD < 7) and are

in good agreement with earlier studies.¹⁶Therefore, the

hydrolysis kinetics of the compounds studied have to be

discussed separately for the acidic and basic regime.

Compounds 8a, 8b, and 12a were synthesized according to

Scheme 10 by reaction of MeOSiMe₂CH₂NMe₂(5a),

MeOSiMe₂(CH₂)₃NMe₂(5b), and EtOSiMe₂CH₂NMe₂

(10a), respectively, with an excess of iodomethane in

acetonitrile.¹⁴

Scheme 10. Syntheses of Compounds 8a, 8b, and 12a

As shown in Scheme 13, under basic conditions OH⁻is the

nucleophile, and the nonprotonated silane ROSiMe₂(CH₂)_nX is

Scheme 13. Hydrolysis of α- and γ-Silanes of the Formula

Type ROSiMe₂(CH₂)_nX (R = Me, Et; n = 1, 3; X =

Functional Group) under Basic Conditions

Compounds 9a and 9b were synthesized according to

Scheme 11 by reaction of MeOSiMe₂CH₂Cl (2a) and

MeOSiMe₂(CH₂)₃Cl (2b), respectively, with “Mg[N(H)-

Ph]₂”¹⁵in aniline.

the electrophile. Assuming that a vast excess of water is present,

the kinetics of this S_N2 reaction can be described in terms of

pseudo-ﬁrst-order kinetics by eq 2, which can be transformed

into eq 3:

Scheme 11. Syntheses of Compounds 9a and 9b

r = k[OH⁻][ROSiMe₂(CH₂)_nX]

= k_exp[ROSiMe₂(CH₂)_nX]

(2)

k_exp= k[OH⁻] (consequence: log(k_exp) ≈ +1·pH)

(3)

where r = reaction rate, k = “microscopic” rate constant, and

k_exp= experimental (“macroscopic”) rate constant.

As can be seen from eq 3, the experimental (“macroscopic”)

rate constant k_expdepends on both the pH value and the

“microscopic” rate constant k. At a given pH value, k depends

only on the functional group X and n (length of the spacer

between the silicon atom and X).

As shown in Scheme 14, under acidic conditions H₂O is the

nucleophile, and the protonated silane (ROH⁺)SiMe₂(CH₂)_nX

(protonation at the alkoxy leaving group) is the electrophile:

Compound 11a was synthesized according to Scheme 12 by

treatment of MeOSiMe₂CH₂N(H)C(O)OMe (6a) with an

excess of ethanol, in the presence of catalytic amounts of

orthophosphoric acid.

At ﬁrst glance, some of the product yields obtained in the

syntheses described in this section may appear somewhat

unsatisfactory; however, it is important to note that we aimed at

Scheme 14. Hydrolysis of α- and γ-Silanes of the Formula

Type ROSiMe₂(CH₂)_nX (R = Me, Et; n = 1, 3; X =

Functional Group) under Acidic Conditions

Scheme 12. Synthesis of Compound 11a

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Again assuming that a vast excess of water is present, the

kinetics of this S_N2 reaction can be described in terms of

pseudo-ﬁrst-order kinetics according to eq 4:

three diﬀerent silanes (comparison of either α- or γ-silanes with

diﬀerent functional groups X). This experimental design

allowed for an exact comparison of the hydrolysis rates of

diﬀerent silanes at a given pD value. In order to avoid

overlapping of the mechanisms for the acidic and basic regime,

no experiments were performed at neutral pD values. A series

of test experiments have shown that buﬀer systems are not

suitable for adjusting the pD values, because these buﬀer

systems can interfere with the hydrolysis reactions. The rate of

hydrolysis can be inﬂuenced by the type of acid or base used,

but this eﬀect is assumed to be negligible compared to the

eﬀect of the pD value.¹⁷Therefore, the acidic pD values (4.5−

6.5) were adjusted by the addition of formic acid, and the basic

pD values (9.0−12.5) were adjusted by the addition of

triethylamine. In the case of the silanes 5a, 5b, 10a, and 10b,

the basic pD values resulted from the basic amino groups of

these compounds and were adjusted by the addition of DCl. To

prevent any interference during the kinetic experiments, the pD

values were measured outside the NMR tubes in separate

reaction vessels that contained identical reaction mixtures as in

the NMR tubes. The pD values were determined with a glass

electrode. As the measurements were performed in CD₃CN/

D₂O mixtures, the values obtained do not represent exact pD

values but can be regarded as suitable approximate values to

characterize the acidic or basic nature of the reaction mixture

studied.^18,19The hydrolysis reactions investigated were found

to proceed with pseudo-ﬁrst-order kinetics. Figure 1 shows a

typical example of the results obtained in these kinetic

competition experiments (for analogous ﬁgures for all the

other experiments performed, see the Supporting Information).

The experimental kinetic data are summarized in Figures

2−5, in which the decadic logarithms of the rate constants (log

r = k[H₂O][(ROH⁺)SiMe₂(CH₂)_nX]

= k_exp[ROSiMe₂(CH₂)_nX]

(4)

Combination of eq 4 with the acid constant K_Aof the

protonated α- or γ-silane (ROH⁺)SiMe₂(CH₂)_nX (eq 5) results

in eq 6:

[H₃O⁺]·[ROSiMe₂(CH₂)_nX]

K_A=

[(ROH⁺)SiMe₂(CH₂)_nX]

(5)

k[H₂O]

K_A

k_exp

=

[H₃O⁺] (consequence: log(k_exp) ≈ −1·pH)

(6)

As can be seen from eq 6, the experimental (“macroscopic”)

rate constant k_expdepends on the pH value, the “microscopic”

rate constant k (impact of the functional group X and n (length

of the spacer between the silicon atom and X)), and the

thermodynamic parameter K_A, which describes the degree of

protonation of the alkoxy leaving group RO; that is, k_expalso

depends on the functional group X, which may also act as

proton acceptor (→ competition of RO and X as proton

acceptors; impact of the diﬀerent proton aﬃnities of RO and

X).

Provided that the proton aﬃnity of X is signiﬁcantly higher

than that of RO (protonation at the functional group X instead

of the alkoxy leaving group RO), the pseudo-ﬁrst-order kinetics

of hydrolysis can be described on the basis of eqs 7−9:

r = k[H₂O][ROSiMe₂(CH₂)_nXH⁺]

= k_exp[ROSiMe₂(CH₂)_nX]

(7)

[H₃O⁺][ROSiMe₂(CH₂)_nX]

K_A′

k_exp

=

[ROSiMe₂(CH₂)_nXH⁺]

(8)

k[H₂O]

[H₃O⁺] (consequence: log(k_exp) ≈ −1·pH)

K_A′

(9)

Kinetics of Hydrolysis. Experimental Method and

Results. To investigate the dependence of the rate of

hydrolysis on (i) the functional group X, (ii) the spacer

between the silicon atom and the functional group X (CH₂vs

(CH₂)₃; α- vs γ-silane), and (iii) the pH value (acidic and basic

conditions; here pD value; see below), a ¹H NMR

spectroscopic study was carried out in order to monitor the

hydrolysis of the model compounds studied. All measurements

were performed in NMR tubes at 23 °C. To guarantee

homogenic phases for all the reaction systems investigated, a

mixture of CD₃CN and D₂O was used as the reaction medium.

The molar ratio silane:D₂O:CD₃CN was 1:90:150 (D₂O in vast

excess) to allow for an analysis in terms of pseudo-ﬁrst-order

kinetics and to prevent condensation of the resulting silanols to

the corresponding disiloxanes.

Figure 1. (a) Kinetics of the co-hydrolysis of compounds 3a (α-OMe,

red), 4a (α-OAc, green), and 5a (α-NMe₂, purple) in CD₃CN/D₂O

[5:1 (v/v)] at 23 °C; [3a] = [4a] = [5a] = 0.1 mol L⁻¹, [DCl] = 1 ×

10⁻⁵mol L⁻¹; average pD value 11.7 0.2. (b) Determination of the

k_expvalues as the gradient of the linear functions. Due to its very fast

hydrolysis (complete hydrolysis at t < 2 min), the k_expvalue of

compound 5a could not be determined.

Preliminary studies have shown that the rates of hydrolysis

depend extremely strongly on both the structure of the silane

and the pD value. Therefore, all kinetic measurements were

performed as competition experiments with two (comparison

of an α- and a γ-silane with identical functional groups X) or

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Figure 2. log k_expvalues of the γ-silanes 1b (γ-H, black), 2b (γ-Cl,

blue), 3b (γ-OMe, red), 4b (γ-OAc, green), 6b (γ-N(H)COOMe,

aqua), 8b (γ-NMe₃, magenta), and 9b (γ-N(H)Ph, brown) as a

function of the pD values. Due to its very fast hydrolysis (complete

hydrolysis at t < 2 min), the k_expvalues of compound 5b (γ-NMe₂)

could not be determined at pD 4.5−6.5. The k_expvalues of 5b at pD

8.0−12.5 are given in Figure 4. The linear functions of the γ-silanes are

shown as dashed lines. The yellow areas, which are also depicted in

Figures 3−5, summarize the linear functions of the γ-silanes, except 1b

(pD 9.0−12.5), 8b (pD 9.0−12.5), and 9b (pD 4.5−6.5).

Figure 4. log k_expvalues of the γ-silanes 5b (γ-NMe₂, SiOMe) and 10b

(γ-NMe₂, SiOEt) in diﬀerent CD₃CN/D₂O mixtures as a function of

the pD values. (a) Co-hydrolysis of compounds 5a (α-NMe₂, SiOMe)

+

○

and 5b ( ) in CD₃CN/D₂O [5:1 (v/v)]; (b) co-hydrolysis of

△

compounds 10a (α-NMe₂, SiOEt) and 10b ( ) in CD₃CN/D₂O [5:1

□

(v/v)]; (c) co-hydrolysis of compounds 5a and 5b ( ) in CD₃CN/

◇

D₂O [59:1 (v/v)]; (d) co-hydrolysis of compounds 10a and 10b (

)

in CD₃CN/D₂O [59:1 (v/v)]. For reasons of clarity, the linear

functions of the γ-silanes (except 1b (pD 9.0−12.5) and 8b (pD 9.0−

12.5); for log k_expvalues of 1b (pD 9.0−12.5) and 8b (pD 9.0−12.5),

see Figure 2) are summarized by the yellow area taken from Figure 2.

Due to their very fast hydrolysis (complete hydrolysis at t < 2 min),

the k_expvalues of compounds 5a and 10a (pD 8.0−12.5) could not be

determined.

Figure 3. log k_expvalues of the α-silanes 1a (α-H, black), 2a (α-Cl,

blue), 3a (α-OMe, red), 4a (α-OAc, green), 6a (α-N(H)COOMe,

+

aqua), 7a (α-N(Me)COOMe, blue with outline), 8a (α-NMe₃,

magenta), and 9a (α-N(H)Ph, brown) and γ-silanes as a function of

the pD values. For reasons of clarity, the linear functions of the γ-

silanes (except 1b (pD 9.0−12.5), 8b (pD 9.0−12.5), and 9b (pD

4.5−6.5); for log k_expvalues of 1b (pD 9.0−12.5), 8b (pD 9.0−12.5),

and 9b (pD 4.5−6.5), see Figure 2) are summarized by the yellow

areas taken from Figure 2. Due to their very fast hydrolysis (complete

hydrolysis at t < 2 min), the k_expvalues of compounds 5a (α-NMe₂;

pD 4.5−6.5 and 9.0−12.5) and 7a (pD 4.5−6.5) could not be

determined. The linear function of compound 7a is shown as a

dashed-dotted line. The linear functions of the γ-silanes are shown as

dashed lines.

●

○

▲

Figure 5. log k_expvalues of the silanes 1a (α-H, ), 1b (γ-H, ), 3b

○

▲

(γ-OMe, red ), 13a (α-OnPr, red ), and 13b (γ-Et, black ) as a

function of the pD values. The linear functions of compound 1b and

3b are shown as dashed lines. For reasons of clarity, the linear

functions of the γ-silanes (except 1b (pD 9.0−12.5), 8b (pD 9.0−

12.5), and 9b (pD 4.5−6.5); for log k_expvalues of 1b (pD 9.0−12.5),

8b (pD 9.0−12.5), and 9b (pD 4.5−6.5), see Figure 2) are

summarized by the yellow areas taken from Figure 2.

acidic regime. This procedure allows for a clear documentation

of the kinetic data.

Kinetics of Hydrolysis. Computational Methods and

Results. All calculations were performed with the ADF²⁰and

QUILD²¹programs at the ZORA-OLYP/TZ2P level^22,23of

relativistic density functional theory (DFT), with technical

settings as follows: CORE small; ACCINT 8.0 8.0; SCFCONV

k_exp; the k_expvalues were transformed into [s⁻¹] before

converting them to logarithms) are plotted as a function of

the pD. Taking into account the direct proportionality between

k_expand [OD⁻] (eq 3) or [D₃O⁺] (eq 9), the gradient of the

linear functions is set to +1 in the basic regime and to −1 in the

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Figure 6. Shape of reaction proﬁles for S_N2 reactions of a nucleophile Nu⁻with an electrophilic substrate SiR₂R′L with leaving group L along the

reaction coordinate ζ: (a) single-step double-well potential; (b) single-step unimodal proﬁle; (c) two-step triple-well potential; (d) two-step bimodal

proﬁle occurring in our model reactions (see also text).

Table 1. k_expValues at pD 11 and Calculated Reaction Barriers and Energies Relative to Reactants of Intermediate Transition

a

Complexes (TC) for Hydroxide-Induced Hydrolysis of the Silanes 1a−9a, 1b−6b, 8b, 9b, 11a−13a, and 13b

k_exp[h⁻¹

]

reaction barrier [kcal mol⁻¹

]

TC [kcal mol⁻¹

]

compound

(pD = 11)

(in H₂O)

(in CH₃CN)

(in H₂O)

(in CH₃CN)

1a (α-H)

1b (γ-H)

2a (α-Cl)

2b (γ-Cl)

0.0582 0.0144

0.003 26 0.0019

1.09 0.18

14.1

14.6

12.6

14.1

14.5

12.0

6.8

7.8

3.7

6.9

7.8

3.8

0.0189 0.0043

0.599 0.149

0.0172 0.0011

3.50 0.56

3a (α-OMe)

12.7

14.5

11.2

13.8

15.3

13.7

14.4

12.0

13.3

14.5

5.6

7.5

3.6

8.1

9.1

5.6

7.8

3.8

8.0

8.8

3b (γ-OMe)

4a (α-OAc)

4b (γ-OAc)

0.0178 0.0040

5a (α-NMe₂)

5a (α-NMe₂H⁺)

d

>90

−7.0

−8.7

b

5b (γ-NMe₂)

0.129

c

6a (α-N(H)COOMe, SiOMe)

6b (γ-N(H)COOMe)

7a (α-N(Me)COOMe)

12.8 5.2

0.0204 0.0042

0.486 0.522

84.7 26.9

6.3

5.9

2.6

2.0

+

8a (α-NMe₃, SiOMe)

1.2

2.1

−0.2

1.5

+

8b (γ-NMe₃)

0.149 0.060

3.10 2.08

c

9a (α-N(H)Ph)

8.9

8.4

4.1

3.4

9b (γ-N(H)Ph)

0.0165 0.0007

4.81 0.17

11a (α-N(H)COOMe, SiOEt)

+

12a (α-NMe₃, SiOEt)

20.5 4.0

13a (α-OnPr)

13b (γ-Et)

0.248 0.004

0.004 84 0.000 63

12.8

14.4

14.5

14.4

5.5

8.1

6.9

8.2

a

b

Electronic energies computed at ZORA-OLYP/TZ2P using COSMO for solvation in H₂O and CH₃CN. This k_expvalue was obtained at pD 11.1

c

(for details, see Figure S57). This reaction barrier was calculated considering hydrogen bonds between the functional group and the nucleophile.

d

The intermediate is not a thermodynamic trap: the correction ΔG − ΔE is estimated to be 8−9 kcal mol⁻¹, leading to a relative Gibbs free energy

of the TC around 0−2 kcal mol⁻¹.

0.000001; GEOMCONV 0.0001. This approach was previously

shown to agree satisfactorily with highly correlated ab initio

benchmarks.²⁴Stationary points were characterized as minima

or transition states through vibrational analyses and intrinsic

reaction coordinate (IRC) calculations.²⁵In selected cases,

enthalpies and entropies at 298.15 K and 1 atm (ΔH₂₉₈and

ΔS₂₉₈) were calculated using standard thermodynamic relation-

ships and the computed partition functions, assuming an ideal

gas.²⁶Solvent eﬀects were taken into account in all condensed-

phase calculations using the COSMO model.²⁷This model

describes bulk solvation as well as the average electrostatic

eﬀects of the ﬁrst solvation shell, but it neglects covalent

bonding with the solvent. Thus, the COSMO model is well

suited for describing the average interaction of the reaction

G

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system with the dynamically ﬂuctuating solvent environment,

including the weakening of nucleophile−substrate and hydro-

gen-bonding interactions. Also, solvent molecules participating

in the reaction cannot be treated with this approach and have

been included explicitly in the quantum mechanical treatment.

Preliminary explorations suggest that mechanisms involving

multiple discrete water molecules are less likely, e.g., for

entropic reasons. We wish to point out, however, that such

mechanisms cannot be completely ruled out. At the same time,

the qualitative agreement between the order of computed and

measured relativities indicates that we catch the major features

of the systems under investigation. For further details, see the

Supporting Information.

Scheme 15. Model Structures for the Protonation of the α-

and γ-Silanes at the Alkoxy Leaving Group (IIIa, IIIb, IVa,

and IVb) or at the Functional Group X (Va, Vb, and VI)

a

Under basic conditions, the nucleophile in our computational

model systems that induces silane hydrolysis is OH⁻, whereas

the leaving group is OMe⁻. Based on our earlier work on S_N2

reactions at carbon,²⁸silicon,²⁹and phosphorus,³⁰the

expectation was that nucleophilic attack of OH⁻at the silicon

center in solution may proceed via a unimodal reaction

proﬁle,^29cas shown in Figure 6b for the S_N2 reaction of Cl⁻+

SiH₃Cl. However, for the reactants (R) that are investigated

herein, the reaction proﬁle has a pretransition state (preTS), an

intermediate transition complex (TC), and a post-transition

state (postTS), as shown in Figure 6d. Note that the

intermediate TC is the pentacoordinate species in this stepwise

(A_N+ D_N) substitution process and corresponds structurally

and electronically with the saddle point or transition state (TS)

in concerted (A_ND_N) substitution reactions (“A_N” =

nucleophilic association; “+” = occurrence of intermediate;

“D_N” = nucleofugic dissociation). The ﬁrst elementary reaction,

from R via preTS to TC, is in general the rate-determining step.

In the gas phase and in poorly solvating media, reactants and

products form, in general, reactant complexes (RC) and

product complexes (PC), as illustrated for one- and two-step

substitution processes in Figure 6a and c, respectively.

However, extensive computational explorations of our model

reactions reveal that, in water and acetonitrile solution, there

are essentially no stable reactant complexes and product

complexes. The computed reaction barriers associated with the

ﬁrst and rate-determining reaction step and the calculated

energies relative to reactants of the intermediate transition

complexes are listed in Table 1, together with the experimental

rate constants of the hydrolysis under basic conditions.

For the hydrolysis under acidic conditions, the nucleophile in

our computational model systems that induces silane hydrolysis

is water, and the leaving species is methanol. The reaction

barrier under acidic condition appears to be unimodal and has a

remarkably broad and shallow maximum (see Figure 6b). This

makes it very diﬃcult to determine the precise position of the

highest point along the potential energy surface (PES). In the

range of about 1.0 Å around the transition state (TS) the

energy varies by less than half a kcal mol⁻¹. We have carried out

TS optimizations at various points on the PES plateau resulting

from a linear transit, all of which return a transition vector with

a positive frequency. The reaction barriers resulting from

choosing the highest point of this PES scan are in a range of 6−

7 kcal mol⁻¹(not contained in Table 1).

a

Some of these species contain an intramolecular hydrogen bond

between the protonated leaving group and the functional group or

between the protonated functional group and the leaving group.

CH₃CN. The results of these studies and the experimental rate

constants of the hydrolysis under acidic conditions are

summarized in Table 2.

Kinetics of Hydrolysis under Basic Conditions.

Discussion of Experimental and Theoretical Results.

Trends in reactivity evolving from the hydrolysis experiments

and those from the computations are qualitatively in nice

agreement. Figure 7 shows the log k_expvalues of all the model

compounds studied at pD 11 (except for 11a and 12a), ranked

in ascending order. The pD-dependence of the hydrolysis rate

under basic conditions is positive for all silanes, which is in

general agreement with OH⁻being the nucleophile attacking

the silicon atom (see Scheme 13). As the electron-withdrawing

eﬀect of the functional groups of the γ-silanes 2b−4b, 6b, and

9b on the silicon center is similar, these compounds show

almost identical rates of hydrolysis (k_exp= 1.65−2.04 × 10⁻²

h⁻¹). However, their rate is slower than that of the sterically less

demanding silane 1a (α-H, no functional group; k_exp= 5.82 ×

10⁻²h⁻¹) but somewhat faster than those of the long-chain

alkylsilanes 1b (k_exp= 3.26 × 10⁻³h⁻¹) and 13b (4.84 × 10⁻³

h⁻¹). Notable exceptions are the γ-silanes 5b (k_exp= 1.29 ×

10⁻¹h⁻¹; γ-NMe₂) and 8b (k_exp= 1.49 × 10⁻¹h⁻¹; γ-NMe₃;

+

positively charged, strongly electron-withdrawing functional

group), which hydrolyze signiﬁcantly faster, separating them

from all the other γ-silanes studied. All α-silanes studied not

only hydrolyze signiﬁcantly faster than any γ-silane but also

show a strong variation of their rate of hydrolysis compared to

the γ-silanes, strongly depending on the functional group and

spanning more than 3 orders of magnitude (k_exp= 5.82 × 10⁻²

to >90 h⁻¹). Interestingly, some exceptionally fast hydrolyses

We have also computed the electronic proton aﬃnity

energies of most of the silanes studied, both for the alkoxy

leaving group and the functional group. The diﬀerent model

structures used for these calculations are shown in Scheme 15.

The calculations were performed at the ZORA-OLYP/TZ2P

level of theory using COSMO for solvation in H₂O and

are observed for compounds 5a (α-NMe₂; k_exp> 90 h⁻¹), 6a

(α-N(H)COOMe; k_exp= 12.8 h⁻¹), and 8a (α-NMe₃; k_exp

=

+

84.7 h⁻¹), whereas a signiﬁcantly slower hydrolysis is observed

H

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a

Table 2. k_expValues at pD 5 and Calculated Proton Aﬃnities of the Silanes 1a−9a, 1b−6b, 8b, 9b, 11a−13a, and 13b

a

proton aﬃnity [kcal mol⁻¹

]

k_exp[h⁻¹

]

alkoxy leaving group

functional group

(in CH₃CN)

compound

(pD = 5)

(in H₂O)

(in CH₃CN)

(in H₂O)

b

1a (α-H)

1b (γ-H)

2a (α-Cl)

2b (γ-Cl)

11.2 2.1

5.15 2.61

1.02 0.09

5.08 1.50

5.76 0.49

6.16 2.04

5.23 0.76

6.88 1.66

>90

9.9

10.0

6.7

12.1

c

12.7

d

e

d

9.2

e

9.4

12.0

d

b

c

d

f

3a (α-OMe)

9.1

11.6

6.7

11.3

b

3b (γ-OMe)

14.7

6.6

17.4

b

h

i

4a (α-OAc)

9.1

11.7

13.7

13.9

42.5

41.2

19.5

19.9

22.0

c

i

4b (γ-OAc)

9.5

12.0

11.2

d

f

5a (α-NMe₂)

8.3

10.9

39.9

g

h

g

h

5b (γ-NMe₂)

>90

39.3

b

h

6a (α-N(H)COOMe, SiOMe)

6b (γ-N(H)COOMe)

7a (α-N(Me)COOMe)

49.6 11.8

5.20 2.72

>90

5.0

7.1

16.6

g

17.1

h

20.2

b

c

b

+

8a (α-NMe₃, SiOMe)

0.132 0.019

7.83 0.95

4.55 0.63

11.7 1.9

1.6

8.2

7.3

9.7

2.0

c

+

8b (γ-NMe₃)

9.5

d

e

d

f

9a (α-N(H)Ph)

9.3

27.5

28.4

31.4

e

g

9b (γ-N(H)Ph)

12.0

29.7

k

11a (α-N(H)COOMe, SiOEt)

128

k

+

12a (α-NMe₃, SiOEt)

74.0

d

c

d

f

13a (α-OnPr)

13b (γ-Et)

3.24 0.34

3.85 1.74

9.7

10.9

10.5

12.8

c

10.4

13.5

a

Electronic proton aﬃnity energy ΔE_PAcomputed at ZORA-OLYP/TZ2P using COSMO for solvation in H₂O and CH₃CN. In this context, see also

b

c

d

e

Scheme 15. Calculated for model structure IIIa. Calculated for model structure IIIb. Calculated for model structure IVa. Calculated for model

structure IVb. Calculated for model structure Va. Calculated for model structure Vb. Calculated for model structure VI. Calculated for the

protonation at the carbonyl oxygen atom without an intramolecular hydrogen bond. This k_expvalue was obtained by using one k_expvalue at pD 6.1

f

g

h

i

k

(for details, see Figure S56).

hydrolysis at t < 2 min). Generally, sterically more demanding

substituents of comparable functional groups, such as α-OnPr

(13a; k_exp= 2.48 × 10⁻¹h⁻¹) < α-OMe (3a; k_exp= 5.99 × 10⁻¹

h⁻¹) or γ-Et (13b; k_exp= 4.84 × 10⁻³h⁻¹) ≈ γ-H (1b; k_exp

=

3.26 × 10⁻³h⁻¹) < α-H (1a; k_exp= 5.82 × 10⁻²h⁻¹), decrease

the rate of hydrolysis.

The experimental results described above can be rationalized

as follows: under basic conditions, the rate of hydrolysis is

mainly controlled by the electrophilicity of the silicon center.

Thus, the rate of the hydrolysis of the γ-silanes should be less

inﬂuenced by the generally electronegative functional groups

than that of their corresponding α-silanes. Indeed, most of the

γ-silanes show very similar rates of hydrolysis, which is in

agreement with the reaction barriers calculated for 3b and 4b

(in H₂O: 13.8−14.5 kcal mol⁻¹; in CH₃CN: 13.3−14.4 kcal

mol⁻¹). In contrast, the rate constants k_expfor the α-silanes 1a−

4a and 8a are signiﬁcantly higher by a factor of 18 (1a/1b), 58

(2a/2b), 35 (3a/3b), 211 (4a/4b), and 570 (8a/8b),

respectively, than those for the corresponding γ-silanes 1b−

4b and 8b. The strength of this eﬀect correlates with the

strength of the electron-withdrawing power of the functional

group (NMe₃⁺> OAc > Cl > OMe > H), which is also reﬂected

by the reaction barriers calculated for 1a−4a (in H₂O: 11.2−

14.1 kcal mol⁻¹; in CH₃CN: 12.0−14.1 kcal mol⁻¹) as well as

by the energies relative to reactants of the intermediate

transition complexes (TC) calculated for 1a−4a and 8a (in

H₂O: 1.2−6.8 kcal mol⁻¹; in CH₃CN: −0.2−6.9 kcal mol⁻¹).

Nevertheless, there are some experimental observations that

need to be explained: (i) compounds 5b (γ-NMe₂) and 8b (γ-

Figure 7. log k_expvalues (including the error bars) of the silanes 1a−

■

□

9a, 1b−6b, 8b, 9b, 13a, and 13b at pD 11 ( α-silanes; gray γ-

silanes). Due to its very fast hydrolysis (complete hydrolysis at t < 2

min; k_exp> 90 h⁻¹), the k_expvalue of compounds 5a (α-NMe₂) could

not be determined (log k_exp> −1.60).

for compounds 7a (α-N(Me)COOMe; k_exp= 4.86 × 10⁻¹h⁻¹)

and 9a (α-N(H)Ph; k_exp= 3.10 h⁻¹). The rate constants of the

ethoxysilanes 11a (α-N(H)COOMe; k_exp= 4.81 h⁻¹) and 12a

(α-NMe₃; k_exp= 20.5 h⁻¹) are lower than those of the

+

corresponding methoxysilanes 6a (k_exp= 12.1 h⁻¹) and 8a (k_exp

= 84.7 h⁻¹). As observed for the methoxysilane 5a (α-NMe₂),

the rate constant of the corresponding ethoxysilane 10a (α-

NMe₂) could not be determined (k_exp> 90 h⁻¹; complete

+

NMe₃) show a signiﬁcantly faster hydrolysis than all the other

γ-silanes studied; (ii) due to a very fast hydrolysis, the rate

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constant for the hydrolysis of 5a (α-NMe₂) could not be

determined (k_exp> 90 h⁻¹; complete hydrolysis at t < 2 min);

(iii) the related (aminomethyl)silanes 5a (α-NMe₂) and 9a (α-

N(H)Ph) diﬀer signiﬁcantly in their rate constants; and (iv) the

NH/NMe analogues 6a (α-N(H)COOMe) and 7a (α-

5b and 10b are similar to the general range observed for the γ-

silanes (yellow bar in Figure 4) and ﬁt into the general

explanation given for the basic hydrolysis of γ-silanes, with

OH⁻being the nucleophile attacking a nonprotonated silane.

The very high reactivity of the α-silane 5a at pD > 11 is due to

the high electrophilicity at the silicon atom combined with

hydrogen-bond assistance toward the attacking hydroxide anion

similar to that described for 6a (see above). On the contrary,

the hydrolysis of 5a, 5b, 10a, and 10b (X = NMe2) up to the

pD of ∼11 is obviously governed by a diﬀerent mechanism

(with water being the nucleophile attacking a protonated

silane), which will be discussed below.

N(Me)COOMe) show very diﬀerent k_expvalues (6a: k_exp

=

12.8 h⁻¹; 7a: k_exp= 4.86 × 10⁻¹h⁻¹).

To understand the high reactivity of compound 8b (γ-

+

NMe₃), it is important to note that permanently positively

charged cationic species exist as ion pairs in solution. Under

basic conditions, both the original counteranion (I⁻) and the

hydroxide anion will be randomly mixed, so a signiﬁcant

fraction of all hydroxide anions present in solution will be

concentrated as ion pairs close to the electrophilic silicon

centers of 8b. This high local concentration of hydroxide can

explain the high rate of hydrolysis of the γ-silane 8b (k_exp= 1.49

× 10⁻¹h⁻¹). The same holds true for the α-silane 8a (α-

Due to their very high electrophilicity at the silicon center,

the α-silanes generally show much higher rates of hydrolysis

than the corresponding γ-silanes. Thus, the rate of hydrolysis of

the essentially unprotonated compounds 9a (α-N(H)Ph) and

9b (γ-N(H)Ph) is controlled by the electrophilicity of the

silicon center, which is much higher for the α-silane (k_exp= 3.10

NMe₃; k_exp= 84.7 h⁻¹), which is even more reactive due to its

+

h⁻¹) when compared with the corresponding γ-silane (k_exp

=

much higher electrophilicity compared with 8b. This is also

reﬂected by the relatively low energy of the intermediate

transition complex for 8a (in H₂O: 1.2 kcal mol⁻¹; in CH₃CN:

−0.2 kcal mol⁻¹) as compared to those of other α-silanes, such

as 4a (α-OAc; in H₂O: 3.6 kcal mol⁻¹; in CH₃CN: 3.8 kcal

mol⁻¹).

The α-silane 6a contains an NH moiety in its functional

group (N(H)COOMe), enabling the formation of hydrogen

bonds to the entering nucleophile (see Figure 8), which results

1.65 × 10⁻²h⁻¹). In addition, the NH moiety of the α-silane 9a

seems to assist the attacking hydroxide anion through hydrogen

bonds similarly to the α-silane 6a, which also results in a

signiﬁcantly lower reaction barrier (in H₂O: 8.9 kcal mol⁻¹; in

CH₃CN: 8.4 kcal mol⁻¹), thus explaining the higher hydrolysis

rate of 9a (α-N(H)Ph; k_exp= 3.10 h⁻¹) compared to the more

electronegatively substituted α-silane 2a (α-Cl; k_exp= 1.09 h⁻¹).

This is supported by the computed reaction barriers, which are

lower for 9a (in H₂O: 8.9 kcal mol⁻¹; in CH₃CN: 8.4 kcal

mol⁻¹) than for 2a (in H₂O: 12.6 kcal mol⁻¹; in CH₃CN: 12.0

kcal mol⁻¹).

Kinetics of Hydrolysis under Acidic Conditions.

Discussion of Experimental and Theoretical Results.

Figure 9 shows the log k_expvalues of all the model compounds

studied at pD 5 (except for 11a and 12a), ranked in ascending

order. The pD-dependence of the hydrolysis rate under acidic

conditions is negative for all silanes, which is in general

agreement with H₂O being the nucleophile attacking the

protonated silane (see Scheme 14). Generally, most γ-silanes

Figure 8. Calculated structure of the pretransition state in the S_N2@Si

reaction of OH⁻+ 6a in acetonitrile, stabilized by an N−H···O

hydrogen bond (for details, see the Supporting Informations).

in a signiﬁcantly increased reactivity (k_exp= 12.8 h⁻¹). This

experimental result is in good agreement with the calculated

low reaction barrier (in H₂O: 6.3 kcal mol⁻¹; in CH₃CN: 5.9

kcal mol⁻¹). Such kind of hydrogen bond assisted nucleophilic

substitution is excluded by the exchange of the NH moiety of

6a by an NMe moiety (→ 7a), and indeed, compound 7a (α-

N(Me)COOMe; k_exp= 4.86 × 10⁻¹h⁻¹) is much less reactive

than 6a.

The amines investigated in this study are characterized by a

vastly diﬀerent basicity (5a, α-NMe₂: pK_A9.3; 5b, γ-NMe₂: pK_A

9.0; 9a, α-N(H)Ph: pK_A3.2; 9b, γ-N(H)Ph: pK_A3.0).³¹

According to their basicities, silanes 5a and 5b will be present in

signiﬁcant fractions (∼50% even at pD 9) as protonated

species, while 9a and 9b will be essentially unprotonated. This

behavior is in agreement with the rate constants of the γ-silanes

5b (γ-NMe₂, SiOMe) and 10b (γ-NMe₂, SiOEt) under basic

conditions (Figure 4), which show a changing dependency on

the pD value in the range pD 8−12, clearly indicating a change

in the dominating mechanism. At pD > 11, the rate constants of

Figure 9. log k_expvalues (including the error bars) of the silanes 1a−

■

□

γ-

9a, 1b−6b, 8b, 9b, 13a, and 13b at pD 5 ( α-silanes; gray

silanes). Due to their very fast hydrolysis (complete hydrolysis at t < 2

min; k_exp> 90 h⁻¹), the k_expvalues of compounds 5a (α-NMe₂), 5b (γ-

NMe₂), and 7a (α-N(Me)COOMe) could not be determined (log k_exp

> −1.60).

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(1b−4b, 6b, and 8b) show almost identical rates of hydrolysis

(k_exp= 5.08−7.83 h⁻¹). Their rate is consistently slower than

that of methylsilane 1a (α-H; k_exp= 11.2 h⁻¹) but somewhat

10⁻¹to 11.2 h⁻¹) and the calculated proton aﬃnities of the

methoxy leaving group (1.6−9.9 kcal mol⁻¹(H₂O); 2.0−12.1

kcal mol⁻¹(CH₃CN)) are signiﬁcantly more pronounced than

faster than that of the long-chain alkylsilane 13b (γ-Et; k_exp

3.85 h⁻¹). Notable exceptions are compounds 9b (γ-N(H)Ph;

_exp= 11.7 h⁻¹) and 5b (γ-NMe₂; k_exp> 90 h⁻¹), which show a

=

those for the corresponding γ-silanes 1b−4b and 8b (k_exp

=

5.08−7.83 h⁻¹; proton aﬃnity = 8.2−10.0 kcal mol⁻¹(H₂O)

and 9.5−12.7 kcal mol⁻¹(CH₃CN)). The slow hydrolysis of

k

+

the α-silanes 2a (α-Cl; k_exp= 1.02 h⁻¹) and 8a (α-NMe₃; k_exp

=

much faster hydrolysis than all the other γ-silanes. The α-silanes

show a strong variation of their hydrolysis rate depending on

the functional group and spanning more than 3 orders of

magnitude (k_exp= 1.32 × 10⁻¹to >90 h⁻¹). Contrary to the

basic regime, where all α-silanes hydrolyze faster than the

corresponding γ-silanes, the rates of acidic hydrolysis of the α-

silanes do not show such a general correlation; for example, α is

1.32 × 10⁻¹h⁻¹), with their strong electron-withdrawing

functional group, can be correlated with the proton aﬃnities of

the methoxy leaving group (2a: proton aﬃnity = 6.7 (H₂O) or

9.2 (CH₃CN) kcal mol⁻¹; 8a: proton aﬃnity = 1.6 (H₂O) or

2.0 (CH₃CN) kcal mol⁻¹). Compared to the α-silane 8a, the

cationic functional group of the γ-silane 8b seems to have only

little inﬂuence on the protonation of the methoxy leaving

group.

+

slower than γ for X = NMe₃, α is similar to γ for X = OMe and

OAc, and α is faster than γ for X = N(H)COOMe. In contrast

to the behavior observed under basic conditions, the cationic α-

While the above-described mechanism applies for all silanes

that are mainly protonated at the alkoxy leaving group (see eqs

3 and 4), it cannot explain the fast hydrolysis of the α-silanes

5a−7a and 9a and the γ-silanes 5b and 9b. For α-silanes with

basic functional groups (NMe₂, N(H)Ph, N(H)COOMe, or

N(Me)COOMe), an additional mechanism has to be

considered. For these silanes, a small or minor (N(H)COOMe

< N(Me)COOMe < N(H)Ph) or even complete (NMe₂)

amount of the silane concentration is protonated at the

functional group, which results in a cationic substituent bearing

a hydrogen atom being available for hydrogen bonding to the

leaving group. As hydrogen bonds preferably constitute an

approximately linear bond between the heteroatoms involved,

this gives rise to the formation of (i) distorted seven-membered

rings for the γ-silanes 5b and 9b (see model structure Vb,

Scheme 15) and for the α-silanes 6a and 7a (see model

structure VI, Scheme 15) and (ii) a distorted (less favorable)

ﬁve-membered ring for the α-silanes 5a and 9a (see model

structure Va, Scheme 15). This hydrogen bond toward the

leaving group, which in this case is not protonated (no doubly

protonated species are expected), should strongly assist the

release of the leaving group, preventing the generation of an

anionic methoxy group.

When considering this particular mechanism, the rate of

hydrolysis should depend on (i) the equilibrium concentration

of the protonated substituent, (ii) the equilibrium concen-

tration of the protonated silane forming a hydrogen bond

toward the leaving group, and (iii) the electrophilicity at the

silicon center, which is expected to be signiﬁcantly higher for

the α-silanes than for the corresponding γ-silanes.

Under acidic conditions, compounds 5a (α-NMe₂; pK_A9.3)

and 5b (γ-NMe₂; pK_A9.0)³¹are completely protonated. Thus,

compounds 5a and 5b show extremely high rates of hydrolysis

(k_exp> 90 h⁻¹). The protonation leads to ﬁve- and seven-

membered rings, respectively, with an intramolecular N−H···O

hydrogen bond to the nonprotonated leaving group (model

structures Va and Vb, Scheme 15). Although the distorted ﬁve-

membered ring of the α-silane 5a is disadvantageous compared

with the seven-membered ring of the corresponding γ-silane 5b,

the proton aﬃnities of 5a (39.9 kcal mol⁻¹(H₂O); 42.5 kcal

mol⁻¹(CH₃CN)) are slightly higher than those of 5b (39.3 kcal

mol⁻¹(H₂O); 41.2 kcal mol⁻¹(CH₃CN)). Together with the

higher electrophilicity of 5a this explains the higher reactivity of

the α-silane 5a compared to the γ-silane 5b. It should be noted

that this mechanism with water attacking the protonated silane

also applies for basic pD values as even at pD well above pK_Aa

large fraction of 5a and 5b will be present as protonated silane.

silane 8a (α-NMe₃; k_exp= 1.32 × 10⁻¹h⁻¹) is the least reactive

+

silane under acidic conditions. Surprisingly, the related α-silane

5a (α-NMe₂; k_exp> 90 h⁻¹), being completely protonated at the

functional group and therefore being cationic as well, is the

most reactive silane. The corresponding ethoxysilane 10a (α-

NMe₂; SiOEt) shows also a very fast hydrolysis (k_exp> 90 h⁻¹;

for details, see Figure S56). These ﬁndings indicate that the

proton at the functional group of 5a (and of 5b as well) is

directly involved in the reaction mechanism, which is

responsible for the extremely fast hydrolysis of compounds

5a and 5b. Interestingly, exceptionally fast hydrolyses are also

observed for the α-silanes 6a (α-N(H)COOMe; k_exp= 49.6

h⁻¹) and 7a (α-N(Me)COOMe; k_exp> 90 h⁻¹), whereas a

signiﬁcantly slower hydrolysis is observed for 2a (α-Cl; k_exp

=

1.02 h⁻¹). Generally, sterically more demanding substituents of

comparable functional groups, such as α-OnPr (13a; k_exp= 3.24

h⁻¹) < α-OMe (3a; k_exp= 5.76 h⁻¹) or γ-Et (13b; k_exp= 3.85

h⁻¹) ≈ γ-H (1b; k_exp= 5.15 h⁻¹) < α-H (1a; k_exp= 11.2 h⁻¹),

decrease the rate of hydrolysis, as also observed under basic

conditions. As pointed out above, in previous theoretical

analyses, the retarding eﬀect of bulky groups has been traced to

an earlier onset, along the reaction coordinate, of Pauli

repulsion between closed-shell orbitals of nucleophile and

substrate.^{28 29a}

a,

The experimental results described above can be rationalized

as follows: under acidic conditions, the rate of hydrolysis

depends on both the electrophilicity of the silicon center

(determining the molecular reactivity) and the concentration of

the (protonated) reactive species (Scheme 14). Both, the

electrophilicity of the silicon center and the proton aﬃnity of

the alkoxy leaving group depend on the strength of the

electron-withdrawing eﬀect of the functional group. These two

inverse eﬀects of the functional group may compensate each

other. As the silane protonated at the leaving group is the

reactive species, the electrophilicity of the protonated silane is

dominated by the protonated leaving group, whereas the

electronegativity of the functional group plays a minor role.

Therefore, only little variation of the microscopic reactivity of

the α-silanes compared with the corresponding γ-silanes is to be

expected. The concentration of the reactive protonated silane

will be strongly inﬂuenced by the basicity (proton aﬃnity) of

the leaving group, which is aﬀected by the electron-withdrawing

eﬀect of the functional group (which can also be protonated in

some cases). Indeed, both the experimental rate constants k_exp

and the calculated proton aﬃnities for the compounds studied

support this assumption. Thus, in the case of the α-silanes 1a−

4a and 8a, the diﬀerences in the rate constants (k_exp= 1.32 ×

K

dx.doi.org/10.1021/om500073m | Organometallics XXXX, XXX, XXX−XXX

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Contrarily, compounds 9a (α-N(H)Ph; pK_A3.2) and 9b (γ-

N(H)Ph; pK_A3.0)³¹are signiﬁcantly less protonated, resulting

in a signiﬁcantly lower tendency to form a hydrogen bond

toward the leaving group. Again, the ﬁve-membered ring of the

protonated α-silane 9a is disadvantageous compared to the

more stable seven-membered ring of the γ-silane 9b, but in this

case the proton aﬃnities of 9b are signiﬁcantly higher (29.7

kcal mol⁻¹(H₂O); 31.4 kcal mol⁻¹(CH₃CN)) than those of 9a

(27.5 kcal mol⁻¹(H₂O); 28.4 kcal mol⁻¹(CH₃CN)), resulting

in an increase of the hydrolysis rate of the γ-silane 9b compared

above-mentioned speciﬁc interactions of the functional group

with the alkoxy leaving group or the attacking nucleophile,

which gives them a signiﬁcantly increased reactivity over a

broad range of pH both for α- and (to a lesser extent) for γ-

silanes. The insight gained from this work allows for a much

clearer conceptual understanding of the reactivity of α- and γ-

silanes in moisture-curing applications. Due to the broad

variation of functional groups and resulting reactivity eﬀects,

our experimental and computational study of the mechanisms

of hydrolysis of selected model compounds constitutes a

general understanding of the rate of hydrolysis as a function of

both type and position of the functional group and pH value.

Therefore, this knowledge will be a valuable guidance for the

further development of moisture-curing silane cross-linking and

adhesion technology and their application, leading to tailor-

made materials and properties.

to the corresponding α-silane (9a: k_exp= 4.55 h⁻¹; 9b: k_exp

=

11.7 h⁻¹). Similarly, the α-silanes 6a (α-N(H)COOMe) and 7a

(α-N(Me)COOMe) are partially protonated at the carbonyl-

oxygen atom (see model structure VI, Scheme 15), giving rise

to the formation of a more favored seven-membered ring,

combined with a more electrophilic silicon center, thus leading

to very fast (6a: k_exp= 49.6 h⁻¹; 7a: k_exp> 90 h⁻¹) rates of

hydrolysis for these α-silanes contrary to the corresponding γ-

silane 6b. Substitution of the N(H)COOMe group (6a) by the

more basic N(Me)COOMe moiety (7a) results in an increase

of the rate of hydrolysis, which is in agreement with the

diﬀerent proton aﬃnities of the functional groups (6a: proton

aﬃnity =16.6 (H₂O) or 19.5 (CH₃CN) kcal mol⁻¹; 7a: proton

aﬃnity = 20.2 (H₂O) or 22.0 (CH₃CN) kcal mol⁻¹).

EXPERIMENTAL SECTION

■

Syntheses and Characterization. General Procedures. All

reactions involving chemicals sensitive to water and/or oxygen were

carried out under a dry argon atmosphere. The organic solvents used

were dried and puriﬁed according to standard procedures and stored

under dry nitrogen. Starting materials, reagents, and solvents were

purchased from Aldrich, ABCR, or Acros. The ¹H, ¹³C, and ²⁹Si NMR

spectra were recorded at 23 °C using a Bruker DRX-300 (¹H, 300.1

MHz; ¹³C, 75.5 MHz; ²⁹Si, 59.6 MHz) or Bruker Avance 500 NMR

spectrometer (¹H, 500.1 MHz; ¹³C, 125.8 MHz; ²⁹Si, 99.4 MHz).

C₆D₆or [D₆]DMSO served as the solvent. Chemical shifts (ppm)

were determined relative to internal C₆HD₅(¹H, δ 7.28), [D₅]DMSO

(¹H, δ 2.49), C₆D₆(¹³C, δ 128.0), or [D₆]DMSO (¹³C, δ 39.5), or

CONCLUSION

■

To understand the silicon α-eﬀect in terms of an enhanced

hydrolysis reactivity of the Si−OC bond of α-silanes of the

formula type ROSiMe₂CH₂X compared to analogous γ-silanes

ROSiMe₂(CH₂)₃X (R = Me, Et; X = functional group), a

systematic experimental and computational study of the

kinetics and mechanisms of hydrolysis of such compounds

was performed. For this purpose, a series of suitable model

compounds was synthesized and studied for their hydrolysis

kinetics in CD₃CN/D₂O under basic and acidic conditions,

using ¹H NMR spectroscopy as the analytical tool. To get more

information about the impact of (i) the functional group X, (ii)

the spacer between the silicon atom and the functional group

(CH₂vs (CH₂)₃), and (iii) the pD value on the mechanisms of

hydrolysis, the experimental investigations were complemented

by computational studies.

external TMS (²⁹Si, δ 0.0). Analysis and assignment of the H NMR

1

spectroscopic data were supported by H,¹H COSY, ¹³C,¹H HMQC,

1

¹³C,¹H HMBC, and ²⁹Si,¹H HMQC (optimized for J_SiH= 7 Hz)

2

experiments. The AA′M₃XX′, A₃M₂X₂, AA′FF′MM′X, and

AA′MM′XX′ spin systems observed for the compounds synthesized

were analyzed by using the WIN-DAISY 4.05 software package.³²

Assignment of the ¹³C NMR spectroscopic data was supported by

DEPT-135 and the aforementioned ¹³C,¹H correlation experiments.

Elemental analyses were performed using a VarioMicro apparatus

(Elementar Analysensysteme GmbH). ESI-HRMS spectra were

recorded on a Bruker MicrOTOF instrument using solutions in

acetonitrile.

Methoxytrimethylsilane (1a). This compound was commercially

Generally, the trends in hydrolysis reactivity evolving from

the kinetic experiments and those obtained from the

computations agree well. The results of these studies clearly

demonstrate that the silicon α-eﬀect cannot be rationalized in

terms of a special single eﬀect. Instead, the diﬀerent hydrolysis

reactivities observed are the result of a number of components,

such as electronic and steric eﬀects, the strong impact of the pD

value, and hydrogen bonds between the functional group (or

even protonated functional group) and the alkoxy leaving

group, which aﬀect the hydrolysis reactivity of the α- and γ-

silanes in an analogous or inverse manner. That is, the observed

hydrolysis reactivity is the result of the summation of all these

eﬀects and cannot be correlated with one particular silicon α-

eﬀect. Therefore, the term “silicon α-eﬀect” should not be used

at all furthermore to explain the hydrolysis reactivity of α- or γ-

silanes, and the many decades long lasting debate on this issue

should be ﬁnished, at least as far as the hydrolysis reactivity at

the silicon atom is concerned.

available and was puriﬁed by distillation prior to use.

Methoxydimethyl(n-propyl)silane (1b). A 2.0 M solution of n-

propylmagnesium bromide in diethyl ether (62.4 mL, 125 mmol of n-

PrMgBr) was added dropwise at 0 °C within 2 h to a stirred solution

of 14 (15.0 g, 125 mmol) in diethyl ether (250 mL). After the mixture

had been stirred at 20 °C for 17 h, the solvent was removed by

distillation at ambient pressure. Subsequently, n-pentane (300 mL)

was added, the mixture was stirred for 30 min, and the solid was

ﬁltered oﬀ, washed with n-pentane (3 × 25 mL), and discarded. The

solvent of the ﬁltrate (including the wash solutions) was removed by

distillation at ambient pressure, and the residue was puriﬁed by

distillation at ambient pressure to furnish 1b as a colorless liquid (14.0

g, 106 mmol; 85% yield). Bp: 98 °C. ¹H NMR (300.1 MHz, C₆D₆): δ

0.19 (s, 6 H; SiCH₃), 0.68 (δ_A), 1.09 (δ_M), and 1.50 (δ_X) (AA′M₃XX′

system, ²J_AA′= 14.1 Hz, ³J_AX= 11.1 Hz, ³J_AX′= 5.4 Hz, ²J_XX′= 15.2 Hz,

3

³J_MX= J_MX′= 7.3 Hz, 7 H; SiCH_AH_A′CH_XH_X′C(H_M)₃), 3.39 (s, 3 H;

OCH₃). ¹³C NMR (75.5 MHz, C₆D₆): δ −2.5 (SiCH₃), 17.1

(CH₂CH₃), 18.3 (CH₂CH₃), 18.9 (SiCH₂), 49.9 (OCH₃). ²⁹Si NMR

(59.6 MHz, C₆D₆): δ 17.7. Anal. Calcd for C₆H₁₆OSi (132.28): C,

54.48; H, 12.19. Found: C, 54.69; H, 12.13.

Not only is this ﬁnding important for basic research, it also

has consequences for the silicone industry. It is interesting to

note that the commercially relevant organofunctional α- and γ-

silanes with amino, urea, and carbamato groups show the

(Chloromethyl)methoxydimethylsilane (2a). This compound was

commercially available and was puriﬁed by distillation prior to use.

(3-Chloroprop-1-yl)methoxydimethylsilane (2b). This compound

was commercially available and was puriﬁed by distillation prior to use.

L

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Methoxydimethyl(methoxymethyl)silane (3a). A solution of

ethylmethylamine (9.95 g, 1.68 mmol) in THF (20 mL) was added

dropwise at 0 °C within 2 h to a stirred solution of 16 (30.2 g, 168

mmol) and ethyldimethylamine (12.3 g, 168 mmol) in THF (200

mL). After the mixture had been stirred at 0 °C for 2 h and at 20 °C

for 2 h, a 3.0 M solution of methylmagnesium bromide in diethyl ether

(140 mL, 420 mmol of MeMgBr) was added dropwise at 0 °C within

1 h. The reaction mixture was stirred at 20 °C for 16 h and was then

concentrated to a volume of ca. 50 mL by distillation at ambient

pressure, n-pentane (250 mL) was added, and the mixture was stirred

at 20 °C for 30 min. The resulting solid was ﬁltered oﬀ, washed with n-

pentane (3 × 25 mL), and discarded. The solvent of the ﬁltrate

(including the wash solutions) was removed by distillation at ambient

pressure, and the residue was distilled at ambient pressure to give three

fractions (20, 1.97 g, bp 93−105 °C; 18, 4.47 g, bp 110−121 °C; 19,

3.17 g, bp 140−147 °C (discarded)).

Transformation A: A 1.25 M solution of hydrogen chloride in

methanol (1.14 mL, 1.43 mmol of HCl, 29.6 mmol methanol) was

added dropwise at 20 °C within 15 min to a stirred solution of 18

(4.47 g, 27.7 mmol) in n-pentane (30 mL), and the resulting mixture

was stirred at 20 °C for 30 min. The resulting solid was ﬁltered oﬀ,

washed with n-pentane (3 × 5 mL), and discarded. The solvent of the

ﬁltrate (including the wash solutions) was removed by distillation at

ambient pressure, and the residue was distilled at ambient pressure to

give 2.12 g of 3a as a crude distillate (bp 87−91 °C).

at 20 °C for 30 min. The resulting solid was ﬁltered oﬀ, washed with n-

pentane (3 × 20 mL), and discarded. The solvent of the ﬁltrate

(including the wash solutions) was removed by distillation at ambient

pressure, and the residue was puriﬁed by distillation in vacuo to furnish

4a as a colorless liquid (8.68 g, 53.5 mmol; 74% yield). Bp: 67 °C/20

mbar). ¹H NMR (300.1 MHz, C₆D₆): δ 0.19 (s, 6 H; SiCH₃), 1.81 (s,

3 H; C(O)CH₃), 3.36 (s, 3 H; OCH₃), 3.92 (s, 2 H; SiCH₂). ¹³C

NMR (75.5 MHz, C₆D₆): δ −3.7 (SiCH₃), 20.2 (C(O)CH₃), 50.3

(OCH₃), 56.2 (SiCH₂), 170.7 (C(O)CH₃). ²⁹Si NMR (59.6 MHz,

C₆D₆): δ 12.5. Anal. Calcd for C₆H₁₄O₃Si (162.26): C, 44.41; H, 8.70.

Found: C, 44.53; H, 8.77.

(3-Acetoxyprop-1-yl)methoxydimethylsilane (4b). Sodium acetate

(14.8 g, 180 mmol) was added at 20 °C in a single portion to a stirred

solution of 2b (10.0 g, 60.0 mmol) and tetra-n-butylphosphonium

chloride (1.77 g, 6.00 mmol) in THF (200 mL). The reaction mixture

was heated under reﬂux for 3 d and was then concentrated to a volume

of ca. 50 mL by distillation at ambient pressure, n-pentane (250 mL)

was added, and the mixture was stirred at 20 °C for 30 min. The

resulting solid was ﬁltered oﬀ, washed with n-pentane (3 × 20 mL),

and discarded. The solvent of the ﬁltrate (including the wash

solutions) was removed by distillation at ambient pressure, and the

residue was puriﬁed by distillation in vacuo to furnish 4a as a colorless

1

liquid (8.90 g, 46.8 mmol; 78% yield). Bp: 91 °C/12 mbar. H NMR

(500.1 MHz, C₆D₆): δ 0.12 (s, 6 H; SiCH₃), 0.55 (δ_A), 1.68 (δ_M), and

4.10 (δ_X) (AA′MM′XX′ system, ²J_AA′= 14.7 Hz, ³J_AM= 5.0 Hz, ³J_AM′

=

2

3

2

Transformation B: Methanol (455 mg, 14.2 mmol) was added

dropwise to a stirred solution of 20 (1.97 g, 14.2 mmol) and

ethyldimethylamine (1.04 g, 14.2 mmol) in n-pentane (15 mL) at 20

°C within 15 min, and the mixture was stirred at 20 °C for 30 min.

The resulting solid was ﬁltered oﬀ, washed with n-pentane (3 × 3 mL),

and discarded. The solvent of the ﬁltrate (including the wash

solutions) was removed by distillation at ambient pressure, and the

residue was distilled at ambient pressure to give 1.18 g of 3a as a crude

distillate (bp 84−91 °C).

12.0 Hz, J_MM′= 13.7 Hz, J_MX= 6.4 Hz, J_MX′= 7.5 Hz, J_XX′= 10.8

Hz, 6 H; SiCH_AH_A′CH_MH_M′CH_XH_X′), 1.82 (s, 3 H; C(O)CH₃), 3.33

(s, 3 H; OCH₃). ¹³C NMR (125.8 MHz, C₆D₆): δ −2.7 (SiCH₃), 12.1

(SiCH₂), 20.5 (C(O)CH₃), 22.9 (SiCH₂CH₂), 50.0 (SiOCH₃), 66.7

(CH₂O), 170.1 (C(O)CH₃). ²⁹Si NMR (99.4 MHz, C₆D₆): δ 17.8.

Anal. Calcd for C₈H₁₈O₃Si (190.31): C, 50.49; H, 9.53. Found: C,

50.15; H, 9.54.

[(Dimethylamino)methyl]methoxydimethylsilane (5a). This com-

pound was synthesized according to ref 11, with slight modiﬁcations. A

mixture of 2a (19.2 g, 138 mmol) and dimethylamine (15.6 g, 346

mmol) was heated in an autoclave at 100 °C for 8 h. The reaction

mixture was allowed to cool to 20 °C within 30 min, n-pentane (400

mL) was added, and the resulting precipitate was ﬁltered oﬀ, washed

with n-pentane (3 × 20 mL), and discarded. The solvent of the ﬁltrate

(including the wash solutions) was removed by distillation at ambient

pressure, and the residue was puriﬁed by distillation at ambient

pressure to furnish 5a as a colorless liquid (15.3 g, 104 mmol; 75%

The product fractions obtained after the transformations A and B

were combined and ﬁnally puriﬁed by distillation at ambient pressure

to furnish 3a as a colorless liquid (1.93 g, 14.4 mmol; 9% yield). Bp: 90

1

°C. H NMR (500.1 MHz, C₆D₆): δ 0.28 (s, 6 H; SiCH₃), 3.14 (s, 2

H; SiCH₂), 3.29 (COCH₃), 3.47 (s, 3 H; SiOCH₃). ¹³C NMR (125.8

MHz, C₆D₆): δ −3.7 (SiCH₃), 50.5 (SiOCH₃), 63.0 (COCH₃), 66.2

(SiCH₂). ²⁹Si NMR (99.4 MHz, C₆D₆): δ 12.1. Anal. Calcd for

C₅H₁₄O₂Si (134.25): C, 44.73; H, 10.51. Found: C, 45.12; H, 10.30.

Methoxy(3-methoxyprop-1-yl)dimethylsilane (3b). Methanol

(4.10 g, 128 mmol) was added dropwise within 20 min to sodium

(1.39 g, 60.5 mmol), and the resulting mixture was heated under reﬂux

for 1 h and was then allowed to cool to 20 °C, followed by the

addition of methanol (25 mL). Subsequently, compound 2b (9.15 g,

54.9 mmol) was added dropwise under reﬂux within 20 min to the

stirred MeONa/MeOH mixture, which was heated under reﬂux for a

further 4 d and then allowed to cool to 20 °C. n-Pentane (100 mL)

was added, and the resulting precipitate was ﬁltered oﬀ, washed with n-

pentane (3 × 15 mL), and discarded. The solvent of the ﬁltrate

(including the wash solutions) was removed by distillation at ambient

pressure, and the residue was puriﬁed by distillation in vacuo to furnish

3b as a colorless liquid (4.80 g, 29.6 mmol; 54% yield). Bp: 45 °C/4

mbar. ¹H NMR (500.1 MHz, C₆D₆): δ 0.18 (s, 6 H; SiCH₃), 0.74 (δ_A),

1

yield). Bp: 125 °C. H NMR (300.1 MHz, C₆D₆): δ 0.27 (s, 6 H;

SiCH₃), 1.94 (s, 2 H; SiCH₂), 2.32 (s, 6 H; NCH₃), 3.41 (s, 3 H;

OCH₃). ¹³C NMR (75.5 MHz, C₆D₆): δ −2.6 (SiCH₃), 49.7 (NCH₃),

50.1 (OCH₃), 50.9 (SiCH₂). ²⁹Si NMR (59.6 MHz, C₆D₆): δ 14.6.

Anal. Calcd for C₆H₁₇NOSi (147.29): C, 48.93; H, 11.63; N, 9.51.

Found: C, 48.65; H, 11.59; N, 9.30.

[3-(Dimethylamino)prop-1-yl]methoxydimethylsilane (5b). A

mixture of 2b (10.0 g, 60.0 mmol) and dimethylamine (13.5 g, 299

mmol) was heated in an autoclave at 100 °C for 8 h. The reaction

mixture was allowed to cool to 20 °C within 30 min, n-pentane (500

mL) was added, and the resulting precipitate was ﬁltered oﬀ, washed

with n-pentane (3 × 30 mL), and discarded. The solvent of the ﬁltrate

(including the wash solutions) was removed by distillation at ambient

pressure, and the residue was puriﬁed by distillation in vacuo to furnish

5b as a colorless liquid (6.80 g, 38.8 mmol; 65% yield). Bp: 57 °C/5

mbar. ¹H NMR (500.1 MHz, C₆D₆): δ 0.20 (s, 6 H; SiCH₃), 0.74 (δ_A),

1.77 (δ_M), and 3.33 (δ_X) (AA′MM′XX′ system, ²J_AA′= 15.4 Hz, ³J_AM

=

5.1 Hz, ³J_AM′= 11.8 Hz, ²J_MM′= 14.4 Hz, ³J_MX= 6.6 Hz, ³J_MX′= 6.6 Hz,

²J_XX′= 11.9 Hz, 6 H; SiCH_AH_A′CH_MH_M′CH_XH_X′), 3.27 (s, 3 H;

COCH₃), 3.38 (s, 3 H; SiOCH₃). ¹³C NMR (125.8 MHz, C₆D₆): δ

−2.6 (SiCH₃), 12.4 (SiCH₂), 23.8 (SiCH₂CH₂), 50.0 (SiOCH₃), 58.2

(COCH₃), 75.4 (CH₂O). ²⁹Si NMR (99.4 MHz, C₆D₆): δ 18.2. Anal.

Calcd for C₇H₁₈O₂Si (162.30): C, 51.80; H, 11.18. Found: C, 51.81;

H, 11.30.

1.67 (δ_M), and 2.32 (δ_X) (AA′MM′XX′ system, ²J_AA′= 14.7 Hz, ³J_AM

=

5.2 Hz, ³J_AM′= 11.6 Hz, ²J_MM′= 13.5 Hz, ³J_MX= 8.5 Hz, ³J_MX′= 5.9 Hz,

²J_XX′= 11.7 Hz, 6 H; SiCH_AH_A′CH_MH_M′CH_XH_X′), 2.24 (s, 6 H;

NCH₃), 3.40 (s, 3 H; OCH₃). ¹³C NMR (125.8 MHz, C₆D₆): δ −2.5

(SiCH₃), 13.7 (SiCH₂), 21.8 (SiCH₂CH₂), 45.5 (NCH₃), 50.0

(OCH₃), 63.1 (CH₂N). ²⁹Si NMR (99.4 MHz, C₆D₆): δ 18.2. Anal.

Calcd for C₈H₂₁NOSi (175.35): C, 54.80; H, 12.07; N, 7.99. Found:

C, 54.62; H, 12.19; N, 8.02.

(Acetoxymethyl)methoxydimethylsilane (4a). Sodium acetate

(17.7 g, 216 mmol) was added at 20 °C in a single portion to a

stirred solution of 2a (10.0 g, 72.1 mmol) in THF (200 mL). The

reaction mixture was heated under reﬂux for 1 d and was then

concentrated to a volume of ca. 50 mL by distillation at ambient

pressure, n-pentane (250 mL) was added, and the mixture was stirred

Methoxy{[(methoxycarbonyl)amino]methyl}dimethylsilane (6a).

A solution of 2a (10.0 g, 72.1 mmol) and methanol (2.43 g, 75.8

mmol) in N,N-dimethylformamide (10 mL) was added dropwise to a

boiling suspension of potassium cyanate (6.14 g, 75.7 mmol) in N,N-

M

dx.doi.org/10.1021/om500073m | Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

dimethylformamide (20 mL) at 145 °C ( 5 °C, temperature

measurement within the ﬂask) within 30 min. The reaction mixture

was allowed to cool to 20 °C within 1 h and was then stirred at this

temperature for 17 h, diethyl ether (150 mL) was added, and the

mixture was stirred at 20 °C for 30 min. The resulting precipitate was

ﬁltered oﬀ, washed with diethyl ether (3 × 30 mL), and discarded. The

solvent of the ﬁltrate (including the wash solutions) was removed by

distillation at ambient pressure, and the residue was puriﬁed by

distillation in vacuo to furnish 6a as a colorless liquid (3.60 g, 20.3

mmol; 28% yield). Bp: 52 °C/0.02 mbar. ¹H NMR (300.1 MHz,

Calcd for C₇H₁₇NO₃Si (191.30): C, 43.95; H, 8.96; N, 7.32. Found: C,

43.82; H, 9.01; N, 7.50.

[(Methoxydimethylsilyl)methyl]trimethylammonium Iodide (8a).

Freshly distilled iodomethane (14.5 g, 102 mmol) was added dropwise

at 20 °C within 30 min under exclusion of light to a stirred solution of

5a (3.00 g, 20.4 mmol) in acetonitrile (50 mL). The reaction mixture

was heated under reﬂux for 2 h, cooled to 20 °C within 30 min, and

then stirred at this temperature for 18 h. The solvent and the excess

iodomethane were removed under reduced pressure, the residue was

washed with n-pentane (4 × 15 mL), and the wash solutions were

discarded. Acetonitrile (15 mL) was added to the residue, and the

resulting suspension was heated under reﬂux until a clear solution was

obtained, which was kept undisturbed at 20 °C for 3 h and then at −20

°C for a further 17 h. The product was isolated by ﬁltration at −20 °C,

washed with n-pentane (5 mL), and dried in vacuo (0.01 mbar, 40 °C,

4 h) to give 8a as a colorless solid (4.89 g, 16.9 mmol; 83% yield). ¹H

NMR (300.1 MHz, [D₆]DMSO): δ 0.28 (s, 6 H; SiCH₃), 3.17 (s, 9 H;

NCH₃), 3.21 (s, 2 H; SiCH₂), 3.43 (s, 3 H; OCH₃). ¹³C NMR (75.5

MHz, [D₆]DMSO): δ −1.7 (SiCH₃), 50.4 (OCH₃), 56.1 (t, ¹J_NC= 3.6

3

C₆D₆): δ 0.12 (s, 6 H; SiCH₃), 2.78 (d, J_HH= 5.3 Hz, 2 H; SiCH₂),

3.27 (s, 3 H; SiOCH₃), 3.60 (s, 3 H; C(O)OCH₃), 4.65 (br s, 1 H;

NH). ¹³C NMR (75.5 MHz, C₆D₆): δ −3.5 (SiCH₃), 30.3 (SiCH₂),

50.1 (SiOCH₃), 51.8 (C(O)OCH₃), 157.9 (C(O)OCH₃). ²⁹Si NMR

(59.6 MHz, C₆D₆): δ 14.6. Anal. Calcd for C₆H₁₅NO₃Si (177.28): C,

40.65; H, 8.53; N, 7.90. Found: C, 40.41; H, 8.44; N, 8.00.

Methoxy{3-[(methoxycarbonyl)amino]prop-1-yl}dimethylsilane

(6b). A solution of 2b (10.0 g, 60.0 mmol) and methanol (3.46 g, 108

mmol) in N,N-dimethylformamide (10 mL) was added dropwise to a

boiling suspension of potassium cyanate (5.11 g, 63.0 mmol) in N,N-

dimethylformamide (20 mL) at 145 °C ( 5 °C, temperature

measurement within the ﬂask) within 30 min. The reaction mixture

was allowed to cool to 20 °C within 1 h and was then stirred at this

temperature for 17 h, diethyl ether (100 mL) was added, and the

mixture was stirred at 20 °C for 30 min. The resulting precipitate was

ﬁltered oﬀ, washed with diethyl ether (3 × 30 mL), and discarded. The

solvent of the ﬁltrate (including the wash solutions) was removed by

distillation at ambient pressure, and the residue was puriﬁed by

distillation in vacuo. The relevant fractions (GC analysis) were

combined, methanol (200 mg, 6.24 mmol) was added, and the

resulting mixture was stirred at 20 °C for 30 min to furnish 6b as a

colorless liquid (9.12 g, 44.4 mmol; 74% yield). Bp: 127 °C/5 mbar.

¹H NMR (500.1 MHz, C₆D₆): δ 0.11 (s, 6 H; SiCH₃), 0.45 (δ_A), 1.43

1

Hz; SiCH₂), 57.9 (t, J_NC= 2.3 Hz; NCH₃). ²⁹Si NMR (59.6 MHz,

[D₆]DMSO): δ 12.0. Anal. Calcd for C₇H₂₀INOSi (289.23): C, 29.07;

H, 6.97; N, 4.84. Found: C, 29.23; H, 7.08; N, 4.72.

[3-(Methoxydimethylsilyl)prop-1-yl]trimethylammonium Iodide

(8b). Freshly distilled iodomethane (10.5 g, 74.0 mmol) was added

dropwise at 20 °C within 30 min under exclusion of light to a stirred

solution of 5b (2.60 g, 14.8 mmol) in acetonitrile (40 mL). The

reaction mixture was heated under reﬂux for 4 h, cooled to 20 °C

within 30 min, and then stirred at this temperature for 18 h. The

solvent and the excess iodomethane were removed under reduced

pressure, the residue was washed with n-pentane (4 × 15 mL), and the

wash solutions were discarded. Acetonitrile (15 mL) was added to the

residue, and the resulting suspension was heated under reﬂux until a

clear solution was obtained, which was kept undisturbed at 20 °C for 3

h and then at −20 °C for a further 17 h. The product was isolated by

ﬁltration at −20 °C, washed with n-pentane (5 mL), and dried in vacuo

(0.01 mbar, 40 °C, 4 h) to give 8a as a colorless solid (2.94 g, 9.27

(δ_F), 3.11 (δ_M), and 4.48 (δ_X) (AA′FF′MM′X system, ²J_AA′= 11.1 Hz,

³J_AF= 11.7 Hz, ³J_AF′= 5.1 Hz, ²J_FF′= 9.9 Hz, ³J_FM= 6.4, ³J_F′M= 7.9 Hz,

3

²J_MM′

=

14.6 Hz, J_MX

=

³J_M′X

=

6.3 Hz,

7

H; SiC-

1

mmol; 63% yield). H NMR (300.1 MHz, [D₆]DMSO): δ 0.10 (s, 6

H_AH_A′CH_FH_F′CH_MH_M′NH_X′), 3.33 (s, 3 H; SiOCH₃), 3.60 (s, 3 H;

C(O)OCH₃). ¹³C NMR (125.8 MHz, C₆D₆): δ −2.7 (SiCH₃), 13.1

(SiCH₂), 24.1 (SiCH₂CH₂), 44.0 (CH₂N), 50.0 (SiOCH₃), 51.6

(C(O)OCH₃), 156.9 (C(O)OCH₃). ²⁹Si NMR (99.4 MHz, C₆D₆): δ

18.1. Anal. Calcd for C₈H₁₉NO₃Si (205.33): C, 46.80; H, 9.33; N,

6.82. Found: C, 46.79; H, 9.24; N, 6.94.

H; SiCH₃), 0.48 (δ_A), 1.66 (δ_M), and 2.35 (δ_X) (AA′MM′XX′ system,

²J_AA′= 11.5 Hz, ³J_AM= 4.9 Hz, ³J_AM′= 12.0 Hz, ²J_MM′= 13.0 Hz, ³J_MX

=

3

2

4.7 Hz, J_{M X ′}

=

12.4 Hz, J_{X X ′}

=

12.7 Hz,

6

H;

SiCH_AH_A′CH_MH_M′CH_XH_X′), 3.05 (s, 9 H; NCH₃), 3.33 (s, 3 H;

OCH₃). ¹³C NMR (75.5 MHz, [D₆]DMSO): δ −2.7 (SiCH₃), 11.7

1

(SiCH₂), 16.3 (SiCH₂CH₂), 49.8 (OCH₃), 52.1 (t, J_NC= 3.6 Hz;

Methoxy{[(methoxycarbonyl)methylamino]methyl}-

dimethylsilane (7a). Methyl chloroformate (5.53 g, 58.5 mmol) was

added dropwise at 20 °C within 30 min to a stirred solution of 27

(7.79 g, 58.5 mmol) and triethylamine (5.92 g, 58.5 mmol) in n-

pentane (300 mL), and the reaction mixture was stirred at 20 °C for

30 min. The resulting precipitate was ﬁltered oﬀ, washed with n-

pentane (3 × 50 mL), and discarded. The solvent of the ﬁltrate

(including the wash solutions) was removed by distillation at ambient

pressure, and the residue was puriﬁed by distillation in vacuo to furnish

7a as a colorless liquid (7.72 g, 40.4 mmol; 69% yield). Bp: 77 °C/4

1

NCH₃), 67.7 (t, J_NC= 1.9 Hz; CH₂N). ²⁹Si NMR (59.6 MHz,

[D₆]DMSO): δ 18.5. Anal. Calcd for C₉H₂₄INOSi (317.29): C, 34.07;

H, 7.62; N, 4.41. Found: C, 34.01; H, 7.62; N, 4.35.

Methoxydimethyl[(phenylamino)methyl]silane (9a). A stirred

mixture of magnesium turnings (911 mg, 37.5 mmol) and aniline

(60.4 g, 649 mmol) was heated under reﬂux for 2 h and was then

concentrated to a volume of ca. 40 mL by distillation at ambient

pressure and heated under reﬂux for a further 2 h. The resulting

suspension was cooled to 125 °C within 15 min, and 2a (10.0 g, 72.1

mmol) was added dropwise at this temperature within 30 min,

followed by the addition of aniline (30.0 g, 32.2 mmol). The reaction

mixture was stirred at 120 °C for 1 h and was then cooled to 20 °C

within 30 min. The resulting precipitate was ﬁltered oﬀ, washed with

toluene (3 × 30 mL), and discarded. The solvent of the ﬁltrate

(including the wash solutions) and the excess aniline were removed by

distillation under reduced pressure, and the residue was puriﬁed by

distillation in vacuo to furnish 9a as a colorless liquid (9.58 g, 49.0

mmol; 68% yield). Bp: 132 °C/12 mbar. ¹H NMR (300.1 MHz,

C₆D₆): δ 0.14 (s, 6 H; SiCH₃), 2.49 (s, 2 H; SiCH₂), 3.31 (s, 3 H;

OCH₃), 3.64 (br s, 1 H; NH), 6.68 (δ_A), 6.90 (δ_M), and 7.32 (δ_X)

1

mbar. H NMR (500.1 MHz, C₆D₆): data for two rotational isomers

(molar ratio, 9:5); main isomer: δ 0.12 (s, 6 H; SiCH₃), 2.76 (s, 3 H;

NCH₃), 2.83 (s, 2 H; SiCH₂), 3.39 (s, 3 H; SiOCH₃), 3.58 (s, 3 H;

C(O)OCH₃); minor isomer: δ 0.35 (s, 6 H; SiCH₃), 2.72 (s, 2 H;

SiCH₂), 2.95 (s, 3 H; NCH₃), 3.29 (s, 3 H; SiOCH₃) 3.60 (s, 3 H;

1

C(O)OCH₃). H NMR (500.1 MHz, C₆D₆, 70 °C): δ 0.26 (s, 6 H;

SiCH₃), 2.81 (s, 2 H; SiCH₂), 2.86 (s, 3 H; NCH₃), 3.38 (s, 3 H;

SiOCH₃), 3.61 (s, 3 H; C(O)OCH₃). ¹³C NMR (75.5 MHz, C₆D₆):

data for two rotational isomers (molar ratio, 9:5); main isomer: δ −2.4

(SiCH₃), 36.4 (NCH₃), 40.3 (SiCH₂), 50.0 (SiOCH₃), 52.4 (C(O)-

OCH₃), 157.0 (C(O)OCH₃); minor isomer: δ −3.0 (SiCH₃), 37.0

(NCH₃), 39.0 (SiCH₂), 50.0 (SiOCH₃), 52.0 (C(O)OCH₃), 156.5

(C(O)OCH₃). ¹³C NMR (75.5 MHz, C₆D₆, 70 °C): δ −2.6 (SiCH₃),

36.7 (NCH₃), 40.2 (SiCH₂), 50.0 (SiOCH₃), 52.1 (C(O)OCH₃),

157.0 (C(O)OCH₃). ²⁹Si NMR (59.6 MHz, C₆D₆): data for two

rotational isomers (molar ratio, 9:5); main isomer: δ 13.2; minor

isomer: δ 14.6. ²⁹Si NMR (59.6 MHz, C₆D₆, 70 °C): δ 13.5. Anal.

(AA′MXX′ system, ³J_AX= ³J_A′X′= 8.2 Hz, ³J_MX= ³J_MX′= 7.3 Hz, ⁴J_AA′

=

4

5

2.6 Hz, J_AM= J_A′M= 1.1 Hz, J_XX′= 1.7 Hz, J_AX′= J_A′X= 0.5 Hz, 5

H; H-2_A/H-6_A′, H-3_X/H-5_X′, and H-4_M, C₆H₅). ¹³C NMR (75.5 MHz,

C₆D₆): δ −3.5 (SiCH₃), 32.5 (SiCH₂), 50.2 (OCH₃), 113.0 (C-2/C-6,

C₆H₅), 117.5 (C-4, C₆H₅), 129.4 (C-3/C-5, C₆H₅), 150.7 (C-1, C₆H₅).

²⁹Si NMR (59.6 MHz, C₆D₆): δ 15.3. Anal. Calcd for C₁₀H₁₇NOSi

N

dx.doi.org/10.1021/om500073m | Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

(195.34): C, 61.49; H, 8.77; N, 7.17. Found: C, 61.52; H, 8.92; N,

7.30.

removed by distillation at ambient pressure. Ethanol (7.80 g, 169

mmol) was added to the residue, and the reaction mixture was heated

under reﬂux for 30 min. The resulting ethanol/methanol mixture was

removed by distillation at ambient pressure, and the residue was

puriﬁed by distillation in vacuo to furnish 11a as a colorless liquid (13.3

Methoxydimethyl[3-(phenylamino)prop-1-yl]silane (9b). A stirred

mixture of magnesium turnings (1.52 g, 62.5 mmol) and aniline (101

g, 1.08 mol) was heated under reﬂux for 2 h and was then

concentrated to a volume of ca. 60 mL by distillation at ambient

pressure and heated under reﬂux for a further 2 h. The resulting

suspension was cooled to 125 °C within 15 min, and 2b (20.0 g, 120

mmol) was added dropwise at this temperature within 30 min,

followed by the addition of aniline (50.0 g, 53.7 mmol). The reaction

mixture was stirred at 120 °C for 1 h and was then cooled to 20 °C

within 30 min. The resulting precipitate was ﬁltered oﬀ, washed with

toluene (3 × 50 mL), and discarded. The solvent of the ﬁltrate

(including the wash solutions) and the excess of aniline were removed

by distillation under reduced pressure, and the residue was puriﬁed by

distillation in vacuo to furnish 9b as a colorless liquid (17.6 g, 78.8

mmol; 66% yield). Bp: 145 °C/8 mbar. ¹H NMR (300.1 MHz, C₆D₆):

δ 0.15 (s, 6 H; SiCH₃), 0.57 (δ_A), 1.54 (δ_M), and 2.96 (δ_X)

(AA′MM′XX′ system, ²J_AA′= 15.9 Hz, ³J_AM= 11.6 Hz, ³J_AM′= 5.2 Hz,

1

g, 69.5 mmol; 62% yield). Bp: 110 °C/12 mbar. H NMR (300.1

3

MHz, C₆D₆): δ 0.14 (s, 6 H; SiCH₃), 1.13 (t, J_HH= 7.0 Hz, 3 H;

CH₂CH₃), 2.81 (d, ³J_HH= 5.2 Hz, 2 H; SiCH₂), 3.54 (q, ³J_HH= 7.0 Hz,

2 H; OCH₂), 3.61 (s, 3 H; OCH₃), 4.69 (br s, 1 H; NH). ¹³C NMR

(75.5 MHz, C₆D₆): δ −3.0 (SiCH₃), 18.6 (CH₂CH₃), 30.7 (SiCH₂),

51.8 (C(O)OCH₃), 58.5 (OCH₂), 157.9 (C(O)OCH₃). ²⁹Si NMR

(59.6 MHz, C₆D₆): δ 12.2. Anal. Calcd for C₇H₁₇NO₃Si (191.30): C,

43.95; H, 8.96; N, 7.32. Found: C, 44.03; H, 9.03; N, 7.50.

[(Ethoxydimethylsilyl)methyl]trimethylammonium Iodide (12a).

Freshly distilled iodomethane (15.5 g, 109 mmol) was added dropwise

at 20 °C within 30 min under exclusion of light to a stirred solution of

10a (4.41 g, 27.3 mmol) in acetonitrile (60 mL). The reaction mixture

was heated under reﬂux for 4 h, cooled to 20 °C within 30 min, and

then stirred at this temperature for 18 h. The solvent and the excess

iodomethane were removed under reduced pressure, the residue was

washed with n-pentane (4 × 15 mL), and the wash solutions were

discarded. Acetonitrile (17 mL) was added to the residue, and the

resulting suspension was heated under reﬂux until a clear solution was

obtained, which was kept undisturbed at 20 °C for 3 h and then at −20

°C for a further 17 h. The product was isolated by ﬁltration at −20 °C,

washed with n-pentane (3 × 5 mL), and dried in vacuo (0.01 mbar, 40

°C, 4 h) to give 12a as a colorless solid (6.70 g, 22.1 mmol; 81%

3

2

²J_MM′= 14.6 Hz, J_MX= 6.1 Hz, J_MX′= 8.1 Hz, J_XX′= 13.3 Hz, 6 H;

SiCH_AH_A′CH_MH_M′CH_XH_X′), 3.32 (br s, 1 H; NH), 3.36 (s, 3 H;

OCH₃), 6.63 (δ_A), 6.88 (δ_M), and 7.32 (δ_X) (AA′MXX′ system, ³J_AX

=

³J_A′X′= 8.2 Hz, ³J_MX= ³J_MX′= 7.3 Hz, ⁴J_AA′= 1.7 Hz, ⁴J_AM= ⁴J_A′M= 1.1

5

Hz, J_AX′= J_A′X= 0.5 Hz, 5 H; H-2_A/H-6_A′, H-3_X/H-5_X′and H-4_M,

C₆H₅). ¹³C NMR (75.5 MHz, C₆D₆): δ −2.6 (SiCH₃), 13.6 (SiCH₂),

23.4 (SiCH₂CH₂), 46.9 (CH₂N), 50.0 (OCH₃), 113.0 (C-2/C-6,

C₆H₅), 117.3 (C-4, C₆H₅), 129.5 (C-3/C-5, C₆H₅), 148.9 (C-1, C₆H₅).

²⁹Si NMR (59.6 MHz, C₆D₆): δ 18.0. Anal. Calcd for C₁₂H₂₁NOSi

(223.39): C, 64.52; H, 9.48; N, 6.27. Found: C, 64.59; H, 9.51; N,

6.26.

1

yield). H NMR (300.1 MHz, [D₆]DMSO): δ 0.27 (s, 6 H; SiCH₃),

1.13 (t, ³J_HH= 7.0 Hz, 3 H; CH₂CH₃), 3.18 (s, 9 H; NCH₃), 3.20 (s, 2

3

H; SiCH₂), 3.71 (q, J_HH= 7.0 Hz, 2 H; OCH₂). ¹³C NMR (75.5

1

[(Dimethylamino)methyl]ethoxydimethylsilane (10a). A mixture

of 24 (45.1 g, 295 mmol) and dimethylamine (66.6 g, 1.48 mol) was

heated in an autoclave at 100 °C for 16 h. The reaction mixture was

allowed to cool to 20 °C within 30 min, n-pentane (600 mL) was

added, and the resulting precipitate was ﬁltered oﬀ, washed with n-

pentane (3 × 100 mL), and discarded. The solvent of the ﬁltrate

(including the wash solutions) was removed by distillation at ambient

pressure, and the residue was puriﬁed by distillation at ambient

pressure to furnish 10a as a colorless liquid (37.5 g, 232 mmol; 79%

MHz, [D₆]DMSO): δ −1.1 (SiCH₃), 18.1 (CH₂CH₃), 56.1 (t, J_NC

=

3.4 Hz; NCH₃), 58.1 (t, J_NC= 1.9 Hz; SiCH₂), 58.4 (OCH₂). ²⁹Si

NMR (59.6 MHz, [D₆]DMSO): δ 9.6. Anal. Calcd for C₈H₂₂INOSi

(303.26): C, 31.69; H, 7.31; N, 4.62. Found: C, 31.42; H, 7.28; N,

4.81.

1

Methoxydimethyl(n-propoxymethyl)silane (13a). A solution of

ethylmethylamine (12.8 g, 217 mmol) in THF (30 mL) was added

dropwise at 0 °C within 2 h to a stirred solution of 17 (45.0 g, 217

mmol) and ethyldimethylamine (17.4 g, 238 mmol) in THF (200

mL). After the mixture had been stirred at 0 °C for 2 h and at 20 °C

for 2 h, a 3.0 M solution of methylmagnesium bromide in diethyl ether

(181 mL, 543 mmol of MeMgBr) was added dropwise at 0 °C within

1 h. The reaction mixture was stirred at 20 °C for 16 h and was then

concentrated to a volume of ca. 70 mL by distillation at ambient

pressure, n-pentane (300 mL) was added, and the mixture was stirred

at 20 °C for 30 min. The resulting solid was ﬁltered oﬀ, washed with n-

pentane (3 × 30 mL), and discarded. The solvent of the ﬁltrate

(including the wash solutions) was removed by distillation at ambient

pressure, and the residue was distilled in vacuo to give three fractions

(23, 5.87 g, bp 38−46 °C/9 mbar; 21, 5.60 g, bp 63−72 °C/9 mbar;

22, 9.07 g, bp 90−97 °C/9 mbar (discarded)).

Transformation A: A 1.25 M solution of hydrogen chloride in

methanol (2.37 mL, 2.96 mmol of HCl, 29.6 mmol of methanol) was

added dropwise at 20 °C within 15 min to a stirred solution of 21

(5.60 g, 29.6 mmol) in n-pentane (40 mL), and the mixture was stirred

at 20 °C for 30 min. The resulting solid was ﬁltered oﬀ, washed with n-

pentane (3 × 5 mL), and discarded. The solvent of the ﬁltrate

(including the wash solutions) was removed by distillation at ambient

pressure, and the residue was distilled in vacuo to give 5.37 g of 13a as

crude distillate (bp 108−115 °C/300 mbar).

1

yield). Bp: 137 °C. H NMR (300.1 MHz, C₆D₆): δ 0.29 (s, 6 H;

SiCH₃), 1.24 (t, ³J_HH= 7.0 Hz, 3 H; CH₂CH₃), 1.96 (s, 2 H; SiCH₂),

2.33 (s, 6 H; NCH₃), 3.68 (q, ³J_HH= 7.0 Hz, 2 H; OCH₂). ¹³C NMR

(75.5 MHz, C₆D₆): δ −2.0 (SiCH₃), 18.7 (CH₂CH₃), 49.8 (NCH₃),

51.4 (SiCH₂), 58.3 (OCH₂). ²⁹Si NMR (59.6 MHz, C₆D₆): δ 12.2.

Anal. Calcd for C₇H₁₉NOSi (161.32): C, 52.12; H, 11.87; N, 8.68.

Found: C, 52.01; H, 11.98; N, 9.00.

[3-(Dimethylamino)prop-1-yl]ethoxydimethylsilane (10b). A mix-

ture of 25 (19.9 g, 110 mmol) and 41.5 g (920 mmol) of

dimethylamine was heated in an autoclave at 120 °C for 17 h. The

reaction mixture was allowed to cool to 20 °C within 30 min, n-

pentane (400 mL) was added, and the resulting precipitate was ﬁltered

oﬀ, washed with n-pentane (3 × 50 mL), and discarded. The solvent of

the ﬁltrate (including the wash solutions) was removed by distillation

at ambient pressure, and the residue was puriﬁed by distillation in

vacuo to furnish 10b as a colorless liquid (13.1 g, 69.2 mmol; 63%

1

yield). Bp: 65 °C/5 mbar. H NMR (500.1 MHz, C₆D₆): δ 0.23 (s, 6

H; SiCH₃), 0.77 (δ_A), 1.69 (δ_M), and 2.33 (δ_X) (AA′MM′XX′ system,

²J_AA′= 14.8 Hz, ³J_AM= 11.6 Hz, ³J_AM′= 5.1 Hz, ²J_MM′= 13.5 Hz, ³J_MX

=

3

2

8.5 Hz, J_MX′

=

5.9 Hz, J_XX′

=

11.7 Hz,

6

H; SiC-

3

H_AH_A′CH_MH_M′CH_XH_X′), 1.24 (t, J_HH= 7.0 Hz, 3 H; CH₂CH₃),

2.25 (s, 6 H; NCH₃), 3.66 (q, ³J_HH= 7.0 Hz, 2 H; OCH₂). ¹³C NMR

(125.8 MHz, C₆D₆): δ −1.9 (SiCH₃), 14.2 (SiCH₂), 18.8 (CH₂CH₃),

21.9 (SiCH₂CH₂), 45.6 (NCH₃), 58.2 (OCH₂), 63.2 (CH₂N). ²⁹Si

NMR (99.4 MHz, C₆D₆): δ 15.7. Anal. Calcd for C₉H₂₃NOSi

(189.37): C, 57.08; H, 12.24; N, 7.40. Found: C, 56.75; H, 12.29; N,

7.75.

Transformation B: Methanol (1.69 g, 52.7 mmol) was added

dropwise to a stirred solution of 23 (5.87 g, 35.2 mmol) and

ethyldimethylamine (2.58 g, 35.3 mmol) in n-pentane (40 mL) at 20

°C within 15 min, and the mixture was stirred at 20 °C for 30 min.

The resulting solid was ﬁltered oﬀ, washed with n-pentane (3 × 5 mL),

and discarded. The solvent of the ﬁltrate (including the wash

solutions) was removed by distillation at ambient pressure, and the

residue was distilled in vacuo to give 4.61 g of 13a as crude distillate

(bp 93−102 °C/200 mbar).

Ethoxy{[(methoxycarbonyl)methylamino]methyl}dimethylsilane

(11a). A stirred mixture of 6a (20.0 g, 113 mmol), ethanol (15.6 g, 339

mmol), and orthophosphoric acid (166 mg, 1.69 mmol) was heated

under reﬂux for 1 h, and the resulting ethanol/methanol mixture was

O

dx.doi.org/10.1021/om500073m | Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

The product fractions obtained after the transformations A and B

were combined and ﬁnally puriﬁed by distillation in vacuo to furnish

13a as a colorless liquid (9.48 g, 58.4 mmol; 27% yield). Bp: 95 °C/

200 mbar. ¹H NMR (300.1 MHz, C₆D₆): δ 0.30 (s, 6 H; SiCH₃), 1.00

to cool to 20 °C within 30 min, diethyl ether (300 mL) was added, and

the resulting precipitate was ﬁltered oﬀ, washed with diethyl ether (3 ×

30 mL), and discarded. The solvent of the ﬁltrate (including the wash

solutions) was removed by distillation at ambient pressure, and the

residue was puriﬁed by distillation at ambient pressure to furnish 27 as

(δ_A), 1.64 (δ_M), and 3.37 (δ_X) (A₃M₂X₂system, ³J_AM= 7.5 Hz, ³J_MX

=

1

6.5 Hz, 7 H; OC(H_X)₂C(H_M)₂C(H_A)₃), 3.21 (s, 2 H; SiCH₂), 3.50 (s,

3 H; OCH₃). ¹³C NMR (125.8 MHz, C₆D₆): δ −3.6 (SiCH₃), 10.8

(OCH₂CH₂CH₃), 23.3 (OCH₂CH₂), 50.5 (OCH₃), 63.7 (SiCH₂O),

77.4 (OCH₂CH₂). ²⁹Si NMR (99.4 MHz, C₆D₆): δ 12.3. Anal. Calcd

for C₇H₁₈O₂Si (162.30): C, 51.80; H, 11.18. Found: C, 51.87; H,

11.30.

a colorless liquid (11.8 g, 88.5 mmol; 65% yield). Bp: 127 °C. H

NMR (500.1 MHz, C₆D₆): δ 0.22 (s, 6 H; SiCH₃), 0.55 (br s, 1 H;

NH), 2.14 (s, 2 H; SiCH₂), 2.46 (s, 3 H; NCH₃), 3.42 (s, 3 H; OCH₃).

¹³C NMR (125.8 MHz, C₆D₆): δ −3.3 (SiCH₃), 41.3 (NCH₃), 42.3

(SiCH₂), 50.2 (OCH₃). ²⁹Si NMR (59.6 MHz, C₆D₆): δ 14.5. Anal.

Calcd for C₅H₁₅NOSi (133.27): C, 45.06; H, 11.35; N, 10.51. Found:

C, 44.97; H, 11.47; N, 10.44.

Methoxydimethyl(n-pentyl)silane (13b). A 2.0 M solution of n-

pentylmagnesium bromide in diethyl ether (41.6 mL, 83.2 mmol of n-

pentylMgBr) was added dropwise at 0 °C within 45 min to a stirred

solution of 14 (10.0 g, 83.2 mmol) in diethyl ether (250 mL). After

the mixture had been stirred at 20 °C for 17 h, the solvent was

removed by distillation at ambient pressure. Subsequently, n-pentane

(300 mL) was added, the mixture was stirred at 20 °C for 30 min, and

the solid was ﬁltered oﬀ, washed with n-pentane (3 × 25 mL), and

discarded. The solvent of the ﬁltrate (including the wash solutions)

was removed by distillation at ambient pressure, and the residue was

puriﬁed by distillation in vacuo to furnish 13b as a colorless liquid

1

Kinetic Studies. All H NMR spectra for the kinetic studies were

recorded at 23 °C using a Bruker DRX-300 NMR spectrometer (¹H,

300.1 MHz). The respective CD₃CN/D₂O mixture (1.0 mL) was

added to 2a (41.6 mg, 300 μmol) in a 5 mm screw-cap NMR tube

(Wilmad). After tuning and matching the probe to this CD₃CN/D₂O

mixture, the spectrometer was also shimmed with this standard

sample. Subsequently, the NMR tube was replaced by a freshly

prepared sample for a co-hydrolysis reaction, and the ¹H NMR

measurement of the locked sample was started using an aquisition time

of 10.0 s and a sweep width of 3004.8 Hz. At least four scans (in the

case of slow hydrolysis reactions eight scans) per spectrum were

recorded with a 30° pulse width and a delay D1 of 30 s. The starting

points of acquisition were extracted automatically from the audit ﬁles

and were used as the value on the time axis. For the preparation of the

samples for the hydrolysis reactions, a mixture of two or three diﬀerent

silanes (500 mmol each; for the respective compositions, see Figures

S1−S60) was added under a dry argon atmosphere to a Schlenk ﬂask

(25 mL). The CD₃CN/D₂O mixture including acid or base (5.0 mL;

for the respective compositions, see Figures S1−S60) was added to the

reaction vessel in a single portion, and the resulting solution was

stirred for 20 s. Subsequently, 1.0 mL of the reaction mixture were

removed with a syringe and then transferred into a clean screw-cap

NMR tube. Further samples (0.5 mL each) were removed at regular

intervals and were used for the measurement of the pD values. For the

measurement of the pD values, a combination of a Mettler Toledo MP

220 pH meter and a Mettler Toledo InLab Science Pro was used. The

glass electrode was conditioned for 7 days in D₂O prior to use. The

pH meter was standardized with conventional buﬀer mixtures with a

pD close to the range of the pD measurements.

1

(9.57 g, 59.7 mmol; 72% yield). Bp: 41 °C/5 mbar. H NMR (500.1

MHz, C₆D₆): δ 0.18 (s, 6 H; SiCH₃), 0.68 (m, 2 H; SiCH₂), 1.00 (m, 3

H; Si(CH₂)₄CH₃), 1.40 (m, 4 H; CH₂CH₂CH₃), 1.46 (m, 2 H;

SiCH₂CH₂), 3.40 (s, 3 H; OCH₃). ¹³C NMR (125.8 MHz, C₆D₆): δ

−2.5 (SiCH₃), 14.2 (CH₂CH₃), 16.3 (SiCH₂), 22.8 (CH₂CH₃), 23.3

(SiCH₂CH₂), 36.1 (SiCH₂CH₂CH₂), 49.9 (OCH₃). ²⁹Si NMR (59.6

MHz, C₆D₆): δ 18.0. Anal. Calcd for C₈H₂₀OSi (160.33): C, 59.93; H,

12.57. Found: C, 59.78; H, 12.66.

Dimethoxydimethylsilane (14). This compound was commercially

available and was puriﬁed by distillation prior to use.

Trichlorosilane (15). This compound was commercially available

and was puriﬁed by distillation prior to use.

Trichloro(methoxymethyl)silane (16). This compound was synthe-

sized according to ref 8b.

Trichloro(n-propoxymethyl)silane (17). A mixture of trichlorosi-

lane (119 g, 879 mmol) and chloromethyl n-propyl ether^33,34(85.8 g,

790 mmol) was added dropwise at 20 °C within 1 h to a mixture of

triethylamine (80.0 g, 791 mmol) and copper(I) chloride (3.80 g, 38.4

mmol) in diethyl ether (450 mL). The reaction mixture was heated

under reﬂux for 2 h and then cooled to 20 °C, and the resulting

precipitate was ﬁltered oﬀ, washed with diethyl ether (3 × 40 mL), and

discarded. The solvent of the ﬁltrate (including the wash solutions)

was removed by distillation, and the residue was puriﬁed by distillation

at ambient pressure to furnish 17 as a colorless liquid (49.0 g, 236

ASSOCIATED CONTENT

* Supporting Information

■

S

Plots of −ln{[silane(t)]/[silane(t = 0)]} vs t for the hydrolyses

of compounds 1a−9a, 1b−6b, 10b, 11a−13a, and 13b (Figures

1

mmol; 30% yield). Bp: 148 °C. H NMR (300.1 MHz, C₆D₆): δ 0.87

1

(δ_A), 1.45 (δ_M), and 3.13 (δ_X) (A₃M₂X₂system, ³J_AM= 7.5 Hz, ³J_MX

=

S1−S60), H, ¹³C, and ²⁹Si NMR spectra of compound 26

6.5 Hz, 7 H; OC(H_X)₂C(H_M)₂C(H_A)₃), 3.20 (s, 2 H; SiCH₂). ¹³C

NMR (75.5 MHz, C₆D₆): δ 10.4 (OCH₂CH₂CH₃), 22.7 (OCH₂CH₂),

65.2 (SiCH₂), 76.6 (OCH₂). ²⁹Si NMR (59.6 MHz, C₆D₆): δ 3.7. Anal.

Calcd for C₄H₉Cl₃OSi (207.56): C, 23.15; H, 4.37. Found: C, 23.31;

H, 4.46.

(Figures S61−63), and Cartesian coordinates of all stationary

points occurring along the reaction coordinate of the model

reactions of compounds 1a−6a, 1b, 3b, 4b, 9a, 13a, and 13b.

This material is available free of charge via the Internet at

http://pubs.acs.org.

(Chloromethyl)ethoxydimethylsilane (24). This compound was

commercially available and was puriﬁed by distillation prior to use.

(3-Chloroprop-1-yl)ethoxydimethylsilane (25). This compound

was commercially available and was puriﬁed by distillation prior to use.

1-Methoxycarbonyl-2,2-dimethyl-2-silapyrrolidine (26). Com-

pound 26 was obtained as an impurity during the distillation of 6b.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: f.m.bickelhaupt@vu.nl.

*E-mail: r.tacke@uni-wuerzburg.de.

■

¹H NMR (500.1 MHz, C₆D₆): δ 0.25 (s, 6 H; SiCH₃), 0.58 (t, ³J_HH

=

3

Notes

7.3 Hz, 2 H; SiCH₂), 1.58 (tt, J_HH= 7.3 Hz, J_HH= 6.4 Hz, 2 H;

3

The authors declare no competing ﬁnancial interest.

SiCH₂CH₂), 3.45 (t, J_HH= 6.4 Hz, 2 H; NCH₂), 3.59 (s, 3 H;

C(O)OCH₃). ¹³C NMR (125.8 MHz, C₆D₆): δ −1.5 (SiCH₃), 12.4

(SiCH₂), 23.0 (SiCH₂CH₂), 47.4 (CH₂N), 51.8 (C(O)OCH₃), 157.8

(C(O)OCH₃). ²⁹Si NMR (99.4 MHz, C₆D₆): δ 17.3; HRMS (ESI),

m/z calcd for [C₇H₁₅NO₂Si+Na]⁺: 196.0764 [M + Na⁺]. Found:

196.0763.

ACKNOWLEDGMENTS

■

We thank The Netherlands Organization for Scientiﬁc

Research (NWO), the National Research School Combina-

tion−Catalysis (NRSC-C), the State of Bavaria, and Wacker

Chemie AG for ﬁnancial support, and Dr. V. Stanjek and Dr. J.

Methoxy[(methylamino)methyl]dimethylsilane (27). A mixture of

2a (19.0 g, 137 mmol) and methylamine (38.3 g, 1.23 mol) was heated

in an autoclave at 100 °C for 16 h. The reaction mixture was allowed

Kohler (Wacker Chemie AG) for many helpful and inspiring

̈

P

dx.doi.org/10.1021/om500073m | Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

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discussions and support of our work. This article is dedicated to

Professor Rudolf Staudigl (Wacker Chemie AG) on the

occasion of his 60th birthday.

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dx.doi.org/10.1021/om500073m | Organometallics XXXX, XXX, XXX−XXX

Article Doi

Silicon α-effect: A systematic experimental and computational study of the hydrolysis of Cα- and Cγ-functionalized alkoxytriorganylsilanes of the formula type ROSiMe2(CH 2)nX (R = Me, Et; N = 1, 3; X = functional group)

DOI: 10.1021/om500073m

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