Hydrosilane σ-Adduct Intermediates in an Adaptive Zinc-Catalyzed Cross-dehydrocoupling of Si?H and O?H Bonds

Article abstract of DOI:10.1002/chem.202101146

Three-coordinate ^PhBOX (Formula presented.) ZnR (^PhBOX (Formula presented.) =phenyl-(4,4-dimethyl-oxazolinato; R=Me: 2 a, Et: 2 b) catalyzes the dehydrocoupling of primary or secondary silanes and alcohols to give silyl ethers and hydrogen, with high turnover numbers (TON; up to 10⁷) under solvent-free conditions. Primary and secondary silanes react with small, medium, and large alcohols to give various degrees of substitution, from mono- to tri-alkoxylation, whereas tri-substituted silanes do not react with MeOH under these conditions. The effect of coordinative unsaturation on the behavior of the Zn catalyst is revealed through a dramatic variation of both rate law and experimental rate constants, which depend on the concentrations of both the alcohol and hydrosilane reactants. That is, the catalyst adapts its mechanism to access the most facile and efficient conversion. In particular, either alcohol or hydrosilane binds to the open coordination site on the ^PhBOX (Formula presented.) ZnOR catalyst to form a ^PhBOX (Formula presented.) ZnOR(HOR) complex under one set of conditions or an unprecedented σ-adduct ^PhBOX (Formula presented.) ZnOR(H?SiR′₃) under other conditions. Saturation kinetics provide evidence for the latter species, in support of the hypothesis that σ-bond metathesis reactions involving four-centered electrocyclic 2σ–2σ transition states are preceded by σ-adducts.

Full text of DOI:10.1002/chem.202101146

Accepted Article
Accepted Article
Title: Hydrosilane σ-Adduct Intermediates in an Adaptive Zinc-
Catalyzed Cross-Dehydrocoupling of Si–H and O–H Bonds
Authors: Smita Patnaik, Uddhav Kanbur, Arkady Ellern, and Aaron
David Sadow
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202101146
Link to VoR: https://doi.org/10.1002/chem.202101146
01/2020

10.1002/chem.202101146
Chemistry - A European Journal
RESEARCH ARTICLE
Hydrosilane σ-Adduct Intermediates in an Adaptive Zinc-
Catalyzed Cross-Dehydrocoupling of Si–H and O–H Bonds
Smita Patnaik,[a,b] Uddhav Kanbur,[a,b] Arkady Ellern,[a] Aaron D. Sadow[a,b]
*
[a]
Dr. S. Patnaik, Mr. U. Kanbur, Dr. A. Ellern, Prof. A. D. Sadow
Department of Chemistry
Iowa State University
Ames, IA 50011
E-mail: sadow@iastate.edu
[b]
Dr. S. Patnaik, Mr. U. Kanbur, Prof. A. D. Sadow
US Department of Energy Ames Laboratory
Iowa State University
Ames, IA 50011
Supporting information for this article is given via a link at the end of the document.
Abstract: Three-coordinate PhBOXMe2ZnR (PhBOXMe2 = phenyl-(4,4-
products by influencing reaction rates, the H2 by-product is inert,
and alkyl- and arylsilanes can be stored in air prior to
dimethyl-oxazolinato;
R
=
Me 2a, Et 2b) catalyzes the
dehydrocoupling of primary or secondary silanes and alcohols to
give silyl ethers and hydrogen, with high turnover numbers (TON),
up to 107, under solvent-free conditions. Primary and secondary
silanes react with small, medium, and large alcohols to give varying
degrees of substitution, from mono-alkoxylation to tri-alkoxylation,
while tri-substituted silanes do not react with MeOH under these
conditions. The effect of coordinative unsaturation on the behavior of
the Zn catalyst is revealed through a dramatic variation of both rate
law and experimental rate constants, which depend on the
dehydrocoupling. Late-transition metal complexes based on
18]
Re,[16] Rh,[17,
Ni,[19] and Ir[20] are catalysts for these cross
dehydrocoupling reactions; however, in some cases these
systems also mediate isomerization or hydrosilylation of C=C or
C=O moieties. Basic catalysts such as sodium hydroxide
overcome this limitation but are restricted to secondary and
tertiary silanes,[21, 22] as are B(C6F5)3-catalyzed reactions.[23]
Hydridozinc
species
also
catalyze
these
cross-
dehydrocouplings;[24-29] however, a divergent picture of the
concentrations of both the alcohol and hydrosilane reactants. That is, fundamental nature of hydridozinc catalysts has emerged,
the catalyst adapts its mechanism access the most facile and
efficient conversion. In particular, either alcohol or hydrosilane bind
to the open coordination site on the PhBOXMe2ZnOR catalyst to form
obscuring design principles. In particular, catalytic product
formation is observed with coordinatively saturated (ZnX2L2, 8-
electron) hydride and super-saturated (ZnX2L3, 10-electron)
alkoxide precatalysts, as well as with dimeric hydride-bridged N-
heterocyclic carbene-coordinated zinc precatalysts[24] which
a
PhBOXMe2ZnOR(HOR) complex under one set of conditions or an
unprecedented σ-adduct PhBOXMe2ZnOR(H–SiR'3) under other
conditions. Saturation kinetics provide evidence for the latter species, likely access lower coordinate catalytic sites (ZnX2L, 6-electron).
in support of the hypothesis that σ-bond metathesis reactions,
involving four-centered electrocyclic 2σ-2σ transition states, are
preceded by σ-adducts.
For example, the interconverting four-coordinate [κ3-Tptm]ZnH
and five-coordinate (Tptm tris(2-
[κ4-Tptm]ZnOR'
=
pyridylthio)methyl) catalyzes the methanolysis of phenylsilane
with high turnover number (TON) of 105 and turnover frequency
of 106 h–1.[26] These precatalysts also mediate carbonyl
hydrosilylation, which involves related Si–O bond formations. In
hydrosilylations, however, TON and rates do not necessarily
benefit from coordinatively unsaturated zinc precatalysts.[30]
Such behavior suggests that complex reaction pathways
underpin deceptively simple transformations.
Introduction
Silicon-oxygen bond formation has wide-ranging impact in
synthetic applications, ranging from the construction of organic-
inorganic hybrid materials[1-3] to the assembly of complex
molecules.[4] Silyl ethers themselves have important roles in
A two-step catalytic cross-dehydrocoupling mechanism has
been proposed based on kinetic studies of conversions
cross-coupling,[5] as templates for cyclization,[6] as protecting
catalyzed by the four-coordinate ToMZnH (ToM
dimethyl-2-oxazolinyl)phenylborate),[25]
=
tris(4,4-
and
8]
groups,[7,
and even improving the efficacy of medicinal
structurally
compounds.[9, 10] These moieties are conventionally formed from
alcohols and chlorosilanes; this approach, however, is hindered
by the formation of HCl or salts as byproducts, moisture
sensitivity and competing hydrolysis of chlorosilanes, as well as
the limited reactivity of bulky tertiary alcohols and bulky
chlorosilanes,[11] incompatibilities with base-sensitive groups,[12,
13] and difficulties selecting for a desired stoichiometry needed to
assemble multiple components into synthesis-enabling scaffolds.
Related reactions of alcohols with labile silyl ethers or silazanes
also involve hydrolytically sensitive reactants.[14, 15]
spectroscopically characterized hydridozinc and alkoxyzinc
intermediates, and kinetically well-defined elementary steps
(Scheme 1). The key turnover-limiting silicon-oxygen bond
formation is proposed to occur by σ-bond metathesis,[31]
involving cleavage of Zn–OR and Si–H bonds and formation of
Zn–H and Si–O bonds via a 4-centered, concerted, electrocyclic
transition state. Related steps are generally accepted for C–H
and Si–H bond activations by d0 early transition metal and rare
earth organometallic compounds,[32-34] and these elementary
steps are ubiquitous in catalytic transformations including alkane
hydrogenolysis,[35, 36] silane polymerization,[37] hydrosilylation,[38-
41] alkane silylation,[42] and hydromethylation of alkenes.[43]
Alternatively, catalytic dehydrogenative cross-coupling of
hydrosilanes and alcohols can provide partly substituted
1
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RESEARCH ARTICLE
HO
Results and Discussion
Rate expression of proposed cycle:
H2
– d[PhMeSiH2]
= k1 [ToMZnH]1[PhMeSiH2]1
Pre-catalyst synthesis and characterization. Reactions of
Ph,HBOXMe2 and dimethylzinc or diethylzinc provide the
heteroleptic compounds PhBOXMe2ZnMe (2a) and PhBOXMe2ZnEt
(2b) after 12 h at room temperature in high yield (90-92%, eq 1).
Singlets assigned to methyl and methylene groups on the
oxazoline in the 1H and 13C{1H} NMR spectra of 2a and 2b were
consistent with C2v symmetric species. The oxazoline methyl
resonances correlated to a single 15N NMR resonance at ca. –
215 ppm in 1H-15N HMBC experiments, much closer to the more
shielded chemical shift for the iminoenamine tautomer than to
that of the diimine form of the proligand. The IR spectra showed
dt
k2
O
O
N
N
Zn
O
Zn
H
PhB
OxMe2
PhB
OxMe2
N
N
O
O
k1
PhMeSiH2
Aryl
SiPhMeH
O
O
Experimental rate law:
Si
[Zn]
H
– d[PhMeSiH2]
= kobs [ToMZnH]1[PhMeSiH2]1
turnover limiting
step
dt
a
band, assigned to the carboximidate moiety, at lower
frequency (νCN = 1609 cm–1) than in the free proligand (νCN
=
Scheme 1. Proposed Cycle for Zn-catalyzed Cross Dehydrocoupling of
Hydrosilanes and Alcohols.
1648 cm–1). X-ray quality crystals of 2a (Figure 1) and 2b (see
Supporting Information) were obtained from pentane solutions
cooled at –30 °C.
Theoretical results suggest that Si–H or C–H σ-coordination
precedes the 4-centered transition state,[44-46] although
experimental evidence for σ-complexes as intermediates is
limited to kinetic isotope effects (KIEs).
O
O
N
N
C6H6
N
N
Ph
H
+
ZnR2
(1)
Ph
Zn–R
r.t., 12 h
– HR
1.5 equiv
KIEs in σ-bond metathesis-type E–E' bond formations vary
considerably, with Si–N and Si–C bond forming steps
characterized by kH/kD = 1,[47, 48] whereas the kH/kD for Si–Si and
P–P bond forming steps are typically ~3.[37, 49] The latter, non-
unity KIEs likely result from the long M–E and E–E bonds in the
cyclic transition state, involving a M–H–E angle that approaches
linearity. In contrast, either transfer of H in a bent geometry or a
rate-determining step which does not break the E–H bond could
lead to the KIE ~ 1. For example, the nearly unitary primary
kH/kD in the reaction of Cp2HfHCl and the stannane Mes2SnH2
(Mes = 2,4,6-C6Me3H2) or Mes2SnD2, to form a Hf–Sn bond, was
interpreted in terms of a σ-coordinated intermediate prior to H–H
bond formation.[50] Also, the near unity isotope effect in reactions
of ToMMgNHtBu and PhMeSiH2 or PhMeSiD2 were instead
interpreted as the resulting from rate-controlling N–Si bond
formation, prior to H migration to Mg via β-H elimination-like step,
in an asynchronous σ-bond metathesis sequence on the basis of
a companion Hammett study.[47]
O
O
(1.0 M in toluene)
Ph,HBOXMe2
PhBOXMe2Zn–R
2a R = Me (90%)
2b R = Et (92%)
Single-crystal X-ray diffraction studies reveal that the
approximately C2-symmetric alkylzinc compounds 2a (Figure 1)
and 2b are three-coordinate, with Zn1, two oxazoline C=N and
the backbone C11 as vertices in a nearly planar six-membered
ring. All torsion angles of this planar and symmetrical chelate
ring are less than 10°, with equivalent distances for Zn–N pairs,
N=C pairs, and C–N pairs. The C1 and C6 dimethyl substituents
of the oxazoline rings are pseudo-equatorial, whereas C3 and
C8 are pseudo-axial in 2a, as defined by transannular torsion
angles ∠ C1–C2–C7–C6 (–70(1)°) and ∠ C3–C2–C7–C8
(165.0(9)°).
Noting that ToMZnH is an 8-electron species, a strategy for
increasing catalytic performance could involve electronically
unsaturated, three-coordinate zinc centers. To investigate this
idea, we targeted zinc species supported by bidentate,
monoanionic bis(4-R-oxazolinato) ligands (BOX),[51-54] which are
the LX analogues of common neutral bis(oxazoline) ligands.[55]
The C1-phenyl ligand was chosen to impede undesired ancillary
ligand redistribution reactions, observed for BOXZnOR and
56]
diketiminatozinc in the presence of alcohols,[54,
without
hindering access to the active site. Here, we report the catalytic
properties of PhBOX alkylzinc compounds in dehydrocoupling of
alcohols and hydrosilanes. The straightforward syntheses of the
alkylzinc pre-catalysts, mild catalytic conditions, solvent-free
reactions and high turnover numbers (TON) make this
methodology
attractive.
Furthermore,
detailed
kinetic
investigations reveal that multiple catalytic pathways become
accessible under varying conditions, as a consequence of three-
coordinate electronically unsaturated zinc pre-catalysts.
Figure 1. Thermal ellipsoid plot of PhBOXMe2ZnMe (2a) at 50% probability. H
atoms are excluded for clarity. Selected interatomic distances (Å): Zn1–C18,
1.93(1); Zn1–N1, 1.947(9); Zn1–N2, 1.947(9); N1–C5, 1.31(2); N2–C10,
1.31(2); C5–C11, 1.44(2); C10–C11, 1.39(2).
2
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Reactions of 2a with methanol, isopropanol, or 3,5-
Catalytic alcohol/hydrosilane dehydrocoupling reactions.
dimethylphenol provide PhBOXMe2ZnOR' compounds (R' = Me 3a, The monomeric species 2a is
a
pre-catalyst for the
iC3H7 3b, C6Me2H3 3c) in fewer than 5 mins in benzene-d6 or
chloroform-d, as determined by 1H NMR spectroscopy. The
dimeric products (3a-c)2 precipitate from the benzene reaction
mixture over 1 h at room temperature and are easily isolated in
excellent yield (Scheme 2). The qualitatively trend in
precipitation rate depends on the alcohol substituent (Me > iC3H7
> C6Me2H3). The solids are insoluble in benzene-d6, but dissolve
in methylene chloride-d2 or chloroform-d. In addition, the
compounds may be generated and used in situ in benzene-d6.
1H and 13C{1H} NMR spectra of isolated species, dissolved in
chloroform-d, consisted of singlet resonances assigned to
oxazoline methyl and methylene groups. These spectra are
identical to those obtained by in situ reaction of 2a and R'OH in
chloroform-d. 2D DOSY NMR measurements performed on a
mixture of zinc ethyl 2b and zinc methoxy 3a in benzene-d6
revealed their similar diffusion constants (ca. 2.1 × 10–8 m2/s and
1.6 × 10–8 m2/s, respectively). Because the molecular weights of
2b differs by only 2 amu from that of the monomeric form of 3a,
these results suggest that the soluble form of 3a is mostly
monomeric.
dehydrocoupling of primary, secondary, or tertiary alcohols and
primary or secondary organosilanes to give trialkoxy, dialkoxy,
or monoalkoxy organosilanes and H2 as the byproduct (eq 2;
Table 1). Catalytic conversions occur readily at room
temperature, although a few examples are improved upon mild
heating. In addition, solvent-free reactions provide products
efficiently.
Figure 2. Thermal ellipsoid plot of {PhBOXMe2Zn(μ-OMe)}2 (3a)2. H atoms are
not included in the plot. Selected interatomic distances (Å): Zn1–O3, 1.976(2);
Zn1–N1, 1.973(2); Zn1–N2, 1.989(3).
O
O
O
N
N
N
N
1.5 equiv. R'OH
Ph
Zn–Me
Ph
Zn–OR'
benzene
< 5 min
– CH4
O
10 mol % PhBOXMe2Zn–Me
2a
PhBOXMe2ZnOR'
(2)
+
H2
+
R'OH
Si
Si
25 - 60 °C, 10 min - 24 h
benzene or neat
R' = Me, iC3H7,
tBu, C6Me2H3
X
X
H
OR'
Y
X = Y = H
X = Me, Y = H
X = Ph, Y = H
Y
R'
O
O
O
N
r.t., 1 h
0.5
Ph
N
Zn
Ph
Zn
Methanol gives quantitative substitution of all silicon hydrides
with 10 mol % 2a, producing PhSi(OMe)3, PhMeSi(OMe)2, or
Ph2Si(OMe)2 within 10 min at room temperature in benzene or
under solvent-free conditions. 2a provides a long-lived and
effective catalyst, resulting in up to 107 turnovers (3 × 106 equiv.
of PhSi(OMe)3 are formed). The reaction of PhSiH3 and excess
O
N
N
(3a)2 OR' = OMe (95%)
(3b)2 OR' = iPr (92%)
O
O
R'
(3c)2 OR' = 3,5-C6H4Me2 (95%)
{
PhBOXMe2Zn(μ-OR')}2
Scheme 2. Synthesis and Dimerization/precipitation of PhBOXMe2ZnOR'.
iC3H7OH provides PhSi(OiC3H7)3 after
1 day under neat
conditions at room temperature or after 4 h in benzene at 60 °C.
Attempts to synthesize mono-methoxy PhH2SiOMe by Zn-
catalyzed reactions of one equiv. of MeOH and PhSiH3 afford a
mixture of PhH2SiOMe and PhHSi(OMe)2. Instead, monoalkoxy
species such as PhH2SiOtBu, PhMeHSiOiPr, PhMeHSiOtBu,
Ph2HSiOiPr, and Ph2HSiOtBu are synthesized by Zn-catalyzed
reactions of iPrOH or tBuOH. Secondary silanes easily provide
monoalkoxy silane products, such as PhMeHSiOiPr. Even in the
presence of excess iPrOH, the tertiary silane Ph2HSiOiPr is
obtained after 1 d at room temperature rather than Ph2Si(OiPr)2.
Remarkably, the reactions involving tBuOH can be performed
solvent-free to access partially substituted silanes directly.
Tertiary silanes such as triethylsilane and methyldiphenylsilane
are inert toward dehydrocoupling reactions with methanol using
2a under these conditions. Clearly, tri-substituted silane
intermediates such as Ph(MeO)2SiH, PhMe(MeO)SiH, and
Ph2(MeO)SiH are reactive toward MeOH in the presence of
catalytic zinc. Because there are only minor differences in the
steric properties at the silicon centers in Ph2MeSiH and
Ph2(MeO)SiH, the greater reactivity of the monoalkoxy silane is
most likely the result of its greater electrophilicity.
Single-crystal X-ray diffraction studies of (3a)2 and (3b)2 reveal
that the isolated dimeric species contain planar Zn2O2 cores
composed of four-coordinate zinc centers bridged by alkoxide
groups (see Figure 2 for (3a)2 and Supporting Information for
(3b)2). Both structures contain
a crystallographic inversion
center at the center of the Zn2O2 core, relating the two
PhBOXMe2Zn units and the two alkoxide ligands. The Zn–O
distances (mean 1.975 Å) within both compounds are equivalent
within 3σ error, giving rhombus-shaped cores. In addition, the
PhBOXMe2Zn moieties form a similar planar six-member chelate
ring as in the zinc alkyls; however, the methyl substituents are
more symmetrically disposed about the plane of the chelate ring
compared to the twisted conformations in 2a and 2b. That is, the
torsion angles in (3a)2 ∠C1–C2–C7–C8 (115.7(4)°) and ∠C3–
C2–C7–C6 (121.5(3)°) provide an approximate C2v conformation.
These different conformations likely reflect low energy barriers
(flexibility) in the BOXZn motif, despite the rigid six-member
chelate ring.
3
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RESEARCH ARTICLE
Table 1. 2a-Catalyzed Dehydrogenative Cross-Coupling of
Hydrosilanes and Alcohols Table Caption.
distinguished by coordination of the aryl alcohol (Kinetic Regime
1: phenol first) or hydrosilane (Kinetic Regime 2: silane first) to
the zinc aryloxide catalytic species (Scheme 3). The relative
concentration of aryl alcohol and silane substrates appears to be
the primary factor that determines which pathway is dominant. In
addition, the observed rate constants in both mechanistic
regimes are affected by saturation behavior by one or both of
the reactants, leading to several concentration dependences.
The saturation in organosilane leads to the important conclusion
that a σ-complex Zn↼H–Si forms prior to the σ-bond metathesis
step forming the Si–O bond. Saturation kinetic in aryl alcohol
concentration indicates that a ZnOAryl(HOAryl) adduct is formed
prior to formation of the Si–O bond under conditions of excess
Reactants
Products
Conditions[a]
% Yield[b]
15:50
PhSiH3 + 1.1
MeOH
PhH2SiOMe +
PhHSi(OMe)2
rt, <10 min
PhSiH3 + 3.5
MeOH
PhSi(OMe)3
PhSi(OMe)3
PhSi(OMe)3
rt, <10 min
100 (98)
100
PhSiH3 + 3.5
MeOH
rt, neat, <10 min
PhSiH3 + 3.5
MeOH
0.00001 mol %
2a,
100
rt, neat, 1 h
arylalcohol. We propose that these mechanisms are
a
consequence of the three-coordinate nature of the monomeric
PhBOXMe2Zn–X catalytic species. This coordinative unsaturation
allows the catalyst to adapt its structure to the reaction
conditions, to maintain high reactivity.
PhSiH3 + 3.5
iPrOH
PhSi(OiPr)3
PhSi(OiPr)3
PhH2SiOtBu
PhH2SiOtBu
60 °C, 4 h
rt, neat, 24 h
rt, 24 h
95 (90)
98
PhSiH3 + 3.5
iPrOH
First, initiation of the pre-catalyst 2a involves its rapid
reaction with ArylOH to produce the aryloxido-zinc 3c, or with
MeOH or iC3H7OH to give 3a or 3b, respectively, as described
above. In contrast, solutions of 2a and PhMeSiH2 contained only
starting materials after standing at room temperature for 4 d. In
addition, NMR spectra and gas chromatograms of reaction
mixtures lacking the PhBOXMe2ZnMe pre-catalyst, revealed only
PhMeSiH2 and ArylOH at their initial concentrations, and
PhMeHSi–OAryl and H2 products were not detected. The rate
laws determined under a wide range of reactant concentration
also show first-order dependence on the initial [2a], suggesting
the active catalytic species is monomeric.
PhSiH3 + 1.1
tBuOH
88
PhSiH3 + 1.1
tBuOH
rt, neat, 24 h
rt, 1 h
100
PhMeSiH2 +
1.1 MeOH
PhMeHSiOMe
+
PhMeSi(OMe)2
42:30
PhMeSiH2 +
3.5 MeOH
PhMeSi(OMe)2
PhMeSi(OMe)2
PhMeHSiOiPr
PhMeHSiOtBu
PhMeHSiOtBu
PhMeHSiOAryl
rt, <10 min
rt, neat, 30 min
60 °C, 4 h
88
PhMeSiH2 +
3.5 MeOH
100 (92)
73 (68)
80
Kinetic Regime 1.
In the presence of excess of 3,5-dimethylphenol
([ArylOH]:[PhMeSiH2] = 1.55:1, with [ArylOH] = 0.18 ± 0.01 M,
60 °C) second-order plots of [PhMeSiH2] vs time (Figure 3)
analyzed by non-linear least squares regression provide rate
PhMeSiH2 +
1.1 iPrOH
PhMeSiH2 +
1.1 tBuOH
60 °C, 4 h
constants k1obs. A plot of second-order rate constants k1
vs
obs
PhMeSiH2 +
1.1 tBuOH
rt, neat, 24 h
60 °C, 3 h
100 (82)
100 (65)
[2a] from 2.1 to 25.1 mM reveals a linear correlation (Figure 3,
inset), with the slope corresponding to the observed ternary rate
constant k1'obs = 0.086 ± 0.005 M–2s–1. The non-zero (but small)
value for the y-intercept of 6 × 10–4 M–1s–1 suggests a catalyst-
free background reaction, in conflict with the lack of background
reaction under catalyst-free conditions. Instead, the dependence
of this y-intercept value on [ArylOH] results from a [ArylOH]-
dependent displacement of the equilibrium between
PhBOXMe2ZnOAryl and PhBOXMe2ZnOAryl(HOAryl). As evidence,
a subsequent series of kinetic experiments revealed that the
PhMeSiH2 +
1.1 ArylOH
Aryl = C6Me2H3
Ph2SiH2 +
3.5 MeOH
Ph2Si(OMe)2
Ph2HSiOiPr
Ph2HSiOiPr
Ph2HSiOtBu
rt, neat, 10 min
rt, 24 h
100 (94)
100
Ph2SiH2 +
1.1 iPrOH
slope and intercept of the plot of k1 vs [2a] were affected by
obs
Ph2SiH2 +
3.5 iPrOH
rt, 24 h
100 (89)
100 (85)
the
concentration
of
3,5-dimethylphenol,
with
[ArylOH]:[PhMeSiH2] = 8:1 giving a flatter slope (k1'obs = 0.015
M–2s–1) and smaller y-intercept (7.6 × 10–5 M–1s–1).
Ph2SiH2 +
1.1 tBuOH
rt, 24 h
Together, this observation and the unlikely represention of
the ternary rate law by a single termolecular elementary step
suggest a mechanism involving a two-step sequence, in which
the first reactant and the catalyst form an adduct in a reversible
step, followed by reaction of the complex intermediate with the
second reactant.[57] This two-step reaction mechanism is further
supported by kinetic saturation of initial rates at high [ArylOH].
The initial rates of product formation (d[PhMeHSiOAryl]/dt)
increase with increasing concentrations of 3,5-dimethylphenol
[a] Standard conditions unless specified: 10 mol % 2a pre-catalyst, benzene (2
mL) or solvent-free (neat), 0.9 mmol hydrosilane. Reactions performed with
3.15 mmol MeOH, 3.15 mmol iC3H7OH, 0.99 mmol iC3H7OH, 0.99 mmol
tBuOH. [b] (Isolated yield).
Kinetics and mechanism of zinc-catalyzed dehydrocoupling
reactions. The kinetic behavior of 2a-catalyzed dehydrocoupling
of 3,5-dimethylphenol and PhMeSiH2, as described below,
indicates that 2a-catalyzed dehydrocoupling reactions follow two
distinct mechanisms. The two proposed mechanisms are
4
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RESEARCH ARTICLE
A. Catalytic Cycles
B. Derived Rate Expressions
H2
ArylOH
H2
Aryl
O
PhMeHSi–OAryl
H
[Zn] H
[Zn]
H
OAryl
Phenol First
k1Ak1B Ph
[
BOXMe2ZnMe]1[PhMeSiH2]1[ArylOH]1
Si
Ph
H
rate =
Me
Kinetic Regime 2:
three-coordinate Zn
pathway,
Kinetic Regime 1:
k1A[ArylOH] + k–1A + k1B[PhMeSiH2] + Ki[ArylOH]2
[Zn]
O
four-coordinate Zn
pathway
Phenol First
Aryl
k2B
Silane First
k1B
Silane First
k2Ak2B PhBOXMe2ZnMe]1[PhMeSiH2]1[ArylOH]0
[
PhMeSiH2
rate =
k–1A
k1A
k2A[PhMeSiH2] + k–2A + k2B
k–2A
Aryl
O H
OAryl
Aryl
[Zn]
H
O
Ki
HOAryl
[Zn]
H
H
k2A
PhMeSiH2
[Zn]
OAryl
Si
HOAryl
Me
Ph
OAryl
O
H
Aryl
Scheme 3. Proposed Catalytic Mechanisms and Rate Expressions for Zinc-Catalyzed Dehydrocoupling.
until 0.44 M, at which point the reaction rate decreases
(Figure 4) attributed to catalyst inhibition by coordination of
the phenol reactant. This observation of inhibition by ArylOH
provides additional important evidence reinforcing the
proposed catalytically-relevant sequence involving reversible
coordination of one arylalcohol molecule to zinc, followed by
reaction with hydrosilane. In particular, the reverse order
(coordination by hydrosilane then Si–O bond formation by
reaction with ArylOH) would require, unreasonably, ArylOH to
simultaneously inhibit the intermediate and react with that
same intermediate in a productive catalytic step.
A
0.12
0.1
∆
[
PhMeSiH ]
2
0
0
[
PhMeSiH ]
=
2
t
1
{k
∆ t
0
}
obs
[
ArylOH
]
0 × e
– [PhMeSiH ]
2
0
0.08
0.06
0.04
0.02
0
0
4
4
4
4
4
4
6×10
1×10
2×10
3×10
4×10
5×10
time (s)
0.003
0.0025
0.002
0.0015
0.001
0.0005
0
B
k1’
obs = 0.086 ± 0.005 M–2s–1
Figure 4. Plot of initial rate of PhMeHSiOAryl formation vs. [ArylOH]
showing decreasing rate constant.
An experimental rate law combining these observations is
given in equation 3, where k1cat corresponds to the product of
the two forward steps, K1m is related to the Michaelis constant
(rates consuming the intermediate divided by the rate
constant for the first step), and K1
is the inhibition
inhib
equilibrium constant.
1
⎡
⎤1 ⎡
⎤
⎦
1
Me
1
kcat Ph BOX 2ZnMe PhMeSiH2 ⎦ ⎣ArylOH
⎡
⎤
d ⎡PhMeHSiOAryl⎤
⎣
⎥
⎦
⎢
⎣
⎦
⎣
0
0.005
0.01
0.015
0.02
0.025
0.03
=
(3)
1+ Km1 ⎡⎣ArylOH⎤ + Ki1nhib ⎣⎡ArylOH⎤2
Ph
[
Me
BOX 2ZnMe] M
dt
⎦
⎦
At low [ArylOH], a ternary rate law is observed, and the
observed order of [ArylOH] dependence becomes zero and
then inverse as its concentration increases.
Figure 3. A. Plots of [PhMeSiH2] vs time for its dehydrocoupling reaction
with ArylOH catalyzed by PhBOXMe2ZnMe at 60 °C, with catalytic
concentration ranging from 1.14 mM to 25.1 mM. [ArylOH]ini = 0.18 ± 0.01
M, [PhMeSiH2]ini = 0.115 ± 0.008 M. B Plot of second order rate constants
k1 vs. [2a]. The slope corresponds to the ternary rate constant k1'obs
=
obs
0.086 ± 0.005 M–2s–1
.
5
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RESEARCH ARTICLE
Kinetic Regime 2.
At lower ArylOH concentrations, the time dependences of
both [PhMeSiH2] and [ArylOH] follow an exponential decay
follows the second-order rate law of equation 4 (k2'obs = 9.8 ±
0.4 × 10–3 M–1s–1) in Kinetic Regime 2.
d ArylOH
ꢀ
ꢂ
ꢃ
1 ꢀ
= k2 'obs ꢀ2aꢂ PhMeSiH2
ꢂ1
ꢁ
−
(4)
ꢁ
ꢃ ꢁ
ꢃ
(Figure 5A; 1.4
> [ArylOH]/[PhMeSiH2] > 1.2; average
dt
[ArylOH]ini = 0.12 M, average [PhMeSiH2]ini = 0.096 M),
indicating that the transformation is first-order in only one of
the reactants. These data indicate that either PhMeSiH2 or
For comparison, the observed second-order rate constant for
ToMZnH-catalyzed dehydrocoupling is 0.014 M–1s–1 at
60 °C.[25] Thus, the experimental rate law, rate constant, and
reactivity, as well as Si–O bond formation as the turnover-
limiting step for 2a-catalyzed dehydrocoupling of ArylOH and
PhMeSiH2 in Kinetic Regime 2 are comparable to the catalytic
ArylOH is present in the turnover-limiting step, in
a
remarkable contrast to the behavior in Kinetic Regime 1.
Experiments varying [2a], with [PhMeSiH2]ini
features of four-coordinate ToMZnH.
A
first major
0.14
A
consequence, then, of the coordinative unsaturation of 2a is
the creation of the new catalytic mechanism in Kinetic
Regime 1, rather than increasing the catalytic rate.
0.12
2
–k
t
obs
[PhMeSiH ] = [PhMeSiH ]
2 t 2 0
e
Zero-order dependence on [ArylOH] in Figure 5b,
however, is ambiguous with respect to the dependence of
rate on [PhMeSiH2] at that high concentration. Unexpectedly,
initial rates of product formation (d[PhMeHSiOAryl]/dt) reveal
saturation behavior as [PhMeSiH2]ini is increased, giving zero-
order dependence on [PhMeSiH2] at high initial
concentrations (Figure 6). Thus, equation 4 represents the
lower-concentration limiting case of the bimolecular
Michaelis-Menten-type description of the catalytic kinetics in
equation 5.
2
–k
t
obs
[ArylOH] = [ArylOH]
e
0.1
0.08
0.06
0.04
0.02
0
t
0
2
–4 –1
= 2.01 ± 0.006 10
k
s
obs
[ArylOH]
t
2
–4 –1
= 2.59 ± 0.002 10 s
⎤1 ⎡PhMeSiH ⎤1
k
obs
⎡
d ⎡PhMeHSiOAryl⎤ kcat 2a
⎣
⎦
⎣
⎦ ⎣
2
⎦
[PhMeSiH ]
2 t
=
(5)
dt
⎡
⎤
Km + PhMeSiH2
⎣
⎦
0
2000
4000
6000
8000
10000
This rate law is consistent with the bimolecular reaction of
time (s)
PhBOXMe2ZnOAryl and PhMeSiH2 occurring in two steps:
reversible association of PhMeSiH2 and PhBOXMe2ZnOAryl to
give an adduct, followed by extrusion of the product in an
irreversible step (Scheme 3, right). These data, which lead to
the revised mechanism of Kinetic Regime 2, reveal this
second consequence of the coordinative unsaturation of
precatalyst 2a.
B
7×10–2
6×10–2
5×10–2
k2obs = 0.007(2) s–1
4×10–2
3×10–2
2×10–2
[
PhBOXMe2ZnMe] = 0.011 M
[PhMeSiH2] = 1.8 M
1×10–2
0
0
200
400
600
time (s)
800
1000
Figure 5. Plots of [ArylOH] and [PhMeSiH2] vs. time in 2a-catalyzed
dehydrocoupling reactions in Kinetic Regime 2 follow (A) exponential time
dependence indicative of first-order behavior or (B) linear time dependence
indicative of zero-order behavior, with the observed rate constant k2
.
obs
and [ArylOH]ini kept constant, reveal first-order dependence
on catalyst concentration.
At high [PhMeSiH2]ini, 2a-catalyzed dehydrocoupling
shows zero-order kinetic dependence on the ArylOH limiting
reactant (Figure 5B). These data indicate that under
conditions of moderate [PhMeSiH2]ini, the catalytic reaction
Figure 6. Initial rates vs. [PhMeSiH2]ini reveals saturation behavior. k2SAT
(often defined as kcat) is equal to the rate at saturation (Vmax) divided by
catalyst concentration.
obs
6
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RESEARCH ARTICLE
The structure of the alkoxyzinc·silane adduct could
involve hydrosilane coordination to the zinc center (Scheme
4a). As noted in the Introduction, M↼H–Si adducts prior to
the four-centered electrocyclic transition have been
disfavor this kind of intermediate. The formation of such a
hyper-coordinated silane intermediate does not require a
coordinatively unsaturated metal center, which appears to be
the significant feature of the chemistry of PhBOXMe2ZnX.
Moreover, the saturation in hydrosilane in Kinetic Regime 2
parallels saturation with excess ArylOH in Kinetic Regime 1,
where coordination of phenol to zinc is reasonably
established. Finally, the distinct rate laws of the two Kinetic
Regimes suggest that their mechanisms are inequivalent,
postulated in σ-bond metathesis-type on the basis of DFT
[46]
calculations,
but these species are typically fleeting.[31]
Although hydrosilane adducts to dn transition metal centers (n
≠ 0) are well established,[58-62] including in silane/alcohol
dehydrocoupling reactions,[16] the core-like 3d orbitals of zinc
are unable to provide the stabilizing back-donating interaction
important to transition metal σ-adducts.
which would also imply that
reasonable.
a zinc-silane adduct is
DFT calculations favor the Zn↼H–Si adduct over a hyper-
coordinate silane adduct preceding the four-centered
transition state during σ-bond metathesis. In particular, the
energy of the system increases as the alkoxyzinc oxgen
approaches the silicon center of PhMeSiH2, and a local
minimum involving a ZnO➝SiPhMeH2 interaction could not
be located. In contrast, the Zn↼H–Si, involving a side-on
interaction, is a local minimum (Figure 7). In particular, the
Zn…H distance of 2.23 Å in this adduct is longer than Zn–H in
three coordinate (DIPP-nacnac)ZnH (1.46(2) Å; DIPP-nacnac
= HC{CHN(2,6-C6H3iPr2)}2)[63] and in four-coordinate ToMZnH
(1.52(2) Å)[64] and [κ3-Tptm]ZnH (1.51(3) Å).[65] The calculated
Zn…H, as well as Zn…Si and Si…O distances (3.32 and 2.95 Å,
respectively), are also longer than the sum of covalent
radii.[66] The ∠Zn↼H–Si angle (124.5°) is far from linear, the
∠ Zn–O…Si–H torsion is –11.8°, and the silicon center is
tetrahedral. In addition, the terminal (1.48 Å) and bridging
(1.50 Å) Si–H distances are very similar, although the
calculated vibrational frequencies of 2147 and 2072 cm–1,
respectively, are not equivalent.
(a)
O
O
R
PhMeSiH2
N
N
O
H
N
N
H
Ph
Zn
Ph
Zn–OR
Me
Ph
Si
O
O
O
(b)
O
O
PhMeSiH2
N
N
N
R
H
Ph
Zn–OR
Ph
Zn
O
N
Si
Me
Ph
O
H
Scheme 4. Possible Equilibria That Could Lead to Saturation Kinetics in
[PhMeSiH2].
Alternatively, coordination of the nucleophilic alkoxide to
silicon would give a five-coordinate silane adduct (Scheme
4b). Formation of this intermediate could also rationalize the
higher reactivity of alkoxy-substituted silanes, via stabilization
of the higher-coordinate silicon center by electronegative
substituents. Several experimental observations, however,
Figure 7. The calculated zinc·silane adduct is an intermediate leading to a four-centered transition state associated with σ-bond metathesis. The phenyl group
and all H atoms bonded to carbon were included in the calculation but not in the illustration for clarity.
7
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RESEARCH ARTICLE
This Zn↼H–Si adduct is connected by an IRC calculation to
the four-centered transition state, characterized by one negative
vibrational mode. The calculated energy coordinate diagram for
Si–OMe bond formation is shown in Scheme 5. In the transition
state, the Zn…H and O…Si distances are shortened (2.12 and
2.30 Å, respectively) and the Si–H and Zn–O distances are
slightly longer (1.52 and 1.89 Å, respectively) than in the adduct.
The calculated νSiH (1912 cm–1) of the moiety participating in the
electrocyclic transition state is greatly reduced with respect to
the non-bridging νSiH (2117 cm–1). Remarkably, the negative
mode corresponds to motion of O and Si along a vector
connecting the two atoms. The Zn and H atoms are also moving
along a vector connecting the two atoms. Although the Zn⟷H
motion is smaller in magnitude than the O⟷Si motion, the two
sets of motion are in phase.
intermediates at high alcohol concentrations, and formation of
alkoxyzinc·silane
intermediates
at
high
hydrosilane
concentrations. Kinetic experiments, corroborated by DFT
calculations, provide powerful evidence in support of hydrosilane
σ-complexes as intermediates prior to the σ-bond metathesis
transition state.
We also note that phenol is a catalyst inhibitor at high
concentration, as depicted in the Phenol First cycle in Scheme 3.
This inhibition either by coordinating to zinc or hydrogen-bonding
to the reactive phenoxide group, likely blocks interaction with the
hydrosilane reactant. Such structures undoubtably affect related
catalytic transformations involving organozinc and coordinating
reactants, because the zinc center is able to effectively adapt its
coordination sphere to reaction conditions.
Conflict of Interest
The authors declare no conflict of interest.
Acknowledgements
This research was supported by the U.S. Department of Energy
(DOE), Office of Science, Basic Energy Sciences, Division of
Chemical Sciences, Geosciences, and Biosciences. Ames
Laboratory is operated for the DOE by Iowa State University
under Contract No. DE-AC02-07CH11358.
Scheme 5. Reaction Free Energies (298.16 K) Showing the σ-Bond
Metathesis Step for Si–O Bond Formation.
Keywords: σ-adduct • silyl ether • dehydrogenative cross-
coupling • adaptive zinc catalysis • saturation kinetics
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RESEARCH ARTICLE
Entry for the Table of Contents
σ-Silane adducts and alcohol adducts are kinetically-detected intermediates in zinc-catalyzed σ-bond metathesis-type
dehydrocoupling reactions. These intermediates feature prominently in the reactivity of coordinatively unsaturated zinc centers, which
efficiently mediate Si–O bond formations. The catalyst is responsive to reaction conditions, adapting as a function of concentration to
follow the kinetically preferred mechanism.
Institute and/or researcher Twitter usernames: @SadowISUChem
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