Electrophilic alkylations of vinylsilanes: A comparison of α- And β-silyl effects

Article abstract of DOI:10.1002/chem.201303215

Kinetics of the reactions of benzhydrylium ions (Aryl₂CH ⁺) with the vinylsilanes H₂C=C(CH₃)(SiR ₃), H₂C=C(Ph)(SiR₃), and (E)-PhCH=CHSiMe ₃ have been measured photometrically in dichloromethane solution at 20 °C. All reactions follow second-order kinetics, and the second-order rate constants correlate linearly with the electrophilicity parameters E of the benzhydrylium ions, thus allowing us to include vinylsilanes in the benzhydrylium-based nucleophilicity scale. The vinylsilane H₂C= C(CH₃)(SiMe₃), which is attacked by electrophiles at the CH₂ group, reacts one order of magnitude faster than propene, indicating that α-silyl-stabilization of the intermediate carbenium ion is significantly weaker than α-methyl stabilization because H ₂C=C(CH₃)₂ is 10³ times more reactive than propene. trans-β-(Trimethylsilyl)styrene, which is attacked by electrophiles at the silylated position, is even somewhat less reactive than styrene, showing that the hyperconjugative stabilization of the developing carbocation by the β-silyl effect is not yet effective in the transition state. As a result, replacement of vinylic hydrogen atoms by SiMe₃ groups affect the nucleophilic reactivities of the corresponding C=C bonds only slightly, and vinylsilanes are significantly less nucleophilic than structurally related allylsilanes. Copyright

Full text of DOI:10.1002/chem.201303215

DOI: 10.1002/chem.201303215
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&
Nucleophilicity
Electrophilic Alkylations of Vinylsilanes:
A Comparison of a- and b-Silyl Effects
Hans A. Laub and Herbert Mayr*[a]
Dedicated to Professor Heinz Langhals on the occasion of his 65th birthday
Abstract: Kinetics of the reactions of benzhydrylium ions
weaker than a-methyl stabilization because H2C=C(CH3)2 is
(Aryl2CH+) with the vinylsilanes H2C=C(CH3)(SiR3), H2C= 103 times more reactive than propene. trans-b-(Trimethylsi-
C(Ph)(SiR3), and (E)-PhCH=CHSiMe3 have been measured
photometrically in dichloromethane solution at 208C. All re-
actions follow second-order kinetics, and the second-order
rate constants correlate linearly with the electrophilicity pa-
rameters E of the benzhydrylium ions, thus allowing us to in-
clude vinylsilanes in the benzhydrylium-based nucleophilicity
scale. The vinylsilane H2C=C(CH3)(SiMe3), which is attacked
by electrophiles at the CH2 group, reacts one order of mag-
nitude faster than propene, indicating that a-silyl-stabiliza-
tion of the intermediate carbenium ion is significantly
lyl)styrene, which is attacked by electrophiles at the silylated
position, is even somewhat less reactive than styrene, show-
ing that the hyperconjugative stabilization of the developing
carbocation by the b-silyl effect is not yet effective in the
transition state. As a result, replacement of vinylic hydrogen
atoms by SiMe3 groups affect the nucleophilic reactivities of
the corresponding C=C bonds only slightly, and vinylsilanes
are significantly less nucleophilic than structurally related al-
lylsilanes.
Introduction
Vinylsilanes represent important reagents in organic synthesis
and commonly undergo electrophilic substitution reactions
with replacement of the silyl group.[1] It has been shown that
they can be combined with a large variety of electrophiles, in-
cluding carbonyl compounds, Michael acceptors, acyl deriva-
tives, halogens, and stabilized carbocations.[1] Reactions involv-
ing reagents of low electrophilicity, like carbonyl compounds,
commonly need activation of either the electrophile by Lewis
acids or the vinylsilanes by nucleophiles, for example, fluoride
ions. The vinylsilane functionality has also been utilized as an
internal nucleophile for trapping intermediate carbocations.[2]
Carbenes and peracids have been applied for the vinylsilane-
based synthesis of silylated cyclopropanes[3] and epoxysila-
nes,[1c,i,4] respectively.
Scheme 1. Electrophilic substitution of vinyl- (a+c) and allylsilanes (b);
E=electrophile, EDG=electron-donating group.
Although the reactions in Scheme 1a,b proceed via inter-
mediates with a carbocationic center in the position b to the
silyl group, allylsilanes are known to react under milder condi-
tions and with weaker electrophiles than the corresponding vi-
nylsilanes,[1i] indicating that allylsilanes are more nucleophilic
than vinylsilanes. The lower efficiency of the organosilicon
moiety to enhance the nucleophilic reactivity of vinylsilanes
can also be deduced from the fact that the introduction of
electron-donating substituents, for example, alkoxy, alkyl, or
aryl groups, in the position a to the silyl group of vinylsilanes
directs the electrophilic attack to the b-position of the vinylsi-
lanes, thus giving rise to the formation of a-silyl stabilized
carbocations (Scheme 1c).[5]
Electrophilic attack at vinylsilanes commonly occurs at the
olefinic carbon atom a to the silicon moiety, eventually leading
to the direct displacement of the silyl group (ipso substitution,
Scheme 1a). Electrophilic substitutions of allylsilanes usually
follow an SE2’ mechanism, where the electrophilic attack
occurs in the position g to the silyl substituent, giving the
product with allyl rearrangement (Scheme 1b).[1i]
[a] H. A. Laub, Prof. Dr. H. Mayr
Soderquist and Hassner[6] studied the rates of formation of
such a-silyl stabilized carbocations through the deuterolyses of
a-silylated methyl vinyl ethers. We now report on the magni-
tude of a- and b-silyl effects on the electrophilic alkylations of
vinylsilanes by studying the kinetics of the reactions of the
Department Chemie, Ludwig-Maximilians-Universitꢀt Mꢁnchen
Butenandtstr. 5–13 (Haus F), 81377 Mꢁnchen (Germany)
Fax: (+49)89-2180-77717
E-mail: herbert.mayr@cup.uni-muenchen.de
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201303215.
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For this study, benzhydrylium ions 2a–j with values of elec-
trophilicity parameter E from ꢁ4.72 to +3.63 (Table 1) were
used as reaction partners for organosilanes 1a–h.
Table 1. Reference electrophiles ArAr’CH+ used for quantifying the
nucleophilicities of 1a–h.
Scheme 2. Organosilanes studied for the quantification of the a- and b-silyl
effects in vinylsilanes.
ArAr’CH+[a]
X
Y
E[b]
3.63
(tol)2CH+ (2a)
CH3
CH3
(Ph)(pop)CH+ (2b)
(tol)(pop)CH+ (2c)
(Ph)(ani)CH+ (2d)
(tol)(ani)CH+ (2e)
(ani)(pop)CH+ (2 f)
(ani)2CH+ (2g)
H
CH3
H
CH3
OMe
OMe
N(Ph)CH2CF3
N(Me)CH2CF3
NPh2
OPh
OPh
OMe
OMe
OPh
OMe
N(Ph)CH2CF3
N(Me)CH2CF3
NPh2
2.90
2.16
2.11
1.48
0.61
propene- (1a–c) and styrene-derived organosilanes (1d–h,
Scheme 2) with carbenium ions.
By utilizing the benzhydrylium method[7] for the quantifica-
tion of the nucleophilic reactivities of 1a–h, it will be possible
to compare their nucleophilicities with those of previously in-
vestigated compounds, including allylsilanes,[7a,f,8] silyl enol
ethers,[7a,8b,9] and ordinary alkenes.[7a,b,f,g] All of these com-
pounds are part of a comprehensive nucleophilicity scale,
which has been derived by the method of overlapping correla-
tion lines by using benzhydrylium ions with variable para and
meta substituents as reference electrophiles.[7a] The correlations
are based on Equation 1, where electrophiles are characterized
by the electrophilicity parameter E and nucleophiles by the sol-
vent-dependent parameters sN (sensitivity) and N (nucleophili-
city).
0.00
(pfa)2CH+ (2h)
ꢁ3.14
ꢁ3.85
ꢁ4.72
(mfa)2CH+ (2i)
(dpa)2CH+ (2j)
[a] tol=p-tolyl; pop=p-phenoxyphenyl; ani=p-anisyl; pfa=4-(phenyl(tri-
fluoroethyl)amino)phenyl; mfa=4-(methyl(trifluoroethyl)amino)phenyl;
dpa=4-(diphenylamino)phenyl. [b] Empirical electrophilicities
E from
ref. [7a].
Results and Discussion
Product analysis
lg kð20 ꢀCÞ ¼ sNðN þ EÞ
ð1Þ
For the attack of electrophiles at vinylsilanes, two sites of
attack are conceivable—in position a or b to silicon (Sche-
me 1a,c). As shown in Scheme 3, vinylsilanes 1a–f with a termi-
nal double bond are attacked at the b-position with formation
of the a-silyl stabilized carbocations, which undergo different
subsequent reactions.
Abstract in German: Die Kinetik der Reaktionen von Benzhy-
drylium-Ionen (Aryl2CH+) mit den Vinylsilanen H2C=C(CH3)(SiR3),
H2C=C(Ph)(SiR3) und (E)-PhCH=CHSiMe3 wurde in Dichlormethan
bei 208C photometrisch bestimmt. Alle Reaktionen verlaufen
nach einer Kinetik 2. Ordnung, und die Geschwindigkeitskonstan-
ten 2. Ordnung korrelieren linear mit der Elektrophilie E der Benz-
hydrylium-Ionen, wodurch es mçglich wird, Vinylsilane in die auf
Reaktivitꢀten gegenꢁber Benzhydrylium-Ionen aufgebaute Nucle-
ophilieskala aufzunehmen. Das Vinylsilan H2C=C(CH3)(SiMe3), das
von Elektrophilen an der CH2-Gruppe angegriffen wird, reagiert
um eine Grçßenordnung schneller als Propen, was darauf hin-
weist, dass die Stabilisierung des intermediꢀren Carbokations
durch den a-Silyl-Effekt wesentlich schwꢀcher ist als die Stabili-
sierung durch den a-Methyl-Effekt, da H2C=C(CH3)2 103 mal reak-
tiver ist als Propen. trans-b-(Trimethylsilyl)styrol, das von Elektro-
philen an der silylierten Position angegriffen wird, ist sogar etwas
weniger reaktiv als Styrol, was zeigt, dass die hyperkonjugative
Stabilisierung des entstehenden carbokationischen Zentrums im
ꢂbergangszustand noch nicht wirksam ist. Somit beeinflusst der
Austausch eines vinylischen Wasserstoffs durch eine SiMe3-
Gruppe die nucleophile Reaktivitꢀt der betrachteten C=C Doppel-
bindung nur marginal, und Vinylsilane sind betrꢀchtlich weniger
nucleophil als strukturell verwandte Allylsilane.
While the formation of 4a can be explained by a 1,2-hydride
shift in 3a, followed by a chloride-induced desilylation,[10a] car-
bocations 3b and 3 f are probably intercepted by chloride ions
to give a-chlorosilanes, which hydrolyze with formation of the
alcohols 4b and 4 f during workup. The formation of 4 f’ as
a side product is probably due to the oxidation of 4 f by at-
mospheric oxygen; an analogous conversion of a-silyl substi-
tuted benzyl alcohols by tBuONO into arylalkylketones has pre-
viously been reported.[11] 1,2-Silyl migration in 3c followed by
fluoride trapping of the resulting silylium ion can explain the
formation of the fluorosilane 4c. In analogy to previously re-
ported reactions of benzhydrylium ions with ordinary al-
kenes,[12] the a-silyl substituted carbocation 3d cyclizes with
formation of the indane 4d with high diastereoselectivity (d.r.
98:2, main diastereoisomer shown in Scheme 3).
In contrast to the vinylsilanes with terminal double bonds,
trans-b-(trimethylsilyl)styrene 1g was attacked at the silyl-sub-
stituted vinylic carbon yielding the phenyl-stabilized carbocat-
ion 3g, which gives 4g by desilylation.[10b] For comparison,
also the allylsilane 1h was investigated and found to give the
common SE2’ substitution product 4h.
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Scheme 3. Reactions of silyl substituted propenes (1a–c) and styrenes 1d,f–h with benzhydrylium salts (DTBP=2,6-di-tert-butylpyridine; dma=4-(dimethyl-
amino)phenyl).
Kinetic measurements
dividing the corresponding first-order rate constants kobs by
the corresponding mean nucleophile concentrations
[Nu]av =[Nu]0ꢁ0.5[2-MXn+1]0. Figure 2 shows exemplarily the
linear Eyring plot for the reaction of 1d with 2b-GaCl4 be-
tween ꢁ698C and ꢁ308C. From analogous correlations, the ac-
tivation parameters DHꢀ and DSꢀ and the second-order rate
constants k2 at 208C given in Table 2 were calculated.
Table 2 shows that most reactions studied at variable tem-
perature possess activation entropies between ꢁ112 and
ꢁ138 Jmolꢁ1 Kꢁ1, similar to those previously reported for the
reactions of benzhydrylium ions with terminal alkenes[12b,13]
and allylsilanes.[8a,13b] It is not clear, why the reaction of 1e
with 2d proceeds with significantly more negative activation
entropy.
Since the benzhydrylium ions 2a–j possess absorption maxima
in the visible spectrum, while the products arising from the nu-
cleophilic attack are colorless, we were able to study the kinet-
ics of these bimolecular reactions in CH2Cl2 spectrophotometri-
cally. First-order conditions were obtained by employing at
least 9 equivalents of the unsaturated organosilanes 1a–h with
respect to the benzhydrylium ions 2a–j. From the resulting
mono-exponential decays of the absorbances of the benzhy-
drylium ions, the corresponding first-order rate constants kobs
were derived.
The second-order rate constants k2 given in Table 2 and
Table 3 were derived as the slopes of the linear correlations of
the kobs values against the corresponding nucleophile concen-
trations [Nu]0 (Figure 1).
Table 3 shows that the second-order rate constants for the
reactions of the organosilanes 1b, 1 f, and 1g with the benz-
hydrylium ions 2d, 2e, and 2g are only slightly affected by the
counterions. Therefore, the second-order rate constants given
in Table 2 can be assigned to the formation of the initial CꢁC
bond, while the subsequent reactions of the intermediates 3b,
The reactions with the highly reactive benzhydrylium ions
2a,b and the reaction of 1e with 2d-GaCl4 were studied in the
temperature range between ꢁ708C and ꢁ208C. In these cases,
the second-order rate constants k2 were calculated by
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Table 2. Second-order rate constants k2 for the reactions of organosi-
lanes 1a–h with benzhydrylium ions 2 (counterion GaCl4ꢁ) in dichlorome-
thane and the nucleophilicity parameters for 1a–h derived thereof.
Nucleophile
(N, sN)[a]
2
k2 (208C)
DHꢀ
[kJmolꢁ1
DSꢀ
[Jmolꢁ1 Kꢁ1
]
[mꢁ1 sꢁ1
]
]
2a
2b
2c
2d
2e
223[b]
30.6[b]
4.28
4.89
1.30
25.9
27.1
–
–
–
ꢁ112
ꢁ124
–
–
–
Figure 1. Exponential decay of the absorbance at 488 nm during the reac-
tion of 1c (c=3.18ꢁ10ꢁ4 m) with 2e-GaCl4 (c=2.43ꢁ10ꢁ5 m) at 208C in
CH2Cl2. Inset: k2 =13.6mꢁ1 sꢁ1 is obtained as the slope of the linear correla-
tion of the first-order rate constants kobs against [1c]0.
2d
2e
2g
53.7
15.2
0.557
–
–
–
–
–
–
2d
2e
2g
63.6
13.6
0.507
–
–
–
–
–
–
2a
2b
2d
2e
2 f
4.10ꢁ103[b]
403[b]
28.7
3.75
16.1
16.8
–
–
–
ꢁ121
ꢁ138
–
–
–
0.153
2d
2e
2 f
177[b]
28.3
13.3
–
–
ꢁ156
Figure 2. Eyring plot for the reaction of 1d with 2b-GaCl4 in CH2Cl2 between
ꢁ698C and ꢁ308C.
–
–
0.777
2e
2 f
2g
137
14.8
4.49
–
–
–
–
–
–
ꢁ
3 f, and 3g with the halometallates MXn+1 have no or only
little influence on the overall rate. As counterion independence
of the rate constants has already been shown in previous stud-
ies of allylsilanes, we assume that the measured rate constants
for the reactions of allylsilanes 1h with benzhydrylium ions
also refer to the CꢁC bond-forming step.[8–9]
2b
2d
2e
2 f
407[b]
66.1
13.3
1.32
18.1
ꢁ133
–
–
–
–
–
–
–
–
2g
0.387
2h
2i
2j
102
22.1
3.94
–
–
–
–
–
–
Linear free-energy relationships
Plots of lgk2 for the reactions of the benzhydrylium ions 2
with the propene derivatives 1a–c (Figure 3) and styrene deriv-
atives 1d–h (Figure 4) against the electrophilicity parameters E
[a] For determination, see next section. [b] Calculated by using the Eyring
activation parameters DHꢀ and DSꢀ which were determined between
ꢁ708C and ꢁ208C, see the Supporting Information for details.
Table 3. Counterion effects on the second-order rate constants for the
reactions of vinylsilanes 1 with benzhydrylium salts 2-MXn+1 in dichloro-
methane.
MXn+1
k2 (208C) [mꢁ1 sꢁ1
]
1b+2d
53.7
45.9
57.2
–
1 f+2e
137
–
141
–
1g+2d
66.1
–
1g+2e
13.3
–
1g+2g[a]
0.387
0.468
0.402
–
ꢁ
GaCl4ꢁ
SnCl5ꢁ
FeCl4
–
–
ꢁ
BCl4
73.4[b]
15.0[c]
[a] With triflate as counterion the kinetics do not follow a first-order rate
law. [b,c] Calculated by using the following Eyring activation parameters
(for determination see the Supporting Information): [b] DHꢀ =
Figure 3. Plots of lgk2 for the reactions of the propene derivatives 1a–c and
5b–d (from ref. [7a]) with benzhydrylium ions in CH2Cl2 at 208C versus the
electrophilicity parameters E of the benzhydrylium ions.
23.2 kJmolꢁ1 and DSꢀ =ꢁ130 Jmolꢁ1 Kꢁ1
;
[c] DHꢀ =26.7 kJmolꢁ1 and
DSꢀ =ꢁ131 Jmolꢁ1 Kꢁ1
.
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owing to the fact that the stabilization of the intermediate car-
bocation by a b-silyl substituent is much more effective than
that by an a-silyl substituent. Comparison of propene (5a)
with 1a and of styrene (6a) with 1d shows different silyl ef-
fects in both reaction series: while the trimethylsilyl group in
a-position of the new carbenium center activates propene by
a factor of 10 (1a/5a), it deactivates styrene by a factor of 18
(1d/6a). These opposing effects can be assigned to the pertur-
bation of conjugation in a-substituted styrenes, in line with
the observation that introduction of an a-methyl group acti-
vates propene by a factor of 3100 (5b/5a) and styrene by only
55 (6b/6a). Analogously, an a-CH2SiMe3 group activates pro-
pene by 3.5ꢁ106 (5c/5a) and styrene by only 8.7ꢁ103 (1h/
6a); for the same reason, OSiMe3 also activates propene 102
times more (5d/5a=1.4ꢁ107) than styrene (6d/6a=1.9ꢁ105).
Perturbation of p-conjugation in a-substituted styrenes also
explains why SiMe2SiMe3 activates by a factor of 102 in the pro-
pene series (1b/5a), but has no effect in the styrene series
(1 f/6a). The perturbation of the conjugation between vinyl
and phenyl group by SiMe3, CH3, CH2SiMe3, and OSiMe3 in the
a-position is also reflected by the UV spectra of the styrenes[14]
listed in Table S1 of the Supporting Information.
Figure 4. Plots of lgk2 for the reactions of styrene derivatives 1d,f–h and
6a,d (from ref. [7a,b]) with benzhydrylium ions in CH2Cl2 at 208C versus the
electrophilicity parameters E of the benzhydrylium ions.
of the reference electrophiles show linear correlations. Equa-
tion 1 allowed us to calculate the N and sN parameters for the
organosilanes 1a–h, which are given in Table 2.
These correlations do not only confirm the consistency of
the rate constants determined in this work but also allow to
extrapolate rate constants for reactions that cannot be directly
measured. Such rate constants are needed for the structure-re-
activity correlations in Table 4. Because the sN parameters differ
slightly for the various p-systems, the relative reactivities
depend somewhat on the nature of the attacking electrophile.
For that reason, small reactivity differences should only be dis-
cussed when systems with equal sN are compared. In order to
avoid ambiguity, the following discussion will focus on the re-
activities toward the p-methoxy substituted benzhydrylium ion
2d, for which most directly measured rate constants are avail-
able.
In the propene as in the styrene series, the SiMe2SiMe3
group activates one order of magnitude more than the SiMe3
group (1b/1a and 1 f/1d), while the supersilyl group[15]
(Si(SiMe3)3) activates by the same extent as SiMe2SiMe3 (1c/
1b), because only one SiꢁSi s-bond of the Si(SiMe3)3 group
can be aligned coplanar with the empty p orbital of the devel-
oping carbenium center. The vinyl silanes 1b, 1c, and 1 f may
thus be considered as sila-allylsilanes as illustrated in Figure 5
While an allylic SiMe3 group activates isobutylene by 3
orders of magnitude (5c/5b), the introduction of an analo-
gously positioned SiMe3 group in 1a accelerates only by one
order of magnitude (1b/1a). Therefore, the hyperconjugative
stabilization of a carbocation by a CꢁSi bond (Figure 5, left) is
Table 4 shows that all vinylsilanes (1a–f) investigated are sig-
nificantly less nucleophilic than the allylsilanes 5c and 1h
Table 4. Comparison of absolute (in mꢁ1 ꢁ1) and relative rate constants for the reactions of silyl substituted propenes (upper part) and styrenes (lower
s
part) with the p-methoxy substituted benzhydrylium ion 2d in CH2Cl2 at 208C.
[a] Taken from ref. [12b]. [b,c] Calculated by using Eq. 1, E(2d)=2.11 and the corresponding nucleophilicity parameters reported in: [b] ref. [7a];
[c] Table 2. [d] Calculated by using the Eyring equation, k2(ꢁ708C)=1.45ꢁ103 mꢁ1 sꢁ1 reported in ref. [12b], and a value of DSꢀ =ꢁ110 Jmolꢁ1 Kꢁ1 (estimat-
ed by taking activation parameters of structurally related compounds into account; ref. [7a,b]).
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Figure 5. Comparison of b-silyl stabilization of the intermediate carbocations
obtained by electrophilic attack at allylsilanes (left) and SiMe2SiMe3 (middle)
as well as supersilyl (right) substituted vinylsilanes.
Scheme 5. Comparison of absolute (in mꢁ1
s
ꢁ1) and relative rate constants
for the reactions of 4-methyl substituted styrenes with the benzhydrylium
ion 2e in CH2Cl2 at 208C (tol=p-tolyl).[7a]
significantly larger than the hyperconjugative stabilization of
a carbocation by a SiꢁSi bond (Figure 5, middle).
Nevertheless, replacement of the SiMe3 group by the super-
silyl group has an effect in the vinylsilane series, as 1c is one
order of magnitude more reactive than 1a. In contrast, earlier
work has shown that replacement of the SiMe3 group by the
supersilyl group Si(SiMe3)3 has almost no effect in allylsilanes
and silylated enol ethers (Table 5) due to the fact that in these
species the SiꢁSi bond is not in a suitable position for hyper-
conjugative interaction with the new carbenium center.[8b]
ene by only 7.4 (1e/1d). This reduction of the electron-releas-
ing effect of the 4-methyl substituent can again be rationalized
by the perturbation of conjugation in a-substituted styrenes.
Vinylsilanes without electron-donating substituents at Ca or
with electron donors at Cb are usually attacked at Ca by elec-
trophiles to give b-silyl stabilized carbenium ions. Despite this
stabilization, 1g reacts almost one order of magnitude more
slowly with the benzhydrylium ion 2d than styrene
(Scheme 6). This retardation of the electrophilic attack can be
Table 5. Supersilyl effects in allylsilanes and silyl enol ethers (absolute
and relative rate constants toward 2i in CH2Cl2, 208C, from ref. [8b]).
Scheme 6. Absolute (in mꢁ1 ꢁ1) and relative rate constants for the reactions
s
of styrene (6a) and trans-b-(trimethylsilyl)styrene (1g) toward the p-methoxy
substituted benzhydrylium ion 2d in CH2Cl2 at 208C (ref. [7a]).
[a] Toward 2g. [b] Calculated by using Eq. 1, N and sN from ref. [7a] and
E(2i)=ꢁ3.85.
explained by the steric shielding of the ipso position and the
fact that the hyperconjugative stabilization of the intermediate
carbocation requires rotation around the former C=C bond
and is obviously not yet effective in the transition state of elec-
trophilic attack.
Soderquist and Hassner[6] reported that the silylated vinyl
ether 7b is deuterated only 1.8 times faster than the vinyl
ether 7a, while we found an activation by a factor of 9.6 when
comparing the electrophilic alkylations of the analogous pro-
pene derivatives 1a and 5a with the benzhydrylium ion 2d
(Scheme 4). The diminished a-silyl effect observed for the vinyl
ether 7b may not only be due to the different electrophile
considered but could also result from the smaller electron
demand of the alkoxy-substituted carbenium ion, which is gen-
erated from vinyl ethers in the rate-determining step.
As shown in Scheme 7, the situation in vinylsilanes differs
from that in allylsilanes, where the hyperconjugative b-silyl sta-
bilization can already become effective in the transition state.
Related silyl effects have previously been observed in the
furan and thiophene series (Scheme 8).[16] Comparison of the
first and third number column of Scheme 8 shows that
a methyl group in 2-position increases the nucleophilicity by
In contrast, Scheme 5 shows that the introduction of a 4-
methyl substituent activates styrene for the reaction with
benzhydrylium ion 2e by a factor of 17 (8/6a), while a 4-
methyl group activates the less reactive a-(trimethylsilyl)styr-
Scheme 4. Comparison of the relative rates for the deuterolyses of vinyl
ethers 7a,b[6] with the relative rates for the alkylation of the propene deriva-
tives 5a and 1a with the benzhydrylium ion 2d.
Scheme 7. Differences of the stereoelectronic effects in reactions of electro-
philes with vinylsilanes (top) and allylsilanes (bottom).
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Scheme 8. Relative rate constants k2 for the reactions of furan and thio-
phene derivatives toward benzhydrylium ions in CH2Cl2 at 208C ([a] towards
the ferrocenylphenylcarbenium ion, [b] partial rate constant, [c] towards the
bis(p-anisyl)carbenium ion 2g; ref. [16]).
three orders of magnitude, similar to the isobutylene/propene
(5b/5a) ratio (Table 4). From the almost equal reactivities of
non-silylated and silylated compounds in columns 3 and 4 of
Scheme 8, one can derive that in the heteroarene series the
weak electronic activation of the silyl-substituted position is
fully compensated by the steric retardation. The stereoelec-
tronic arguments (Scheme 7, top) which explain the (even re-
tarding) silyl effect in the comparison 1g/6a (Scheme 6) apply
analogously.
The weak activation of the 5-position in furan and thiophene
by a 2-trimethylsilyl group (compare columns 1 and 2 in
Scheme 8) is in the same order of magnitude as in the compar-
ison 1a/5a (Table 4), which also measures the effect of SiMe3
in a-position of the resulting carbenium ion. As this activation
is stronger than the ipso activation, 2-(trimethylsilyl)furan and
2-(trimethylsilyl)thiophene react with electrophiles in the 5-po-
sition and not in the 2-position, as indicated by the arrows in
Scheme 8.[16] The substituent effects found in the vinylsilane
series are, therefore, fully consistent with those previously ob-
served in the furan and thiophene series.
Figure 6. Nucleophilicities of propene (left) and styrene derivatives (right)
compared to those of other p-nucleophiles (ref. [7b]); sN values given in pa-
rentheses.
Conclusion
The kinetic measurements reported in this article allow us to
include vinylsilanes in our comprehensive nucleophilicity scale
(Figure 6), which shows that vinylsilanes are significantly less
reactive than structurally related allylsilanes.
Scheme 9. Approximate changes of nucleophilicity due to replacement of
the marked hydrogens by different groups.
Scheme 9 compares the effects on nucleophilicity when allyl-
ic and vinylic hydrogens (marked by shaded circles) are re-
placed by different groups. Replacement of the marked allylic
hydrogen atom in isobutylene by the trimethylsilyl or the su-
persilyl group activates the p-bond for electrophilic attack by
a factor of 103 (Scheme 9A), which is explained by the well-
known hyperconjugative stabilization of the intermediate car-
benium ion by the CꢁSi s-bond.[1,8a] The equal effects of SiMe3
and Si(SiMe3)3 can be explained by the large separation of the
developing carbenium center from the SiꢁSi bond. Replace-
ment of the allylic hydrogen atom by an alkyl group has little
SiꢁSi hyperconjugation, thus prompting us to consider vinylsi-
lanes incorporating the supersilyl group (or the SiMe2SiMe3
group) as sila-allylsilanes.
From the comparison 5a<1a<1b<5b’ꢂ5b !5c in
Figure 6 we can derive the following series of carbenium-sta-
bilizing effects: H<SiꢁC<SiꢁSi<CꢁCꢂCꢁH !CꢁSi. At first
glance, it may be surprising that the hyperconjugative stabili-
zation of a carbenium center through a SiꢁC bond is so much
weaker than that through a CꢁSi bond. One reason is the
smaller overlap between the empty p-orbital with the SiꢁC
bond than with the CꢁSi bond because of the longer C+ꢁSi
effect on nucleophilicity owing to the similar magnitude of Cꢁ bond compared with the C+ꢁC bond (Scheme 10).[17] A second
H and CꢁC hyperconjugation.[12b]
Replacement of the marked vinylic hydrogen atom in pro-
pene by SiMe3 activates by one order of magnitude showing
that the stabilizing effect of silicon in the a-position of the de-
veloping carbocation is significantly smaller than the effect of
a methyl group (103, Scheme 9B). The larger effect of a supersil-
yl group at the developing carbenium center (102) can be ex-
Scheme 10. Comparison between hyperconjugative stabilization of a carbeni-
plained by stabilization of the intermediate carbenium ion by
um center by a SiꢁC (left) and a CꢁSi (right) bond.
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111; f) L. A. Paquette, Science 1982, 217, 793–800; g) S. Pawlenko, Orga-
nosilicon Chemistry, de Gruyter, Berlin, 1986; h) E. W. Colvin, Silicon Re-
agents in Organic Synthesis, Academic Press, London, 1988; i) I. Fleming,
J. Dunoguꢂs, R. Smithers in Organic Reactions, Vol. 37, Wiley, Hoboken,
NJ, 1989, pp. 57–575; j) J. S. Panek in Comprehensive Organic Synthesis,
Vol. 1 (Eds.: B. M. Trost, I. Fleming, S. L. Schreiber), Pergamon, Oxford,
1991, pp. 579–627; k) I. Fleming, A. Barbero, D. Walter, Chem. Rev.
1997, 97, 2063–2192; l) M. A. Brook, Silicon in Organic, Organometallic,
and Polymer Chemistry, Wiley, New York, 2000; m) K. Oshima in Science
of Synthesis, Vol. 4 (Eds.: I. Fleming), Thieme Verlag, Stuttgart, 2002,
pp. 713–756; n) T. K. Sarkar in Science of Synthesis, Vol. 4 (Eds.: I. Flem-
ing), Thieme Verlag, Stuttgart, 2002, pp. 837–925; o) L. Chabaud, P.
James, Y. Landais, Eur. J. Org. Chem. 2004, 3173–3199; p) M. J. Curtis-
Long, Y. Aye, Chem. Eur. J. 2009, 15, 5402–5416.
reason is the polarization of the carbon–silicon bond which is
reflected by the larger AO coefficient at the carbon atom in
a sCꢁSi bond.
Whereas methyl groups in the b-position of styrene have
a weak activating or deactivating effect on the reactivity to-
wards electrophiles,[13b] b-trimethylsilyl groups deactivate
slightly (Scheme 9C), indicating that the steric retardation
overcompensates the weak electronic activation because the
perpendicular orientation between developing carbenium
center and the CꢁSi s-bond inhibits significant hyperconjuga-
tive stabilization of the transition state of electrophilic attack.
As a result, vinylsilanes are generally only slightly activated or
deactivated compared to the corresponding alkenes, inde-
pendent of the position of the silyl group in a- or b-position of
the intermediate carbocation.
[2] T. A. Blumenkopf, L. E. Overman, Chem. Rev. 1986, 86, 857–873.
[3] a) J. Cudlꢃn, V. Chvalovsky, Collect. Czech. Chem. Commun. 1962, 27,
1658–1665; b) J. Cudlꢃn, V. Chvalovsky, Collect. Czech. Chem. Commun.
1963, 28, 3088–3095; c) I. A. D’Yakonov, G. V. Golodnikov, I. B. Repin-
skaya, Zh. Obshch. Khim. 1965, 35, 2181–2189; d) D. Seyferth, H. M.
Cohen, Inorg. Chem. 1962, 1, 913–916; e) D. Seyferth, H. Dertouzos, J.
Organomet. Chem. 1968, 11, 263–270.
Experimental Section
[4] a) H. Sakurai, N. Hayashi, M. Kumada, J. Organomet. Chem. 1969, 18,
351–354; b) G. S. Patil, G. Nagendrappa, J. Chem. Soc. Perkin Trans. 2
2001, 1099–1102.
Chemicals
[5] a) L. H. Sommer, F. J. Evans, J. Am. Chem. Soc. 1954, 76, 1186–1187;
b) L. H. Sommer, D. L. Bailey, G. M. Goldberg, C. E. Buck, T. S. Bye, F. J.
Evans, F. C. Whitmore, J. Am. Chem. Soc. 1954, 76, 1613–1618.
[6] J. A. Soderquist, A. Hassner, Tetrahedron Lett. 1988, 29, 1899–1902.
[7] a) H. Mayr, T. Bug, M. F. Gotta, N. Hering, B. Irrgang, B. Janker, B. Kempf,
R. Loos, A. R. Ofial, G. Remennikov, H. Schimmel, J. Am. Chem. Soc.
2001, 123, 9500–9512; b) H. Mayr, B. Kempf, A. R. Ofial, Acc. Chem. Res.
2003, 36, 66–77; c) H. Mayr, A. R. Ofial in Carbocation Chemistry (Eds.:
G. A. Olah, G. K. S. Prakash), Wiley, Hoboken, NJ, 2004, pp. 331–358;
d) H. Mayr, A. R. Ofial, Pure Appl. Chem. 2005, 77, 1807–1821; e) H.
Mayr, A. R. Ofial, J. Phys. Org. Chem. 2008, 21, 584–595; f) J. Ammer, C.
Nolte, H. Mayr, J. Am. Chem. Soc. 2012, 134, 13902–13911; g) for a com-
prehensive database of reactivity parameters, see http://www.cup.uni-
muenchen.de/oc/mayr/DBintro.html.
[8] a) G. Hagen, H. Mayr, J. Am. Chem. Soc. 1991, 113, 4954–4961; b) H. A.
Laub, H. Yamamoto, H. Mayr, Org. Lett. 2010, 12, 5206–5209.
[9] J. Burfeindt, M. Patz, M. Mꢄller, H. Mayr, J. Am. Chem. Soc. 1998, 120,
3629–3634.
[10] a) G. Hagen, Ph.D Thesis, Medizinische Universitꢅt zu Lꢄbeck (Germany),
1990; b) M. Herrlich, Ph.D Thesis, Ludwig-Maximilians-Universitꢅt Mꢄn-
chen (Germany), 2001.
[11] M. D. Paredes, R. Alonso, J. Org. Chem. 2000, 65, 2292–2304.
[12] a) H. Mayr, R. Pock, Chem. Ber. 1986, 119, 2473–2496; b) H. Mayr, R.
Schneider, B. Irrgang, C. Schade, J. Am. Chem. Soc. 1990, 112, 4454–
4459.
[13] a) H. Mayr, R. Schneider, C. Schade, J. Bartl, R. Bederke, J. Am. Chem. Soc.
1990, 112, 4446–4454; b) H. Mayr, R. Schneider, U. Grabis, J. Am. Chem.
Soc. 1990, 112, 4460–4467.
[14] a) H. Suzuki, Bull. Chem. Soc. Jpn. 1960, 33, 619–628; b) H. Suzuki, Elec-
tronic Absorption Spectra and Geometry of Organic Molecules: An Appli-
cation of Molecular Orbital Theory, Academic Press, New York, 1967; c) I.
Benito, H. Seidl, H. Bock, Rev. Fac. Cienc., Univ. Oviedo 1973, 14, 95–110.
[15] a) H. Bock, J. Meuret, K. Ruppert, Angew. Chem. 1993, 105, 413–415;
Angew. Chem. Int. Ed. Engl. 1993, 32, 414–416; b) M. B. Boxer, H. Yama-
moto, J. Am. Chem. Soc. 2007, 129, 2762–2763.
CH2Cl2 (p.a. grade) used for kinetic experiments was purchased
from Merck and successively treated with concentrated sulfuric
acid, water, 10% NaHCO3 solution, and water. After drying with
CaCl2, it was freshly distilled over CaH2 under exclusion of moisture
(N2 atmosphere). GaCl3, TiCl4, FeCl3, and BCl3 were purchased and
used as obtained. SnCl4 was purchased and distilled prior to use.
Ethereal HBF4·Et2O was purchased from Aldrich. For the syntheses
of nucleophiles and electrophiles as well as the product studies,
see the Supporting Information.
Kinetics
All kinetics were followed using UV/Vis spectroscopy. The kinetic
experiments conducted below 208C were carried out by using the
method described previously.[13a] The rates of the reactions studied
at 208C were determined by using a J&M TIDAS diode array spec-
trophotometer controlled by Labcontrol Spectacle Software and
connected to a Hellma 661.502-QX quartz Suprasil immersion
probe (5 mm light path) through fiber optic cables and standard
SMA connectors.
Acknowledgements
The authors thank Dr. Mirjam Herrlich-Loos for studying the re-
actions of 1g with 2d,e-BCl4 and Dr. Gisela Hagen-Bartl for
a product study (1a+2a). Helpful discussions with Dr. Armin
Ofial are gratefully acknowledged. We thank the Deutsche For-
schungsgemeinschaft (SFB 749, Project B1) for financial sup-
port.
[16] M. Herrlich, N. Hampel, H. Mayr, Org. Lett. 2001, 3, 1629–1632.
[17] Similar effects have also been reported for hyperconjugative interac-
tions with CꢁS vs. SꢁC bonds: a) I. V. Alabugin, T. A. Zeidan, J. Am.
Chem. Soc. 2002, 124, 3175–3185; b) I. V. Alabugin, K. M. Gilmore, P. W.
Peterson, WIREs Comput. Mol. Sci. 2011, 1, 109–141.
Keywords: electrophilic substitution
kinetics · linear free-energy relationships · organosilanes
· hyperconjugation ·
[1] a) C. Eaborn, Organosilicon Compounds, Butterworth, London, 1960;
b) E. W. Colvin, Chem. Soc. Rev. 1978, 7, 15–64; c) T. H. Chan, I. Fleming,
Synthesis 1979, 761–786; d) E. W. Colvin, Silicon in Organic Synthesis,
Butterworth, London, 1981; e) I. Fleming, Chem. Soc. Rev. 1981, 10, 83–
Received: August 14, 2013
Published online on December 16, 2013
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1110
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