The β-silicon effect. II. Substituent effects on the solvolysis of 1-aryl-2-(aryldimethylsilyl)ethyl 3,5-dinitrobenzoates

Article abstract of DOI:10.1246/bcsj.79.1091

Solvolysis rates of 2-(dimethylphenylsilyl)-1-(Y-phenyl)ethyl 3,5-dinitrobenzoates were determined conductimetrically in 60% (v/v) aqueous ethanol. In order to clarify the nature of the β-Si participation quantitatively, the effects of α-aryl substituents on the rates were analyzed by means of the Yukawa-Tsuno Eq. The α-aryl (Y)-substituent effect at 25°C was correlated with r ? 1:0 and ρ = -3:0, which is significantly reduced compared with that of -5:45 for the non-silylated 1-arylethyl system. There is a linear relationship between log k _Y=k_H of silylated and non-silylated substrates: log(k _Y/k_H)_Si = 0:52 log(k_Y=k _H)_non-Si. This is the same form as the extended Bronsted relationship. The Bronsted coefficient α = 0:52 appeared to be consistent with the neighboring silyl-participation in the silyl-bridged transition state.

Full text of DOI:10.1246/bcsj.79.1091

ꢀ 2006 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 79, No. 7, 1091–1099 (2006) 1091
The ꢀ-Silicon Effect. II. Substituent Effects on the Solvolysis
of 1-Aryl-2-(aryldimethylsilyl)ethyl 3,5-Dinitrobenzoates
ꢀ1
Mizue Fujio, Yuzo Umezaki,2 Md. Ashadul Alam,1
Kiyoshi Kikukawa,2 Ryoji Fujiyama,3 and Yuho Tsuno1
1Institute for Materials Chemistry and Engineering, Kyushu University, Fukuoka 812-8581
2Graduate School of Advanced Technology, Kinki University, Iizuka 820-8555
3Department of Material Science, Faculty of Science, Kochi University, Kochi 780-8520
Received January 5, 2006; E-mail: fujio@ms.ifoc.kyushu-u.ac.jp
Solvolysis rates of 2-(dimethylphenylsilyl)-1-(Y-phenyl)ethyl 3,5-dinitrobenzoates were determined conductimet-
rically in 60% (v/v) aqueous ethanol. In order to clarify the nature of the ꢀ-Si participation quantitatively, the effects of
ꢁ-aryl substituents on the rates were analyzed by means of the Yukawa–Tsuno Eq. The ꢁ-aryl (Y)-substituent effect at
ꢁ
ꢂ
25 C was correlated with r 1:0 and ꢂ ¼ ꢃ3:0, which is significantly reduced compared with that of ꢃ5:45 for the
¼
non-silylated 1-arylethyl system. There is a linear relationship between log kY=kH of silylated and non-silylated sub-
strates: logðkY=kHÞSi ¼ 0:52 logðkY=kHÞnon-Si. This is the same form as the extended Brønsted relationship. The Brønsted
coefficient ꢁ ¼ 0:52 appeared to be consistent with the neighboring silyl-participation in the silyl-bridged transition
state.
It is well known that the ꢀ-silicon substituent greatly facil-
itates the heterolysis of the C–Lg bond, which is caused by a
trimethylsilyl group in an antiperiplanar conformation.2 Stabi-
lization of this electron deficiency was interpreted in terms of
either the open form (1aþ) stabilized by ꢃ–ꢄ hyperconjuga-
tion or by the Si-bridged form (1bþ) of the cationic intermedi-
ate,2–5 as shown in Scheme 1.
systems, the silyl-bridged pathway (1b) was instead support-
ed.1 Thus, the relative importance of these two pathways still
remains substantially unresolved. A question then arises as
to whether or not the kC pathway 1a can be distinguished from
the silyl-bridged kꢀ pathway 1b via the silyl-bridged TS for
the ionization of ꢀ-silylated substrates.
Whatever the mechanism of enormous ꢀ-silyl participation,
the overall participation measured by the reactivity ratio of si-
lylated/non-silylated substrates is anti-proportional to the cat-
ion-stabilities of the non-silylated substrates. Nevertheless, the
silyl-assisted reactivities are proportional to the stabilities of
non-silylated cations. In order to deal quantitatively with such
a relationship as regards the ꢀ-silyl-assisted reactions, we have
chosen for study the solvolysis of 2-trimethylsilyl-1-(Y-phen-
yl)ethyl trifluoroacetates (2),5 and for the present further ex-
tended study, the solvolysis of 2-[dimethyl(X-phenyl)silyl]-1-
The conformational dependence of the ꢀ-silicon effect in
the solvolysis of cyclic secondary systems2,4b was taken as
convincing evidence for the Si–C hyperconjugation mecha-
ꢀ
nism. Numerous results can be explained satisfactorily by
either the non-classical (bridged) or the classical (open cation)
transition state (TS). For stable substrate systems (sec. and tert.
cations, and also ꢁ-phenyl-substrates), in fact, various mecha-
nistic criteria2,4,5 suggested the simple ionization mechanism
(kC) via the open carbocation 1aþ, while in the case of primary
R
MeSiMe
R4
R3
R1
R2
R
MeSiMe
R4
(1a+)
R1
R2
R3
R
E1 or SN1 products
Lg
MeSiMe
(1)
R1
R2
R4
R3
(1b+)
Scheme 1. ꢀ-Silicon effect in the carbocationic solvolysis.
Published on the web July 4, 2006; doi:10.1246/bcsj.79.1091

1092 Bull. Chem. Soc. Jpn. Vol. 79, No. 7 (2006)
The ꢀ-Silicon Effect on Solvolysis Rates
In an earlier study concerning the ꢀ-Me3Si-ꢁ-phenyl sub-
strate system 2, correlation analysis studies on the substituent
effect lent some support for Lambert’s interpretation, in which
the mechanism via the open cationic TS is stabilized by en-
Me
Me Si
Me
Me
Si
X
Lg
Y
ODNBz
Y
Me
hanced C –Si hyperconjugation.5
ꢀ
In this paper, we will deal with the ꢁ-aryl (Y)-substituent
effects on the silicon-participation solvolysis of 4 with a fixed
silyl moiety (X ¼ H), especially aiming at the distinction be-
tween the silyl-hyperconjugation and the neighboring silyl-
group participation mechanisms (1a) and (1b).
(2)
(3) X and Y
(4) X = H fixed
(5) X = 3,5-(CF ) Y variable
variable
Y variable
3 2
Chart 1. Structures of ꢀ-Si-ꢁ-Ar-ethyl systems.
Results
(Y-phenyl)ethyl 3,5-dinitrobenzoates (3) (Chart 1).
One of the major interests underlying our investigation is to
establish how to use the structure–reactivity relationship and
relevant parameters as mechanistic criteria in order to clarify
the structure(s) of the TS(s). The ꢀ-silyl-assisted reaction is
especially attractive for us, because of its enormous rate-accel-
eration as well as the high dependency on substrate structures.
Furthermore, it is most interesting if we can distinguish be-
tween the two silyl-assisted pathways, the vertical (open) ion
and the nonvertical (silyl-bridged) pathways, (1a) and (1b),
respectively.
Rates of Solvolysis. The solvolysis rates of 2-(dimethyl-
phenylsilyl)-1-(Y-phenyl)ethyl 3,5-dinitrobenzoates (4) were
determined conductimetrically in 60% (v/v) aqueous ethanol
(60E) for a wide range of aryl substituents from p-MeO to
3,5-(CF3)2. The rate for the highly reactive 1-(p-methoxyphen-
yl) derivative was obtained for the less reactive leaving ben-
zoate and was converted to kODNBz by the kODNBz=kOBz ratio
of 76.8 derived from the rate ratio for the 1-(p-methylphenyl)-
ethyl derivative. The solvolysis rates of 1-aryl-2-[3,5-bis(tri-
fluoromethyl)phenyldimethylsilyl]ethyl 3,5-dinitrobenzoates
(5) were also measured for several electron donating Y-deriv-
atives. Kinetic data are listed in Table 1. The solvolysis rates
for the parent 3 (X ¼ Y ¼ H) 3,5-dinitrobenzoate were deter-
mined in various solvents at 25.0 ꢁC (in Table 2). Activation
parameters in the present data were of quite normal magnitude
for the SN1 reaction mechanism.
The detailed analysis of the substituent effects in these sys-
tems was carried out by the Yukawa–Tsuno (Y–T) Eq. 1:6
0
ꢁ þ
logðk=k0Þ ¼ ꢂðꢃ þ rꢀꢃR Þ;
ð1Þ
where k is the rate constant for a given reaction of a ring-sub-
stituted derivative and k0 is the corresponding value of the un-
substituted one. Symbol ꢃ0 is the normal substituent constant
that involves no additional ꢄ-electronic interaction between
Product Analysis. The product analysis for the solvolyses
of 2-(dimethylphenylsilyl)-1-phenylethyl 3,5-dinitrobenzoate
was carried out in CD3CD2OD in the presence of excess
2,6-lutidine at 55 ꢁC by using a 1H NMR method. The products
of complete reaction were found to be styrene (64%) accompa-
ꢁ þ
the substituent and the reaction center, and ꢀꢃR is the reso-
nance substituent constant, defined by ꢃþ ꢃ ꢃ0, measuring
the capability for ꢄ-delocalization of a p-ꢄ-donor substituent.
Table 1. Solvolysis Rates of 2-[Dimethyl(X-substituted phenyl)silyl]-1-(Y-substituted phenyl)ethyl 3,5-
Dinitrobenzoate in 60% Aqueous Ethanol
a)
a)
Substituent
H
105 ꢄ k/sꢃ1
(Temp/ꢁC)
ꢀH2z5 ꢁ
ꢀSz25 ꢁ
C
C
Y
X
25 ꢁC
/kcal molꢃ1
/e.u.
p-MeO
2:3 ꢄ 104 b)
301c)
33.0 (5)c)
454 (45)c)
27.3 (5)
3,5-(CF3)2
3:5 ꢄ 103 b)
46.0c)
1:9 ꢄ 103
363
m-Cl-p-MeO
p-Me
H
3,5-(CF3)2
H
20.7
ꢃ0.2
676
8.80c)
68.9
3,5-(CF3)2
H
H
3,5-(CF3)2
H
H
782(45)
10.56(5)
652(45)
70.8(45), 198.7(55)
313(45)
61.6(45)
22.3
21.2
20.1
22.2
21.1
22.9
22.9
24.2
24.9
24.2
1.8
ꢃ0.3
ꢃ5.6
ꢃ3:3
ꢃ3.8
ꢃ1.2
ꢃ2.5
ꢃ3.6
0.8
m-Me
H
149.1
72.9
5.96
31.3
5.07
p-Cl
m-Cl
m-CF3
H
3,5-(CF3)2
2.69
32.7(45)
0.169
0.528
0.0897d)
2.44(45), 7.72(55)
7.88(45)
4.14(55), 36.9(75)
3,5-Cl2
3,5-(CF3)2
H
H
ꢃ4.9
a) 1 cal ¼ 4:184 J. b) Estimated value from the rate data for the benzoate based on the 3,5-dinitro-
benzoate/benzoate ratio for Y ¼ p-Me. c) Rate data for the benzoate. d) Extrapolated from the rate
data at other temperatures.

M. Fujio et al.
Bull. Chem. Soc. Jpn. Vol. 79, No. 7 (2006) 1093
2.0
1.5
Table 2. Solvolysis Rates of 2-(Dimethylphenylsilyl)-1-
phenylethyl 3,5-Dinitrobenzoate at 25 ꢁC
97T
80T
50M
Me
ODNBz
Si
105 ꢄ k
105 ꢄ k
Me
Solventa)
Solventa)
60M
/sꢃ1 at 25 ꢁC
/sꢃ1 at 25 ꢁC
1.0
50E
90E
80E
70E
60E
50E
80A
70A
60A
8.19
22.5b)
39.9
50A
40A
97Tb)
80T
MeOH
80M
60M
50M
56.8
142.2
1200
1143
10.35
64.0
40A
60E
80M
0.5
50A
72.9
70E
0.0
MeOH
60A
112.4
2.22
6.60
80E
-0.5
-1.0
-1.5
70A
90E
264
555
aq EtOH
aq Acetone
aq MeOH
aq TFE
22.1
80A
a) Volume percent (v/v) of first named organic component un-
less otherwise noted. Abbreviations: E = EtOH, M = MeOH,
A = Acetone, T = 2,2,2-trifluoroethanol. b) Weight % (w/w)
of organic component.
-2
-1
0
1
2
3
Y
Cl
Fig. 1. The W–G plots against YCl for the solvolysis of
2-(dimethylphenylsilyl)-1-phenylethyl 3,5-dinitrobenzoate
at 25 ꢁC.
nied with the substitution product, ethyl ether (36%), and the
composition of the products did not change during the course
of reaction. This indicates that the rate-determining ionization
leads to the benzylic cation followed by the elimination of the
phenylsilyl moiety or displacement of the solvent. The substi-
tution product is sufficient to support that this reaction pro-
ceeds through the ꢀ-Si carbocation, not through the nucleo-
philic solvent attack to a Si atom. In the solvolysis in 60E, sty-
rene was obtained as the sole product.
The Solvent Effect. The studies of the solvent effects pro-
vide important information to estimate the TS structure, espe-
cially regarding the charge-delocalization. The solvent effect
on the solvolyses of 2-(dimethylphenylsilyl)-1-phenylethyl
3,5-dinitrobenzoate at 25 ꢁC was analyzed in terms of the
Winstein–Grunwald (W–G) equation, Eq. 2:7
3
p-OCH
3
Me
Si
ODNBz
Me
2
1
Y
p-OCH -m-Cl
3
60E, 25 °C
p-CH
3
m-CH
3
H
0
p-Cl
-1
-2
-3
logðk=k80EÞ ¼ mYLg
;
ð2Þ
m-Cl
m-CF
3
where YLg is the ionizing power parameter of respective sol-
vents based on rates of kc solvolysis of the common reference
substrate 1-adamantyl–Lg, and m is its susceptibility of a given
solvolyzing substrate. Because of the lack of an appropriate set
of Y parameters for leaving 3,5-dinitrobenzoate (YODNB), we
employed YCl instead of YODNB. As shown in Fig. 1, the sol-
vent effect exhibited a poor correlation against YCl with signif-
icant splittings for the respective binary solvent series and an
m value of 0.54, indicating a significant decrease in cationic
charge at the reaction center carbon in the TS. The scatterings
from the correlation line were observed for the respective bina-
ry solvent series, indicating the dissimilarity of the present de-
localized carbocationic TS to the standard 1-adamantyl system
with the localized cationic TS. We made no further efforts to
delineate the overall plot in this paper. For a precise discus-
sion, it is necessary to compare the W–G correlations for relat-
ed systems with the common leaving group –ODNB. Never-
theless, it is enough to conclude the absence of nucleophilic
solvent assistance in the rate-determining step.
3,5-Cl
2
3,5-(CF
)
3 2
-1.0
-0.5
0.0
0.5
1.0
σ
Fig. 2. Substituent effect on the solvolysis of 1-aryl-2-(di-
methylphenylsilyl)ethyl 3,5-dinitrobenzoates (4) in 60%
(v/v)þaq EtOH at 25 ꢁC: Closed circles for ꢃ0, open circles
ꢁ
for ꢃ , and open squares for ꢃ with r ¼ 1:04.
ꢁ þ
0
logðkY=kHÞ4 ¼ ðꢃ2:95 ꢅ 0:06Þ½ꢃ þ ð1:04 ꢅ 0:05ÞꢀꢃR ꢆ: ð3Þ
The Y–T correlation for the corresponding ꢀ-Me3Si series 2
was reported5a to have closely the same ꢂ and r values as those
in the correlation (3) for the ꢀ-dimethylphenylsilyl series 4,
while the leaving group and also the solvent were different.
0
ꢁ þ
logðk=k0Þ2 ¼ ꢃ3:05ðꢃ þ 1:05ꢀꢃR Þ:
There existed a linear log–log relationship (5) between 4
(X ¼ H) and 5 (X = 3,5-(CF3)2) subsets as shown in Fig. 3,
with a slope of 1.12 (R ¼ 0:998, SD ¼ ꢅ0:11):
ð4Þ
ꢁ-Aryl Substituent Effects. The substituent effects of
the ꢁ-aryl group of 4 and the related systems were analyzed
routinely based on the Y–T Eq. 1. As shown in Fig. 2, the
substituent effects of the ꢁ-aryl group in 4 at 25 ꢁC in 60E
was linearly correlated with r ¼ 1:04 and ꢂ ¼ ꢃ2:95: R ¼
0:9995 and SD ¼ ꢅ0:055,
logðkY=kHÞ5 ¼ ð1:12 ꢅ 0:04Þ logðkY=kHÞ4 ꢃ 0:04;
ð5Þ

1094 Bull. Chem. Soc. Jpn. Vol. 79, No. 7 (2006)
The ꢀ-Silicon Effect on Solvolysis Rates
3
2
4
p-OCH
3
p-OCH
3
p-OCH -m-Cl
3
3
p-CH
p-OCH -m-Cl
1
slope = 0.52
3
3
m-CH
3
slope=1.12
0
H
2
p-Cl
p-CH
3
-1
-2
-3
m-Cl
m-CF
3
1
H
3,5-Cl
2
3,5-(CF
)
3 2
0
-6
-4
-2
0
2
4
6
log (k/k
)
o non-silyl
m-CF
3
Fig. 4. Brønsted plot for 1-aryl-2-(dimethylphenylsilyl)-
ethyl solvolyses.
-1
0
1
2
3
4
5
5 + log k
at 25 °C
X=H
with a coefficient ꢁ ¼ 0:52 for 4; practically the same ꢁ value
was generally obtained for other ꢀ-silyl subsets 2 and 5. Equa-
tion 6 is exactly the same form as the extended Brønsted rela-
tionship,10 provided that the logðkY=kHÞ3n is practically identi-
cal to ꢀpKR+ of the non-silylated system 3n (and further to
that of the silylated system 3 without silyl-participation).
The overall silyl participation was manifested by 1) the ꢃ–ꢄ
hyperconjugation mechanism (1a) between the highly polariz-
Fig. 3. Linear logarithmic plots ðkYÞ4 vs ðkYÞ5 between 4
(X ¼ H) and 5 (X = 3,5-(CF3)2) for 2-[dimethyl(X-sub-
stituted phenyl)silyl]-1-(Y-substituted phenyl)ethyl solvol-
yses.
indicating that the ꢂY value for the X = 3,5-(CF3)2 subset is
ꢃ3:29, as the r remains the same.
The ꢂ values vary less significantly (from ꢃ2:95 to ꢃ3:29)
with a change of silyl moieties, but all were different distinctly
from the reference ꢂ value (ꢂ3n ¼ ꢃ5:45) for the non-silylated
1-phenylethyl system (3n).8
able C –Si bond and the empty p-orbital, and 2) the silyl-
ꢀ
bridging mechanism (1b), where the silicon atom forms a par-
tial C–Si bond to the carbon atom from which the nucleofuge
departs. The bridging (Si–C bond-forming) may be in concert
ꢁ
Discussion
with nucleofuge departure, so that the reaction pathway 1b
should be analogous to the so-called neighboring group partic-
ipation process.
The solvolysis rates of ꢀ-silylated ꢁ-arylethyl substrates
were found to be remarkably higher than the rates of the
non-silylated 3n or the corresponding ꢀ-t-Bu-derivative. The
ꢀ-silicon effects or the overall silyl participation may be as-
sessed most reasonably by rate accelerations, kSi=kt-Bu or
kSi=kH, for the respective Y-derivatives. The ꢀ-silicon effects
in the ꢁ-phenyl system 4 were assessed to be at least of 105
order of magnitude or ca. 7 kcal molꢃ1 for the parent substrate
4 (Y ¼ H).5 Nevertheless, the silyl participation in the ꢁ-phen-
yl system 4 is distinctly less important compared with the par-
ticipation of up to 1012-fold rate acceleration observed in the
other ꢀ-silylated systems 1.2
The neighboring ꢄ-participation of a remote double bond
occurs in the solvolyses of syn-7-aryl-anti-7-norbornenyl p-
nitrobenzoates (6).11 While the corresponding saturated system
6n gave a linear Y–T correlation for the whole substituent
range with a ꢂ value of ꢃ5:3 (r ¼ 1:0) pointing to the ordinary
kC mechanism,6b,11 the unsaturated 6 gave a clearly bisected
plot; one falling on the 6n line with the same ꢂ for strong elec-
tron donating (ED) p-Me2N, and another for the range less ED
than the p-MeO group with a reduced ꢂ value of ꢃ2:3 against
ꢁ
the same ꢃ scale (at r ¼ 1:0) (Fig. 5), indicating competition
from the C=C assisted mechanism (ꢁ ¼ 0:43 ¼ ꢃ2:3=ðꢃ5:3Þ)
(Scheme 2).
As shown in Fig. 2, the substituent (Y) effect in regards to
the substrate 4 was linearly correlated with a ðꢂYÞSi value of
Similar participation of a neighboring electron-rich group
occurs significantly in the solvolysis of 1-aryl-5-methyl-
hepten-5-yl chlorides (7).12 In the hexen-5-yl series 7, the p-
anisyl derivative solvolyzes practically at the same rate as that
in the corresponding saturated series 7n (Scheme 3).
Thus, if the stabilization of the TS is enhanced by the elec-
tron-donating p-MeO group, the neighboring group participa-
tion of the 5-double bond becomes insignificant. Below this
leveling point of reactivity, the compounds of the unsaturated
series 7 solvolyze faster than the corresponding compounds of
the saturated series 7n (Fig. 5). The acceleration, logðkꢀ=kCÞ,
for the respective Y derivatives appears to be significant and
becomes more important as Y becomes more electron with-
drawing. Equation 6 applies also to the ꢄ-participation of
the remote double bond with a similar ꢁ-value of 0.4.
ꢁ
ꢃ2:95 against ꢃ (r ¼ 1:04). Essentially, the same correlation
(ðꢂYÞSi ¼ ꢃ3:05 and r ¼ 1:05) was found for the ꢀ-trimethyl-
silyl subset 2.5a The excellent linearity of the correlation indi-
cates the absence of a significant mechanistic change for the
whole range of Y-substituents. However, the ꢂY value of the
ꢀ-silylated system is distinctly lower than that in the non-silyl-
ated 1-phenylethyl system (3n). The size of ꢂ is commonly be-
lieved to reflect the extent of charge development at the reac-
tion site directly involved in the bond-breaking process of Lg
at the TS.9
As shown in Fig. 4, there is a precise linear relationship
between the silylated (Si) and non-silylated (n) reactivities,
logðkY=kHÞSi and logðkY=kHÞ3n:
logðkY=kHÞSi ¼ ꢁlogðkY=kHÞ3n;
ð6Þ

M. Fujio et al.
Bull. Chem. Soc. Jpn. Vol. 79, No. 7 (2006) 1095
Y
Y
Y
δ−
OPNB
OPNB
OPNB
k∆
δ+
δ+
δ+
(6n)
(6)
Scheme 2. Solvolysis of syn-7-aryl-anti-7-norbornenyl system.
k∆
+
Y
Y
Y
Cl
[7]
10
[7+]
kc
A
+
5
[7n+]
B
Scheme 3. Solvolysis of 1-arylhexen-5-yl substrates.
C
unimolecular (ꢂ1 ¼ ꢃ5:0, r ¼ 1:15) and bimolecular (ꢂ2 ¼
ꢃ2{ ꢃ3, r ¼ 1:1) processes, respectively.14
0
D
The Brønsted-type treatment (Eq. 6) is employed as the
most useful tool for mechanistic investigation of the concerted
nucleophilic substitution reactions, which involve varying
degrees of nucleophilic participation at the TS. The criterion
for the change in mechanism should be only a significant
change in the ꢁ-coefficient or the magnitude of ꢂ. Thus, it is
most essential of either the Brønsted-type treatment (Eq. 6)
or Brown’s tool of increasing electron demand that the sub-
stituent effect correlations are based on essentially the same
E
-5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
σ Y
Fig. 5. Y–T plots for 1-aryl-2-(dimethylphenylsilyl)ethyl
(A), 1-aryl-2-[3,5-bis(trifluoromethyl)phenyldimethylsi-
lyl]ethyl (B), syn-7-aryl-anti-7-norbornenyl (C), and 1-
arylhexen-5-yl (D) substrates, for the reference (E), sol-
volyses of 3n and 6n.
ꢁ
ꢃ scale as for the kC solvolysis of the parent substrate, regard-
less of whether or not the participation of the neighboring
group (or nucleophile) takes place.
In earlier studies,5 the linear ꢃþ correlation of 2 was taken
as evidence for the kC mechanism without nonvertical silyl-
ꢂ
In order to detect the neighboring group participation,
Brown applied the tool of increasing electron demand to the
related systems to the above,13 so that the ꢀꢂ (¼ꢂC ꢃ ꢂꢀ)
was used as the criteria instead of the coefficient ꢁ in Eq. 6.
Both criteria can be related for the present system as Eq. 7,
participation; the observed r 1 was considered to be not in
¼
line with the silyl-bridged mechanism via a siliconium ion
(2bþ) in which the resonance interaction should be largely
or totally inhibited. Hence, the ꢁ-phenyl-ꢀ-silylethyl system
3 was referred to as a system of an open cation mechanism
and the aryl (Y)-substituent effect will offer a measure of the
kC mechanism via (1aþ) without bridging.5 However, this ear-
lier conclusion was grossly misleading.
ðꢀꢂÞSi ¼ ð1 ꢃ ꢁÞꢂ3n:
ð7Þ
In Fig. 5, the Brønsted-type correlations are shown as in the
form of the Y–T correlations for the above ꢀ-silyl and neigh-
boring electron-rich group participation systems.
In the general case of the concerted nucleophilic substitu-
tion reactions, the benzylic substituent effects are correlated
though appreciably concave by the same ꢃ scale,
14,15
ꢁ
In the overall ꢀ-silyl participation mechanism in Scheme 1,
the bridged (nonvertical) pathway (1b) can be referred to the
concerted nucleophilic displacement mechanism with an inter-
nal nucleophile (Si-moiety), while the vertical pathway (1a)
may be referred to the internal nucleophile (Si)-unassisted
mechanism. The latter pathway involving the ꢃ–ꢄ hyperconju-
gative stabilization will presumably be compatible with the
so-called kC mechanism in the solvolysis of the corresponding
non-silylated substrate, ꢁ-arylethyl precursor (3n). On the oth-
er hand, the silyl-bridged pathway 1b may be just compared
with a concerted (SN2) mechanism, such as the Menschutkin
reaction of ꢁ-phenylethyl (3n) substrates with pyridine in ace-
tonitrile, for which the Y–T correlations were assigned for
which
þ
is referred to that for pKR of the substrate cations. The struc-
ture of the concerted displacement TS changes from the loose
structure (8az), where a large positive charge ꢀþ resides on ꢁ-
Cþ, to the tight one (8bz), where a small positive charge ꢅþ
resides on ꢁ-Cþ; 8az resulted in a large negative ꢂ ¼ ꢃ5
and 8bz a much reduced ꢂ ¼ ꢃ2{ ꢃ3.14,15 The former TS
involving weak nucleophilic participation indicates an ꢁ-coef-
ficient close to 1, but the latter involving strong nucleophilic
participation indicates ꢁ as small as 0.4–0.5 (Scheme 4).
The ꢁ-coefficient of 0.52 in the present ꢀ-silyl system 3 is
consistent with the tight structure of the displacement TS, im-
plying that an appreciable degree of Si–C bond-forming is
ꢁ

1096 Bull. Chem. Soc. Jpn. Vol. 79, No. 7 (2006)
The ꢀ-Silicon Effect on Solvolysis Rates
aq NaHCO3 and aq saturated sodium chloride, and then dried over
anhydrous magnesium sulfate. Purification by column chroma-
tography afforded 1.1 g (yield, 40%) of the alcohol as a colorless
liquid: 1H NMR ꢅ 0.20 (6H, s, SiCH3), 1.39 (1H, dd, J ¼ 14:5,
6.7 Hz, CH2), 1.51 (1H, dd, J ¼ 14:5, 8.0 Hz, CH2), 1.73 (1H, d,
J ¼ 3:0 Hz, OH), 4.80 (1H, ddd, J ¼ 8:0, 6.7, 3.0 Hz, CH), 7.29–
7.51 (10H, m, Ph-H).
By the same procedure, the following ꢁ-aryl derivatives (all
colorless liq.) were prepared from the corresponding substituted
benzaldehyde.
δδ+
Nu
∆+
∆−
∆−
C
Lg
C δδ+ Lg
Nu
∆+
Y
Y
α ≅ 1
α ≅ 0.4 ~ 0.5
loose TS (8a+)
tight TS (8b+)
2-(Dimethylphenylsilyl)-1-(p-methoxyphenyl)ethanol:
1H NMR ꢅ 0.19 (6H, s, SiCH3), 1.38 (1H, dd, J ¼ 14:3, 7.3 Hz,
CH2), 1.51 (1H, dd, J ¼ 14:3, 7.6 Hz, CH2), 1.67 (1H, brs, OH),
3.79 (3H, s, OCH3), 4.76 (1H, dd, J ¼ 7:6, 7.3 Hz, CH), 6.81–
7.49 (9H, m, Ar-H).
1-(3-Chloro-4-methoxyphenyl)-2-(dimethylphenylsilyl)etha-
nol: 1H NMR ꢅ 0.21 and 0.23 (6H, ss, SiCH3), 1.34 (1H, dd, J ¼
14:5, 7.0 Hz, CH2), 1.47 (1H, dd, J ¼ 14:5, 7.9 Hz, CH2), 1.67
(1H, d, J ¼ 3:0 Hz, OH), 3.87 (3H, s, OCH3), 4.80 (1H, ddd, J ¼
7:9, 7.0, 3.0 Hz, CH), 6.81–7.47 (8H, m, Ar-H).
Since the p-methoxy-substituted alcohols were decomposed to
the p-methoxy-substituted styrenes during SiO2 column chroma-
tography, these were esterified without further purification.
2-(Dimethylphenylsilyl)-1-(p-methylphenyl)ethanol:
1H NMR ꢅ 0.19 (6H, s, SiCH3), 1.37 (1H, dd, J ¼ 14:5, 6.9 Hz,
CH2), 1.50 (1H, dd, J ¼ 14:5, 7.9 Hz, CH2), 1.65 (1H, d, J ¼
3:3 Hz, OH), 2.32 (3H, s, CH3), 4.78 (1H, ddd, J ¼ 7:9, 6.9, 3.3
Hz, CH), 7.09–7.49 (9H, m, Ar-H).
2-(Dimethylphenylsilyl)-1-(m-methylphenyl)ethanol:
1H NMR ꢅ 0.21 and 0.22 (6H, ss, SiCH3), 1.35 (1H, dd, J ¼ 14:5,
6.6 Hz, CH2), 1.50 (1H, dd, J ¼ 14:5, 8.0 Hz, CH2), 1.69 (1H, brs,
OH), 2.32 (3H, s, CH3), 4.77 (1H, dd, J ¼ 8:0, 6.6 Hz, CH), 7.04–
7.50 (9H, m, Ar-H).
Scheme 4. The structure of transition state.
achieved at the TS in the silyl-assisted reactions of ꢀ-silylated
systems. Consequently, we prefer to conclude that the ꢀ-silyl-
assisted reaction should proceed by the silyl-bridged pathway
1b. A sufficiently low ꢁ-coefficient may not be decisive evi-
dence for the silyl-bridging, unless it is confirmed that the
ꢁ-coefficient for the ꢃ–ꢄ hyperconjugation process is close
to unity. Perhaps, we have to still be careful in interpreting
the magnitudes of the ꢁ value and linking it with the degree
of bond-formation in the TS.
Experimental
Column chromatography was performed by using Silica gel 60
(230–400 mesh, Merck) for flash column chromatography or
LiChroprep Si 60 (25–40 mm, Merck) for middle-pressure liquid
chromatography. The 1H NMR spectra were taken in CDCl3 on
a JEOL JNM-A500 (or -ECA500) FT-NMR spectrometer operat-
ing at 500 MHz and the chemical shifts were recorded in ppm (ꢅ)
downfield from TMS as an internal standard. All air- and mois-
ture-sensitive reactions were carried out under nitrogen or argon.
Ether and tetrahydrofuran were distilled from sodium/benzophe-
none under nitrogen.
Material. The following 1-aryl-2-(dimethylphenylsilyl)eth-
anols or 1-aryl-2-[3,5-bis(trifluoromethyl)phenyldimethylsilyl]-
ethanols were obtained by the reactions of dimethylphenylsilyl-
methylmagnesium chloride or [3,5-bis(trifluoromethyl)phenyldi-
methylsilyl]methylmagnesium chloride reported previously1 and
substituted benzaldehydes, which are commercially available ex-
cept for 3-chloro-4-methoxybenzaldehyde. The alcohols were
converted to 3,5-dinitrobenzoate or benzoate using butyllithium
and acid chloride.
3-Chloro-4-methoxybenzaldehyde. According to the litera-
ture method,16 a solution of N-methylformanilide (4.8 g, 35.5
mmol) in 10 cm3 of ether was added dropwise to the Grignard so-
lution prepared from 3-chloro-4-methoxybromobenzene (8 g, 35.5
mmol) and Mg (0.86 g, 35.5 mmol) in 60 cm3 of ether at 0 ꢁC, and
the reaction mixture was stirred at room temperature for 3 h. The
solution was treated with H2O (40 cm3) and then with 10% HCl
(40 cm3), and extracted with ether. After the usual work-up, puri-
fication by SiO2 column chromatography gave the colorless crys-
tal, mp 55–56 ꢁC: 1H NMR ꢅ 4.00 (3H, s, OCH3), 7.05 (1H, d, J ¼
8:6 Hz, Ar-H), 7.78 (1H, dd, J ¼ 8:6, 2.0 Hz, Ar-H), 7.92 (1H, d,
J ¼ 2:0 Hz, Ar-H), 9.86 (1H, s, CHO). Anal. Found: C, 56.08; H,
4.13%. Calcd for C8H7ClO2: C, 56.32; H, 4.14%.
1-(p-Chlorophenyl)-2-(dimethylphenylsilyl)ethanol:
1H NMR ꢅ 0.21 and 0.23 (6H, ss, SiCH3), 1.34 (1H, dd, J ¼ 14:5,
6.7 Hz, CH2), 1.47 (1H, dd, J ¼ 14:5, 8.0 Hz, CH2), 1.72 (1H, brs,
OH), 4.77 (1H, dd, J ¼ 8:0, 6.7 Hz, CH), 7.18–7.48 (9H, m, Ar-H).
1-(m-Chlorophenyl)-2-(dimethylphenylsilyl)ethanol:
1H NMR ꢅ 0.22 and 0.25 (6H, ss, SiCH3), 1.33 (1H, dd, J ¼ 14:5,
6.4 Hz, CH2), 1.45 (1H, dd, J ¼ 14:5, 8.0 Hz, CH2), 1.81 (1H, brs,
OH), 4.75 (1H, dd, J ¼ 8:0, 6.4 Hz, CH), 7.13–7.49 (9H, m, Ar-H).
2-(Dimethylphenylsilyl)-1-[m-(trifluoromethyl)phenyl]etha-
nol: 1H NMR ꢅ 0.22 and 0.26 (6H, ss, SiCH3), 1.33 (1H, dd, J ¼
14:3, 6.4 Hz, CH2), 1.45 (1H, dd, J ¼ 14:3, 8.2 Hz, CH2), 1.81
(1H, d, J ¼ 2:7 Hz, OH), 4.85 (1H, ddd, J ¼ 8:2, 6.4, 2.7 Hz, CH),
7.34–7.53 (9H, m, Ar-H).
1-(3,5-Dichlorophenyl)-2-(dimethylphenylsilyl)ethanol:
1H NMR ꢅ 0.26 and 0.30 (6H, ss, SiCH3), 1.30 (1H, dd, J ¼ 14:5,
6.0 Hz, CH2), 1.42 (1H, dd, J ¼ 14:5, 9.0 Hz, CH2), 1.74 (1H, brs,
OH), 4.73 (1H, dd, J ¼ 9:0, 6.0 Hz, CH), 7.14–7.49 (8H, m, Ar-H).
1-[3,5-Bis(trifluoromethyl)phenyl]-2-(dimethylphenylsilyl)-
ethanol: 1H NMR ꢅ 0.26 and 0.33 (6H, ss, SiCH3), 1.37 (1H, dd,
J ¼ 14:7, 6.0 Hz, CH2), 1.48 (1H, dd, J ¼ 14:7, 8.6 Hz, CH2),
4.92 (1H, dd, J ¼ 8:6, 6.0 Hz, CH), 7.34–7.70 (8H, m, Ar-H).
The following 1-aryl-2-[3,5-bis(trifluoromethyl)phenylsilyl]-
ethanols were obtained in a similar way by the [3,5-bis(trifluoro-
methyl)phenylsilyl]methylmagnesium chloride1 and substituted
benzaldehydes.
2-(Dimethylphenylsilyl)-1-phenylethanol. To the Grignard
solution prepared from (chloromethyl)dimethylphenylsilane (2.0
g, 10.83 mmol) and Mg (0.25 g, 10.83 mmol) in 40 cm3 of ether
was added dropwise a solution of benzaldehyde (1.16 g, 10.93
mmol) in 20 cm3 of ether with stirring at 0 ꢁC.5c After stirring
overnight, the reaction mixture was treated with H2O (50 cm3),
and extracted with ether. The ethereal extract was washed with
2-[3,5-Bis(trifluoromethyl)phenyldimethylsilyl]-1-phenyleth-
anol: 1H NMR ꢅ 0.31 and 0.32 (6H, ss, SiCH3), 1.43 (1H, dd,
J ¼ 14:7, 7.0 Hz, CH2), 1.55 (1H, dd, J ¼ 14:7, 7.9 Hz, CH2),
1.79 (1H, brs, OH), 4.81 (1H, brt, J ¼ 7:0 Hz, CH), 7.26–7.27

M. Fujio et al.
Bull. Chem. Soc. Jpn. Vol. 79, No. 7 (2006) 1097
(5H, m, Ph-H), 7.80 (1H, s, Ar-H), 7.83 (2H, s, Ar-H).
2-[3,5-Bis(trifluoromethyl)phenyldimethylsilyl]-1-(p-meth-
oxyphenyl)ethanol: 1H NMR ꢅ 0.29 and 0.31 (6H, ss, SiCH3),
1.43 (1H, dd, J ¼ 14:5, 7.5 Hz, CH2), 1.55 (1H, dd, J ¼ 14:5,
7.5 Hz, CH2), 1.69 (1H, d, J ¼ 2:6 Hz, OH), 3.77 (3H, s, OCH3),
4.76 (1H, ddd, J ¼ 7:5, 7.5, 2.6 Hz, CH), 6.77 (2H, d, J ¼ 8:6 Hz,
Ar-H), 7.16 (2H, d, J ¼ 8:6 Hz, Ar-H), 7.80 (3H, s, Ar-H).
2-[3,5-Bis(trifluoromethyl)phenyldimethylsilyl]-1-(p-methyl-
phenyl)ethanol: 1H NMR ꢅ 0.30 and 0.32 (6H, ss, SiCH3), 1.43
(1H, dd, J ¼ 14:5, 7.5 Hz, CH2), 1.54 (1H, dd, J ¼ 14:5, 7.5 Hz,
CH2), 1.74 (1H, d, J ¼ 2:6 Hz, OH), 2.28 (3H, s, CH3), 4.77 (1H,
ddd, J ¼ 7:5, 7.5, 2.6 Hz, CH), 7.04 (2H, d, J ¼ 8:0 Hz, Ar-H),
7.12 (2H, d, J ¼ 8:0 Hz, Ar-H), 7.80 (3H, s, Ar-H).
(1H, t, J ¼ 2:1 Hz, Ar-H).
1-(p-Chlorophenyl)-2-(dimethylphenylsilyl)ethyl 3,5-Dini-
trobenzoate: 1H NMR ꢅ 0.21 and 0.25 (6H, ss, SiCH3), 1.62
(1H, dd, J ¼ 14:3, 7.6 Hz, CH2), 1.86 (1H, dd, J ¼ 14:3, 7.9 Hz,
CH2), 6.11 (1H, dd, J ¼ 7:9, 7.6 Hz, CH), 7.21–7.41 (9H, m,
Ar-H), 8.89 (2H, d, J ¼ 2:1 Hz, Ar-H), 9.15 (1H, t, J ¼ 2:1 Hz,
Ar-H).
1-(m-Chlorophenyl)-2-(dimethylphenylsilyl)ethyl 3,5-Dini-
trobenzoate: 1H NMR ꢅ 0.22 and 0.27 (6H, ss, SiCH3), 1.61
(1H, dd, J ¼ 14:3, 7.3 Hz, CH2), 1.90 (1H, dd, J ¼ 14:3, 8.3 Hz,
CH2), 6.11 (1H, dd, J ¼ 8:3, 7.3 Hz, CH), 7.17–7.41 (9H, m,
Ar-H), 8.89 (2H, d, J ¼ 2:1 Hz, Ar-H), 9.15 (1H, t, J ¼ 2:1 Hz,
Ar-H).
2-(Dimethylphenylsilyl)-1-[m-(trifluoromethyl)phenyl]ethyl
3,5-Dinitrobenzoate: 1H NMR ꢅ 0.23 and 0.25 (6H, ss, SiCH3),
1.64 (1H, dd, J ¼ 14:5, 7.6 Hz, CH2), 1.90 (1H, dd, J ¼ 14:5,
7.9 Hz, CH2), 6.19 (1H, dd, J ¼ 7:9, 7.6 Hz, CH), 7.17–7.59
(9H, m, Ar-H), 8.90 (2H, d, J ¼ 2:1 Hz, Ar-H), 9.15 (1H, t,
J ¼ 2:1 Hz, Ar-H).
2-[3,5-Bis(trifluoromethyl)phenyldimethylsilyl]-1-(3-chloro-
4-methoxyphenyl)ethanol: 1H NMR ꢅ 0.33 and 0.35 (6H, ss,
SiCH3), 1.39 (1H, dd, J ¼ 14:6, 7.2 Hz, CH2), 1.52 (1H, dd,
J ¼ 14:6, 7.8 Hz, CH2), 1.75 (1H, d, J ¼ 2:6 Hz, OH), 3.86 (3H,
s, OCH3), 4.74 (1H, ddd, J ¼ 7:5, 7.5, 2.6 Hz, CH), 6.80 (1H, d,
J ¼ 8:6 Hz, Ar-H), 7.09 (1H, dd, J ¼ 8:6, 2.0 Hz, Ar-H), 7.25
(1H, d, J ¼ 2:0 Hz, Ar-H), 7.82 (3H, s, Ar-H).
1-(3,5-Dichlorophenyl)-2-(dimethylphenylsilyl)ethyl 3,5-Di-
nitrobenzoate: 1H NMR ꢅ 0.28 (6H, s, SiCH3), 1.65 (1H, dd,
J ¼ 14:5, 7.5 Hz, CH2), 1.91 (1H, dd, J ¼ 14:5, 8.2 Hz, CH2),
6.20 (1H, dd, J ¼ 8:2, 7.5 Hz, CH), 6.96–7.77 (8H, m, Ar-H),
8.90 (2H, d, J ¼ 2:0 Hz, Ar-H), 9.17 (1H, t, J ¼ 2:0 Hz, Ar-H).
1-[3,5-Bis(trifluoromethyl)phenyl]-2-(dimethylphenylsilyl)-
ethyl 3,5-Dinitrobenzoate: 1H NMR ꢅ 0.28 and 0.31 (6H, ss,
SiCH3), 1.65 (1H, dd, J ¼ 14:5, 7.5 Hz, CH2), 1.91 (1H, dd,
J ¼ 14:5, 8.2 Hz, CH2), 6.20 (1H, dd, J ¼ 8:2, 7.5 Hz, CH),
7.18–7.77 (8H, m, Ar-H), 8.90 (2H, d, J ¼ 2:1 Hz, Ar-H), 9.17
(1H, t, J ¼ 2:1 Hz, Ar-H).
2-[3,5-Bis(trifluoromethyl)phenyldimethylsilyl]-1-[m-(trifluo-
romethyl)phenyl]ethanol: 1H NMR ꢅ 0.35 and 0.38 (6H, ss,
SiCH3), 1.41 (1H, dd, J ¼ 14:7, 6.3 Hz, CH2), 1.54 (1H, dd, J ¼
14:7, 8.5 Hz, CH2), 1.88 (1H, s, OH), 4.89 (1H, brt, J ¼ 7:3 Hz,
CH), 7.34–7.82 (7H, m, Ar-H).
1-Aryl-2-(aryldimethylsilyl)ethanols with butyllithium in ether
were converted into the corresponding 3,5-dinitrobenzoate or ben-
zoate by the reaction of 3,5-dinitrobenzoyl chloride or benzoyl
chloride.
2-(Dimethylphenylsilyl)-1-phenylethyl 3,5-Dinitrobenzoate.
The alcohol (0.2 g, 0.78 mmol) in 5 cm3 of ether was esterified
with the addition of 0.62 cm3 (0.93 mmol) of butyllithium (1.5
mol dmꢃ3 hexane solution), and then 0.20 g (0.85 mmol) of 3,5-di-
nitrobenzoyl chloride in 10 cm3 of ether at 0 ꢁC under a N2 atmo-
sphere. Water was added to the reaction mixture and extracted
with ether. The ether extract was washed with NaHCO3 and sat
NaCl solution, and then dried over anhydrous MgSO4. The crude
ester was recrystallized from ether–hexane; mp 95–96 ꢁC:
1H NMR ꢅ 0.16 and 0.24 (6H, ss, SiCH3), 1.66 (1H, dd, J ¼ 14:5,
8.0 Hz, CH2), 1.89 (1H, dd, J ¼ 14:5, 7.6 Hz, CH2), 6.18 (1H, dd,
J ¼ 8:0, 7.6 Hz, CH), 7.20–7.50 (10H, m, Ph-H), 8.91 (2H, d,
J ¼ 2:1 Hz, Ar-H), 9.14 (1H, t, J ¼ 2:1 Hz, Ar-H).
2-[3,5-Bis(trifluoromethyl)phenyldimethylsilyl]-1-(3-chloro-
4-methoxyphenyl)ethyl 3,5-Dinitrobenzoate: 1H NMR ꢅ 0.35
(6H, s, SiCH3), 1.81 (1H, dd, J ¼ 14:3, 9.8 Hz, CH2), 1.87 (1H,
dd, J ¼ 14:3, 6.3 Hz, CH2), 3.86 (3H, s, OCH3), 6.05 (1H, dd,
J ¼ 9:8, 6.3 Hz, CH), 6.80 (1H, d, J ¼ 8:3 Hz, Ar-H), 7.21 (1H,
dd, J ¼ 8:3, 2.3 Hz, Ar-H), 7.30 (1H, d, J ¼ 2:3 Hz, Ar-H), 7.71
(2H, s, Ar-H), 7.77 (1H, s, Ar-H), 9.01 (2H, d, J ¼ 2:1 Hz, Ar-
H), 9.19 (1H, t, J ¼ 2:1 Hz, Ar-H).
2-[3,5-Bis(trifluoromethyl)phenyldimethylsilyl]-1-(p-methyl-
phenyl)ethyl 3,5-Dinitrobenzoate: 1H NMR ꢅ 0.311 and 0.315
(6H, ss, SiCH3), 1.85 (1H, d, J ¼ 9:2 Hz, CH2), 1.86 (1H, d,
J ¼ 6:9 Hz, CH2), 2.27 (3H, s, CH3), 6.10 (1H, dd, J ¼ 9:2,
6.9 Hz, CH), 7.05 (2H, d, J ¼ 8:0 Hz, Ar-H), 7.21 (2H, d,
J ¼ 8:0 Hz, Ar-H), 7.69 (2H, s, Ar-H), 7.75 (1H, s, Ar-H), 9.02
(2H, d, J ¼ 2:0 Hz, Ar-H), 9.18 (1H, t, J ¼ 2:0 Hz, Ar-H).
2-[3,5-Bis(trifluoromethyl)phenyldimethylsilyl]-1-phenylethyl
3,5-Dinitrobenzoate: 1H NMR ꢅ 0.29 and 0.32 (6H, ss, SiCH3),
1.83 (1H, dd, J ¼ 14:3, 9.5 Hz, CH2), 1.90 (1H, dd, J ¼ 14:3, 6.4
Hz, CH2), 6.13 (1H, dd, J ¼ 9:5, 6.4 Hz, CH), 7.26–7.35 (5H, m,
Ph-H), 7.72 (2H, s, Ar-H), 7.75 (1H, s, Ar-H), 9.02 (2H, d, J ¼ 2:1
Hz, Ar-H), 9.18 (1H, t, J ¼ 2:1 Hz, Ar-H).
The following 3,5-dinitrobenzoates or benzoates were obtained
in a similar way by the corresponding alcohols and 3,5-dinitroben-
zoyl chloride or benzoyl chloride.
1-(3-Chloro-4-methoxyphenyl)-2-(dimethylphenylsilyl)ethyl
3,5-Dinitrobenzoate: 1H NMR ꢅ 0.22 and 0.26 (6H, ss, SiCH3),
1.63 (1H, dd, J ¼ 14:5, 8.0 Hz, CH2), 1.86 (1H, dd, J ¼ 14:5,
7.6 Hz, CH2), 3.88 (3H, s, OCH3), 6.09 (1H, dd, J ¼ 8:0, 7.6 Hz,
CH), 7.13–7.42 (8H, m, Ar-H), 8.90 (2H, d, J ¼ 2:1 Hz, Ar-H),
9.14 (1H, t, J ¼ 2:1 Hz, Ar-H).
2-(Dimethylphenylsilyl)-1-(p-methylphenyl)ethyl 3,5-Dini-
trobenzoate: 1H NMR ꢅ 0.16 and 0.24 (6H, ss, SiCH3), 1.65
(1H, dd, J ¼ 14:7, 7.9 Hz, CH2), 1.87 (1H, dd, J ¼ 14:7, 7.6 Hz,
CH2), 2.33 (3H, s, CH3), 6.15 (1H, dd, J ¼ 7:9, 7.6 Hz, CH),
7.13–7.42 (9H, m, Ar-H), 8.89 (2H, d, J ¼ 2:0 Hz, Ar-H), 9.13
(1H, t, J ¼ 2:0 Hz, Ar-H).
2-[3,5-Bis(trifluoromethyl)phenyldimethylsilyl]-1-[m-(trifluo-
romethyl)phenyl]ethyl 3,5-Dinitrobenzoate: 1H NMR ꢅ 0.35
(6H, s, SiCH3), 1.82 (1H, dd, J ¼ 14:3, 9.2 Hz, CH2), 1.93 (1H,
dd, J ¼ 14:3, 6.7 Hz, CH2), 6.17 (1H, dd, J ¼ 9:2, 6.7 Hz, CH),
7.41–7.56 (4H, m, Ar-H), 7.70 (2H, s, Ar-H), 7.75 (1H, s, Ar-H),
9.00 (2H, d, J ¼ 2:1 Hz, Ar-H), 9.20 (1H, t, J ¼ 2:1 Hz, Ar-H).
2-(Dimethylphenylsilyl)-1-(p-methoxyphenyl)ethyl Benzo-
ate: 1H NMR ꢅ 0.14 and 0.19 (6H, ss, SiCH3), 1.59 (1H, dd,
J ¼ 14:5, 8.5 Hz, CH2), 1.80 (1H, dd, J ¼ 14:5, 7.0 Hz, CH2),
3.76 (3H, s, OCH3), 6.04 (1H, dd, J ¼ 8:5, 7.0 Hz, CH), 6.81
(2H, d, J ¼ 8:5 Hz, Ar-H), 7.28–7.48 (10H, m, Ph-H), 7.93 (2H,
2-(Dimethylphenylsilyl)-1-(m-methylphenyl)ethyl 3,5-Dini-
trobenzoate: 1H NMR ꢅ 0.18 and 0.25 (6H, ss, SiCH3), 1.63
(1H, dd, J ¼ 14:5, 7.6 Hz, CH2), 1.87 (1H, dd, J ¼ 14:5, 7.9 Hz,
CH2), 2.33 (3H, s, CH3), 6.15 (1H, dd, J ¼ 7:9, 7.6 Hz, CH),
7.11–7.42 (9H, m, Ar-H), 8.91 (2H, d, J ¼ 2:1 Hz, Ar-H), 9.14

1098 Bull. Chem. Soc. Jpn. Vol. 79, No. 7 (2006)
The ꢀ-Silicon Effect on Solvolysis Rates
Table 3. Physical and Analytical Data of 2-[Dimethyl(X-substituted phenyl)silyl]-1-(Y-substituted phenyl)ethanols
and the 3,5-Dinitrobenzoates
Substituents
Mp/ꢁC
Carbon/%
Hydrogen/%
Nitrogen/%
Y
X
Found
Calcd
Found
Calcd
Found
Calcd
Alcohols
p-Me
m-Me
H
H
H
H
H
H
H
H
H
Liq
Liq
Liq
Liq
Liq
Liq
Liq
Liq
Liq
Liq
Liq
Liq
Liq
75.38
75.56
75.16
66.19
65.87
63.16
59.19
55.11
54.11
56.19
49.76
55.21
49.59
75.50
75.50
74.95
66.07
66.07
62.94
59.07
55.09
54.02
56.15
49.95
55.09
49.57
8.22
8.28
7.81
6.71
6.65
5.92
5.68
4.83
4.80
5.02
4.14
4.77
3.82
8.20
8.20
7.86
6.58
6.58
5.90
5.58
4.62
4.77
4.96
4.19
4.62
3.72
p-Cl
m-Cl
m-CF3
3,5-Cl2
3,5-(CF3)2
p-MeO
p-Me
m-Cl-p-MeO
H
m-CF3
3,5-(CF3)2
3,5-(CF3)2
3,5-(CF3)2
3,5-(CF3)2
3,5-(CF3)2
3,5-Dinitrobenzoates
H
p-Cl
m-CF3
3,5-(CF3)2
m-Cl-p-MeO
p-Me
H
H
H
H
95–96
Liq
96–97
Liq
Liq
61.42
56.58
55.39
51.52
47.70
51.79
48.11
61.32
56.96
55.59
51.20
47.97
52.00
47.71
4.96
4.37
4.08
3.59
3.29
3.80
2.95
4.92
4.36
4.13
3.44
3.25
3.69
2.93
6.16
5.56
5.40
4.76
4.02
4.39
4.28
6.22
5.78
5.42
4.78
4.30
4.66
4.28
3,5-(CF3)2
3,5-(CF3)2
3,5-(CF3)2
Liq
Liq
m-CF3
d, J ¼ 8:5 Hz, Ar-H).
(dimethylphenylsilyl)-1-phenylethyl 3,5-dinitrobenzoate buffered
with ca. 2 equiv of 2,6-lutidine in deuterated ethanol (CD3CD2-
OD) or deuterated 60% (v/v) aqueous ethanol (CD3CD2OD–
D2O) was allowed to react at 55 ꢁC in an NMR tube, and the sol-
2-[3,5-Bis(trifluoromethyl)phenyldimethylsilyl]-1-(p-meth-
oxyphenyl)ethyl Benzoate: 1H NMR ꢅ 0.28 and 0.32 (6H, ss,
SiCH3), 1.68 (1H, dd, J ¼ 14:5, 8.9 Hz, CH2), 1.84 (1H, dd, J ¼
14:5, 6.9 Hz, CH2), 3.75 (3H, s, OCH3), 6.05 (1H, dd, J ¼ 8:9,
6.9 Hz, CH), 6.75 (2H, d, J ¼ 8:9 Hz, Ar-H), 7.25 (2H, d, J ¼
8:9 Hz, Ar-H), 7.35–7.52 (3H, m, Ph-H), 7.73 (3H, s, Ar-H),
7.88–7.91 (2H, m, Ph-H).
1
volysis products at ten half-lives were identified by the H NMR
spectra. The relative amounts of reaction products, styrene and
2-(dimethylphenylsilyl)-1-phenylethyl ethyl ether, were deter-
mined from the integral areas of the corresponding peaks, as fol-
lows: styrene 1: the ether 0.56 in absolute ethanol and styrene only
in 60% ethanol. Olefin peaks of 1H NMR in ethanol-d6 were ꢅ
5.18 (1H, d, J ¼ 10:9 Hz), 5.72 (1H, d, J ¼ 17:8 Hz), 6.69 (1H,
dd, J ¼ 17:8, 10.9 Hz) and characteristic peaks of 2-(dimethyl-
phenylsilyl)-1-phenylethyl ethyl ether were ꢅ 0.16 and 0.21 (6H,
ss, SiCH3), 1.22 (1H, dd, J ¼ 14:9, 6.3 Hz, CH2), 1.43 (1H, dd,
J ¼ 14:9, 8.6 Hz, CH2), 4.23 (1H, dd, J ¼ 8:6, 6.3 Hz, CH).
2-(Dimethylphenylsilyl)-1-(p-methylphenyl)ethyl Benzoate:
1H NMR ꢅ 0.14 and 0.19 (6H, ss, SiCH3), 1.57 (1H, dd, J ¼
14:3, 8.2 Hz, CH2), 1.80 (1H, dd, J ¼ 14:3, 7.6 Hz, CH2), 2.31
(3H, s, CH3), 6.05 (1H, t, J ¼ 7:8 Hz, CH), 7.09 (2H, d, J ¼ 7:9
Hz, Ar-H), 7.09–7.51 (10H, m, Ph-H), 7.93 (2H, d, J ¼ 7:9 Hz,
Ar-H).
Physical constants and elemental analysis data of the alcohols
and the esters are listed in Table 3.
Solvents. Solvents were purified and binary solvents were pre-
pared by mixing appropriate volumes or weights of pure solvents
at 25 ꢁC, as previously described.1
References
1
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unit and printer, and those of CM-60V, CM-40G, and CM-40V
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