γ-Trimethylsilylcyclobutyl carbocation stabilization

Article abstract of DOI:10.1021/jo502691t

A series of isomeric 3-trimethylsilyl-1-arylcyclobutyl carbocations, 10 and 11, where the cross-ring 3-trimethylsilyl group has the potential to interact with the cationic center, have been generated under solvolytic conditions. When the cationic center can interact with the rear lobe of the carbon-silicon bond, rate enhancements become progressively larger as the substituent on the aryl group becomes more electron-withdrawing. When the potential interaction with the trimethylsilyl group is via a front lobe interaction, there is minimal rate enhancement over the range of substituents. Computational studies have also been carried out on these cations 10 and 11. Calculated trimethylsilyl stabilization energies progressively increase with electron-withdrawing character of the aryl groups when the trimethylsilyl interaction is via the rear lobe. By way of contrast, there are minimal changes in stabilization energies when the potential trimethylsilyl interaction is via the front lobe of the carbon-silicon bond. These computational studies, along with the solvolytic studies, point to a significant rear lobe 3-trimethylsilyl stabilization of arylcyclobutyl cations. They also argue against any front lobe stabilization of the isomeric arylcyclobutyl cations.

Full text of DOI:10.1021/jo502691t

Article
pubs.acs.org/joc
γ‑Trimethylsilylcyclobutyl Carbocation Stabilization
Xavier Creary,* Anna Heffron, Gabrielle Going, and Mariana Prado
Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States
S
* Supporting Information
ABSTRACT: A series of isomeric 3-trimethylsilyl-1-arylcyclobutyl carbo-
cations, 10 and 11, where the cross-ring 3-trimethylsilyl group has the
potential to interact with the cationic center, have been generated under
solvolytic conditions. When the cationic center can interact with the rear
lobe of the carbon−silicon bond, rate enhancements become progressively
larger as the substituent on the aryl group becomes more electron-
withdrawing. When the potential interaction with the trimethylsilyl group is via a front lobe interaction, there is minimal rate
enhancement over the range of substituents. Computational studies have also been carried out on these cations 10 and 11.
Calculated trimethylsilyl stabilization energies progressively increase with electron-withdrawing character of the aryl groups when
the trimethylsilyl interaction is via the rear lobe. By way of contrast, there are minimal changes in stabilization energies when the
potential trimethylsilyl interaction is via the front lobe of the carbon−silicon bond. These computational studies, along with the
solvolytic studies, point to a significant rear lobe 3-trimethylsilyl stabilization of arylcyclobutyl cations. They also argue against
any front lobe stabilization of the isomeric arylcyclobutyl cations.
INTRODUCTION
■
Carbocations directly adjacent to the trimethylsilyl group, as
in 1, are greatly stabilized by an interaction with the adjacent
C−Si σ-bond. This β-silyl effect has been extensively studied.1
We and others have been interested in the effect of silicon
further removed from a cationic center and, in particular, the
γ-silyl effect. Shiner and co-workers2 have studied this γ-effect
in solvolysis reactions that lead to carbocation 2. This
carbocation forms in solvolysis reactions at a moderately
enhanced rate that is attributed to an interaction of the cationic
center with the rear lobe of the carbon−silicon σ-bond.
Additional studies by Grob and Sawlewicz3 on cation 3 and
Adcock et al.4 on cation 4 support this suggestion. The rate
enhancement in formation of cation 3 is a modest factor of 8.6,
whereas the rate enhancement in formation of 4 is a more
substantial value of 1300. In a recent synthetic application of
this phenomenon, Tilley and co-workers5 have observed the
formation of trifluoromethyl substituted cyclopropanes by
desilylation of carbocation 5 and analogues, with increased
yields of cyclopropanes with electron-withdrawing aryl groups.
In our laboratory, we have found remarkable rate-enhancing
effects (factors of 105) in solvolysis reactions that form
carbocations 6a6 and 7.7 A rate enhancement of over 106
has been observed in formation of 6b.5 Computational
studies indicate that cation 6a is more stable than the
2-trimethylsilylcyclobutyl cation; i.e., the γ-silyl effect in cation
6a is even greater than the β-silyl effect.
We have also been interested in the effects of silicon on other
electron-deficient reactive intermediates such as singlet
carbenes.8 Among such carbenes that have been studied in our
laboratory are the carbenes 8 and 9.8b These carbenes are
characterized by their facile rearrangement via trimethylsilyl
migration to the carbene center. This was suggestive of a long-
range interaction of the γ-trimethylsilyl group with the carbene
center. This γ-interaction of silicon with carbenes is quite
different from the γ-stabilization of carbocations 2−7. Whereas
stabilization of these carbocations occurs via the “rear lobe” of the
carbon−silicon σ-bond, our carbene studies suggested a “front
lobe” interaction of carbenes analogous to the β-silyl effect seen
in carbocations of type 1. In light of these studies, it seemed
reasonable to expect that γ-silyl carbocations could also be
Received: November 26, 2014
Published: January 6, 2015
© 2015 American Chemical Society
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Scheme 1. Synthesis of Substrates for Solvolytic Studies
stabilized by a long-range “front lobe” interaction as shown in 11.
We, therefore, wanted to progressively destabilize cations 10 and
11 and to probe for the magnitude of long-range silicon
stabilization of these carbocations.9 We wanted to study such
carbocations both experimentally and computationally. Re-
ported here are our results comparing “rear lobe” silyl stabilized
carbocations 10 with potential “front lobe” silyl stabilized
carbocations 11.
Table 1. Solvolysis Rates for Substrates 15, 16, and 18 in
Various Solvents
compound
solvent
T (°C)
k (s−1
)
a
a
15 (Ar = C6H4-p-OCHs)
15 (Ar = C6H4-p-CH3)
15 (Ar = C6H5)
CD3OD
25.0
25.0
25.0
25.0
25.0
25.0
25.0
25.0
25.0
25.0
25.0
25.0
25.0
25.0
25.0
25.0
25.0
25.0
25.0
25.0
25.0
9.31 × 10−6
5.87 × 10−4
9.37 × 10−3
2.26 × 10−3
7.51 × 10−4
4.16 × 10−4
2.24 × 10−4
6.26 × 10−6
7.00 × 10−5
5.85 × 10−4
1.98 × 10−4
1.42 × 10−5
4.69 × 10−6
1.19 × 10−6
1.63 × 10−6
9.46 × 10−6
4.54 × 10−5
1.23 × 10−5
9.19 × 10−7
2.77 × 10−7
8.08 × 10−8
CF3CH2OH
CD3CO2D
CD3CO2D
CD3CO2D
CD3CO2D
CD3CO2D
CD3OD
15 (Ar = C6H4-p-CI)
15 (Ar = C6H4-m-Cl)
15 (Ar = C6H4-m-CF3)
15 (Ar = C6H4-p-CF3)
16 (Ar = C6H4-p-OCH3)
16 (Ar = C6H4-p-CH3)
16 (Ar = C6H5)
a
CF3CH2OH
CD3CO2D
CD3CO2D
CD3CO2D
CD3CO2D
CD3CO2D
CD3OD
16 (Ar = C6H4-p-CI)
16 (Ar = C6H4-m-Cl)
16 (Ar = C6H4-m-CF3)
16 (Ar = C6H4-p-CF3)
18 (Ar = C6H4-p-OCH3)
18 (Ar = C6H4-p-CH3)
18 (Ar = C6H5)
RESULTS AND DISCUSSION
■
b
The syntheses of precursors to cations 10 and 11 (Scheme 1)
began with addition of a variety of Grignard reagents to
3-trimethylsilylcyclobutanone, which gave mixtures of alcohols
13 and 14. These alcohols were, in turn, converted to
trifluoroacetate or acetate derivatives 15 and 16 for solvolytic
studies. We have previously reported6 on the solvolytic behavior
of 15 (Ar = C6H5), but we now have obtained data on substrates
15 with a range of aryl substituents. These substrates reacted in
the solvents studied by first-order processes to give substitution
products 17 with net retention of configuration (Scheme 2).
ab
,
a
CF3CH2OH
CD3CO2D
CD3CO2D
CD3CO2D
CD3CO2D
18 (Ar = C6H4-p-CI)
18 (Ar = C6H4-m-Cl)
18 (Ar = C6H4-m-CF3)
b
b
18 (Ar = C6H4-p-CF3)
CD3CO2D
a
b
Rate of acetate derivative. Extrapolated from data at higher
temperatures.
Scheme 2. Solvolyses of Substrates 15 in CD3CO2D and
CD3OD
derivatives. Introduction of a p-CF3 substituent into 15 (Ar =
C6H4-p-CF3) slows the rate by only a factor of 42 relative to 15
(Ar = C6H5). A more “normal” value is seen for the model
compounds 18 (ρ+ = −4.34), where introduction of a p-CF3
group slows the rate by a factor of 562. Also apparent is the
variable rate-enhancing effect of the trimethylsilyl group, which is
summarized in Table 2. This table gives the 15:18 rate ratio as a
function of substituent, and also includes our previously
determined data for 19 and 20. There is a steady increase in
rate ratio as the substituent becomes more electron-withdrawing
(less carbocation stabilizing). These rate effects are completely
consistent with a rear lobe trimethylsilyl stabilized carbocation
intermediate 10 in solvolyses of 15. With increased demand for
stabilization as the substituent becomes more electron-with-
drawing, the γ-trimethylsilyl substituent in 10 interacts more
strongly, resulting in a larger rate ratio.
The exception was 15 (Ar = C6H4-p-OCH3), where solvolysis of
the acetate derivative in CD3OD gave 91% retention along with
9% of a product with inverted stereochemistry. Kinetic data for
15 and 16, as well as data for the unsubstituted model cyclobutyl
compounds 18, are reported in Table 1.
The first thing that is apparent is that there are relatively small
substituent effects in solvolyses of 15 as the aryl group is changed.
The Hammett ρ+ value is only −2.40 for the five trifluoroacetate
The behavior of the p-OCH3 derivative 15 (Ar = C6H4-
p-OCH3) begins to deviate from that of the other substrates 15.
The 9% inverted solvolysis product in CD3OD indicates that the
bridged ion 10 is not the sole intermediate in this reaction. It is
suggested that the powerful cation stabilizing effect of the
methoxy group is now beginning to offset the cross-ring silicon
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Table 2. Solvolysis Rate Ratios for 15:18 as a Function
of Substituent
Table 3. Solvolysis Rate Ratios for 16:18 as a Function
a b
,
of Substituent
stabilizing effect in the cation 10. Substrate 15 (Ar = C6H4-
p-OCH3) appears to be reacting by a competing silicon assisted
and an unassisted pathway, which results in some inverted
product.
The substituent effects in the isomeric substrates 16 contrast
with the behavior in 15. Table 3 summarizes these substituent
effects. While the silylated derivatives 16 all react slightly faster
than the desilylated derivatives 18, the effect is small in all cases.
The Hammett ρ+ value for 16 is −4.25, which very similar to the
value of −4.34 seen in the desilylated derivatives 18. There is no
systematic increase in the 16:18 rate ratio as the substituent
becomes more electron-withdrawing. As the demand for cation
stabilization increases, there is little response from the
trimethylsilyl group. This implies very little interaction of the
developing cationic center with the trimethylsilyl group, i.e.,
there is no kinetic evidence for stabilization of the cation 11 by an
interaction with the front lobe of the carbon−silicon bond. It is
suggested that substrates 16 react via cyclobutyl cations 11 that
are stabilized by standard interactions with the bent bonds of the
cyclobutane ring. The trimethylsilyl group may provide small
inductive stabilization of these carbocations. The cyclobutyl bent
bond stabilization in 11 is not sufficient to prevent subsequent
ring inversion to give rear lobe silyl stabilized cation 10, which
results in acetates 17 as the major products of solvolyses of 16 in
CD3CO2D (Scheme 3).10
a
b
Rates of acetate derivatives in CD3OD. Rates of acetate derivatives
in CF3CH2OH.
compare the stability of cation 10 with that of the desilylated
analogue, cation 24. Table 4 gives a summary of ΔE values for
this reaction, as well as the calculated C1−C3 and C3−Si
distances. As the group on the cation becomes more electron-
withdrawing, there are large increases in the calculated
stabilization of cation 10 by the trimethylsilyl group. The
calculation suggests significant stabilization even in the p-OCH3
substituted cation 10 (Ar = C6H4-p-OCH3), where kinetic data
indicate minimal cation stabilization in the solvolytic reaction.
The stabilization of 10 (Ar = C6H4-p-OCH3) amounts to 5.4
kcal/mol and increases to 16.5 kcal/mol in 10 (Ar = C6H4-
p-NO2). It is even larger when the aryl group is replaced with
H (cation 6) and with the electron-withdrawing cyano group
(cation 25).
Computational Studies. Density functional calculations11
at the M062X/6-311+G** level were also used as a probe for
trimethylsilyl stabilization of carbocations 10 and 11. The value
of ΔE for the isodesmic reaction in Scheme 4 was used to
The distance between the cationic carbon, C1, and C3 is
indicative of a bonding interaction in all of these cations. There is
a systematic shorting of the C1−C3 bond distance as the group
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Scheme 3. Reaction of Substrates 16 in CD3CO2D
Scheme 4. Isodesmic Reactions of Cation 10
Scheme 5. Isodesmic Reactions of Cation 11
Table 4. M062X/6-311+G** Calculated Stabilization
Energies and Bond Lengths in Cations 10
10 (Ar = C6H4-p-CF3). The respective bond distances of 1.845
and 1.710 Å in these structures compare to the value of 2.184 Å in
the neutral molecule 23. These structures imply a significant
bonding interaction across C1−C3. There is a corresponding
lengthening of the C3−Si bond (from 1.946 to 2.032 Å) in 10 as
the substituent becomes more electron-withdrawing. This com-
pares to a C3−Si bond length of about 1.88 Å in the neutral
molecules 23. These trends are all consistent with increasing rear
lobe silyl stabilization of cations 10 as the substituent becomes less
cation stabilizing. In valence bond terms, the importance of form
10a increases as substituents on the aryl group become less cation
stabilizing.
It is of interest to note that the p-methoxy stabilized cation 10
(R = C6H4-p-OCH3) shows a shortened C1−C3 distance, as well
as a lengthening of the C3−Si bond, indicative of a significant rear
lobe cation stabilizing interaction. This computational result
contrasts with the kinetic study, which shows minimal rate
enhancement in solvolysis of 10 (Ar = C6H4-p-OCH3). As in our
previous study,12 gas phase computational studies appear to
overestimate the importance of neighboring group participation
as a carbocation stabilizing feature.
Attention was next focused on the isodesmic reactions of the
isomeric cations 11 in Scheme 5. Results are summarized in
Table 5, which gives the ΔE values as well as C1−C3 distances
and C3−Si bond lengths. In stark contrast to the behavior of
cations 10, cations 11 show minimal changes in calculated
trimethylsilyl stabilization energies as substituents become
electron-withdrawing. The increase from 3.2 to 4.8 kcal/mol as
the substituent is changed from p-OCH3 to p-CF3 can hardly be
becomes less cation stabilizing. This is illustratedinFigure1, which
shows the calculated structures of 10 (Ar = C6H4-p-OCH3) and
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Figure 1. M062X/6-311+G** calculated structures of cations 10 (Ar = C6H4-p-OCH3) and 10 (Ar = C6H4-m-CF3).
Table 5. M062X/6-311+G** Calculated Stabilization
The calculated structures of 11 (Ar = C6H4-p-OCH3) and 11
(Ar = C6H4-p-CF3) (Figure 2) are also informative. The C1−C3
distances show minimal changes as a function of substituent.
Indeed, the slight changes in 11 parallel those seen in the
desilylated cyclobutyl cations 24. These C1−C3 bond distances
give no indication of a significant C1−C3 bonding interaction.
Finally, the C3−Si bond lengths also show minimal changes as a
function of substituent. There is no significant lengthening
of the C3−Si bond with electron-withdrawing groups as ob-
served in cations 10. There appears to be no response of the
C3−Si bond to increasing demand as cations 11 become more
destabilized.
a
Energies and Bond Lengths in Cations 11
CONCLUSIONS
■
Solvolytic studies, as well as computational studies, provide
convincing evidence for rear lobe silyl stabilization of carbo-
cations of type 10. By way of contrast, there is no compelling
experimental or computational evidence for a cross-ring front
lobe trimethylsilyl stabilizing interaction in cations 11. Cations
11 appear to derive stabilization by the same mechanisms (bent
bond interactions) that stabilize simple aryl substituted cyclo-
butyl cations. The trimethylsilyl group in 11 provides no
significant additional stabilization. The validity of our previously
suggested front lobe stabilization of carbenes 8 and 9 must await
further scrutiny.
EXPERIMENTAL SECTION
■
General. NMR spectra were recorded on a 600 MHz spectrometer.
HRMS measurements were carried out using either an electrospray
ionization source with time-of-flight mass analyzer or a GC-mass
spectrometer with an electron impact ionization source and time-of-
flight mass analyzer. NMR analyses were carried out on an instrument
operating at 600 MHz for 1H NMR.
a
C1−C3 bond length in 24.
taken as evidence for an interaction of the trimethylsilyl group
with the cationic center of 11.
Figure 2. M062X/6-311+G** calculated structures of cations 11 (Ar = C6H4-p-OCH3) and 11 (Ar = C6H4-p-CF3).
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Preparation of Alcohols 13 and 14. Alcohols 13 and 14 were
prepared by addition of aryl Grignard reagents13 to 3-trimethylsilyl-
cyclobutanone.14 The following procedure is representative. A solution
of 1.60 mL of 0.90 M 3-chlorophenylmagnesium bromide (1.44 mmol)
in ether was cooled in an ice bath, and 170 mg (1.20 mmol) of
3-trimethylsilylcyclobutanone in 3 mL of ether was added dropwise. The
mixture was then warmed to room temperature for 15 min and then
quenched with approx 2.5 M aqueous NH4Cl solution. The ether phase
was separated, washed with water and saturated NaCl solution, and
dried over a mixture of Na2SO4 and MgSO4. After filtration, the solvent
was removed using a rotary evaporator. 1H NMR analysis of the crude
reaction mixture showed alcohols 13 and 14 (Ar = C6H4-m-Cl) in a
40:60 ratio. The crude products were chromatographed on 8 g of silica
gel and eluted with pentane, followed by increasing amounts of ether in
pentane (1% ether increments). Alcohols 13 and 14 (254 mg; 94%
yield) eluted with 6−10% ether in pentane. Early fractions were
enriched with alcohol 13 (Ar = C6H4-m-Cl), while later fractions were
enriched with alcohol 14 (Ar = C6H4-m-Cl). Pure samples of isomeric
alcohols 13 and 14 could be isolated by the protocols described below.
Isolation of Alcohol 13 (Ar = C6H4-m-Cl). A mixture of alcohols 13
and 14 (199 mg of a 52:48 ratio; 0.783 mmol) was dissolved in 4 mL
of ether, and 162 mg of 2,6-lutidine (1.514 mmol) was added. The
solution was cooled to −10 °C, and 260 mg of trifluoroacetic anhydride
(1.238 mmol) was added. After 10 min, the mixture was warmed to
room temperature and then taken up into pentane. The solution was
consecutively washed with cold water, cold dilute HCl solution, water,
and NaHCO3 solution, and then dried over MgSO4. After filtration, the
solvent was removed using a rotary evaporator to give a mixture of
trifluoroacetates 15 and 16 (243 mg, 89% yield) in a 52:48 ratio.
A solution of 72 mg (0.673 mmol) of 2,6-lutidine in 4.5 mL of acetic
acid was added to the mixture of trifluoroacetates 15 and 16 prepared
above. Trifluoroacetate 15 (Ar = C6H4-m-Cl) reacts in acetic acid with a
half-life of about 16 min at 25 °C to form the acetate derivative. After
stirring the solution for 21 h at 25−27 °C, the mixture was taken up into
ether and 15 mL of water was added. The acetic acid was neutralized by
addition of Na2CO3 with stirring. The mixture was transferred to a
separatory funnel, and the ether extract was diluted with pentane and
then washed with water and saturated NaCl solution. After drying over
MgSO4, the organic phase was filtered and the solvent was removed
using a rotary evaporator. The residue was chromatographed on 6 g
of silica gel. The column was eluted with increasing amounts of ether
in pentane, and the product 1-(3-chlorophenyl)-3-(trimethylsilyl)-
cyclobutyl acetate (154 mg) was eluted with 2−3% ether in pentane.
Methanol (4 mL) was added to 138 mg of the acetate (0.466 mmol)
prepared above, and then 0.5 mL of 0.48 M NaOCH3 in methanol
(0.240 mmol) was added. The mixture was stirred for 8 h at room
temperature, and the methanol was then removed using a rotary
evaporator. The residue was taken up into ether, and the mixture was
washed with a small amount of water, and saturated NaCl. The solution
was dried over MgSO4 and filtered, and the ether was removed using a
rotary evaporator to give 116 mg (97% yield) of 13 (Ar = C6H4-m-Cl) as
an oil. 1H NMR of 13 (Ar = C6H4-m-Cl) (CDCl3) δ 7.57 (t, J = 1.9 Hz, 1
H), 7.47 (m, 1 H), 7.32 (t, J = 7.8 Hz, 1 H), 7.28 (m, 1 H), 2.56 (m, 2 H),
2.21 (m, 2 H), 2.03 (s, 1 H), 1.19 (t of t, J = 11.1, 8.8 Hz,) 1 H), 0.01 (s,
9 H). 13C NMR of 13 (Ar = C6H4-m-Cl) (CDCl3) δ 148.0, 134.4, 129.8,
127.5, 125.7, 123.5, 75.5, 38.6, 11.5, −3.4. Exact mass (ESI)(M + Na+)
calcd for C13H19ClNaOSi: 277.0786. Found: 277.0800. Exact mass
(EI)(M − H2O) calcd for C13H17ClSi: 236.0788. Found: 236.0774.
Isolation of Alcohol 14 (Ar = C6H4-m-Cl). A mixture of alcohols 13
and 14 (197 mg of a 26:74 ratio; 0.774 mmol) was dissolved in 4 mL
of ether, and 158 mg of 2,6-lutidine (1.477 mmol) was added. The
solution was cooled to −10 °C, and 260 mg of trifluoroacetic anhydride
(1.238 mmol) was added. After 10 min, the mixture was warmed to
room temperature and then taken up into pentane. The solution was
consecutively washed with cold water, cold dilute HCl solution, water,
and NaHCO3 solution, and then dried over MgSO4. After filtration, the
solvent was removed using a rotary evaporator to give a mixture of
trifluoroacetates 15 and 16 (265 mg, 97% yield) in a 25:75 ratio.
A solution of 51 mg of 2,6-lutidine (0.477 mmol) in 4 mL of methanol
was added to the mixture of trifluoroacetates 15 and 16 prepared above.
Trifluoroacetate 15 (Ar = C6H4-m-Cl) reacts in methanol with a half-life
of about 8 min at 23 °C to form the methyl ether derivative. After stirring
the solution for 70 min at 23 °C, 1.5 mL of 0.48 M NaOCH3 in methanol
was added. After 10 min at room temperature, the methanol was
removed using a rotary evaporator and the residue was taken up into
pentane and water. The pentane extract was dried over MgSO4, and after
solvent removal, the residue was chromatographed on 6 g of silica gel.
The column was eluted with increasing amounts of ether in pentane.
The methyl ether solvolysis product derived from 15 (50 mg) was eluted
with 2−3% ether in pentane. The alcohol 14 (Ar = C6H4-m-Cl) (124
mg; 98% yield based on the amount of trifluoroacetate 16 in the starting
1
mixture) was eluted with 5−8% ether in pentane, mp 59−60 °C. H
NMR of 14 (Ar = C6H4-m-Cl) (CDCl3) δ 7.37 (m, 1 H), 7.28 (m, 1 H),
7.25 (m, 1 H), 7.23 (m, 1 H), 2.40 (m, 2 H), 2.29 (m, 2 H), 2.08 (quin,
J = 9.9 Hz, 1 H), 2.02 (s, 1 H), −0.03 (s, 9 H). 13C NMR of 14 (Ar =
C6H4-m-Cl) (CDCl3) δ 147.8, 134.3, 129.7, 127.3, 125.2, 123.0, 77.9,
36.9, 14.8, −3.5. Exact mass (ESI)(M + Na+) calcd for C13H19ClNaOSi:
277.0786. Found: 277.0790. Exact mass (EI)(M − H2O) calcd for
C13H17ClSi: 236.0788. Found: 236.0780.
Preparation of Trifluoroacetates 15 and 16. Trifluoroacetates
15 and 16 were prepared by reaction of the corresponding alcohols
13 and 14 with trifluoroacetic anhydride (1.8 equiv) and 2,6-lutidine
(2.0 equiv) in ether solvent. The following procedures are
representative.
A solution of 40 mg of 13 (Ar = C6H4-m-Cl) (0.157 mmol) in 2 mL of
ether and 35 mg of 2,6-lutidine (0.327 mmol) was cooled to 0 °C, and
62 mg of trifluoroacetic anhydride (0.295 mmol) in 0.5 mL of ether was
added dropwise. The mixture was warmed to room temperature, and
after 10 min, the mixture was taken up into pentane. The solution was
consecutively washed with cold water, cold dilute HCl solution, water,
and NaHCO3 solution, and then dried over MgSO4. After filtration, the
solvent was removed using a rotary evaporator to give 51 mg (93% yield)
1
of 15 (Ar = C6H4-m-Cl) as an oil. H NMR of 15 (Ar = C6H4-m-Cl)
(CDCl3) δ 7.55 (m, 1 H), 7.44 (m, 1 H), 7.37−7.32 (m, 2 H), 2.78 (m,
2 H), 2.46 (m, 2 H), 1.33 (t of t, J = 11.7, 8.4 Hz, 1 H), 0.02 (s, 9 H).
13C NMR of 15 (Ar = C6H4-m-Cl) (CDCl3) δ 155.5 (q, J = 42 Hz),
141.5, 134.6, 130.0, 128.8, 126.5, 124.4, 114.1 (q, J = 286 Hz), 84.6, 35.8,
13.3, −3.6.
A solution of 56 mg of 14 (Ar = C6H4-m-Cl) (0.220 mmol) in 2 mL of
ether and 48 mg of 2,6-lutidine (0.449 mmol) was cooled to 0 °C, and
84 mg of trifluoroacetic anhydride (0.400 mmol) in 0.5 mL of ether was
added dropwise. The mixture was warmed to room temperature, and
after 10 min, the mixture was taken up into pentane. The solution was
consecutively washed with cold water, cold dilute HCl solution, water,
and NaHCO3 solution, and then dried over MgSO4. After filtration, the
solvent was removed using a rotary evaporator to give 74 mg (97% yield)
1
of 16 (Ar = C6H4-m-Cl) as an oil. H NMR of 16 (Ar = C6H4-m-Cl)
(CDCl3) δ 7.35 (m, 1 H), 7.32−7.26 (m, 3 H), 2.78 (m, 2 H), 2.57 (m,
2 H), 2.03 (quin, J = 10 Hz, 1 H), −0.06 (s, 9 H). 13C NMR of 16 (Ar =
C6H4-m-Cl) (CDCl3) δ 156.1 (q, J = 42 Hz), 142.2, 134.4, 129.7, 128.7,
126.5, 124.3, 114.2 (q, J = 287 Hz), 89.5, 33.9, 15.4, −3.6. Neat
trifluoroacetates 15 and 16 are prone to decomposition at room
temperature and were, therefore, stored in pentane solution at 10 °C.
Preparation of Acetate 15 (Ar = C6H4-p-CH3). A solution of
97 mg of a mixture of alcohols 13 and 14 (Ar = C6H4-p-CH3) (0.415
mmol; 62% alcohol 13, 38% alcohol 14) and 81 mg of 2,6-lutidine
(0.757 mmol) in 3 mL of ether was cooled to −10 °C, and 151 mg of
trifluoroacetic anhydride (0.719 mmol) in a small amount of ether was
added dropwise. The mixture was warmed to room temperature and
recooled to −10 °C, and pentane was added. The mixture was then
rapidly washed with cold water, cold dilute HCl solution, water, and
NaHCO3 solution, and then dried over MgSO4. After filtration, the
solvent was removed using a rotary evaporator to give a mixture of trifluo-
roacetates 15 and 16. A solution of 65 mg of 2,6-lutidine (0.607 mmol) in
3.64 g of acetic acid was immediately added to this trifluoroacetate
mixture. After 17 h at room temperature, the solution was taken up into
pentane (5 parts) and ether (1 part). The solution was washed with
2 portions of water and then with dilute Na2CO3 solution. After drying
over MgSO4, the solvents were removed using a rotary evaporator to
give 105 mg (92% yield) of acetate 15 (Ar = C6H4-p-CH3). 1H NMR of
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Article
15 (Ar = C6H4-p-CH3) (CDCl3) δ 7.44 (d, J = 8.6 Hz, 2 H), 7.18 (d, J =
8.1 Hz, 2 H), 2.70 (m, 2H), 2.34 (s, 3 H), 2.34 (m, 2 H), 1.94 (s, 3 H),
1.29 (t of t, J = 11.6, 8.5 Hz, 1 H), −0.01 (s, 9 H). 13C NMR of 15 (Ar =
C6H4-p-CH3) (CDCl3) δ 169.5, 139.3, 137.1, 128.9, 126.0, 81.0, 36.3,
21.7, 21.1, 13.7, −3.5. Exact mass (EI)(M − CH3) calcd for C15H21O2Si:
261.1311. Found: 261.1325.
ASSOCIATED CONTENT
■
S
* Supporting Information
Complete ref 11, the M062X/6-311+G* calculated structures,
energies, and Cartesian coordinates of cations 6, 10, 11, 24, and
1
25, H and 13C NMR spectra of 13, 14, 15, 16, 17, 18, (Ar =
Preparation of Acetate 15 (Ar = C6H4-p-OCH3). A solution of
57.2 mg of alcohol 13 (Ar = C6H4-p-OCH3) (0.229 mmol) in 1.0 mL of
CH2Cl2 was stirred, and 47.5 mg of acetic anhydride (0.466 mmol) was
added. Dimethylaminopyridine (35 mg; 0.287 mmol) was then added,
and the mixture was stirred at room temperature for 6 h. The mixture
was then taken up into 6 mL of pentane, and the solution was washed
with cold water, dilute HCl solution, water, and NaHCO3 solution, and
then dried over MgSO4. After filtration, the solvent was removed using
a rotary evaporator to give 62.4 mg (93% yield) of 15 (Ar = C6H4-
p-OCH3). 1H NMR of 15 (Ar = C6H4-p-OCH3) (CDCl3) δ 7.49 (d, J =
9 Hz, 2 H), 6.89 (d, J = 9 Hz, 2 H), 3.81 (s, 3 H), 2.70 (m, 2 H), 2.33 (m,
2 H), 1.93 (s, 3H), 1.26 (t of t, J = 11.6, 8.4 Hz, 1 H), −0.01 (s, 9 H). 13C
NMR of 15 (Ar = C6H4-p-OCH3) (CDCl3) δ 169.5, 158.8, 134.2, 127.7,
113.4, 80.9, 55.2, 36.3, 21.7, 13.8, −3.4. Exact mass (EI)(M − HOAc)
calcd for C14H20OSi: 232.1283. Found: 232.1285.
Preparation of Trifluoroacetates 18. Trifluoroacetates 18 were
prepared from the corresponding arylcyclobutanols15 using the
procedure described for preparation of 15 and 16. The following
procedure is representative. Reaction of 57 mg of 1-(3-chlorophenyl)-
cyclobutanol (0.312 mmol) with 105 mg of trifluoroacetic anhydride
(0.500 mmol) and 63 mg of 2,6-lutidine (0.589 mmol) in 2 mL of ether
at 0 °C gave 79 mg (91% yield) of 18 (Ar = C6H4-m-Cl). 1H NMR of 18
(Ar = C6H4-m-Cl) (CDCl3) δ 7.48 (m, 1 H), 7.38 (m, 1 H), 7.36−7.31
(m, 2 H), 2.79−2.67 (m, 4 H), 2.07 (m, 1 H), 1.79 (m, 1 H). 13C NMR
of 18 (Ar = C6H4-m-Cl) (CDCl3) δ 155.6 (q, J = 42 Hz), 141.8, 134.6,
130.0, 128.7, 126.3, 124.1, 114.2 (q, J = 286 Hz), 86.0, 34.3, 13.8.
Solvolyses of Trifluoroacetates 15 and 16 in CD3CO2D. The
following procedure is representative. A solution of 4.5 mg of
trifluoroacetate 15 (Ar = C6H4-m-Cl) and 2.5 mg of 2,6-lutidine
in 475 mg of CD3CO2D was placed in an NMR tube at 25 °C for 3 h
(10 half-lives). The tube was then analyzed by 1H NMR spectroscopy
that showed acetate 17 (Ar = C6H4-m-Cl) as the sole product. Acetate
17 (Ar = C6H4-m-Cl) was identified by 1H NMR spectral comparison
with an authentic sample of 17-H3 in CD3CO2D prepared by acetylation
of alcohol 13 (Ar = C6H4-m-Cl) with acetic anhydride and
dimethylaminopyridine. 1H NMR of 17-H3 (Ar = C6H4-m-Cl)
(CDCl3) δ 7.50 (t, J = 1.9 Hz, 1 H), 7.42 (m, 1 H), 7.30 (t, J =
7.3 Hz, 1 H), 7.26 (m, 1 H), 2.66 (m, 2 H), 2.35 (m, 2 H), 1.97 (s, 3 H),
1.32 (t of t, J = 11.7, 8.6 Hz, 1 H), 0.00 (s, 9 H). 13C NMR of 17 (Ar =
C6H4-m-Cl) (CDCl3) δ 169.4, 144.6, 134.2, 129.5, 127.6, 126.2, 124.0,
80.5, 36.2, 21.6, 13.6, −3.5. Exact mass (EI)(M − Cl) calcd for
C15H21O2Si: 261.1311. Found: 261.1318.
C6H4-m-Cl), 15 (Ar = C6H4-p-CH3), and 15 (Ar = C6H4-p-
OCH3), as well as kinetics studies on 15 (Ar = C6H4-m-Cl), 16
(Ar = C6H4-p-OCH3), and 18 (Ar = C6H4-m-Cl). This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: creary.1@nd.edu (X.C.).
Notes
The authors declare no competing financial interest.
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Solvolyses of Trifluoroacetates and Acetates. Kinetics
Procedures. Rate constants reported in Table 1 were all determined
1
using H or 19F NMR spectroscopy. Kinetic studies on 15 and 16 in
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1
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showed no negative frequencies.
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