β-silyl effects on the stabilities of carbanions and carbon-centered radicals derived from ethyl thionacetates, diethyl malonates, and ethyl acetoacetates

Article abstract of DOI:10.1021/jo951506g

The effects of an α-Me₃SiCH₂ group on the equilibrium acidities in DMSO of the acidic C-H bonds in esters, including ethyl thionacetate, diethyl malonate, and ethyl acetoacetate, were found to differ from that of an α-MeCH₂

Full text of DOI:10.1021/jo951506g

J . Org. Chem. 1996, 61, 51-54
51
â-Silyl Effects on th e Sta bilities of Ca r ba n ion s a n d
Ca r bon -Cen ter ed Ra d ica ls Der ived fr om Eth yl Th ion a ceta tes,
Dieth yl Ma lon a tes, a n d Eth yl Acetoa ceta tes
Shizhong Zhang* and Frederick G. Bordwell*
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113
Received August 14, 1995X
The effects of an R-Me3SiCH2 group on the equilibrium acidities in DMSO of the acidic C-H bonds
in esters, including ethyl thionacetate, diethyl malonate, and ethyl acetoacetate, were found to
differ from that of an R-MeCH2 group by less than 1 kcal/mol, pointing to little or no stabilizing
effect of a â-Me3Si group on carbanion stabilities. On the other hand, substitution of an R-Me3-
SiCH2 group for one of the acidic hydrogen atoms in these esters to give Me3SiCH2CH2C(dS)OEt,
Me3SiCH2CH(CO2Et)2, and Me3SiCH2CH(CO2Et)COCH3 was found to decrease the homolytic bond
dissociation energies (BDEs) of the remaining acidic C-H bonds by 7.1, 8.2, and 7.1 kcal/mol,
respectively, relative to those of the parent esters and by 2.7, 4.5, and 4.4 kcal/mol, respectively,
relative to the BDEs of the acidic C-H bonds in the corresponding MeCH2-substituted esters. The
differences in BDEs observed between the Me3SiCH2- and the MeCH2-substituted ester derivatives
are indicative of the presence of 3-5 kcal/mol C-H bond stabilizing effects of â-Me3Si groups, relative
to those of â-Me groups, on the corresponding carbon radicals. The BDE value of the acidic C-H
bond in Me3CCH2CH2C(dS)OEt was found to be 86.4 kcal/mol, which represents a 0.6 kcal/mol
increase in BDE relative to that of the acidic C-H bond in MeCH2CH2C(dS)OEt. This result
demonstrates the absence of a stabilizing effect of a â-tert-butyl group on an open-chain carbon-
centered radical. This is contrary to an apparent 6.4 kcal/mol C-H bond stabilizing effect of the
â-tert-butyl group in 9-neopentylfluorene, relative to that in fluorene, which is attributed to the
relief of steric strain in forming the radical.
In tr od u ction
times slower than that for the Me3SiCH2• radical toward
(n-Bu)3Ge-H, the rate for the formation of the Me3SiCH2-
Both theory and experiment have shown that R-Me3Si
groups are less stabilizing to carbocations than alkyl
groups, but generally more stabilizing than hydrogens,
whereas a â-Me3Si groups stabilizes, a carbocation.1 On
the other hand, an R-Me3Si group stabilizes a carbanion
as shown by the increases in the acidities of fluorene and
PhSO2CH3 in DMSO upon the introduction of a Me3Si
group at the acidic site.2 Information on the effect of
â-Me3Si groups on carbanion stabilities is limited, but
existing evidence suggests that the effects are small.3
The evidence for stabilizing or destabilizing effects of
R- or â-Me3Si groups on radicals is less clear-cut. The
bond dissociation enthalpy for the C-H bond in Me3-
SiCH2-H has been found to be only 0.4 kcal/mol less than
that in Me3CCH2-H,4 indicating that an R-Me3Si group
has little or no ability to stabilize the corresponding
carbon-centered radical relative to that of an R-t-Bu
group, and recent work in our laboratory has shown that
R-Me3Si groups usually exert small destabilizing effects
on carbon-centered radicals.5
CH2• radical was only 1.2 times slower than that for the
•
Me3SiCH2CH2CH2 radical and only about 27% slower
than that for the CH3CH2CH2CH2 radical.6a Kinetic
•
effects for hydrogen atom abstraction are generally small
in such radical reactions because there is relatively little
bond breaking in the transition state, and the kinetics
are therefore qualitative and difficult to interpret. There
is also thermodynamic evidence to support the stabilizing
effect of â-Me3Si groups on carbon-centered radicals,
however. Thus, the BDE of the C-H bond in Me3SiCMe2-
CH2-H has been found to be weaker by 3 ( 1 kcal/mol
than that in MeCMe2CH2-H.7 (Henceforth, kcal/mol will
be abbreviated as kcal.) This result indicates that a
â-Me3Si group stabilizes a carbon-centered radical by
about 3 kcal, relative to that of a â-methyl group. Also,
pyrolytic cleavages of the C-C bonds in GCH2CH2-CMe3
or GCH2CH2-CH2CHdCH2 to give GCH2CH2• and Me3C•
•
•
or GCH2CH2 and CH2dCHCH2 radicals, respectively,
have been found to be favored by 2.6 ( 1 kcal when G )
Me3Si, compared to when G ) H.8 Furthermore, in the
pyrolytic cleavage of a C-C bond in (trimethylsilyl)-
cyclopropane, the activation enthalpy for the formation
of the â diradical, Me3SiCH(C•H2)2, is 5.4 kcal lower than
that for the formation of the R,â diradical, Me3Si•CHCH2-
There is kinetic evidence from rates of hydrogen atom
abstraction from Et4C, Et4Si, and (n-Bu)3Ge-H showing
that â-silicon stabilizes a carbon-centered radical, relative
to an alkyl group.6 The effects are small, however, and
although the rate for the Me3SiCH2CH2• radical was 7.1
•
CH2 . These experiments also point to a stabilizing effect
of about 3 kcal for a â-Me3Si group on a primary radical,
relative to that of a â-methyl group or a â-hydrogen atom.
X Abstract published in Advance ACS Abstracts, December 15, 1995.
(1) Lambert, J . B. Tetrahedron 1990, 46, 2677 and references cited
therein.
(2) Bordwell, F. G. Acc. Chem. Res. 1988, 21, 456, and references
cited therein.
(3) Engel, W.; Fleming, I.; Smithers, R. H. J . Chem. Soc., Perkin
Trans. 1 1986, 1637.
(4) Doncaster, A. M.; Walsh, R. J . Chem. Soc., Faraday Trans. 1
1976, 72, 2908.
(6) (a) J ackson, R. M.; Ingold, K. U.; Griller, D.; Nazran, A. S. J .
Am. Chem. Soc. 1985, 107, 208. (b) Wilt, J . W.; Lusztyk, J .; Peeran,
M.; Ingold, K. U. J . Am. Chem. Soc. 1988, 110, 281.
(7) Auner, N; Walsh, R.; Westrup, J . Chem. Soc., Chem. Commun.
1986, 207.
(5) Zhang, S.; Zhang, X.-M.; Bordwell, F. G. J . Am. Chem. Soc. 1995,
117, 603.
(8) Davidson, I. M. T.; Barton, T. J .; Hughes, K. J .; Ijadi-Maghsoodi,
S.; Revis, A.; Paul, G. C. Organometallics 1987, 6, 644.
0022-3263/96/1961-0051$12.00/0 © 1996 American Chemical Society

52 J . Org. Chem., Vol. 61, No. 1, 1996
Zhang and Bordwell
Ta ble 1. p KHA a n d BDE Va lu es for th e Acid ic C-H
Bon d s in r-Su bstitu ted Eth yl Th ion a ceta tes,
GCH2C(dS)OEt, in DMSO
On the other hand, the BDE of the acidic C-H bond in
9-((trimethylsilyl)methyl)fluorene, 9-Me3SiCH2FlH, has
been estimated to be nearly the same as that of the acidic
C-H bond in 9-neopentylfluorene, 9-Me3CCH2FlH, which
has led to the conclusion that the corresponding (tertiary)
fluorenyl radicals, 9-Me3SiCH2Fl• and 9-Me3CCH2Fl•, are
stabilized to nearly the same degree by hyperconjugation
involving either a â-Me3Si or a â-Me3C group.10
a
G
pKHA
Eox(A-)b
BDE (kcal)c
∆BDE
H
19.2
18.8
19.1
18.7
18.6
-0.405
-0.590
-0.590
-0.548
-0.680
90.2
85.5
85.8
86.4
83.1
(0.0)
4.7
4.4
3.8
7.1
CH3
CH3CH2
t-BuCH2
Me3SiCH2
In summary, kinetic and thermodynamic studies show
that â-Me3Si groups usually stabilize carbon-centered
radicals, whereas R-Me3Si groups show little or no
tendency to do so. The thermodynamic studies indicate
that the stabilizing effect of a â-Me3Si group on a primary
carbon-centered radical appears to be about 3 kcal,
relative to â-methyl7 or â-hydrogen,8 but appears to be
negligible toward â-Me3C in fluorenyl radicals.10 In the
present study, we have used â-Me3Si-substituted ethyl
thionacetates, Me3SiCH2CH2C(dS)OEt, diethyl mal-
onates, Me3SiCH2CH(COOEt)2, and ethyl acetoacetates,
Me3SiCH2CH(COOEt)COCH3, to compare effects on BDEs
of a â-Me3Si group with those of â-alkyl groups.
a
b
Measured in DMSO (see Table 4). Irreversible oxidation
potentials measured by cyclic voltammetry in DMSO using
platinum working and auxiliary electrodes and a Ag/AgI reference
electrode with 0.1 M Et4N+BF4 electrolyte and ferrocene/ferro-
-
cenium as a standard. c Calculated using eq 1.
Ta ble 2. p KHA a n d BDE Va lu es for th e Acid ic C-H of
9-Su bstitu ted F lu or en es, 9-G-F lH
a
9-G
pKHA
Eox(A-)a
BDE (kcal)c
∆BDE
H (fluorene)
CH3CH2
t-BuCH2
22.6
22.7
20.3
21.3b
-1.050
-1.221
-1.191
-1.135b
80.0
76.2
73.6
76.3
(0.0)
3.8
6.4
Me3SiCH2
3.7
a
b
All the data are from ref 16, except where specified. pKHA’s
were measured in DMSO (see Table 4); irreversible oxidation
potentials measured by cyclic voltammetry in DMSO using
platinum working and auxiliary electrodes and a Ag/AgI reference
Resu lts a n d Discu ssion
For the study of the effect of a â-Me3Si group on the
acidity and BDE of a C-H bond, we first examined
â-substituent effects on the acidic C-H bonds in ethyl
thionacetates, GCH2C(dS)OEt; a class of esters for which
no pKHA or BDE data on substituent effects has previ-
ously been obtained.11 Our method is based on the
combination of pKHA values in DMSO with the oxidation
potentials of their conjugate bases Eox(A-) according to
eq 1. The BDE differences (∆BDEs) between the sub-
electrode with 0.1 M Et4N+BF4 electrolyte and ferrocene/ferro-
-
cenium as a standard. c BDEs were calculated using eq 1.
hyperconjugative effects. The approximately 5 kcal
decrease in the BDE observed for R-methyl substitution
onto ethyl thionacetate is similar in size to the BDE
decreases observed for R-methyl substitution onto ac-
etone,13 acetophenone,13 or nitromethane.14 Analysis of
these effects in terms of eq 1 shows that all of these BDE
decreases are caused by cathodic shifts in the Eox(A-)
values.15 A further cathodic shift of 0.132 V (3 kcal) was
observed when the t-Bu group in t-BuCH2CH2C(dS)OEt
was replaced by a Me3Si group (Table 1). The BDE when
G ) Me3Si is 7.1 kcal lower than that of the parent, G )
H, and 3.3 kcal lower than that of an alkyl model, with
G ) Me3C. The appreciable radical stabilization energies
for a â-Me3Si group can be attributed to â-silyl hyper-
conjugation for this radical, as illustrated by 1a T 1b.
BDE (kcal) ) 1.37pKHA + 23.1Eox(A-) + 73.3 (1)
stituted and parent weak acid can be approximated as
radical stabilization energies in the absence of ap-
preciable changes in ground state energies.
Ethyl thionacetate, which is readily prepared by the
reaction of ethyl acetate with Lawesson’s reagent,12 has
a pKHA value 10.8 units (15 kcal) lower and a BDE value
about 5 kcal lower than the corresponding values of ethyl
acetate. The effects of â-substituents on the acidity and
BDEs of the acidic C-H bonds of selected â-substituted
ethyl thionacetates were therefore accessible and are
summarized in Table 1.
Examination of Table 1 shows that the acidities of the
ethyl thionacetates, GCH2C(dS)OEt, vary over a narrow
range for substituents where G is Me, Et, Me3CCH2, and
Me3SiCH2 (0.14-0.82 kcal), showing that â-alkyl and
â-Me3Si groups have nearly the same small effects on
carbanion stabilities. On the other hand, BDEs of the
R-C-H bonds for these substituted thion esters, relative
to that of the parent with G ) H, were found to be
substantially decreased, pointing to appreciable stabiliz-
ing effects on the corresponding radicals. For the alkyl
groups, the BDEs decrease progressively from 4.7 to 4.4
to 3.8 kcal when G is changed from CH3, to CH3CH2 to
Me3CCH2, respectively, presumably due to decreasing
This evidence for a sizable stabilizing effect of a â-Me3-
Si group on the radical provides support for the earlier
kinetic and thermodynamic evidence.6-8 On the other
hand, the â-tert-butyl group shows little or no stabilizing
effect on the carbon radical, and this is contrary to the
result seen from 9-neopentylfluorene.10
We have also examined the effect of the â-Me3Si and
â-tert-butyl group on fluorene (Table 2). The results
summarized in Table 2 agree with those reported by
Bausch and Gong except for the ∆BDE between 9-((tri-
methylsilyl)methyl)fluorene and fluorene. We have ob-
served a ∆BDE of -3.7 (76.3 - 80.0) kcal instead the
BDE of -7 (69 - 76) kcal reported. According to our
results, the â-Me3Si group has little or no effect on the
(9) Conlin, R. T.; Kwak, Y.-W. Organometallics 1986, 5, 1025.
(10) Bausch, M. J .; Gong, Y. J . Am. Chem. Soc. 1994, 116, 5963.
(11) A study of the acidity and BDE of the acidic C-H bonds in
R-substituted ethyl acetates is precluded because the conjugate base
of ethyl acetate [CH2COOEt]- is a strong base (pKHA ca. 30) and is
unstable, apparently due to facile loss of EtO- ion.2
(13) Bordwell, F. G.; Harrelson, J . A., J r. Can. J . Chem. 1990, 68,
1714.
(14) Bordwell, F. G.; Satish, A. V. J . Am. Chem. Soc. 1994, 116, 8885.
(15) Bordwell, F. G.; Zhang, X.-M.; Filler, R. J . Org. Chem. 1993,
58, 6067.
(12) Pedersen, B. S.; Scheibye, S.; Clausen, K.; Lawessen, S.-O. Bull.
Soc. Chim. Belg. 1978, 87, 293.

Carbanions and Carbon-Centered Radicals
J . Org. Chem., Vol. 61, No. 1, 1996 53
Ta ble 3. p KHA a n d BDE Va lu es for th e Acid ic C-H
Bon d s in r-Su bstitu ted Dieth yl Ma lon a tes a n d Eth yl
Acetoa ceta tes in DMSO
Ta ble 4. Su m m a r y of p KHA Mea su r em en tsa
b
acid
CH3C(S)OEt
indicator (pKHIn
)
pKHA std
CNAH (18.9)
FMY30 (18.1)
CNAH (18.9)
PFH (17.9)
CNAH (18.9)
PFH (17.9)
CNAH (18.9)
PFH (17.9)
CNAH (18.9)
PFH (17.9)
19.22 0.01
19.10 0.02
18.82 0.03
18.77 0.04
19.15 0.01
18.90 0.05
18.69 0.01
18.75 0.05
18.60 0.01
18.59 0.03
18.63 0.01
18.57 0.01
18.65 0.02
15.84 0.05
15.80 0.04
16.42 0.07
16.41 0.05
14.37 0.01
14.23 0.01
21.31 0.02
a
G
pKHA
Eox(A-)a
BDE (kcal)c
∆BDE
[GCH(COOEt)2]
-0.022
CH3CH2C(S)OEt
H
CH3
CH3CH2
Me3SiCH2
16.4
18.7
19.1
18.6b
95.2
90.8
91.5
87.3
(0.0)
4.4
3.7
-0.352
CH3CH2CH2C(S)OEt
t-BuCH2CH2C(S)OEt
Me3SiCH2CH2C(S)OEt
-0.346
-0.480b
8.2
[GCH(COOEt)COCH3]
H
14.3b
0.005b
-0.265b
-0.420b
92.3
89.6
85.2
(0.0)
3.7
7.1
CH3CH2
Me3SiCH2
16.4b
15.8b
FMY30 (18.1)
FMY30 (18.1)
CNAH (18.9)
Me3SiCH2CH(COOEt)2
a
b
Reference 17. pKHA’s were measured in DMSO (see Table 4);
irreversible oxidation potentials measured by cyclic voltammetry
in DMSO using platinum working and auxiliary electrodes and a
Ag/AgI reference electrode with 0.1 M Et4N+BF4- electrolyte and
ferrocene/ferrocenium as a standard. c BDEs were calculated using
eq 1.
CH3C(O)CH(CH2SiMe3)- MCLPHF (16.8)
COOEt
CH3C(O)CH(CH2CH3)-
COOEt
CH3C(O)CH2COOEt
MCLPHF (16.8)
MCLPHF (16.8)
MCLPHF (16.8)
PSFH (15.4)
FMY330 (13.8)
Me3SiCH2FlH
(standard acid)
m-FC6H4CH2SO2Ph
stability of fluorenyl radical, but the â-tert-butyl group
appears to have a stabilizing effect relative to a â-Me
group. We believe, however, that the apparent â-tert-
butyl stabilization is deceptive and is due to a steric effect
rather than an electronic effect, namely a relief of steric
strain upon the formation of the 9-neopentylfluorenyl
radical. This effect becomes smaller in ((trimethylsilyl)-
methyl)fluorene because the Si-C bonds are longer than
C-C bonds. These results were augmented by studies
of the effects of â-Me3Si substituents in diethyl malonates
and ethyl acetoacetate (Table 3).
(21.7)
m-CF3C6H4CH2SO2Ph 21.30 0.01
(21.7)
a
b
Measured by the overlapping indicator method.20 Names of
indicators: CNAH, 4-chloro-2-nitroaniline; FMY30, 2-(phenylsul-
fonyl)fluorene; PFH, 9-phenylfluorene; MCLDHF, 9-(m-chloro-
phenyl)fluorene; PSFH, 9-(phenylthio)fluorene.
â-trimethylsilyl stabilizing effects on carbon-centered
radicals can be as large as 4.5 kcal, but there is little
â-silyl effect on carbanions.
The alkyl effects in the R-position on acidities in the
diethyl malonates and ethyl acetoacetates are subject to
chelation and conformational effects, whereas the alkyl
effects on the BDEs are not.17 The effects of alkyl and
â-Me3SiCH2 substituents on BDEs in Tables 1 and 3 are
remarkably similar. In Table 1 the effects of introducing
Me, Et, and Me3SiCH2 groups for an acidic hydrogen
atom in ethyl thionacetates lead to decreases in BDEs of
the acidic C-H bond of 4.7, 4.4, and 7.1 kcal, respectively.
In Table 3 the effects of introducing Me, Et, and Me3-
SiCH2 groups for an acidic hydrogen atom in diethyl
malonates lead to decreases in BDEs of the acidic C-H
bond of 4.4, 3.7, and 8.2 kcal, respectively, and the effects
of introducing Et and Me3SiCH2 groups for an acidic
hydrogen atom in ethyl acetoacetates lead to decreases
in BDEs of the acidic C-H bond of 3.7 and 7.1 kcal,
respectively. In Table 1 the effects of introducing Et or
t-BuCH2 groups for an acidic hydrogen atom on the BDEs
of the acidic C-H bond differ slightly (4.4 and 3.8 kcal,
respectively).
Su m m a r y a n d Con clu sion s. Substitution of the
Me3SiCH2 group at the acidic site of ethyl thionacetate,
diethyl malonate, and ethyl acetoacetate causes decreases
in BDEs of 7-8 kcal relative to the BDE of the parent
esters and decreases of about 4 kcal relative to the BDEs
of the ethyl-substituted esters. These BDE decreases are
rationalized in terms of hyperconjugation effects that are
larger for the Me3SiCH2 group by 3-4 kcal than for alkyl
groups, including a Me3CCH2 group in an ethyl thionac-
etate. In contrast, the decrease in BDE for a 9-Me3SiCH2
group in fluorene was only 3.7 kcal compared to 6.4 kcal
for a Me3CCH2 group. This reversal is believed to be
associated with the larger steric effect in 9-neopentylfluo-
rene than in the Me3SiCH2 analogue. In conclusion,
Exp er im en ta l Section
The pKHA measurements in Me2SO were carried out as
described in earlier publications;2 details are given in Table
4. Cyclic voltammetry was carried out in the manner previ-
ously described18 using platinum working and auxiliary elec-
-
trodes and a Ag/AgI reference electrode with 0.1 M Et4N+BF4
electrolyte and ferrocene/ferrocenium as a standard. NMR
spectra were recorded on a Varian GEMINI 300 or VRX 300.
Ethyl thionacetate CH3C(dS)OEt, ethyl thionpropionate
CH3CH2C(dS)OEt, and ethyl thionbutyrate CH3CH2CH2C-
(dS)OEt were prepared from the reaction of the appropriate
ethyl ester with Lawesson’s reagent (Aldrich).12 A typical
procedure is as follows.
Lawesson’s reagent, 10 g, and an ethyl ester, 25 mL, were
mixed in a 50 mL round-bottom flask. The two-phase mixture
was refluxed, during which time aliquots were taken and
analyzed by GLC, until the concentration of thionester failed
to increase. This usually took about 2-4 days. The volatiles
were transferred into another round-bottom flask under
vacuum, leaving the solid materials behind. The thionester
solution was then concentrated to 40-60% by fractional
distillation, and the pure compound (>98%) was obtained by
preparative GLC. The crude yield was between 30 and 50%.
Ethyl thionbutyrate CH3CH2CH2C(dS)OEt appears to be a
new compound. Its spectroscopic data are as follows: 1H NMR
(CDCl3) δ 0.942 (3H, t, CH3), 1.386 (3H, t, CH3), 1.758 (2H,
hex, CH2), 2.687 (2H, t, CH2), 4.480 (2H, q, CH2); mass
spectrum m/ z (relative abundance) 132 (100), 89 (25.3), 87
(29.4), 76 (39.2), 71 (76.9), 55 (31.5); high-resolution MS calcd
for C6H12OS 132.0609, found 132.0618.
Eth yl 3-(Tr im eth ylsilyl)th ion p r op ion a te [(Me3Si)CH2-
CH2C(dS)OEt]. Lawesson’s reagent (1.2 g, 3 mmol, 6 mequiv
of sulfur) and 0.8 g (4.6 mmol) of ethyl 3-(trimethylsilyl)-
propionate, prepared according to the literature procedure,19
(16) Bordwell, F. G.; Zhang, X.-M. J . Am. Chem. Soc. 1994, 116,
973.
(17) Zhang, X.-M.; Bordwell, F. G. J . Phys. Org. Chem. 1994, 7, 751.
(18) Bordwell, F. G.; Cheng, J .-P.; J i, G.-Z.; Satish, A. V.; Zhang,
X.-M. J . Am. Chem. Soc. 1991, 113, 9790.
(19) Sommer, L. H.; Marans, N. S. J . Am. Chem. Soc. 1950, 72, 1937.

54 J . Org. Chem., Vol. 61, No. 1, 1996
Zhang and Bordwell
were mixed in 30 mL of toluene. The mixture was refluxed
for 3 days, and the final ratio of ester vs thionester was about
40:60, according to GLC analysis. The mixture was then
passed through a silica gel column with methylene chloride
as the elution solvent. The desired thionester was eluted first.
The early fractions were combined and concentrated by
removing most of the solvent. Ethyl 3-(trimethylsilyl)thion-
propionate, 100 mg (11% yield), was obtained by preparative
GLC (purity >98%): 1H NMR (CDCl3) δ 0.018 (9H, s, Si(CH3)3),
0.983 (2H, m, CH2), 1.404 (3H, t, CH3), 2.678 (2H, m, CH2),
4.507 (2H, q, CH2); mass spectrum m/ z (relative abundance)
190 (1.9), 175 (68.2), 147 (41.4), 129 (17.0), 91 (29), 73 (100);
high-resolution MS calcd for C8H18OSSi 190.0848, found
190.0835.
Eth yl 3-ter t-Bu tyl)p r op ion a te a n d Eth yl 3-ter t-Bu -
tylth ion p r op ion a te. tert-Butylacetylene (25 g, 0.3 mol) and
300 mL of anhydrous Et2O were placed in a 500 mL three-
neck flask, and 120 mL of butyllithium (2.5 M in hexanes) was
added dropwise at -78 °C. After stirring for 20 min, 30 g (0.28
mol) of ethyl chloroformate was added slowly. The solution
was gradually warmed, and a white solid precipitated. The
organic solution was separated from the solid, washed with
50 mL of H2O, and then dried over anhydrous Na2SO4. After
removal of the volatiles, distillation at 80 °C/0.8 mmHg yielded
40 g of ethyl 3-tert-butylpropynoate, t-BuCtCC(dO)OEt (yield
81%). Hydrogenation of the ethyl 3-tert-butylpropynoate in
ethanol with Pd/C catalyst gave a quantitative yield of ethyl
3-tert-butylpropionate, t-BuCH2CH2C(dO)OEt. Refluxing the
t-BuCH2CH2C(dO)OEt with Lawessen’s reagent in toluene for
24 h produced ethyl 3-tert-butylthionpropionate (70% yield).
A pure (>98%) sample was obtained by preparative GLC: 1H
NMR (CDCl3) δ 0.90 (s, 9H), 1.38 (t, 3H), 1.63 (m, 2H), 2.69
(m, 2H), 4.48 (q, 2H); high-resolution MS calcd for C9H18OS
174.1078, found 174.1079.
9-((Tr im eth ylsilyl)m eth yl)flu or en e was prepared from
the reaction of lithium fluorenide with (chloromethyl)trimeth-
ylsilane in THF and purified by recrystallization from ethanol.
1
Colorless crystalline needles were obtained, and their H NMR
spectrum and mp were found to be identical to those reported
by Bausch and Gong.10 Repeated cyclic voltammetry experi-
ments on the oxidation potential of 9-((trimethylsilyl)methyl)-
fluorenyl anion gave the Eox(A-) value as shown in Table 2;
the difference of the oxidation potentials between 9-((trimeth-
ylsilyl)methyl)fluorenyl anion and fluorenyl anion is 0.085 V,
which is 0.135 V smaller than that reported earlier.10
Dieth yl 2-((tr im eth ylsilyl)m eth yl)m a lon a te was pre-
pared from the reaction of (trimethylsilyl)methyl chloride with
diethyl malonate and pyridine in THF and purified by frac-
tional distillation.
Eth yl 2-((tr im eth ylsilyl)m eth yl)a cetoa ceta te a n d eth yl
2-eth yla cetoa ceta te were purchased from Aldrich and puri-
fied by fractional distillation.
Ack n ow led gm en t. We are grateful to the National
Science Foundation for support of this research.
(20) Matthews, W. S.; Bares, J . E.; Bartmess, J . E.; Bordwell, F.
G.; Cornforth, F. J .; Drucker, G. E.; Margolin, Z.; McCallum, R. J .;
McCollum, G. J .; Vanier, N. R. J . Am. Chem. Soc. 1975, 97, 7006-
7014.
J O951506G