Phenyldimethylsilyl-substituted ketenes and bisketenes

Article abstract of DOI:10.1021/jo9605197

The phenyldimethylsilyl-substituted monoketene PhMe₂SiCH=C=O (1) and bisketene (PhMe₂-SiC=C=O)₂ (3) have been prepared and compared to the corresponding Me₃Si- and t-BuMe₂Si-substituted species. The ¹³C, ¹⁷O, and ²⁹Si NMR spectra fit the pattern shown by other silylketenes and provide no evidence for transmission of a substituent effect of the Ph group through the silicon to the ketenyl group, as has been proposed for PhMe₂Si-substituted radicals. The UV spectrum of 1 does show a longer λ and greater ∈ than for t-BuMe₂SiCH=C=O, and this may indicate some interaction of the phenyl group with the ketene chromophore, while the greater reactivity of 1 in hydration compared to t-BuMe₂SiCH=C=O is ascribed to the inductive effect of the phenyl. The very similar ring-opening reactivity of the bis(phenyldimethylsilyl)cyclobutenedione (6) to form 3 compared to the bis(Me₃Si) analogues also provides no evidence of a significant interaction of the phenyl with the ketene. A new type of stabilized 1,8-bisketene based on the arylbis(dimethylsilyl) grouping, namely, 1,4-bis(ketenyldimethylsilyl)benzene (12), has been prepared for the first time.

Full text of DOI:10.1021/jo9605197

J . Org. Chem. 1996, 61, 6227-6232
6227
P h en yld im eth ylsilyl-Su bstitu ted Keten es a n d Bisk eten es
Ronghau Liu, Romeo M. Marra, and Thomas T. Tidwell*
Department of Chemistry, University of Toronto, Toronto, Ontario, Canada M5S 3H6
Received March 18, 1996X
The phenyldimethylsilyl-substituted monoketene PhMe2SiCHdCdO (1) and bisketene (PhMe2-
SiCdCdO)2 (3) have been prepared and compared to the corresponding Me3Si- and t-BuMe2Si-
substituted species. The 13C, 17O, and 29Si NMR spectra fit the pattern shown by other silylketenes
and provide no evidence for transmission of a substituent effect of the Ph group through the silicon
to the ketenyl group, as has been proposed for PhMe2Si-substituted radicals. The UV spectrum of
1 does show a longer λ and greater ꢀ than for t-BuMe2SiCHdCdO, and this may indicate some
interaction of the phenyl group with the ketene chromophore, while the greater reactivity of 1 in
hydration compared to t-BuMe2SiCHdCdO is ascribed to the inductive effect of the phenyl. The
very similar ring-opening reactivity of the bis(phenyldimethylsilyl)cyclobutenedione (6) to form 3
compared to the bis(Me3Si) analogues also provides no evidence of a significant interaction of the
phenyl with the ketene. A new type of stabilized 1,8-bisketene based on the arylbis(dimethylsilyl)
grouping, namely, 1,4-bis(ketenyldimethylsilyl)benzene (12), has been prepared for the first time.
As part of our studies of silylated ketenes and bis-
ketenes, we have examined the properties of different
silyl subsituents, including trimethylsilyl, tert-butyldim-
ethylsilyl, and triisopropylsilyl.1 Extension of these
studies to include the phenyldimethylsilyl (PDMS) sub-
stituent is of interest for a variety of reasons including
the possibility that these derivatives might be more
highly crystalline than those bearing other silyl substit-
uents, the utility of this group in synthesis as a masked
hydroxy group,2 the formation of metal-complexed aryl
derivatives,3 other synthetic applications of the PDMS
group,4 and the possibility of electronic transmission of
the aryl group through the silyl group to affect the
properties of the ketene.5 Specifically it has been
proposed5a that the p orbitals of the phenyl in â-PDMS-
substituted cyclohexanones can enhance photochemical
Norrish Type I cleavage by homoconjugative p-d-p
orbital overlap with the developing radical center (eq 1).
Two mechanisms have been proposed for silyl stabiliza-
tion of ketenes,1a namely, (a) π donation from the ketene
to the d orbitals on silicon5b and (b) σπ-pπ donation from
the carbon-silicon bond to the electron deficient in-plane
p orbital on the carbonyl carbon.5c Thus a study of
PDMS-substituted ketenes could provide evidence re-
garding the viability of dπ-pπ interactions in ketenes, and
by implication in developing radicals as in eq 1 as well.
O
C
O
Bu–t
Bu–t
hν
(1)
SiMe2Ph
SiMe2Ph
Therefore we have examined the properties of ketenes
and bisketenes substituted with the PDMS group includ-
ing their 13C, 17O, and 29Si NMR spectra, which we have
found to be a sensitive probe of electronic effects in these
ketenes.6a We have also previously shown that cy-
clobutenedione precursors to bisketenes can be linked to
aryl groups to give precursors to tetraketenes1f and have
now studied the 1,4-bis(dimethylsilyl)phenyl group as a
linking agent for stabilized 1,8-bisketenes.
Resu lts
Previous reports7a-c of the preparation of PhMe2-
SiCHdCdO (1) have appeared, but none of these gives
a thorough characterization of this ketene. Thus 1 has
been prepared by dehydrochlorination of the correspond-
ing acyl chloride,7a pyrolysis of the alkynyl ether 2 at 115
°C and vacuum distillation (eq 2),7b and pyrolysis of the
anhydride (PhMe2SiCH2CO)2O.7c
115 °C
(2)
PhMe2SiCH
C
O
PhMe2SiC
COEt
CH2
–CH2
2
1
X Abstract published in Advance ACS Abstracts, August 15, 1996.
(1) (a) Tidwell, T. T. Ketenes; Wiley: New York, 1995. (b) Allen, A.
D.; Ma, J .; McAllister, M. A.; Tidwell, T. T.; Zhao, D.-c. Acc. Chem.
Res. 1995, 28, 265-271. (c) Zhao, D.-c.; Allen, A. D.; Tidwell, T. T. J .
Am. Chem. Soc. 1993, 115, 10097-10103. (d) Allen, A. D.; Lai, W.-Y.;
Ma, J .; Tidwell, T. T. J . Am. Chem. Soc. 1994, 116, 2625-2626. (e)
Allen, A. D.; J i, R.; Lai, W.-Y.; Ma, J .; Tidwell, T T. Heteroatom Chem.
1994, 5, 235-244. (f) Liu, R.; Tidwell, T. T. Can. J . Chem. 1995, 73,
1818-1822. (g) Gong, L.; McAllister, M. A.; Tidwell, T. T. J . Am. Chem.
Soc. 1991, 113, 6021-6028.
(2) (a) Fleming, I.; Henning, R.; Parker, D. C.; Plaut, H. E.;
Sanderson, P. E. J . J . Chem. Soc., Perkin Trans. 1 1995, 317-337. (b)
Taber, D. F.; Yet, L.; Bhamidipati, R. S. Tetrahedron Lett. 1995, 36,
351-354.
(3) Do¨tz, K. H. Angew. Chem., Int. Ed. Engl. 1979, 18, 954-955.
(4) (a) Barrett, A. G. M.; Hill, J . M.; Wallace, E. M. J . Org. Chem.
1992, 57, 386-400. (b) Barrett, A. G. M.; Flygare, J . A.; Hill, J . M.;
Wallace, E. M. Org. Synth. 1995, 73, 50-60.
We find that the ketene 1 may be conveniently ob-
tained in pure form directly from 2 by pyrolysis in a
preparative gas chromatograph and collection. The
1
ketene was characterized by H, 13C, 17O, and 29Si NMR
and showed the distinctive bands for silylketenes in 13C
NMR at δ -0.7 (Câ) and 178.9 (CR), an 17O signal at δ
256.1, and a 29Si signal at δ -5.4. These band positions
are consistent with those for other silylketenes.6
(6) (a) Allen, A. D.; Egle, I.; J anoschek, R.; Liu, H. W.; Ma, J .; Marra,
R. M.; Tidwell, T. T. Chem. Lett. 1996, 45-46. (b) Valenti, E.; Pericas,
M. A.; Serratosa, F. J . Org. Chem. 1990, 55, 395-397.
(7) (a) Baukov, Yu. I.; Burlachenko, G. S.; Kostyuk, A. S.; Lutsenko,
I. F. Dokl. Vses. Konf. Khim Atsetilena, 4th 1972, 2, 130-133; Chem.
Abstr. 1973, 79, 78904y. (b) Himbert, G.; Henn, L. Liebigs Ann. Chem.
1984, 1358-1366. (c) Kostyuk, A. S.; Boyadzhan, Zh. G.; Zaitseva, G.
S.; Sergeev, V. N.; Savel'eva, N. I.; Baukov, Yu. I.; Lutsenko, I. F. Zh.
Obshch. Khim. 1979, 49, 1543-14552. (d) Sommer, L. H.; Gold, J . R.;
Goldberg, G. M.; Marans, N. S. J . Am. Chem. Soc. 1949, 71, 1509.
(5) (a) Hwu, J . R.; Chen, B.-L.; Huang, L. W.; Yang, T.-H. J . Chem.
Soc., Chem. Commun. 1995, 299-300. (b) Runge, W. Prog. Phys. Org.
Chem. 1981, 13, 315-484. (c) Brady, W. T.; Cheng, T. C. J . Org. Chem.
1977, 42, 732-734.
S0022-3263(96)00519-1 CCC: $12.00 © 1996 American Chemical Society

6228 J . Org. Chem., Vol. 61, No. 18, 1996
Liu et al.
a
Ta ble 1. Hyd r a tion Ra tes (s-1
)
of RMe2SiCHdCdO in
CCl3COCl
Zn
PhMe2SiC
CSiMe2Ph
H2O/CH3CN a t 25 °C
4
[H2O] (M) R ) Ph (1)
R ) Meb R ) t-Bub Ph/Me Ph/t-Bu
PhMe2Si
O
PhMe2Si
PhMe2Si
O
O
38.9
33.3
27.9
22.2
16.7
11.1
2.23 × 10-2 2.11 × 10-2 5.21 × 10-3
1.03 × 10-2 6.68 × 10-3 2.08 × 10-3
5.40 × 10-3 2.84 × 10-3 9.58 × 10-4
3.10 × 10-3 2.03 × 10-3 5.52 × 10-4
1.77 × 10-3 8.63 × 10-4 2.81 × 10-4
8.24 × 10-4 6.61 × 10-4 1.28 × 10-4
1.06
1.54
1.90
1.53
2.05
1.25
4.28
4.94
5.64
5.62
6.30
6.44
AgO2CCF3
Cl
PhMe2Si
Cl
5
6
(3)
SiH3
SiH3
a
O
Measured by the decrease in absorbance at 243 nm, log kobs
)
C4 C1
C2
0.0498[H2O] - 3.62 (see text). Duplicate rates at each concentra-
tion, maximum deviations (2%. Reference 1c.
b
C3
O
6a
at δ 270. In the 13C spectrum the (CH3)2Si resonances
at δ -2.73 showed coupling to 29Si, J ) 56 Hz, and the
same coupling was seen in the 29Si spectrum.
O
RMe2Si
C
RMe2Si
O
O
∆
(4)
RMe2Si
SiMe2R
C
6 (R = Ph)
7 (R = Me)
O
3 (R = Ph)
8 (R = Me)
The kinetics of the conversion of 6 to 3 were measured
in isooctane by UV spectroscopy, as reported in Table 3.
For comparison with the rates in isooctane the kinetics
for 7, R ) Me, were also measured in this medium.
The (aryldimethylsilyl)ketene motif has also been used
in the preparation of a new type of bisketene, namely,
the bis(silylketene) with a 1,4-disubstituted benzene
spacer. Thus 1,4-bis(bromodimethylsilyl)benzene (9),10
prepared as shown in eq 5, reacted with lithium ethoxy-
acetylene to give the bisalkyne 10. Upon pyrolysis of 10
in a gas chromatograph, two products were obtained in
a variable ratio depending upon the exact conditions. The
product of shortest retention time was tentatively identi-
fied as monoketene 11 resulting from conversion of one
of the alkynyl groups of 10 to a ketenyl function. This
assignment was based upon the 1H NMR spectrum,
including the appearance of separate signals for the
EtOCtCSiMe2 and SiMe2CHdCdO moieties, the obser-
vation of two separate 29Si NMR resonances, and a
ketenyl IR band at 2108 cm-1. Insufficient material was
obtained for measuring the 13C NMR spectrum. The
major product was identified as the bisketene 12 based
upon its complete spectral characterization. Some salient
signals and the comparative values for PhMe2SiCHdCdO
(1) (parentheses) are as follows: IR 2115 cm-1 (2114
cm-1); 1H NMR δ 0.44 (0.42), SiMe2; 2.00 (1.98), CHdCdO;
13C NMR δ -0.7 (-0.7) CâdCdO; 178.9 (178.9), CdCRdO;
17O NMR δ 256.6 (256.1); 29Si NMR δ -5.39 (-5.45).
F igu r e 1. X-Ray structure of 3,4-bis(phenyldimethylsilyl)-
cyclobut-3-ene-1,2-dione (6).
To evaluate the reactivity of 1, kinetics of its hydration
with H2O/CH3CN mixtures were measured at different
H2O concentrations by observing the decrease in the
absorption at 243 nm. This reaction proceeds to the
known acid PhMe2SiCH2CO2H,7d which was isolated and
its structure confirmed from its spectroscopic properties.
The results are collected in Table 1, along with compara-
tive data for other silylketenes.1c
For the preparation of the PDMS-substituted 1,2-
bisketene 3, the sequence shown in eq 3 was followed.
This procedure is analogous to those used for other
silylated 1,2-bisketenes, using bis(phenyldimethylsilyl)-
acetylene (4)8a and activation of the zinc by heating,8b and
utilizes AgO2CCF38c for conversion of the dichloride 5 to
the diketone 6. The use of the latter regent is necessary
instead of the H2SO4-catalyzed hydrolysis procedure used
in other cases1 because cleavage of the phenyl group by
protodesilylation occurs if 5 is treated with acid. The
X-ray crystal structure of the cyclobutenedione 6 was
determined and is shown in Figure 1, and some bond
distances and bond angles are given in Table 2, along
with some ab initio calculated values for bis(SiH3)-
substituted cyclobutenedione (6a ).9
The dione 6 in CDCl3 was converted upon heating at
100 °C for 2 h to the bisketene 3 (eq 4) as the only product
Discu ssion
1
observable by H NMR and was identified by its char-
For consideration of the NMR chemical shifts of the
PhMe2Si-substituted ketenes and bisketenes, data for the
monoketene 1 and the bisketene 3 are collected in Table
4 with data for other ketenes and bisketenes.6 It is
striking from the chemical shifts (Table 4) for silyl groups
R3Si that the 13C shifts for both CR and Câ, and the 17O
shifts, show only very small effects due to the particular
acteristic ketenyl IR band at 2080 cm-1, the distinctive
silylketene bands in the 13C NMR at δ 5.87 (Câ) and
180.88 (CR), the 29Si signal at δ -2.42, and the 17O band
(8) (a) West, R.; Quass, L. C. J . Organomet. Chem. 1969, 18, 55-
67. (b) Stenstrøm, Y. Synth. Commun. 1992, 22, 2801-2810. (c) Wong,
H. N. C.; Sondheimer, F.; Goodlin, R.; Breslow, R. Tetrahedron Lett.
1976, 2715-2718.
(9) McAllister, M. A.; Tidwell, T. T. J . Am. Chem. Soc. 1994, 116,
7233-7238.
(10) (a) Plenio, H. J . Organomet. Chem. 1992, 435, 21-28. (b) Fink,
W. Helv. Chim. Acta 1974, 57, 1010-1015.

Phenyldimethylsilyl-Substituted Ketenes
J . Org. Chem., Vol. 61, No. 18, 1996 6229
Ta ble 2. Com p a r ison of Mea su r ed a n d Ca lcu la ted Bon d Dista n ces (Å) a n d An gles (d eg) for Bis(silyl)cyclobu ten ed ion es
bond distance
C1O
C2O
C1C4
C2C3
C1C2
C3C4
C3Si
C4Si
6 (PhMe2Si)a
6a (SiH3)b
1.185
1.175
1.192
1.175
1.511
1.511
1.512
1.511
1.567
1.554
1.375
1.352
1.886
1.888
1.878
1.888
bond angle
C1C4Si
C2C3Si
OC1C4
OC2C3
C2C1C4
C1C2C3
C1C4C3
C2C3C4
6 (PhMe2Si)a
6a (SiH3)b
130.0
129.6
126.9
129.6
136.8
136.1
136.6
136.1
86.6
86.2
86.1
86.2
93.4
93.8
93.9
93.8
a
b
Measured. Calculated, HF/6-31G*, ref 9.
Ta ble 3. Kin etic Da ta for Con ver sion of
Cyclobu ten ed ion es to Bisk eten es in Isoocta n e
Ta ble 4. NMR Ch em ica l Sh ifts for Keten es, Bisk eten es,
a n d Refer en ce Com p ou n d s
cyclobutenedione
T (°C)
k (s-1
)
6 (R ) Ph)a
79.0
70.7
70.2
61.0
25.0c
79.0
72.6
59.0
25.0c
(2.36 ( 0.06) × 10-4
(8.30 ( 0.47) × 10-5
(8.64 ( 0.15) × 10-5
(2.50 ( 0.45) × 10-5
1.2 × 10-7
7 (R ) Me)b
(1.44 ( 0.03) × 10-4
(6.01 ( 0.01) × 10-5
(1.19 ( 0.01) × 10-5
6.2 × 10-8
ln(k/T) ) -14500/T + 27; Eact ) 29.5 kcal/mol; ∆Hq ) 28.8
a
kcal/mol; ∆Sq ) 6.3 eu. ln (k/T) ) -14100/T + 25; Eact ) 29.5
b
kcal/mol; ∆Hq ) 28.0 kcal/mol; ∆Sq ) 3.1 eu. c Calculated from
data at other temperatures.
Br
SiMe2H
SiMe2Br
1. Mg, THF
2. HMe2SiCl
Br2, CCl4
(5)
(6)
6
Br
SiMe2H
SiMe2Br
COEt
9
Me
Me
CSi
Me
EtOC
CLi
9
EtOC
SiC
Me
a
This work for 1, 3, 6, and 12; others ref 6a except as noted.
Reference 6b.
b
10
donation from the Câ-Si bond to the in-plane p orbital
of the carbonyl carbon.6a
For the cyclobutenediones the effects of R in RMe2Si
on δ 13C are very small, with a total variation of δ 0.3 for
CR and δ 1.4 for Câ. The corresponding variation on the
17O is δ 4.9.
Me
CSi
Me
170 °C
+
O
10
EtOC
Si
C
CH
–CH2
CH2
Me
Me
11
Me
CHSi
Me
Me
There are systematic effects of the groups R in R4Si
on the 29Si chemical shifts (Table 4). Thus for R ) Ph
compared to R ) Me there are upfield shifts in 29Si of δ
3.0-5.6, and for R ) t-Bu compared to R ) Me there are
downfield shifts of δ 7.4-8.8. These trends appear to be
largely independent of the rest of the structure. The
upfield shift caused by phenyl has been noted previously,
as has a similar effect by vinyl groups.11b It was also
noted previously11b that the replacement of the hydrogens
on carbon bonded to silicon by alkyl groups caused
downfield shifts of about 2 ppm per alkyl group and this
is consistent with the trend noted for R ) t-Bu compared
to R ) Me.
Thus for the substrates RMe2SiX there are no specific
interactions of the groups R and ketenyl or cyclobutene-
dione groups X which affect the chemical shifts in R, X,
or Si, at least for the compounds studied. In particular
no transmission of electrical effects or conjugation be-
tween R and X through Si is apparent.
(7)
O
SiCH
Me
C
O
C
12
groups R. Thus for monoketenes R3SiCHdCdO there
are variations of 1.5 and 4.5 for δ CR and Câ, respectively,
while for the bisketenes the corresponding variations are
only 1.3 and 1.6, respectively, although CR and Câ average
2.5 and 7.5 ppm, respectively, to lower field for the
bisketenes compared to monoketenes. The 17O shifts vary
by no more than 1.3 as a function of R for the ketenes
and bisketenes, and the latter are on average to lower
field by 14.4 ppm.
J ust as has been found for other silyl-substituted
ketenes and bisketenes,6a the CR and 17O NMR chemical
shifts of 1, 3, and 12 show upfield shifts of about 20 and
70 ppm, respectively, compared to nonsilylated ketenes,
while the 29Si NMR chemical shifts are downfield by 8.1-
11.1 ppm compared to the cyclobutenedione 6. These
effects in silylketenes were attributed6a to neutral
hyperconjugation,11a in which there is σπ-pπ electron
(11) (a) Lambert, J . B.; Singer, R. A. J . Am. Chem. Soc. 1992, 114,
10246-10248. (b) Scholl, R. L.; Maciel, G. E.; Musker, W. K. J . Am.
Chem. Soc. 1972, 94, 6376-6385.

6230 J . Org. Chem., Vol. 61, No. 18, 1996
Liu et al.
The UV spectrum of PhMe2SiCHdCdO (1) shows that
in addition to the aryl absorption of 1 between 240-270
nm the characteristic ketene absorption is at 309 nm, ꢀ
) 110 L mol-1 cm-1 in isooctane, as opposed to the
absorption for t-BuMe2SiCHdCdO at 292 nm, ꢀ ) 35 L
mol-1 cm-1 in hexane. The shift to longer wavelength
and the increased ꢀ for 1 suggests there is a measureable
interaction between the phenyl group and the ketene
chromophore. A similar increase to longer wavelength
has been reported in the UV spectra upon the addition
of a second and third ketenyl group to silicon in the series
t-BuMe2SiCHdCdO, Me2Si(CHdCdO)2, and MeSi-
(CHdCdO)3.12 The UV spectra are thus more sensitive
for detecting perturbations due to different substitution
patterns in these compounds than are the NMR spectra.
The observed shifts to longer wavelength of the carbonyl
n f π* transition presumably results from a lowering of
the π* levels, as the n orbitals would be relatively
unaffected by the remote substituents.
Thus the reactivity data agree with the analysis of the
NMR spectra and indicate that there is no strong
evidence for a transmission of electrical effects of the
phenyl through the silicon to affect the ketenyl group.
Only the UV spectrum of PhMe2SiCHdCdO compared
that of t-BuMe2SiCHdCdO reveals any significant effect
attributable to phenyl.
Interestingly, the X-ray crystal structure of the cy-
clobutenedione 6 (Figure 1) reveals that the two aryl
groups are found on the same side of the cyclobutenedi-
one ring, in an almost parallel arrangement.13 However
the two groups are too far from one another for any direct
interaction, either favorable or unfavorable, and this
effect apparently results from the intermolecular stacking
of aryl groups seen in the crystal structure. The geom-
etry of 6 obtained from the X-ray crystal structure
determination is in reasonable agreement with that
obtained earlier for the bis(SiH3)-substituted cyclobutene-
dione 6a by HF/6-31G* calculations (Table 2).9 Calcula-
tions of the structures and energies of 1,2-bisketenes and
their cyclobutenedione precursors have played an impor-
tant part in the elucidation of the chemistry of these
novel species,1,9 and this agreement between the calcu-
lated and X-ray structures lends some confidence to the
validity of this approach.
The bisketene 12 is the first example to be prepared
in which two silyl-stabilized ketene units have been
joined by a spacer group to give a stable bisketene and
points the way to the utilization of the aryldimethylsi-
lylketene moiety for the preparation of other new types
of ketenes. The ketenyl NMR chemical shifts of 12 are
essentially identical with the corresponding values for
the monoketene PhMe2SiCHdCdO (1), indicating that
there is no detectable interaction of the two ketenyl units
in 12 through the phenyl spacer group. This structural
unit is of interest because it suggests that a variety of
such bis- and polyketenes could be prepared with various
spacer groups, and additions to these ketenyl groups
could then give further more complex products with
predetermined arrays of repetitive structural types. By
analogy with the designation of 3 as a 1,2-bisketene, then
12 is a 1,8-bisketene.
The UV spectrum of the cyclobutenedione 6 shows the
long wavelength (visible) band at 355 nm, ꢀ ) 41, and
these values are essentially identical to those for bis-
(trimethylsilyl)cyclobutenedione (7) (λmax 354 nm, ꢀ ) 37)
and the bis(tert-butyldimethylsilyl) analogue (λmax 356
nm, ꢀ 27).1c Thus as far as this measure is concerned
there is also no evidence for an interaction between the
aryl group and the cyclobutenedione moiety. The bis-
ketenes 3, 8,1c and (t-BuMe2SiCdCdO)2 all show long
1c
wavelength shoulders in their UV spectra near 325 and
380 nm, but any differences appear too indistinct for
interpretation.
Upon reaction with H2O, 1 gives the acid PhMe2-
SiCHCO2H,7d and the hydration reactivity of 1 is greater
than that of Me3SiCHdCdO and t-BuMe2SiCHdCdO at
different [H2O] in CH3CN by factors of 1.05-2.05, and
4.28-6.44, respectively (Table 1). A lesser decrease in
the reactivity of 1 as the [H2O] is decreased is expressed
in the slopes of plots of log kobs vs [H2O] of 0.0498,
0.0622,1c and 0.0642,1c respectively. We have found such
plots of log kobs vs [H2O] are usually linear and have used
these as an empirical measure of the influence of solvent
polarity on reactivity. A simple explanation of the
greater reactivity of 1 relative to Me3SiCHdCdO and
t-BuMe2SiCHdCdO is that this effect results from
inductive electron withdrawal by phenyl stabilizing a
negatively charged enolate-like transition state (eq 8).1a
In summary the phenyldimethylsilyl group is an ef-
fective substituent for the stabilization of ketenes and
bisketenes and has very little effect on their 13C and 17O
NMR spectroscopic properties or their rates of formation
and reaction compared to the corresponding Me3Si-
substituted derivatives. Only the UV spectra show any
evidence for interaction of the phenyl with more distant
parts of the molecule, and our results provide no evidence
for the p-d-p type of overlap proposed5a for this sub-
stituent in radical systems.
H2O
C
O
PhMe2SiCH
1
O-
(8)
C
PhMe2SiCH
PhMe2SiCH2CO2H
+
OH2
Comparison of the kinetics of ring opening of the
PhMe2Si-substituted cyclobutenedione 6 to the bisketene
3 (Table 3) reveals a very modest acceleration compared
to the corresponding reaction of the Me3Si-substituted
cyclobutenedione 7, by a factor of 1.6 at 79.0 °C. This
small factor could be due to ground state destabilization
of the cyclobutenedione 6 by the PhMe2Si groups, or
stabilization of the transition state leading to the bis-
ketene 3, or a combination of effects, including confor-
mational influences. The magnitude of the net effect is
so small as to preclude any convincing assignment to a
specific cause.
Exp er im en ta l Section
Unless otherwise stated, reagents were obtained from
commercial suppliers and used as received. Ether and THF
for use as reaction solvents were dried by refluxing over Na/
benzophenone and distilling. Glassware was either dried in
an oven at 150 °C and then cooled under N2 or Ar or flame
dried under N2 or Ar before use.
1-(P h en yld im eth ylsilyl)-2-eth oxya cetylen e.7b Ethoxy-
acetylene (0.0285 mol, 50% by weight solution in hexane) was
(13) The authors have deposited atomic coordinates for this struc-
ture with the Cambridge Crystallographic Data Centre. The coordi-
nates can be obtained, on request, from the Director, Cambridge,
Crystallographic Data Centre, 12 Union Road, Cambridge, CB2 1EZ,
U.K.
(12) Sung, K.; Tidwell, T. T. J . Am. Chem. Soc. 1996, 118, 2768-
2769.

Phenyldimethylsilyl-Substituted Ketenes
J . Org. Chem., Vol. 61, No. 18, 1996 6231
added slowly via a syringe to a solution of n-butyllithium (0.03
mol, 20 mL, 1.50 M in hexane) and 30 mL of dry THF at -78
°C under Ar and then HMPA (hexamethylphosphoramide, 5
mL, 1.1 equiv) was added. The mixture was allowed to warm
to room temperature and was stirred at room temperature for
2 h, and then phenyldimethylsilyl chloride (5.26 mL, 5.35 g,
0.0314 mol) was added slowly via a syringe and the solution
was allowed to stir overnight at room temperature. The
mixture was extracted between water and pentane, the pen-
tane layer was washed twice with saturated NaCl solution,
dried over MgSO4, and concentrated under vacuum, and the
residue was then distilled under reduced pressure (41-55 °C,
0.025 mmHg) to yield 1-(phenyldimethylsilyl)-2-ethoxy-
acetylene7b as a clear colorless liquid (4 g, 70%): 1H NMR
(CDCl3) δ 0.41 (s, 6, SiMe2), 1.43 (t, 3, J ) 7 Hz, CH3), 4.20 (q,
2, J ) 7 Hz, CH2), 7.4, -7.7 (m, 5, Ph).
3,4-Bis(p h en yld im eth ylsilyl)cyclobu t-3-en e-1,2-d ion e
(6). Silver trifluoroacetate (0.87 g, 4.0 mol) was dissolved in
ethyl acetate (2 mL) in a round bottom flask equipped with a
magnetic stirrer and a drying tube. The solution was heated
to 65 °C, 5 (291 mg, 0.719 mmol), dissolved in 2 mL of ethyl
acetate, was added dropwise to the stirring mixture using a
transfer pipette, and the mixture was stirred at 65 °C for 1.5
h and then extracted with H2O and saturated NaCl. The
combined aqueous washings were extracted twice with ether
(5 mL), and the combined organic extracts were dried over
MgSO4 and concentrated on a rotary evaporator leaving a
bright yellow oil. The crude oil was purified by radial
chromatography on silica gel to yield 6 (120 mg, 0.342 mmol,
48%) as a pale yellow solid, mp 72.5-81 °C (dec). As an
alternate purification method, the crude dione was recrystal-
lized from ether at -20 °C to yield long clear yellow crystals:
IR (CDCl3), 1764 cm-1; UV λmisoaoxctane 210 (sh, ꢀ ) 68 000), 270
(P h en yldim eth ylsilyl)keten e (1).7a-c A solution of 1-(phe-
nyldimethysilyl)-2-ethoxyacetylene (40 mg) in 30 µL of pentane
was injected into a gas chromatograph (column ) 185 °C), and
1
(sh, ꢀ ) 3400), 355 (ꢀ ) 41); H NMR (CDCl3) δ 0.42 (s, 12,
SiMe2), 7.31-7.38 (m, 10, Ph); 13C NMR (CDCl3) δ -3.38,
128.2, 130.2, 134.0, 201.8, 215.9; 17O NMR (CDCl3) δ 504; 29Si
1 was collected as a clear colorless oil with a retention time of
isooctane
5.5 min: IR (film) 2114 cm-1 (s); UV λ
254, 260, 264,
NMR (CDCl3) δ -13.54, EIMS m/ z 350 (M+, 60), 335 (M+
-
max
1
CH3, 29), 306 (M+ - CO2, 71), 279 (M+ - C2O2, CH3, 81), 135
(PhMe2Si+, 100); HRMS m/ z calcd for C20H22O2Si2, 350.1158,
found 350.1170. Anal. Calcd for C20H22O2Si2: C, 68.52; H,
6.32. Found: C, 68.53; H, 6.62.
270, 309 nm (ꢀ 200, 290, 280, 210, 110); H NMR (CDCl3) δ
0.42 (s, 6, SiMe2), 1.98 (s, 1, CH), 7.38-7.56 (m, 5, Ph); 13C
NMR (CDCl3) δ -0.72, -0.52, 127.9, 129.5, 133.4, 138.0, 178.9;
17O NMR (CDCl3) δ 256.1; 29Si NMR (CDCl3) δ -5.45; EIMS
m/ z 176 (M+, 8), 169 (M+ - CH3, 100), 148 (M+ - CO, 19),
135 (PhMe2Si+, 5).
2,3-Bis(ph en yldim eth ylsilyl)bu ta-1,3-dien e-1,4-dion e (3).
A solution of 6 (50 mg, 14 mmol) in 1.5 mL of CDCl3 in an
NMR tube was degassed for 15 min by bubbling through Ar.
The tube was sealed and heated in an oil bath at 100 °C for 2
h. All NMR signals corresponding to the dione disappeared
and were replaced with signals corresponding to the bisketene
3. Evaporation of the CDCl3 gave 3 as a yellow oil, or
alternatively heating 6 at 120 °C for 2 h in an ampule sealed
under argon also gave neat 3: IR (CDCl3) 2080 cm-1; 1H NMR
Hyd r a tion of 1. Ketene 1 (20 mg, 0.11 mmol) was
dissolved in 1 mL of deuterioacetone in an NMR tube, H2O
(40 µL, 2.2 mmol) was added, and the solution was left for 14
1
h, after which H NMR indicated the complete disappearance
of 1. The solution was dried over MgSO4 and evaporated, and
the residue was dissolved in ether and dried again with MgSO4
and evaporated to leave PhMe2SiCH2CO2H as a white solid,
mp 89 °C (lit.7d mp 90 °C): IR (film) 3500-3300 cm-1 (br), OH,
1693 cm-1 (s), CdO; 1H NMR (CDCl3) δ 0.42 (s, 6, SiMe2), 2.12
(s, 2, CH2), 7.36-7.57 (m, 5, Ph); EIMS m/ z 193 (M+ - H, 2),
135 (PhMe2Si+, 15), 117 (M+ - Ph, 100).
(CDCl3) δ 0.38 (s, 12, Me2Si), 7.35-7.53 (m, 10, Ar); UV
isooctane
max
λ
210 (sh, ꢀ ) 16 000), 250 (sh, ꢀ ) 3,400), 320 (sh, ꢀ )
450), 380 (sh, ꢀ ) 140); 13C NMR (CDCl3) δ -2.73 (J 13C-29Si
) 56 Hz), 5.9, 128.0, 129.9, 133.9, 136.0, 180.9; 17O NMR
(CDCl3) δ 270; 29Si NMR (CDCl3) δ -2.42 (J 29Si-13C) ) 56.4
Hz).; EIMS m/ z 350 (M+), 335 (M+ - CH3), 321 (M+, - CHO),
306 (M+, - CO2), 135 (PhMe2Si+). Anal. Calcd for C20H22O2-
Si2: C, 68.52; H, 6.32. Found: C, 68.41; H, 6.19.
Bis(p h en yld im eth ylsilyl)a cetylen e (4).8a cis-Dichloro-
ethylene (2.43 g, 0.025 mol) in 50 mL of ether was added
dropwise with a pressure-equalizing addition funnel to n-
butyllithium (50 mL, 1.51 M in hexane, 0.075 mol) in 75 mL
of ether at 0 °C, and the mixture was stirred at room
temperature for 2 h. The mixture was then cooled to -78 °C,
a solution of phenyldimethylsilyl chloride (12.8 g, 0.075 mol)
in ether (100 mL) was added dropwise with stirring, and the
mixture was stirred at room temperature for 9 h, filtered, and
concentrated to yield a yellow oil, which was distilled under
vacuum (0.015 mmHg, 110 °C) to yield a clear colorless liquid
which crystallized on standing (5.90 g, 0.0200 mol, 80%).
2,3-Bis(p h en yld im et h ylsilyl)-4,4-d ich lor ocyclob u t -2-
en on e (5). A 3-necked round bottom flask equipped with a
magnetic stirring bar, pressure-equalizing addition funnel, gas
inlet, and septum was flame dried under Ar. Zinc dust (3.71
g, 0.0567 mol) was added and heated with a Bunsen burner
with stirring until it became a fine powder. A solution of 4
(5.30 g, 0.0180 mol) in 120 mL of ether was injected with a
syringe, and trichloroacetyl chloride (4.22 g, 0.0232 mol) in
13 mL of ether was added dropwise to the stirring mixture,
which was then stirred for 3 days at room temperature. The
mixture was quenched with 15 mL of H2O and washed twice
with saturated Na2CO3 and twice with saturated NaCl. The
combined aqueous washings were extracted once with 15 mL
of diethyl ether, and the combined ether extracts were dried
over anhydrous MgSO4 and concentrated on a rotary evapora-
tor, leaving a clear yellow-orange oil. Some of the crude
product (1.05 g) was chromatographed on silica gel by radial
chromatography to yield 890 mg of 5, mp 95-97 °C: IR (KBr)
1770 cm-1; UV λmax 210 nm, shoulder at 230 nm; 1H NMR
(CDCl3) δ 0.25 (s, 6, SiMe2), 0.47 (s, 6, SiMe2), 7.21-7.43 (m,
10, Ph); 13C NMR (CDCl3) δ -3.30, -2.47, 128.1, 128.2, 129.9,
130.2, 134.0, 134.3, 134.4, 135.6, 168.5, 184.0, 195.5; 29Si NMR
(CDCl3) δ -15.85, -11.68; EIMS m/ z 408, 406, 404 (M+, 6),
368 (M+, - HCl, 16), 353 (M+, - CH3, -HCl, 40), 135 (Me2-
PhSi+, 100); HRMS m/ z calcd for C20H22Cl2OSi2, 404.0586,
found 404.0597.
1,4-Bis(br om od im eth ylsilyl)ben zen e (9).10 1,4-Dibro-
mobenzene (5.9 g, 25.0 mmol) in 5 mL of THF was added
dropwise to a mixture of chlorodimethylsilane (5.0 g, 52.5
mmol) in 30 mL of THF and Mg turnings (1.3 g, 52.5 mL)
under N2. The reaction mixture was stirred for 1 h at room
temperature and filtered, and the collected solid was washed
with hexanes. The combined organic layers were evaporated
to give crude 1,4-bis(dimethylsilyl)benzene (4.0 g, 20.4 mmol,
82%). To a sample of this product (1.0 g, 5.1 mmol) in 10 mL
of dry CCl4 was added a Br2/CCl4 solution dropwise until the
bromine color persisted. Evaporation of the solvent gave crude
9 (1.74 g, 4.9 mmol, 96%): 1H NMR (CDCl3) δ 0.78 (s, 12, Me2-
Si), 7.53 (s, 4, Ar).
1,4-Bis(et h oxyet h yn yld im et h ylsilyl)b en zen e
(10).
Ethoxyacetylene (1 g, 5.7 mol, 50% in hexane) was added by
a syringe to n-BuLi (6.0 mmol, 3.8 mL in hexane) in 10 mL of
dry ether at 0 °C. The solution was stirred for 2 h, and crude
9 (1.0 g, 2.8 mmol) in 5 mL of ether was added via a syringe.
The solution was allowed to stir for 3 h at room temperature
and was then extracted with ether, and the ether extracts were
washed with H2O, dried, and evaporated to give crude 10 (0.8
g, 2.4 mmol, 86%), which was purified by column chromatog-
1
raphy giving a colorless oil: IR (CDCl3) 2176 cm-1; H NMR
(CDCl3) δ 0.37 (s, 12, SiMe2), 1.40 (t, 6, OCH2CH3), 4.18 (q, 4,
OCH2CH3), 7.64 (s, 4, Ar); 13C NMR (CDCl3) δ 0.0, 14.2, 35.0,
74.8, 110.7, 132.7, 139.1; EIMS m/ z 330 (M+, 5), 315 (M+
CH3, 40), 301 (M+ - C2H5, 53), 259 (100), 99 (50).
-
P yr olysis of 10. A solution of 10 (50 mg) in 50 µL of
acetone was injected into a gas chromatograph (3 m × 1 cm
OV-17 column, 170 °C), and the products 11 and 12 observed
in a ratio of 1:3 were collected. The first was tentatively
identified as ((4-((ethoxyethynyl)dimethylsilyl)phenyl)dimethyl-
1
silyl)ketene (11) (tR 40 min): IR (CDCl3) 2108 cm-1; H NMR

6232 J . Org. Chem., Vol. 61, No. 18, 1996
Liu et al.
(CDCl3) δ 0.39 (s, 6, SiMe2), 0.45 (s, 6, SiMe2), 1.20 (t, 3, J )
7 Hz, OCH2CH3), 2.00 (s, 1, CHdCdO), 3.68 (q, 2, J ) 7 Hz,
OCH2), 7.59 (s, 4, Ar); 29Si NMR (CDCl3) δ -5.48 (SiMe2-
CHdCdO), 7.42 (SiMe2CtCOEt). 1,4-Bis(k eten yld im eth -
were measured as described previously1c by observing the
change in the UV spectrum with time.
Ack n ow led gm en t. Financial support by the Natu-
ral Sciences and Engineering Research Council of
Canada and the Ontario Centre for Materials Research
is gratefully acknowledged. We thank Dr. Alan Lough
for the X-ray structure of 6.
1
ylsilyl)ben zen e (12) (tR 75 min): IR (CDCl3) 2115 cm-1; H
NMR (CDCl3) δ 0.44 (s, 12, SiMe2), 2.00 (s, 2, CHdCdO), 7.59
(s, 4, Ar); 13C NMR (CDCl3) δ -0.9, -0.7, 132.8, 139.5, 178.9;
17O NMR (CDCl3) 256.6; 29Si NMR (CDCl3) δ -5.39; EIMS m/ z
274 (M+, 15), 259 (M+ - CH3, 100), 246 (M+ - CO, 34), 233
(M+ - CHCO, 9), 99 (SiMe2CHCO+, 17); HRMS m/ z calcd for
C14H18O2Si, 274.0845, found 274.0845.
Kin etic Stu d ies. The rates of hydration of 1 were mea-
sured as described previously1c by injecting aliquots of a
solution of 1 dissolved in CH3CN into solutions of H2O/CH3-
CN equilibrated at 25.0 °C and measuring the decrease in the
absorption at 243 nm. The kinetics of ring opening of 6 and 7
Su p p or tin g In for m a tion Ava ila ble: Copies of NMR
spectra (6 pages). This material is contained in libraries on
microfiche, immediately follows this article in the microfilm
version of the journal, and can be ordered from the ACS; see
any current masthead page for ordering information.
J O9605197