Stabilized and persistent allenylketenes

Article abstract of DOI:10.1021/ja963496m

Photolyses of 2,3-bis(trimethylsilyl)-substituted methylenecyclobutenones 22-26 give essentially quantitative conversion to the allenylketenes 28-32 which have been isolated as long-lived species at room temperature. As predicted by molecular orbital calc

Full text of DOI:10.1021/ja963496m

2832
J. Am. Chem. Soc. 1997, 119, 2832-2838
Stabilized and Persistent Allenylketenes
Wenwei Huang, Decai Fang,† Karen Temple, and Thomas T. Tidwell*
Contribution from the Department of Chemistry, UniVersity of Toronto, Toronto,
Ontario, Canada M5S 3H6
ReceiVed October 7, 1996X
Abstract: Photolyses of 2,3-bis(trimethylsilyl)-substituted methylenecyclobutenones 22-26 give essentially quantitative
conversion to the allenylketenes 28-32 which have been isolated as long-lived species at room temperature. As
predicted by molecular orbital calculations, the methylenecyclobutenones 22 and 26, with carbethoxy and hydrogen
substitution on the methylene group, respectively, undergo thermal equilibration with the corresponding allenylketenes
28 and 32 with equilibrium constants [22]/[28] ) 1.0 and [26]/[32] ) 0.029, at 100 °C. The X-ray structure of the
phenyl-substituted allenylketene 29 confirms the anti-planar conformation as predicted by calculation. The bis-
(allenylketene) 35 has been made by an analogous procedure. Hydration rates of the allenylketenes show acceleration
by the CO2Et substituent, but in all cases these are modestly less reactive than the 1,2-bisketene (Me3SiCdCdO)2.
The interconversion of cyclobutene (1) with 1,3-butadiene
facile intramolecular cyclization with attached unsaturated
substituents (Smith-Hoehn reaction).4c-h These reactions have
added significance because of their relationships to the cycliza-
tion of enediynes, enyneallenes, and enyneketenes, including
derivatives with “skipped” conjugation, which have DNA
cleaving ability and utility as anticancer antibiotics and are also
valuable synthetic intermediates.8
(2) is a reaction of great synthetic and mechanistic interest,1 as
is the corresponding reaction of the analogous pairs methyl-
enecyclobutene (3)/vinylallene (4),2 bis(methylene)cyclobutene
(5)/bisallene (6),3 cyclobutenone (7)/vinylketene (8),4 methyl-
enecyclobutenone (9)/allenylketene (10),5 and cyclobutenedione
(11)/1,2-bisketene (12).6,7 Substituted vinylketenes in particular
Work in our laboratory has concentrated on the reaction
forming bisketene (12) and has made particular use of the
ketene-stabilizing effect of silyl substitution.6,7 We now report
the first studies of stable and persistent allenylketenes and find
that the thermodynamic stability of these species, compared to
their isomeric methylenecyclobutenone precursors, as well as
the structure determined by X-ray, are accurately predicted by
ab initio molecular orbital calculations. These highly function-
alized molecules are readily available and have great promise
as synthetic intermediates.
Allenylketenes have been previously postulated as reactive
intermediates,5 but they have not been observed as long-lived
species at ambient temperatures. The parent 10 was formed
by Wolff rearrangement upon photolysis of 13 (eq 1), observed
in a matrix at 8 K,5a and was also postulated as an intermediate
in the pyrolytic conversion of 14 to 9 (eq 2).5b Photolysis of
(5) (a) Chapman, O. L. Pure Appl. Chem. 1979, 51, 331-339. (b)
Trahanovsky, W. S.; Park, M. J. Am. Chem. Soc. 1973, 95, 5412. (c) Maier,
G.; Lage, H. W.; Reisenauer, H. P. Angew. Chem., Int. Ed. Engl. 1981, 20,
976-977. (d) Boch, R.; Bradley, J. C.; Durst, T.; Scaiano, J. C. Tetrahedron
Lett. 1994, 35, 19-22. (e) Shimada, K.; Akimoto, S.; Takikawa, Y.; Kabuto,
C. Chem. Lett. 1994, 2283-2286. (f) Toda, F.; Todo, E. Bull. Chem. Soc.
Jpn. 1976, 49, 2645-2646. (g) Xu, S. L.; Moore, H. W. J. Org. Chem.
1992, 57, 326-338.
(6) (a) McAllister, M. A.; Tidwell, T. T. J. Org. Chem. 1994, 59, 4506-
4515. (b) Gong, L.; McAllister, M. A.; Tidwell, T. T. J. Am. Chem. Soc.
1991, 113, 6021-6028. (c) McAllister, M. A.; Tidwell, T. T. J. Am. Chem.
Soc. 1994, 116, 7233-7238.
have become major synthetic intermediates because of their
† Permanent address: Chemistry Department, Beijing Normal University,
Beijing, China.
X Abstract published in AdVance ACS Abstracts, March 1, 1997.
(1) (a) Kirmse, W.; Rondan, N. G.; Houk, K. N. J. Am. Chem. Soc. 1984,
106, 7989-7991. (b) Niwayama, S.; Kallel, E. A.; Spellmeyer, D. C.; Sheu,
C.; Houk, K. N. J. Org. Chem. 1996, 61, 2813-2825.
(2) (a) Murakami, M.; Amii, H.; Itami, K.; Ito, Y. Angew Chem., Int.
Ed. Engl. 1995, 34, 1476-1477. (b) Pasto, D. J.; Kong, W. J. Org. Chem.
1989, 54, 4028-4033. (c) Lopez, S.; Rodriquez, J.; Rey, J. G.; de Lera, A.
R. J. Am. Chem. Soc. 1996, 118, 1881-1891. (d) Rey, J. G.; Rodriquez, J.;
de Lera, A. R. Tetrahedron Lett. 1993, 34, 6293-6296.
(7) (a) Tidwell, T. T. Ketenes; Wiley: New York, NY, 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.; Lough, A. J.; Tidwell, T. T. J. Chem. Soc., Chem. Commun. 1996,
2171-2172. (f) Allen, A. D.; Ma, J.; McAllister, M. A.; Tidwell, T. T.;
Zhao, D.-c. J. Chem. Soc., Perkin Trans. 2 1995, 847-851.
(8) (a) Wang, K. K. Chem. ReV. 1996, 96, 207-222. (b) Nicolaou, K.
C.; Dai, W.-M. Angew. Chem., Int. Ed. Engl. 1991, 30, 1387-1415. (c)
Myers, A. G.; Kuo, E. Y.; Finney, N. S. J. Am. Chem. Soc. 1989, 111,
8057-8059. (d) Toshima, K.; Ohta, K.; Ohtsuka, A.; Matsumura, S.; Nakata,
M. J. Chem. Soc., Chem. Commun. 1993, 1406-1407. (e) Wang, K. K.;
Wang, Z.; Sattsangi, P. D. J. Org. Chem. 1996, 61, 1516-1518. (f) Nakatani,
K.; Isoe, S.; Maekawa, S.; Saito, I. Tetrahedron Lett. 1994, 35, 605-608.
(g) Nakatani, K.; Maekawa, S.; Tanabe, K.; Saito, I. J. Am. Chem. Soc.
1995, 117, 10635-10644. (h) Sullivan, R W.; Coghlan, V. M.; Munk, S.
A.; Reed, M. A.; Moore, H. W. J. Org. Chem. 1994, 59, 2276-2278.
(3) (a) Pasto, D. J.; Yang, S.-H. J. Org. Chem. 1989, 54, 3544-3549.
(b) Saalfrank, R. W.; Welch, A.; Haubner, M.; Bauer, U. Liebigs Ann. 1996,
171-181. (c) Hopf, H. Angew. Chem., Int. Ed. Engl. 1970, 9, 432. (d)
Huntsman, W. D.; Wristers, H. J. J. Am. Chem. Soc. 1967, 89, 342-347.
(4) (a) Niwayama, S.; Kallel, E. A.; Sheu, C.; Houk, K. N. J. Org. Chem.
1996, 61, 2517-2522. (b) Bibas, H.; Wong, M. W.; Wentrup, C. J. Am.
Chem. Soc. 1995, 117, 9582-9583. (c) Smith, L. I.; Hoehn, H. H. J. Am.
Chem. Soc. 1939, 61, 2619-2624. (d) Moore, H. W.; Yerxa, B. R. AdV.
Strain Org. Chem. 1995, 4, 81-162. (e) Koo, S.; Liebeskind, L. S. J. Am.
Chem. Soc. 1995, 117, 3389-3404. (f) Danheiser, R. L.; Casebier, D. S.;
Firooznia, F. J. Org. Chem. 1995, 60, 8341-8350. (g) Taing, M.; Moore,
H. W. J. Org. Chem. 1996, 61, 329-340. (h) Petasis, N. A.; Fu, D.-K.
Synlett 1996, 155-156.
S0002-7863(96)03496-8 CCC: $14.00 © 1997 American Chemical Society

Stabilized and Persistent Allenylketenes
J. Am. Chem. Soc., Vol. 119, No. 12, 1997 2833
Table 1. HF/6-31G* Bond Distances (Å), Bond Angles (deg), and Dipole Moments (D) for Allenylketenes
cmpd
C1C2
C2C3
C3C4
C4C5
C1O
C1C2C3
C2C3C4
C3C4C5
OC1C2
C1C2C3C4
µ (D)
E-10
Z-10
TS 10
9
1.311
1.311
1.308
1.498
1.381
1.471
1.478
1.494
1.338
1.402
1.300
1.300
1.299
1.485
1.368
1.295
1.296
1.295
1.315
1.308
1.145
1.144
1.146
1.180
1.151
122.6
123.5
122.0
91.9
124.2
125.9
123.1
95.2a
105.9
179.7
179.8
179.5
138.7
149.9
180.0
179.5
179.8
136.9
159.8
180.0
26.7
104.7
0.0
1.43
1.54
3.74
TS 9
100.8
20.4
a C3C4C1 ) 87.4.
by the isodesmic reactions in eqs 6 and 7, respectively. As
part of these calculations, we obtained the energy and structure
of allenylketene (10), as well as the stabilization energies of
the SiH3 group on ketene (eq 6) and allene (eq 7), of 11.5 (MP2/
6-31G*//MP2/6-31G* level) and 4.1 (HF/6-31G*//HF/6-31G*
level) kcal mol-1, respectively.6a
15 led to 16, as identified by its IR bands at 1890 and 2080
cm-1, and this formed 17 upon further photolysis (eq 3).5c Flash
photolysis of 18 led to a transient species detected by its UV
absorption near 395 nm, which was identified as an allenylketene
and reverted rapidly back to 18 (eq 4).5d Formation of an
allenyl(seleno)ketene has also been proposed.5e
RCHdCdO + CH3CHdCH2 ∆E )8
SE
CH3CHdCdO + RCHdCH2 (6)
RCHdCdCH2 + CH3CHdCH2 ∆E )8
SE
CH3CHdCdCH2 + RCHdCH2 (7)
Results and Discussion
For the determination of the barriers for interconversion and
the relative stabilities of allenylketene (10) and the isomeric
methylenecyclobutenone (9), we have examined the structures
and energies of these intermediates and their connecting
transition structures using ab initio molecular orbital calcula-
tions, as we have done previously (Tables 1 and 2).6,10 Two
minimum energy structures for 10 were found, and the anti
structure E-10 is found to be planar, but the syn structure Z-10
is not, with a dihedral angle for C1C2C3C4 of 26.7°.
Other cases of ring openings of alkylidenecyclobutenones
were, however, proposed not to involve allenylketene intermedi-
ates. For example, 19, upon thermolysis, was suggested to
undergo rearrangement to 19a prior to ring opening to the
unobserved ketene 20, as shown in eq 5.5g There are other
studies of the relative stabilities of vinylallenes and the ring-
closed isomeric methylenecyclobutenes,2 including the effects
of silyl substituents2a and a continuing interest in the synthetic
utility of silylated allenes.9
Thus, allenylketene favors an anti-periplanar geometry (E-
10), as does vinylketene 8,11 but by only 1.1 kcal/mol. By
contrast, 1,2-bisketene is calculated to prefer the twisted
geometry 12,6,7a,b and this has been confirmed by the X-ray
structure of an analogous tetraketene.7e
To examine the methylenecyclobutenone/allenylketene in-
terconversion experimentally, 3,4-bis(trimethylsilyl)cyclobut-
3-ene-1,2-dione (21)7c was reacted with Wittig reagents Ph3-
PCRR1 to give methylenecyclobutenones 22-25. For 22 and
24, mixtures of E/Z isomers were obtained, and the lower field
vinyl hydrogen or methyl group in each pair, which cor-
We have recently reported ab initio calculated substituent
stabilization energies (SE) of ketenes6 and allenes,6a as defined

2834 J. Am. Chem. Soc., Vol. 119, No. 12, 1997
Huang et al.
Table 2. Energiesa of Methylenecyclobutenone 9, TS 9 for Ring Opening, Allenylketenes E-10 and Z-10 and TS 10 for Their
Interconversions
9
TS 9
Z-10
TS 10
E-10
HF/6-31G*
ZPVE
-266.45062 (0)
0.07044
-266.37289 (48.78)
0.07774
-266.43033 (12.73)
0.06773
-266.42702 (14.81)
0.06899
-266.43221 (11.55)
0.06776
HF/6-31G* + Z
MP2//HF/6-31G* + Z
-226.38023 (0)
-267.17134 (0)
-226.30596 (46.56)
-267.11763 (33.66)
-226.36259 (11.04)
-267.15248 (11.80)
-226.35804 (12.93)
-267.14762 (14.86)
-226.36445 (9.91)
-267.15427 (10.68)
a Hartrees, relative energies (kcal/mol) in parentheses.
responded to the minor isomer, was assigned to the Z isomer,
due to deshielding of the syn-hydrogen or methyl by the
carbonyl group (eq 8).12a,b For 23 and 25 only one stereoisomer
was obtained, and these were tentatively assigned the E
configuration on the basis of the preference for this isomer for
22 and 24 and in related examples.12c
δ 179.6, 15.5, 92.4, and 208.2, respectively, of 17O at δ 273,
and of 29Si at δ 3.2 and 2.0. For the analogous 1,2-bisketene
(Me3SiCdCdO)2, the 13CR, 13Câ, 17O, and 29Si shifts are δ
181.8, 0.2, 269, and 3.2, respectively.
Thermolysis of pure E-22 at 100 °C in CDCl3 led to
equilibration with Z-22 and to the formation of 28, and after
2.5 h the relative concentrations of E-22, Z-22, and 28 were
constant at 0.37, 0.14, and 0.49, respectively. This result
indicates that the bis(silylated)allenylketene 28 has essentially
the same thermodynamic stability as the isomeric cyclobutene-
dione 22, and this is in remarkable agreement with the result
calculated from the substituent stabilization parameters (eqs 6
and 7). Thus, the HF/6-31G*//HF/6-31G* ab initio calculated
stabilization energy values for SiH3 relative to H on an allene
and ketene, and of CHdO on an allene, are 3.0, 7.6,14a and 0.2
kcal mol-1, respectively,6a,b and when the sum of 10.8 kcal
mol-1 of these is compared to the 10.7 kcal mol-1 greater
stabilization calculated for E-10, compared to 9 (Table 2), the
prediction that 28 is 0.1 kcal mol-1 more stable than 22 is
obtained, consistent with the observed similar stability of the
species.
Attempted preparation of the methylenecyclobutenone 26 by
a Wittig reaction was unsuccessful, but this compound was
obtained by reaction of 21 with Tebbe reagent12d in 54% yield,
along with 34% of the bis(methylene)cyclobutene 27 (eq 9).
Recently dimethyltitanocene has also been used for the meth-
ylenation of cyclobutenediones.12e
Photolyses of the cyclobutenenones E-23, the E/Z-24 mixture,
E-25, and 26 with 350 nm light led to allenylketenes 29-32,
respectively, in essentially quantitative yields. Allenylketenes
29 and 30 were rather stable solids at room temperature and
were purified by chromatography on silica gel, but 31 was
unstable to chromatography. The terminal methylene compound
32 was also purified by chromatography, but appeared rather
unstable as a neat liquid. The IR and 13C, 17O, and 29Si NMR
spectra of 29-32 all showed the very characteristic signals for
the allenylketene structures. Upon heating, 29-31 reformed
the original cyclobutenones, along with other as yet unidentified
products.
Photolysis of E- or Z-22 with 350 nm light in CDCl3 at 5 °C
gave complete consumption of the reactant and led to the
carbethoxy-substituted allenylketene 28 as 98% of the observ-
able product in solution by 1H NMR (eq 10). Chromatography
gave 28 in 93% yield. The identification of 28 follows from
its IR spectrum with strong ketenyl and allenyl bands at 2086
and 1919 cm-1, respectively. We have recently reported on
the distinctive 13C, 17O, and 29Si NMR spectra of ketenes,13 and
strong spectral evidence for the structure of 28 are the very
characteristic 13C NMR chemical shifts of C1, C2, C3, and C4 at
Heating of 32 at 100 °C in CDCl3 led to clean conversion
back to 26, and after 9 h the ratio of 26/32 as measured by 1H
NMR was constant at 97:3. The group stabilizing energies (eqs
6 and 7) cited above for the pair 22/28 predict 22 is 0.1 kcal/
mol more stable.
Rate constants for the thermal interconversion of 26 and 32
(eq 11) in CDCl3 were obtained by monitoring the decrease in

Stabilized and Persistent Allenylketenes
J. Am. Chem. Soc., Vol. 119, No. 12, 1997 2835
Figure 1. Calculated (MP2/6-31G*) relative energies for reactants,
transition states, and products in ketene-forming reactions (kcal/mol).
b
aReference 6c. Twisted conformation.
Table 3. Rate (×105 s-1) and Equilibrium Constants for the
Interconversion of 26 and 32 (eq 11) in CDCl3
Figure 2. X-ray crystal structure of 29.
are compared. Previous calculations6c showed that for cy-
clobutenedione the substitution of first and second SiH3 groups
lowered the barrier for ring opening by 2.0 and 1.6 kcal/mol
and lowered ∆E for ring opening by 3.7 and 4.4 kcal/mol,
respectively. The experimental Eact for ring opening of the
bis(Me3Si)-substituted cyclobutenedione in CDCl3 of 29.3 kcal/
mol is in rather good agreement with the calculated value for
the bis(SiH3) derivative of 27.2 kcal/mol. Thus, similar
agreement might be expected for the allenylketene. For ring
closure of bisketenes (RCdCdO)2, the effect of two SiH3
substituents relative to hydrogen is calculated to increase the
barrier by 4.5 kcal/mol, and if the effect of two SiH3 groups on
the ring closure of an allenylketene were the same, this would
give a calculated barrier of 27.5 kcal/mol for ring closure of
32, as compared to the experimental value of 26.2 kcal/mol.
kobsd
Keq
T (°C) () kc + ko)
kca
ko
() kc/ko)b
98.8
98.7
87.3
87.1
77.3
77.2
25.0
30.5
28.8
9.61
9.00
3.29
3.39
29.6
28.0
0.85
0.84
0.21
0.19
0.066
0.064
34.7
33.5
44.5
46.6
49.0
51.6
160c
9.40
8.81
3.22
3.32
4.5 × 10-3 c 0.028 × 10-3 c
a ln k ) -13 200/T + 27.3, Eact ) 26.2 kcal/mol, ∆Hq ) 25.6 kcal/
mol, ∆Sq ) -6.3 cal K-1 mol-1 b ∆H ) -4.7 kcal/mol, ∆S ) -5.6
.
cal K-1 mol-1 c Extrapolated.
.
1
the H NMR absorption of the vinyl protons of 32 generated
by photolysis of 26 in an NMR tube, as reported in Table 3.
The rate expression for this process is given in eq 1214b and
from the relations kobsd ) kc + ko and Keq ) kc/ko lead to eq 13.
From the measured value of kobsd and Keq, values of kc and ko
may be obtained (Table 3). For comparison, the measured7c
rate constant for the ring opening of the cyclobutenedione 21
to form the bisketene (Me3SiCdCdO)2 at 99.0 °C in CDCl3 of
9.45 × 10-4 s-1 is 110 times greater than that for the ring
opening of 26 at 98.8 °C.
The structure and the conformation of the phenyl-substituted
allenylketene 29 were confirmed by an X-ray determination,
as shown in Figure 2.15 Not only is the anti-planar conformation
predicted by the molecular orbital calculations confirmed but
also the observed bond distances and bond angles as noted below
are in good agreement with the calculated values from Table 1
(parentheses): C1dC2 1.310 (1.311), C1dO 1.170 (1.145),
C2dC3 1.491 (1.471), C3dC4 1.310 (1.300), C4dC5 1.313
(1.295), C1C2C3 121.5 (122.6), C2C3C4 122.7 (124.2), C2C1O
175.7 (180.0), and C1C2C3C4 175.7 (180.0). The calculated
structure does not possess the bis(trimethylsilyl) and phenyl
substituents of 29, but evidently these have only small effects
of the geometry.
By analogy to the preparation by Ried et al.12c of unsubstituted
1,4-bis(4′-oxo-2′-cyclobuten-1′-ylidene)-1,4-dimethylbenzene the
ln([32] - [32]∞)/([32]o - [32]∞) ) -(ko + kc)t (12)
(11) (a) Brown, R. D.; Godfrey, P. D.; Woodruff, M. Aust. J. Chem.
1979, 32, 2103-2109. (b) Trahanovsky, W. S.; Surber, B. W.; Wilkes, M.
C.; Preckel, M. M. J. Am. Chem. Soc. 1982, 104, 6779-6781.
(12) (a) Ezcurra, J. E.; Moore, H. W. Tetrahedron Lett. 1993, 34, 6177-
6180. (b) Ezcurra, J. E.; Pham, C.; Moore, H. W. J. Org. Chem. 1992, 57,
4787-4789. (c) Ried, W.; Knorr, H.; Knorr, U. Chem. Ber. 1976, 109,
1506-1515. (d) Tebbe, F. N.; Parshall, G. W.; Reddy, G. S. J. Am. Chem.
Soc. 1978, 100, 3611-3613. (e) Petasis, N. A.; Hu, Y.-H.; Fu, D.-K.
Tetrahedron Lett. 1995, 36, 6001-6004.
ko ) kobsd/(Keq + 1)
(13)
In Figure 1 the calculated barriers and energies for intercon-
version of the pairs cyclobutenone/vinylketene, cyclobutenedi-
one/1,2-bisketene, and methylenecyclobutenone/allenylketene
(9) (a) Danheiser, R. L.; Carini, D. J. J. Org. Chem. 1980, 45, 3925-
3927. (b) Danheiser, R. L.; Stoner, E. J.; Koyama, H.; Yamashita, D. S.;
Klade, C. A. J. Am. Chem. Soc. 1989, 111, 4407-4413. (c) Peterson, P.
E.; Jensen, B. L. Tetrahedron Lett. 1984, 25, 5711-5714. (d) Jin, J.; Smith,
D. T.; Weinreb, S. M. J. Org. Chem. 1995, 60, 5366-5367. (e) Billups,
W. E.; Haley, M. M. J. Am. Chem. Soc. 1991, 113, 5084-5085.
(10) Gaussian 92, Revision C. Frisch, M. J.; Trucks, G. W.; Head-Gordon,
M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel,
H. B.; Robb, M. A.; Replogle, E. S.; Goperts, R.; Andres, J. L.;
Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.;
Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. Gaussian, Inc.:
Pittsburgh, PA, 1992.
(13) Allen, A. D.; Colomvakos, J. D.; Egle, I.; Ma, J.; Marra, R.; Tidwell,
T. T. Chem. Lett. 1996, 45-46.
(14) (a) Based on the HF/6-31G*//HF/6-31G* SE values for SiH3 and
H of 10.9 and 3.3 kcal/mol, respectively (ref. 6b). (b) Lowry, T. H.;
Richardson, K. S. Mechanism and Theory in Organic Chemistry; Harper
and Row: New York, NY, 1987; pp 194-195.
(15) X-ray data for 29: C17H24OSi2; formula weight, 300.54; crystal
system, monoclinic; space group, C2/c; color, pale yellow; unit cell
dimensions, a ) 26.625(5) Å, R ) 90°, b ) 6.1358(10) Å, â ) 96.011(13)°,
c ) 22.04(3) Å, γ ) 90°; volume, Z, 3581.8(11) Å3, 8; R ) 0.0370, GOF
) 0.925. Full crystallographic data are given in the Supporting Information.

2836 J. Am. Chem. Soc., Vol. 119, No. 12, 1997
Huang et al.
Table 4. Hydration Rates of Allenylketenes in H2O/CH3CN
Mixtures at 25 °C
range and displayed the empirical correlation of log kobsd with
[H2O] typical of many ketenes.7a-c This behavior has been
interpreted as indicating that the hydration proceeds through a
highly polar transition state. There is a significant variation in
the reactivity of the different substrates in 30% (16.7 M) H2O/
CH3CN, with a maximum rate ratio k(28)/k(30) of 37:1.
Examination of the product resulting from hydration of the
carbethoxy-substituted allenylketene 28 (performed in H2O/t-
BuOH because of the limited solubility of 28) showed that there
was an initial, rather unstable, and not completely pure product
formed in about 70% yield, identified as the desilylated lactone
36 on the basis of its spectral properties. Upon attempted
purification by vapor phase chromatography, this was partially
converted to another isomeric material identified as 37, and upon
heating in CDCl3 and chromatography on silica gel, 36 gave
the lactone 38 in 73% yield (eq 15).
[H2O] (M)
k(28)a,b
k(29)a,c
k(30)a,d
k(32)a,e
38.9
33.3
27.8
22.2
16.7
11.1
72.4
34.7
21.6
12.4
8.28
4.80
0.400
0.225
0.508
a Rates in 104 s-1 (average of duplicate runs, (5%). b Measured at
225 nm, log k ) 0.0412[H2O] - 3.79. c Measured at 251 nm.
d Measured at 257 nm. e Measured at 210 nm.
reaction of 21 with the bis Wittig reagent 33 gave the
tetrasilylated analogue 34, which upon photolysis gave the bis-
(allenylketene) 35 as shown by the appearance of the charac-
teristic IR bands and 13C NMR absorptions (eq 14).
Salient spectral features upon which these assignments are
1
based include distinctive H NMR signals of two allylic CH2
groups in 36 with a mutual coupling of 1.4 Hz and IR
absorptions at 1802, 1741, and 1648 cm-1 consistent with
lactone, ester, and alkenyl groups, respectively. Lactone 37,
in a mixture containing 33% residual 36, showed a UV λmax at
The allene moieties in 35 each possess axial chirality;
therefore, there should be two diastereomeric forms of this
molecule,16a as has been observed for other bis(allenes).16 The
meso form 35a and the chiral aS,aS isomer 35b are depicted,
and there is also a chiral aR,aR form.16a The 1H NMR signals
for both meso 35a and the racemic mixture of 35b are observed
as a 1:1 mixture in the crude product. Recrystallization of the
mixture gave one of the stereoisomers in pure form, with
characteristic IR bands at 2080 (ketene) cm-1 and 1912 (allene)
cm-1 and 13C NMR signals at δ 16.4 and 181.4 (ketene) and δ
92.57, 92.59, and 205.7 (allene).
As a measure of the reactivity of the allenylketenes, the rates
of hydration of 28-30 and 32 were obtained in H2O/CH3CN
mixtures, as we have done previously,7c and are reported in
Table 4. For 28 the dependence of the reactivity on the [H2O]
in CH3CN was determined over the range of 11.1-38.9 M H2O
(10-70%) and the rate increased by a factor of 15 over this
1
217 nm and H NMR signals and couplings for the 3 distinct
protons in the CHCH2 unit and allylic coupling between the
methine and vinyl H. The assignment was verified by decou-
pling of the methine H.
In accord with previous studies7a,c and with the rate evidence
favoring a highly polar transition state, these ketenes may be
interpreted as reacting through rate-limiting nucleophilic attack
of H2O on the ketene.
In summary, long-lived allenylketenes 28-32 and bis-
(allenylketene) 35 have been generated from the readily avail-
able bis(trimethylsilyl)cyclobut-3-ene-1,2-dione 21, and char-
acterized by their distinctive IR and NMR properties. The
stability of 28-32 and 35 and the X-ray structure of 29 are in
remarkable agreement with predictions based on calculated
substituent stabilization energies and calculated structures,
respectively.
Experimental Section
(16) (a) Eliel, E. L.; Wilen, S. H. Stereochemistry of Organic Compounds;
Wiley: New York, NY, 1994; Chapter 14. (b) Garratt, P. J.; Nicolaou, K.
C.; Sondheimer, F. J. Am. Chem. Soc. 1973, 95, 4582-4592. (c) Kleveland,
K.; Skattebøl, L. J. Chem. Soc., Chem. Commun. 1973, 432-433. (d) Lin,
J.; Pang, Y.; Young, V. G., Jr.; Barton, T. J. J. Am. Chem. Soc. 1993, 115,
3794-3795. (e) Johnson, R. P. Chem. ReV. 1989, 89, 1111-1124.
Infrared (IR) spectra were recorded on a Nicolet FTIR-821OE
spectrometer. 1H NMR spectra were recorded on a Varian VXR-200
at 199.975 MHz referenced to residual CHCl3 (7.26 ppm), and are
reported as follows: ppm (multiplicity, number of protons, assignment).

Stabilized and Persistent Allenylketenes
J. Am. Chem. Soc., Vol. 119, No. 12, 1997 2837
13C NMR spectra were recorded on a Varian VXR-200 at 50.289 MHz
or a Varian UNITY-400 at 100.577 MHz and referenced to the center
line of CDCl3 (77.00 ppm). 17O NMR and 29Si NMR spectra were
recorded on a Varian UNITY-400 at 54.219 and 79.459 MHz,
respectively. Thin layer chromatography was performed on precoated
silica gel 60F 254 on aluminum. The products were detected by
ultraviolet light and by iodine vapor. Flash column chromatography
was performed using silica gel (230-400 mesh) and hexanes/EtOAc
as eluent. UV irradiation was performed in a Rayonet Photochemical
Reactor using five RPR 3500 A 8 W lamps at about 5 °C.
2,3-Bis(trimethylsilyl)-4-(carbethoxymethylene)cyclobuten-1-
one (22). A solution of ethyl (triphenylphosphoranylidene)acetate
(0.774, 2.22 mmol) and cyclobutenedione 21 (0.502 g, 2.22 mmol) in
50 mL of CHCl3 was stirred for 18 h under N2 at 25 °C. The solvent
was evaporated, and the viscous product was separated by column
chromatgraphy to give Z-22 (0.070 g, 0.24 mmol, 11%), Rf ) 0.68,
and E-22, (0.529 g, 1.79 mmol, 80%), Rf ) 0.31. For Z-22: IR (neat)
1764 (CdO) 1715 (CO2Et) cm-1; 1H NMR (CDCl3) δ 0.31 (s, 9, Me3-
Si), 0.35 (s, 9, Me3Si), 1.29 (t, 3, J ) 7.2 Hz, CH3), 4.19 (q, 2, J ) 7.2
Hz, CH2), 5.69 (s, 1, CdCH); 13C NMR (CDCl3) δ -0.67, -0.42,
14.3, 60.7, 99.4, 165.2, 169.2, 189.1, 191.1, 201.2; UV λmax (cyclo-
hexane) 253 (ꢀ 15 000), 280 (sh) nm; EIMS m/z 296 (M+, 19), 267
(M+ - C2H5, 39), 73 (Me3Si+, 100); HRMS m/z calcd for C14H24O3Si2
296.1264, found 296.1254. For E-22: IR (neat) 1767 (ketone), 1707
(CO2Et) cm-1; 1H NMR (CDCl3) δ 0.28 (s, 9, Me3Si), 0.33 (s, 9, Me3-
Si), 1.32 (t, 3, J ) 7.1 Hz, CH3), 4.22 (q, 2, J ) 7.1 Hz, CH2), 5.44 (s,
1, CdCH); 13C NMR (CDCl3) δ -1.47, -1.28, 14.0, 60.3, 104.3, 164.6,
172.5, 184.6, 190.2, 198.1; UV λmax (cyclohexane) 248 (ꢀ 21 000), 275
(sh) nm; EIMS m/z 296 (M+, 8), 267 (M+ - C2H5, 31), 73 (Me3Si+,
100); HRMS m/z calcd 296.1264, found, 296.1261.
181.3, 205.3 (one C not seen); 17O NMR (CDCl3) δ 274.0; 29Si NMR
(CDCl3) δ 0.7, 2.2; UV λmax (cyclohexane) 249 nm (ꢀ 26 000); EIMS
m/z 300 (M+, 11), 285 (M+ - CH3, 5), 272 (M+ - CO, 51), 257 (M+
- CO- CH3, 43), 73 (Me3Si+, 100); HRMS m/z calcd for C17H24OSi2
300.1366, found 300.1368. Heating of 29 in CDCl3 led to a complex
mixture.
(E/Z)-2,3-Bis(trimethylsilyl)-4-(1-phenylethylidene)cyclobuten-1-
one (E/Z-24). Reaction of Ph3PCHMePh+Br- (0.781 g, 2.0 mmol)
with 21 (450 mg, 2.0 mmol) and n-BuLi (2.0 mmol), as for E-15, gave
after chromatography a 4.5:1 mixture of E/Z-24 (0.415 g, 66%). For
1
E-24: Rf ) 0.36; H NMR (CDCl3) δ 0.26, (s, 9, Me3Si), 0.42 (s, 9,
Me3Si), 2.21 (s, 3, CH3), 7.2-7.4 (m, 5, Ph). For Z-24: Rf ) 0.47; 1H
NMR (CDCl3) δ -0.14 (s, 9, Me3Si), 0.28 (s, 9, Me3Si), 2.30 (s, 3,
CH3), 7.2-7.4 (m, 5, Ph).
2,3-Bis(trimethylsilyl)-5-phenyl-1,3,4-hexatrien-1-one (30).
A
solution of E/Z-24 (150 mg, 0.50 mmol) in 4 mL of CHCl3 was
irradiated 12 h with 350 nm light at 5 °C and chromatographed (Rf )
0.77), as for E-23, to give 30 (144 mg, 4.6 mmol, 93%): mp 77-78
1
°C (dec); IR (CCl4) 2080 cm-1 (allenyl band not visible); H NMR
(CDCl3) δ 0.14 (s, 9, Me3Si), 0.19 (s, 9, Me3Si), 2.06 (s, 3, CH3), 7.1-
7.4 (m, 5, Ph); 13C NMR (CDCl3) δ -0.99, -0.97, 16.4, 16.6, 90.5,
97.9, 125.3, 126.0, 128.3, 137.3, 182.0, 205.7; 17O NMR (CDCl3) δ
273.2; 29Si NMR (CDCl3) δ 1.76, 1.30; UV λmax (cyclohexane) 257 (ꢀ,
23 000) nm; EIMS m/z 314 (M+, 40), 299 (M+ - CH3, 12), 286 (M+
- CO, 58), 271 (M+ - CO- CH3, 43), 73 (Me3Si+, 100); HRMS m/z
calcd for C18H26OSi2 314.1522, found 314.1521.
(E)-2,3,5-Tris(trimethylsilyl)-4-methylenecyclobuten-1-one (E-25).
To a stirred suspension of Ph3PCH2SiMe3+I- (957 mg, 2.0 mmol) in
40 mL of ether was added n-BuLi (1.2 mL, 1.6 M in hexane, 2 mmol),
the solution was stirred 10 min at 0 °C and 1 h at 25 °C, and dione 21
(450 mg, 2.0 mmol) in 5 mL of ether was added to the yellow
suspension. After 15 h of stirring at 25 °C, the mixture was filtered
through a small plug of silica gel, the solvent was evaporated, and the
product was chromatographed (5% EtOAc in hexanes, Rf ) 0.68) to
give E-25 (0.189 g, 0.64 mmol, 32%); IR (CCl4) 1753 cm-1 (CdO);
1H NMR (CDCl3) δ 0.17 (s, 9, Me3Si), 0.25 (s, 9, Me3Si), 0.31 (s, 9,
Me3Si), 5.26 (s, 1, CdCH); 13C NMR (CDCl3) δ -1.04, -0.80, -0.08,
115.1, 171.3, 181.7, 203.1; UV λmax (cyclohexane) 242 (ꢀ 21 000);
EIMS m/z 296 (M+, 26), 281 (M+ - CH3, 11), 253 (21), 155 (44), 73
(Me3Si+, 100); HRMS m/z calcd for C14H28OSi3 296.1448, found
296.1445.
2,3-Bis(trimethylsilyl)-5-carbethoxy-1,3,4-pentatrien-1-one (28).
A solution of E-22 (110 mg, 0.37 mmol) in 4 mL of CHCl3 in a glass
tube was irradiated with 350 nm light 12 h at 5 °C. The solvent was
evaporated, and the product was chromatographed (10% EtOAc in
hexanes, Rf ) 0.60) to give 28 (102 mg, 0.344 mmol, 93%): IR (CCl4)
1
2086, 1919, 1711 cm-1; H NMR (CDCl3) δ 0.21 (s, 9, Me3Si), 0.22
(s, 9, Me3Si), 1.25 (t, 3, J ) 7.1 Hz, CH3), 4.17 (q, 2, J ) 7.1 Hz,
CH2), 5.42 (s, 1, CdCH); 13C NMR (CDCl3) δ -1.34, -0.78, 14.3,
15.5, 60.5, 85.4, 92.4, 167.1, 179.6, 208.2; 17O NMR (CDCl3) δ 273.2;
29Si (CDCl3) δ 2.0, 3.2; UV λmax (cyclohexane) 220 (ꢀ 28 000), 348 (ꢀ
141) nm; EIMS m/z 296 (M+, 6), 267 (M+ - C2H5, 15), 73 (Me3Si+,
100); HRMS m/z calcd for C14H24O3Si2 296.1264, found 296.1262.
Similar photolysis of Z-22 gave complete conversion to 28. Heating
of E-22 (10 mg, 0.034 mmol) in 0.76 mL of degassed CDCl3 in an
NMR tube at 100 °C for 3 h gave a mixture of Z-22, E-22, and 28 in
a ratio of 0.14:0.37:0.49, as measured by 1H NMR. This ratio did not
change on further heating.
2,3,5-Tris(trimethylsilyl)-1,3,4-pentatrien-1-one (31). A solution
of E-25 (140 mg, 0.50 mmol) was irradiated as above, and the solvent
1
was evaporated to give 31: IR (CDCl3) 2074, 1904 cm-1; H NMR
(CDCl3) δ 0.09 (s, 9, Me3Si), 0.13 (s, 9, Me3Si), 0.21 (s, 9, Me3Si),
4.69 (s, 1, CdCH); 13C NMR (CDCl3) δ -1.20, -0.80, -0.31, 14.1,
78.1, 181.6, 208.3; 17O NMR (CDCl3) δ 270.3; 29Si (CDCl3) δ -4.9,
-0.7, 2.2; EIMS m/z 296 (M+, 17), 253 (10), 147 (45), 73 (Me3Si+,
100). Upon attempted purification by chromatography, 31 decomposed.
(E)-2,3-Bis(trimethylsilyl)-4-(phenylmethylene)cyclobuten-1-
one (E-23). To a stirred suspension of PhCH2PPh3+Cl- (0.78, 2.0
mmol) in 40 mL of ether at -78 °C was added n-BuLi (1.2 mL, 1.6 M
in hexane, 2 mmol), the solution was stirred 10 min at -78 °C and 1
h at 0 °C, and cyclobutenedione 21 (450 mg, 2.0 mmol) in 5 mL of
ether was added to the orange suspension. After 15 h of stirring at 25
°C, the mixture was filtered through a small plug of silica gel, the
solvent was evaporated, and the product was chromatographed (5%
EtOAc in hexanes, Rf ) 0.50) to give E-23 (0.420 g, 1.4 mmol, 70%):
2,3-Bis(trimethylsilyl)-4-methylenecyclobuten-1-one (26). To a
stirred solution of dione 21 (0.401 g, 1.8 mmol) in 8 mL of dry THF
at -100 °C was added µ-chloro-µ-methylenebis(η5-2,4-cyclopentadien-
1-yl)(dimethylaluminum)titanium (Tebbe reagent, 2.0 mL, 0.5 M in
toluene, 1 mmol), and the solution was stirred 6 h at -100 °C and
then poured into 50 mL of pentane and filtered. The filtrate was
concentrated and chromatographed (3% EtOAc in hexanes) to give 1,2-
bis(trimethylsilyl)-3,4-bis(methylene)cyclobutene (27) (0.038 g, 0.17
mmol, 34% based on Tebbe reagent), 26 (0.124 g, 0.554 mmol, 55%
based on Tebbe reagent), and unreacted 21 (0.097 g, 24%). For 26:
Rf ) 0.42; IR (CCl4) 1757 cm-1; 1H NMR (CDCl3) δ 0.25 (s, 9, Me3-
Si), 0.32 (s, 9, Me3Si), 4.63 (d, 1, J ) 1.4 Hz), 4.95 (d, 1, J ) 1.4 Hz);
13C NMR (CDCl3) δ -1.09, -0.76, 94.9, 165.0, 181.1, 191.7, 199.9;
EIMS m/z 224 (M+, 22), 209 (M+ - CH3, 8), 196 (M+ - CO, 34),
181 (M+ - CO - CH3, 65), 155 (62), 147 (34), 108 (31), 73 (Me3Si+,
100). For 27: Rf ) 0.90; 1H NMR (CDCl3) δ 0.26 (s, 18), 4.63 (s, 2),
4.67 (s, 2); 13C NMR (CDCl3) δ -0.26, 92.8, 154.7, 173.0; EIMS m/z
222 (M+, 39), 207 (M+ - CH3, 100), 179 (M+ - CO - CH3, 10), 155
(41), 73 (Me3Si+, 91); HRMS m/z calcd for C12H22Si2 222.1260, found
222.1256.
1
IR (CCl4) 1750 cm-1 (CdO); H NMR(CDCl3) δ 0.28 (s, 9, Me3Si),
0.38 (s, 9, Me3Si), 6.03 (s, 1, CHPh), 7.2-7.8 (m, 5, Ph); 13C NMR
(CDCl3) δ -1.47, -1.27, 116.9, 127.5, 128.4, 129.3, 135.4, 158.6,
177.9, 190.0, 201.4; UV λmax (cyclohexane) 211 (ꢀ 17 000), 283 (ꢀ
25 000), 294 (ꢀ 27 000), 309 (ꢀ 20 000); EIMS m/z 300 (M+, 23), 285
(M+ - CH3, 8), 272 (M+ -CO, 57), 257 (M+ - CO - CH3, 43), 73
(Me3Si, 100); HRMS m/z calcd for C17H24OSi2 300.1366, found,
300.1364.
2,3-Bis(trimethylsilyl)-5-phenyl-1,3,4-pentatrien-1-one (29). A
solution of E-23 (150 mg, 5.0 mmol) in 4 mL of CHCl3 in a glass tube
was irradiated with 350 nm light 12 h at 5 °C. The solvent was
evaporated, and the product was chromatographed (5% EtOAc in
hexanes, Rf ) 0.78) to give 29 (141 mg, 0.47 mmol, 94%): mp 89-
90 °C (dec); IR (CCl4) 2080, 1908 cm-1; 1H NMR (CDCl3) δ 0.18 (s,
9, Me3Si), 0.21 (s, 9, Me3Si), 6.07 (s, 1, CdCH), 7.1-7.4 (m, 5, Ph);
13C NMR (CDCl3) δ -1.0, -0.7, 14.1, 92.5, 92.6, 126.3, 128.5, 135.0,
2,3-Bis(trimethylsilyl)-1,3,4-pentatrien-1-one (32). A solution of
26 (120 mg, 0.54 mmol) in 4 mL of CHCl3 in glass tubes was irradiated

2838 J. Am. Chem. Soc., Vol. 119, No. 12, 1997
Huang et al.
with 350 nm light for 12 h at 5 °C. The solution was concentrated
and chromatographed (3% EtOAc in hexanes, Rf ) 0.80) to give 32
Hydration of Allenylketene 28. A sample of 28 (200 mg, 0.68
mmol) was dissolved in a solution of H2O (1 g) and t-BuOH (9 g) and
stirred overnight at 22 °C. Then, EtOAc (50 mL) was added and the
solution was dried over MgSO4, evaporated, and chromatographed (20%
EtOAc in hexanes) to give slightly impure 36 (114 mg, 0.47 mmol,
69%) as an oil: 1H NMR (CDCl3) δ 0.184 (s, 9, Me3Si), 1.271 (t, 3,
J ) 7.0 Hz, CH3), 3.19 (t, 2, J ) 1.4 Hz, CH2), 3.35 (t, 2, J ) 1.4 Hz,
CH2), 4.19 (q, 2, J ) 7.0 Hz, OCH2); 13C NMR (CDCl3) δ -1.33,
13.83, 34.67, 37.29, 61.16, 111.82, 152.66, 167.75, 176.88; IR (neat)
1802, 1741, 1648 cm-1; EIMS m/z 242 (M+, 5), 227 (M+ - CH3, 100),
199 (79), 171 (47), 147 (24), 129 (46), 73 (Me3Si+, 66).
Injection of 36 (10 mg, 0.04 mmol) in 0.1 mL of hexanes into a gas
chromatograph (OV-17 column, injector 220 °C, column 200 °C,
retention time 35 min) gave a colorless oil showing the 1H NMR signals
of 36 as well as those assigned to 37 in a 33:67 ratio. For 37: 1H
NMR (CDCl3) δ 0.26 (s, 9, Me3Si), 1.28 (t, 3, J ) 7.2 Hz, CH3), 2.50
(dd, 1, J ) 16.8, 8.8 Hz, CHH), 2.85 (dd, 1, J ) 16.8, 3.8 Hz, CHH),
4.195 (q, 2, J ) 7.2 Hz, OCH2), 5.51 (ddd, 1, J ) 8.8, 3.8, 1.9 Hz,
CHO), 6.21 (d, 1, J ) 1.9 Hz, CdCH). Irradiation of the methine H
at 5.51 collapsed the coupling to the alkene proton at 6.21 and the
doublet splittings of the protons at δ 2.50 and 2.85 (UV λmax
(cyclohexane) 217 nm (ꢀ = 3000)).
1
(101 mg, 0.45 mmol, 84%): IR (CCl4) 2080.5, 1912 cm-1; H NMR
(CDCl3) δ 0.157 (s, 9, Me3Si), 0.224 (s, 9, Me3Si), 4.55 (s, 2, CdCH2);
13C NMR (CDCl3) δ -1.48, -0.52, 15.5, 76.4, 86.5, 181.2, 208.3; 17
O
NMR (CDCl3) δ 271.5; 29Si NMR (CDCl3) δ -0.96, 2.40; EIMS m/z
224 (M+, 6), 196 (M+ - CO, 8), 181 (M+ - CO - CH3, 22), 155
(30), 147 (28), 73 (Me3Si+, 100); HRMS m/z calcd for C11H20OSi2
224.1053, found 224.1054.
Thermal Ring Closure of 32. Allenylketene 32 in a degassed
solution of CDCl3 in sealed NMR tubes was heated and periodically
cooled, and the relative amounts of 26 and 32 were measured from the
1H NMR spectrum. Derived rate and equilibrium constants are given
in Table 3.
1,4-Bis[[2′,3′-bis(trimethylsilyl)-4′-oxo-2′-cyclobuten-1′-ylidene]-
methyl]benzene (34). To a stirred solution of 1,4-bis[(triphenylphos-
phoniumyl)methyl]benzene dibromide (876 mg, 1.1 mmol) in 40 mL
of ether at -78 °C was added n-BuLi (1.4 mL, 1.6 M in hexane, 2.2
mmol), and the mixture was stirred for 10 min at -78 °C and 1 h at
0 °C. The dione 21 (502 mg, 2.2 mmol) in 5 mL of ether was added
to the dark orange mixture, which was stirred 12 h at 22 °C and filtered
through a small plug of silica gel. The filtrate was concentrated on a
rotary evaporator and the residue chromatographed (5% EtOAc/hexanes,
Rf ) 0.45) to give 34 (0.356 g, 0.682 mmol, 62%), as a solid that
isomerized/decomposed at 165 °C: 1H NMR (CDCl3) δ 0.278 (s, 18),
0.379 (s, 18), 6.00 (s, 2), 7.75 (s, 4); 13C NMR (CDCl3) δ -1.05, -0.75,
116.8, 129.5, 134.5, 158.7, 177.8, 189.9, 201.6; EIMS m/z 522 (M+,
13), 494 (M+ - CO, 23), 466 (M+ -2CO, 100), 363 (28), 155 (16),
97 (19), 73 (Me3Si+, 97); HRMS m/z calcd for C28H42O2Si4 522.2262,
found 522.2284.
The lactone 36 (36 mg, 0.15 mmol) in 1 mL of CDCl3 in a sealed
NMR tube under Ar was heated for 15 h at 180 °C, the solvent was
evaporated, and purification by chromatography (5% EtOAc/hexanes)
gave 38 (18.7 mg, 0.11 mmol, 73%): mp 96-97 °C; 1
H NMR (CDCl3)
δ 1.287 (t, 3, J ) 7.0 Hz, CH3), 2.70-2.78 (m, 2, CH2), 3.34-3.43
(m, 2, CH2), 4.18 (q, 2, J ) 7.0 Hz, OCH2), 5.71 (t, 1, J ) 2.2 Hz,
CdCH); 13C NMR (CDCl3) δ 14.26, 26.10, 60.14, 97.52, 166.64,
167.36, 173.47 (one C not observed); IR (KBr) 1819, 1699 cm-1; UV
λmax (cyclohexane) 227 nm (ꢀ 11 500); EIMS m/z 170 (M+, 39), 142
(M+ - CO, 64), 125 (100), 115 (26), 96 (39), 87 (55), 69 (60), 55
(41); HRMS m/z calcd for C8H10O4 170.0579, found 170.0574.
1,4-Bis[2′,3′-bis(trimethylsilyl)-1′-oxo-1′,3′,4′-pentatrien-5′-yl]ben-
zene (35). A solution of 34 (10 mg, 0.019 mmol) in CDCl3 (4 mL) in
three NMR tubes was irradiated with 350 nm light 8 h at 5 °C to give
a 1:1 mixture of meso and dl 35: 1H NMR (CDCl3) δ 0.157 (s, 18,
Me3Si), 0.171 (s, 18, Me3Si), 0.197 (s, 18, Me3Si), 0.206 (s, 18, Me3-
Si), 6.02 (s, 4, CHdC), 7.11 (s, 8, Ar). Removal of the solvent and
recrytallization three times from CHCl3 gave one of the diastereoisomers
of 35 as a white solid that isomerized/decomposed near 158 °C upon
fast heating: 1H NMR (CDCl3) δ 0.157 (s, 9, Me3Si), 0.207 (s, 9, Me3-
Si), 6.02 (s, 2, CHdC), 7.11 (s, 4, Ar); 13C NMR (CDCl3) δ -1.05,
-0.74, 16.42, 92.57, 92.59, 126.59, 133.10, 181.43, 205.70; IR (CDCl3)
2080, 1912 cm-1; EIMS m/z 522 (M+, 6), 494 (M+ - CO, 15), 466
(M+ - 2CO, 87), 363 (20), 155 (13), 97 (15), 73 (Me3Si+, 100); HRMS
m/z calcd for C28H42O2Si4 522.2262, found 522.2244.
Acknowledgment. Financial support by the Natural Sciences
and Engineering Research Council of Canada and the Ontario
Centre for Materials Research is gratefully acknowledged. Dr.
Alan Lough is thanked for determining the X-ray structure of
29.
Supporting Information Available: 1H NMR spectra and
full X-ray crystallographic data on 29 (32 pages). See any
current masthead page for ordering and Internet access instruc-
tions.
JA963496M