Table 1. Photoinitiated [6π + 2π] Cycloaddition Reaction of 1
with 1,1-Disubstituted Allenes 2a-c
Table 2. Photoinitiated [6π + 2π] Cycloaddition Reaction of 1
with 1,3-Disubstituted Allenes 4a-c
entry allene
R1
Ph
R2
procedurea product yield (%)b
isomeric ratioc
yield
1
2
3
2a
2b
2c
H
H
A
B
B
3a
3b
3c
50
51
35
entry allene procedurea product (%)b
5 (E/Z)
6 (E/Z)
(CH2)5
(CH2)5 CH2OTBS
1
2
3
4
4a
4a
4b
4c
A
B
A
B
5a
59 100 (>99:<1)
50 100 (85:15d)
56f 100 (>99:<1)
64 88 (10:90)
0
0
0
a Procedure A: a mixture of 1 (1 equiv) and allene (2 equiv) in hexanes
was irradiated at rt in a pyrex tube for periods of 4-6 h. Procedure B: to
a solution of 1 in hexanes in a pyrex tube was introduced under irradiation
at rt slowly a solution of allene (2 equiv) in hexanes via a syringe pump,
then additional irradiation was continued for 4-6 h. See Supporting
Information for more details. b Yield of isolated product after purification
by flash chromatography.
5a
5be
5c, 6c
12 (23:77)
a See footnote a of Table 1. b Overall yield (5 + 6) of isolated product
after purification. c Ratios (5/6, E/Z) determined by GC analysis on the crude
product before purification. d Sometimes E/Z ∼ 98:2. e Isolated as an alcohol
after removal of the THP group. f Yield over two steps after removal of the
THP group.
only [8 + 2]6 or [4 + 2]7 cycloadducts were isolated
depending on the nature of the allene.
Herein, results of a study on the photoinitiated [6π + 2π]
cycloaddition of allenes with (η6-cycloheptatriene)tricarbo-
nylchromium(0) (1) are reported.
syn coplanar with the (C-8)-(C-1) bond and, consequently,
orientated away from the substituent at C-7.10 Subsequent
X-ray crystallographic analysis performed on a derivative
of 5a (p-nitrobenzoate ester) confirmed the structural and
stereochemical assignments (see Supporting Information).
Surprisingly, when the same cycloaddition was performed
using the “slow addition” technique (procedure B), the Z
isomer of 5a with the phenyl group syn to C-7 could also be
detected (E/Z ) 85:15, entry 2). In rare cases, procedure B
led to the virtually exclusive formation of the E isomer. For
the moment, it is not evident how the rate of slow addition
of allene promotes the formation of the Z isomer. The
cycloaddition with 4b (procedure A) behaved similarly to
the cycloaddition with 4a to give, after deprotection of the
THP group, uniquely the alcohol (E)-endo-5b (56% over two
steps, entry 3). Irradiation of 1 with 4c (procedure B) gave
a 88:12 mixture of regioisomers endo-5c and endo-6c in 64%
overall yield (entry 4). Curiously, cycloaddition also occurred
at the π bond bearing the phenyl group. The high regiose-
lectivity observed when an ether group was present is
consistent with a prior coordination of the oxygen atom to
the chromium center which should favor bond formation at
the π bond bearing the ether substituent. With a methyl
group, no coordination is possible, and the steric effects with
1 would be expected to control regioselectivity. Each of the
regioisomers 5c and 6c was in turn isolated as a mixture of
Z and E isomers. For 5c, the Z isomer with the phenyl group
orientated away from the methyl group at C-7 was found to
be the major isomer in line with previous results (Table 2,
entries 1-3), whereas for the major isomer of 6c, the methyl
group was found to be syn to the phenyl group at C-7.
All of the following cycloaddition reactions were per-
formed in a pyrex reactor with a 450 W Canrad-Hanovia
medium pressure Hg vapor lamp. Two different procedures
were evaluated in the study: (a) a mixture of complex 1 (1
equiv) and allene (2 equiv) in hexanes was directly irradiated
(procedure A); (b) a solution of allene (2 equiv) in hexanes
was added slowly to a solution of 1 in hexanes using a
syringe pump under continuous irradiation (procedure B).
A preliminary experiment was performed with complex 1
and 1,1-diphenyl-1,2-propadiene (2a) following procedure
A. The metal-free [6 + 2]-cycloadduct 3a was obtained in
50% yield as a single regioisomer (Table 1, entry 1). Similar
results were observed with vinylidenecyclohexane (2b) and
trisubstituted allene 2c using procedure B, affording cy-
cloadducts 3b and 3c, respectively, in moderate yields
(entries 2 and 3). In each case, cycloaddition occurred at
the less sterically congested double bond of the 2π partner.
Regioisomer 3c was isolated as a single diastereomer
resulting from an exclusive endo approach of the allene,
paralleling the [6π + 2π] cycloaddition with alkenes.8,9
1,3-Disubstituted allenes were then examined. When allene
4a was employed using procedure A, a single isomer 5a was
obtained in 59% yield (Table 2, entry 1). The addition
occurred exclusively at the unsaturation bearing the
CH2OTBS substituent via an endo transition state. The phenyl
group on the exocyclic double bond in 5a was found to be
(6) (a) Gompper, R.; Wolf, U. Liebigs Ann. Chem. 1979, 1388–1405.
(b) Hayakawa, K.; Nishiyama, H.; Kanematsu, K. J. Org. Chem. 1985, 50,
512–517.
(9) When concerned, the relative configuration of the stereocenter at
the C-7 position was established by 1H NMR NOE difference experi-
ments, and in each case studied, only the endo-cycloadduct was obtained.
See Supporting Information for additional discussion of structural
assignments.
(10) All the E and Z geometries discussed in this study were supported
by 1H NMR NOE correlations. See Supporting Information for some
discussed examples.
(7) (a) Gandhi, R. P.; Ishar, M. P. S. Chem. Lett. 1989, 101–104. (b)
Ishar, M. P. S.; Gandhi, R. P. Tetrahedron 1993, 49, 6729–6740.
(8) (a) Rigby, J. H.; Henshilwood, J. A. J. Am. Chem. Soc. 1991, 113,
5122–5123. (b) Rigby, J. H.; Ateeq, H. S.; Charles, N. R.; Henshilwood,
J. A.; Short, K. M.; Sugathapala, P. M. Tetrahedron 1993, 49, 5495–5506
.
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Org. Lett., Vol. 10, No. 24, 2008