Photorearrangement of vinylidenecyclopropanes to 1,2,3-butatriene derivatives

Article abstract of DOI:10.1021/ol000379u

(Matrix presented) Photoirradiation of benzene solutions containing 1-diarylvinylidene-2,2,3,3-tetramethylcyclopropanes (2a-d) afforded rearranged products 1,2,3-butatrienes (3a-d) in good to high yields. Photorearrangement from 2,2,3-trimethyl and 2,2- and 2,3-dimethyl derivatives 2e-g also proceeded but the rates of the rearrangement were lower than those of 2a-d. A singlet mechanism is proposed for this photorearrangement, where alky migration occurs from 1,3-biradical intermediates generated via the homolysis of the C1-C2 bond. Generate of diarylvinylidene carbenes from 1,3-biradicals might be competitive with the formation of 3.

Full text of DOI:10.1021/ol000379u

ORGANIC
LETTERS
2001
Vol. 3, No. 4
581-584
Photorearrangement of
Vinylidenecyclopropanes to
1,2,3-Butatriene Derivatives
Kazuhiko Mizuno,* Hajime Maeda, Hikaru Sugita, Shogo Nishioka,
Takayoshi Hirai, and Akira Sugimoto
Department of Applied Chemistry, Graduate School of Engineering,
Osaka Prefecture UniVersity, 1-1 Gakuen-cho, Sakai, Osaka 599-8531, Japan
mizuno@chem.osakafu-u.ac.jp
Received December 11, 2000
ABSTRACT
Photoirradiation of benzene solutions containing 1-diarylvinylidene-2,2,3,3-tetramethylcyclopropanes (2a−d) afforded rearranged products 1,2,3-
butatrienes (3a−d) in good to high yields. Photorearrangement from 2,2,3-trimethyl and 2,2- and 2,3-dimethyl derivatives 2e−g also proceeded,
but the rates of the rearrangement were lower than those of 2a−d. A singlet mechanism is proposed for this photorearrangement, where alkyl
migration occurs from 1,3-biradical intermediates generated via the homolysis of the C1−C2 bond. Generation of diarylvinylidene carbenes
from 1,3-biradicals might be competitive with the formation of 3.
Photochemical reactivities of methylenecyclopropanes (1)
have attracted much attention from mechanistic, theoretical,
spectroscopic, and synthetic viewpoints since the middle of
the 20th century.1 Carbon-carbon bond cleavage of meth-
ylenecyclopropane derivatives usually occurs at C2-C3
bonds; hence they isomerize via trimethylenemethane bi-
radical intermediates or via trimethylenemethane radical
cations formed by photoinduced electron transfer (Scheme
1). On the other hand, the photochemical behavior of
vinylidenecyclopropanes (2) has been less reported, although
we have recently reported a few examples of thermal2 and
photochemical3 ring cleavage of vinylidenecyclopropanes (2),
where the ring cleavage at C1-C2 bond of 2 might occur.
Here we report an unprecedented photorearrangement from
2 to 1,2,3-butatriene derivatives (3) via C1-C2 bond
cleavage, accompanied by the generation of vinylidene
carbenes.
Irradiation of a benzene solution of 1-(2′,2′-diphenylvi-
nylidene)-2,2,3,3-tetramethylcyclopropane (2a, 0.03 mol
dm-3)4 through a Pyrex filter (>280 nm light) under argon
(1) (a) Miyashi, T.; Ikeda, H.; Takahashi, Y.; Akiyama, K. In AdVances
in Electron-Transfer Chemistry; Mariano, P. S., Ed.; Jai Press Inc.:
Stamford, 1999; Vol. 6, pp 1-39. (b) Dowd, P.; Chow, M. Tetrahedron
1982, 38, 799-807. (c) Kende, A. S.; Goldschmidt, Z.; Smith, R. F. J. Am.
Chem. Soc. 1970, 92, 7606-7607. (d) Takahashi, Y.; Miyashi, T.; Mukai,
T. J. Am. Chem. Soc. 1983, 105, 6511-6513.
Scheme 1
(2) Mizuno, K.; Sugita, H.; Kamada, T.; Otsuji, Y. Chem. Lett. 1994,
449-452.
(3) (a) Mizuno, K.; Sugita, H.; Isagawa, K.; Goto, M.; Otsuji, Y.
Tetrahedron Lett. 1993, 34, 5737-5738. (b) Mizuno, K.; Nire, K.; Sugita,
H.; Otsuji, Y. Tetrahedron Lett. 1993, 34, 6563-6566. (c) Mizuno, K.;
Sugita, H.; Hirai, T.; Maeda, H. Chem. Lett. 2000, 1144-1145.
(4) We have developed a convenient method for the synthesis of
vinylidenecyclopropanes: Isagawa, K.; Mizuno, K.; Sugita, H.; Otsuji, Y.
J. Chem. Soc., Perkin Trans. 1 1991, 2283-2285.
10.1021/ol000379u CCC: $20.00 © 2001 American Chemical Society
Published on Web 01/30/2001

between two central carbons (C2 and C3) is 1.241 Å, which
is shorter than those of ethylene (1.339 Å) and allene (1.308
Å) but longer than that of acetylene (1.202 Å). UV-vis
spectrum of 3 showed a characteristic absorption maximum
at 340 nm assigned to a 1,1-diaryl-1,2,3-butatriene structure.5
The time-dependent UV spectrum for the irradiation of
2a showed spectral change from 2a to 3a with three
isosbestic points at 240, 265, and 310 nm, which indicated
that 3a was the sole product in the photoreaction of 2a
(Figure 2a).7 Similar photorearrangement from 2,2,3-trim-
Scheme 2
atmosphere gave 1,1-diphenyl-4,5,5-trimethyl-1,2,3-hexatriene
(3a)5 in a 55% isolated yield (Scheme 2, Table 1). Similar
Table 1. Photorearrangement of 2a-d to 3a-da
isolated
entry substrate
Ar
C6H5
p-ClC6H4
p-MeOC6H4
p-MeC6H4
product
yield (%)
Φb
1
2
3
4
2a
2b
2c
2d
3a
3b
3c
3d
55
85
45
62
0.008
0.016
0.008
0.017
a [2] ) 3.0 × 10-2 mol dm-3 in benzene. b Quantum yield for the
formation of 3 at 313 nm irradiation ([2] ) 9.5 × 10-5 mol dm-3 in
benzene).
irradiation of p-chloro, p-methoxy, and p-methyl derivatives
2b-d afforded the corresponding 1,2,3-butatrienes 3b-d in
good to high yields.6 Quantum yields for the formation of
3a-d were 0.008-0.017 at 313 nm. Structures of products
were determined by their spectral data and confirmed by
X-ray analysis of p-chloro derivative 3b (Figure 1).
Figure 2. UV spectral changes in the photorearrangement of
vinylidenecyclopropanes 2a, 2e-g to 1,2,3-butatriene derivatives
3a, 3e-g. Direct irradiation of cyclohexane solutions (1.0 × 10-4
mol dm-3) of (a) 2a at 3 min intervals, (b) 2e at 10 min intervals,
(c) 2f at 30 min intervals, and (d) 2g at 30 min intervals.
ethyl and 2,2- and 2,3-dimethyl derivatives 2e-g also
proceeded as indicated by the UV spectral changes (Figure
2b-d), but the rates for the formation of the corresponding
1,2,3-butatrienes were quite low (Φ < 10-3) and products
were not isolated.8
The bicyclic vinylidenecyclopropanes 2h and 2j also
rearranged to the 1,2,3-butatriene derivatives (Scheme 3).
The former gave 3h and 3i in a 4:6 ratio, and the latter gave
3j exclusively. This is reasonably explained in terms of the
higher strain energy of the cyclobutane ring of 3k.
The photorearrangement of 2 was not sensitized by triplet
sensitizers such as benzophenone and Michler’s ketone and
was not quenched by 2-methyl-1,3-butadiene or molecular
dioxygen. Photoirradiation of 2a-d in methanol did not
disturb this rearrangement, and no methanol-incorporated
product was obtained. Irradiation of 2,3-dimethyl derivative
2g in the presence of a large excess of ethyl vinyl ether gave
Figure 1. ORTEP drawing of 3b. Monoclinic, P21/c (No. 14), Z
) 4, R ) 0.043, a ) 7.6341(9) Å, b ) 21.718(2) Å, c ) 11.394(2)
Å, â ) 97.61(1)°, V ) 1872.6(4) Å3. Selected bond lengths: C1-
C2, 1.341(4) Å; C2-C3, 1.241(4) Å; C3-C4, 1.334(4) Å. Selected
bond angles: C1-C2-C3, 177.5(3)°, C2-C3-C4, 177.9(4)°.
The linear structure of 3b is shown in the ORTEP drawing
indicating a 1,2,3-butatriene structure. The bond length
582
Org. Lett., Vol. 3, No. 4, 2001

On the basis of these results, we propose a singlet
mechanism for the formation of 1,2,3-butatriene derivatives
and the generation of vinylidene carbenes as shown in
Scheme 5. Homolysis of the C1-C2 bond of 2 occurs via
Scheme 3
Scheme 5
2-ethoxy-1-(diphenylvinylidene)cyclopropane 4a in a 25%
isolated yield (Scheme 4). Similar irradiation of 2g in the
Scheme 4
the excited singlet state of 2 generated by the direct
irradiation to give singlet 1,3-biradical intermediate 6.3c This
biradical intermediate 6 smoothly rearranges to 1,2,3-
butatriene 3 by the migration of an alkyl group. An
alternative pathway via heterolytic cleavage of the C1-C2
bond of 2 to give dipolar intermediate 7 seems unlikely. The
formation of vinylidene carbene 5 might be a competitive
pathway for this rearrangement, although it is not clear at
this stage whether 5 is produced via 6 or a concerted two
presence of cyclohexene gave the corresponding vinylidenecy-
clopropane 4b, although the yield was low. These results
clearly demonstrate the formation of diphenylvinylidene
carbene 5, which is trapped by alkenes.9 This carbene was
not generated by the triplet sensitization.
(6) Data for 3b: mp 129-130 °C; 1H NMR (270 MHz, CDCl3) δ 7.26-
7.40 (m, 8 H), 2.06 (s, 3 H), 1.20 (s, 9 H); 13C NMR (75 MHz, CDCl3) δ
158.9, 150.4, 137.4, 133.2, 132.7, 129.9, 129.5, 128.7, 128.6, 109.9, 109.3,
38.0, 29.5, 20.4; IR (neat) 2962, 2044, 1903, 1604, 1485 cm-1; UV
(cyclohexane) λmax ) 268, 345 nm; MS (EI) m/z ) 343 (M+).
(7) This is also supported by time-dependent 1H NMR studies in benzene-
d6. The allenyl absorption band of 2 at ca. 1970 cm-1 disappeared upon
irradiation, and a new absorption band appeared at ca. 2050 cm-1
characterized to the 1,2,3-butatriene structure.
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Org. Lett., Vol. 3, No. 4, 2001
583

carbon-carbon bond cleavage of 2. Most of 5 probably
reacts with the eliminated alkene in cage to regenerate 2.
Some escaped carbene 5 attacks the added alkenes to give
4.
tion, Science, Sports, and Culture of Japan. We thank Dr.
T. Tamai for HRMS analysis. We also are indebted to
Emeritus Professors Y. Otsuji and K. Isagawa for helpful
discussion.
In conclusion, we have developed a novel skeletal rear-
rangement from vinylidenecyclopropane derivatives to 1,2,3-
butatrienes via the homolytic cleavage of the C1-C2 bond.
This photorearrangement occurs from the excited singlet
states of vinylidenecyclopropanes accompanied by the
formation of vinylidenecarbenes.
Supporting Information Available: Experimental de-
tails, spectral data of 3a-d, and X-ray report of 3b. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Acknowledgment. This work was supported by a Grand-
in-Aid for Scientific Research from the Ministry of Educa-
OL000379U
584
Org. Lett., Vol. 3, No. 4, 2001