Intermediacy of cyclobutylidene in photochemical methylenecyclopropane rearrangement

Article abstract of DOI:10.1016/j.tetlet.2005.09.149

Methylenecyclopropane 1 undergoes photochemical rearrangement to 2. Intervention of cyclobutylidene 3 explains not only the rearrangement but also newly obtained products such as cyclobutene 4 and cyclobutylidenecyclobutane 6. Experiments designed to generate cyclobutylidene 3 independently have provided some support for the intermediacy of 3.

Full text of DOI:10.1016/j.tetlet.2005.09.149

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 8415–8418
Intermediacy of cyclobutylidene in photochemical
methylenecyclopropane rearrangement
*
Yasutake Takahashi, Yoko Mori, Akiko Nakamura and Hideo Tomioka
Chemistry Department for Materials, Faculty of Engineering, Mie University, Tsu 514-8507, Japan
Received 17 August 2005; revised 19September 2005; accepted 22 September 2005
Available online 11 October 2005
Abstract—Methylenecyclopropane 1 undergoes photochemical rearrangement to 2. Intervention of cyclobutylidene 3 explains not
only the rearrangement but also newly obtained products such as cyclobutene 4 and cyclobutylidenecyclobutane 6. Experiments
designed to generate cyclobutylidene 3 independently have provided some support for the intermediacy of 3.
Ó 2005 Elsevier Ltd. All rights reserved.
1
In contrast to abundant pyrolytic chemistry of methyl-
With ether or cyclohexane as a solvent for photolysis of
1, lower yields of 2 (8% or 2%, respectively) were ob-
tained with significant amount of recovered 1 (79% or
94%, respectively). Similar photoirradiation of 1 in oxy-
gen-saturated acetonitrile resulted in significant increase
in the yield of cyclobutanone 5 (10%) with formation of
2 (39%) and a smaller amount of recovered 1 (10%). It is
by no means obvious that cyclobutene 4 and cyclobuta-
none 5 are structurally connected to 1 by 1,3-C shift or
trimethylenemethane type intermediate.
2
–5
enecyclopropanes, their photochemistry
is rather
sparse and awaits further investigations, particularly
2
,3
for direct excitation. We have revisited the photo-
chemistry of methylenecyclopropanes 1 and 2 in order
to assess their utility as photochemical precursor to
3
vinylidenes, and found that their photochemistry is
more intricate than it appeared. Expansion of our initial
interest has prompted us to develop new facts in the
photochemistry of 1, which suggest the intermediacy
of cyclobutylidene 3 in the photoprocess.
Mechanistically more suggestive results were provided
Photoirradiation (k >250 nm) of a 3 ml acetonitrile solu-
tion of 1 (0.20 mmol) in a quartz tube under nitrogen led
to formation of rearranged product 2 (29%) and 1,1-
diphenylethylene (<1%) with recovery of 1 (45%). This
by solid-state photolysis of 1. Thus, when finely pow-
dered 1 was photoirradiated, a dimeric product (M =
+
412) was obtained in 3% yield together with 2 (4%),
1,1-diphenylethylene, and unreacted 1 (91%). Prolonged
photolysis caused further consumption of the starting
material but resulted in no increase in the yield of the
2
result is virtually consistent with the previous report
and it appears to be an example of photochemical 1,3-
1
7
C shift. On the other hand, careful H NMR analysis
dimeric product. The spectroscopic data suggest the
of the reaction mixture suggested formation of addi-
tional new products albeit in much lower yields. Two
compounds were isolated by repeated preparative TLC
of combined reaction mixtures obtained from multipli-
cative irradiations. They were identified as cyclobutene
new product to be a cyclobutylidenecyclobutane 6.
The unique structure was established based on the spec-
tral comparison with the independently synthesized
material and the geometry was unequivocally deter-
mined to be trans by X-ray crystallographic analysis
(Fig. 1).
4
and cyclobutanone 5 by comparison of their spectro-
scopic data with those of the authentic samples synthe-
6
sized independently. Cyclobutene 4 was confirmed to
These results led us to consider the intermediacy of 2,2-
diphenylcyclobutylidene 3 in addition to a 1,3-shift
pathway in the photoreaction scheme of 1 (Scheme 1).
A simple and feasible mechanism for the generation of
be stable under the photolytic conditions.
2,2-diphenylcyclobutylidene 3 from 1 is the photochem-
*
0
Corresponding author. Tel.: +81 59231 49 17; fax: +81 59231
418; e-mail: ytaka@chem.mie-u.ac.jp
ical 1,2-C-shift with respect to the C1–C2 bond of
1. Since parent cyclobutylidene is known to undergo
9
040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.09.149

8416
Y. Takahashi et al. / Tetrahedron Letters 46 (2005) 8415–8418
pendently by thermolysis and photolysis of typical pre-
1
6
cursors such as tosylhydrazone sodium salt
oxadiazoline.
and
1
5,17
Thus, potential precursors, 8 and 9
were synthesized to examine. When sodium salt 8 was
heated to 150 °C for 15 min, 1 (84%) and 2 (2.6%) were
produced (Table 1). Comparable result was obtained
with diglyme as a solvent. Again, no formation of cyc-
1
lobutene 4 was detected by H NMR. It turned out,
however, that no appropriate judgment could be made
as to the ratio of 1,2-C/1,2-H because control experi-
ments showed that cyclobutene 4 decomposes easily
under the thermolysis conditions. On the other hand,
photolysis of a solution of sodium salt 8 in dimethoxy-
1
8
ethane afforded a mixture of 1 (54%), 2 (9.6%), and
cyclobutene 4 (3.3%). Similar photolysis of oxadiazoline
9 resulted in 1 (97%) and relatively lower yield of 2
(1.6%) and cyclobutene 4 (<1%). In either case, no
formation of dimer 6 was detected.
Figure 1. ORTEP drawing of 6. Triclinic, P-1 (#2), Z = 1, R = 0.088,
˚
˚
˚
a = 7.430(2) A, b = 7.538(1) A, c = 10.781(2) A, a = 84.475(7)°,
˚
3
b = 85.988(6)°, c = 83.07(1)°, V = 595.6(2) A . Hydrogen atoms are
omitted for clarity.
The observed preference for 1,2-C (1+2) over 1,2-H
migration (4) in the photolysis of 8 and 9 is similar or
maybe more pronounced than that observed for a par-
preferential 1,2-C shift to form methylenecyclopropane
1
2–15
over 1,2-H shift to form cyclobutene,
intervention
1
2,13
of 2,2-diphenylcyclobutylidene would rationally
3
ent system.
Indeed, it explains formation of cyclo-
explain the observed rearrangement of 1 to 2 with for-
mation of cyclobutene 4 as a minor product.
butene 4 in much lower yield relative to 2 in the
photoreaction of 1. According to the recent calculation
1
4
by Sulzbach et al., singlet cyclobutylidene possesses a
non-classical bicyclic structure and the transition state
There have been several comparable precedents for pho-
tochemical 1,2-C and 1,2-H migrations of alkenes. For
example, Kropp and co-workers have reported evidence
for carbene intermediates in the photochemistry of tetra-
alkyl alkenes and proposed the involvement of p,R(3s)
À1
for 1,2-C migration is only 10.5 kcal mol above the
ground state. Because of the structural similarity
between the ground-state geometry of singlet cyclobutyl-
idene and the transition state for 1,2-C migration, the
1,2-C rearrangement becomes quite facile with respect
to 1,2-H migration.
8
Rydberg excited state for the occurrence of 1,2-C shift.
Similar rearrangement of cycloheptene to give methyl-
enecyclohexane and bicyclo[4.1.0]heptane through
a photogenerated cyclohexylcarbene intermediate is
In view of considerably high 1/2 ratio in the 1,2-C
migrations (Table 1), significant fraction of cyclobutyl-
idene 3 generated by direct excitation of 1 would rear-
range back to 1. It is evident that the phenyl groups
would lend a strong directing effect to carbene 3 by sta-
bilizing the transition state for formation of 1 from 3,
leading to preferential formation of 1. In contrast, the
other type of 1,2-C migration process, leading to 2,
would be affected rather limitedly by the phenyl groups,
so that the rate can be slightly higher than that of the
parent. Thus, the observed 1/2 ratio ranging from 5.6
9
reported by Inoue and co-workers. Later, Hixson
reported photochemical 1,2-H-shift to generate carbenes
1
0
from phenylalkenes. Recent laser flash photolysis stud-
ies on phenylallenes have indicated that vinylcarbenes,
generated by photochemical 1,2-H shift of phenylallenes,
1
1
are protonated by alcohols to give allyl cations.
Since it is important to know the reactivity of 3 itself in
terms of branching ratio for two types of 1,2-C shifts
and 1,2-H shift, we sought to generate carbene 3 inde-
Ph
Ph
•
•
hν [1,2-C]
1,2-C]
Ph
Ph
[1,2-H]
Ph
Ph
[
1
3
4
hν [1,3-C]
[1,2-C]
Ph
O
Ph Ph
Ph
Ph
Ph
Ph
Ph
Ph Ph
Ph Ph
2
5
6
7
Scheme 1.

Y. Takahashi et al. / Tetrahedron Letters 46 (2005) 8415–8418
Table 1. Photolysis and thermolysis of 8 and 9
8417
OMe
N
NN(Na)Ts
Ph
N
O
Ph
Ph
Ph
8
9
Entry
Precursor
Conditions
Conv. (%)
Products (yield (%))
1
2
3
4
8
8
8
9
150 °C, 5 min, no solvent
150 °C, 5 min, diglyme
hm, 7 h, dimethoxyethane
hm, 5 h, dimethoxyethane
95
99
97
1 (84), 2 (2.6), 5 (<1)
1 (89), 2 (1.8), 5 (<1)
1 (54), 2 (9.6), 4 (3.3), 5 (<1)
100
1 (97), 2 (1.6), 4 (<1), 5 (<1)
1
Precursor (0.20 mmol) was heated or irradiated (k > 300) under nitrogen and the resulting reaction mixture was analyzed by H NMR (300 MHz).
to 61 may lead to a rough estimate that the rate of 1,2-
C migration of 3 to 1 would be at least 5.6–61 times
faster than that of the parent. In other words, 2,2-diphen-
ylcyclobutylidene 3 would have 5.6–61 times shorter
lifetime relative to that of the parent; reportedly 4–
3. (a) Gilbert, J. C.; Butler, J. R. J. Am. Chem. Soc. 1970, 92,
7
4
493–7494; (b) Gilbert, J. C.; Luo, T. J. Org. Chem. 1981,
6, 5237–5239.
4
. Mizuno, K.; Maeda, H.; Sugita, H.; Nishioka, S.; Hirai,
T.; Sugimoto, A. Org. Lett. 2001, 3, 581–584.
5
. (a) Ikeda, H.; Akiyama, K.; Takahashi, Y.; Nakamura, T.;
Ishizaki, S.; Shiratori, Y.; Ohaku, H.; Goodman, J. L.;
Houmam, A.; Wayner, D. D. M.; Tero-Kubota, S.;
Miyashi, T. J. Am. Chem. Soc. 2003, 125, 9147–9157; (b)
Ikeda, H.; Nakamura, T.; Miyashi, T.; Goodman, J. L.;
Akiyama, K.; Tero-Kubota, S.; Houmam, A.; Wayner, D.
D. M. J. Am. Chem. Soc. 1998, 120, 5832–5833; (c)
Miyashi, T.; Kamata, M.; Mukai, T. J. Am. Chem. Soc.
2
0 ns in cyclohexane-d and 0.3–1 ns in acetonitrile,
12
1
5
respectively.
In this regard, the lifetime of 3 may be practically too
short to react with 1 in solution to form an adduct such
as 7, whose spiropentane–methylenecyclobutane-type
skeletal rearrangement can explain the formation of
dimer 6. However, formation of adduct 7 may be
marginally possible in the solid-state photolysis of 1,
particularly when crystal structure of 1 adopts special
molecular orientation and packing.
1
987, 109, 2780–2788; (d) Miyashi, T.; Kamata, M.;
Mukai, T. J. Am. Chem. Soc. 1986, 108, 2755–2757; (e)
Miyashi, T.; Takahashi, Y.; Mukai, T.; Roth, H. D.;
Schilling, M. L. M. J. Am. Chem. Soc. 1985, 107, 1079–
1
081; (f) Takahashi, Y.; Miyashi, T.; Mukai, T. J. Am.
Chem. Soc. 1983, 105, 6511–6513.
Another possibility is the involvement of triplet state of
6
7
. Cyclobutene 4 was synthesized by the reaction of
n-BuLi with tosylhydrazone of cyclobutanone 5. Cyclo-
butanone 5 was synthesized by the oxidation of 1 with
MCPBA.
3
. Recent theoretical calculations for parent cyclobutyl-
idene suggest that its triplet state is thermally accessible
because the singlet–triplet energy separation (DE
=
S–T
À1
1
. Data of 6: mp 176.2–176.4 °C; H NMR (500 MHz,
À5.9kcal mol ) is smaller than the barriers to 1,2-C
CDCl ) 7.34–7.31 (m, 8H), 7.26–7.22 (m, 12H), 2.63 (t,
3
and 1,2-H migrations to methylenecyclopropane and
13
1
9
J = 8.1 Hz, 4H), 2.21 (t, J = 8.1 Hz, 4H); C NMR
125 MHz, CDCl ) 146.34 (4C, Ph), 139.61 (2C, C@C),
28.06 (8C, Ph), 128.02 (8C, Ph), 125.94 (4C, Ph), 60.21
2C, >CPh ), 35.15 (2C, CH ), 25.62 (2C, CH ); MS (EI)
cyclobutene, respectively. It may be an interesting
interpretation that formation of cyclobutanone 5 is also
an indication of intervening triplet state of 3. At this
stage, however, this point is yet to be fully investigated.
(
3
1
(
2
2
2
+
m/z = 412 (M ), 204 (100%). HRMS m/z = 412.2197,
calcd. for C H = 412.2191.
3
2
28
8
. Fields, T. R.; Kropp, P. J. J. Am. Chem. Soc. 1974, 96,
559–7560.
7
Acknowledgements
9
. Inoue, Y.; Takamuku, S.; Sakurai, H. J. Chem. Soc.,
Chem. Commun. 1975, 577–578.
Support of this work by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Science, and
Culture of Japan is greatly acknowledged.
1
0. (a) Hixson, S. S. J. Am. Chem. Soc. 1975, 97, 1981–1982;
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(
Soc. 1975, 97, 3230–3232.
1
1
1
1. Kirmse, W.; Strehlke, I. K.; Steenken, S. J. Am. Chem.
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1
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