Effect of phenyl substitution on the lifetime and product distribution of cyclobutylidene: preference change in the rearrangements via 1,2-carbon shift and 1,2-hydrogen shift

Article abstract of DOI:10.1016/j.tetlet.2006.04.007

Steady state and laser flash photolytic experiments with precursors 6 and 11 revealed that diphenyl substitution affects the lifetime and reaction mode of cyclobutylidene. 2,2-Diphenylcyclobutylidene 3 (τ <0.1 ns) produces methylenecyclopropane 1 via 1,2-carbon in significant preference to the positional isomer 2 or cyclobutene 4. On the other hand, 3,3-diphenylcyclobutylidene 5 (τ = ca. 4 ns) gives 1,2-hydrogen shift product 4 more favorably than 1,2-carbon shift product 2 together with formal carbene dimer 14. MRMP2//MP2 calculations afford useful results to understand the interrelationship among substitution, structure, and reactivity.

Full text of DOI:10.1016/j.tetlet.2006.04.007

Tetrahedron Letters 47 (2006) 3995–3999
Effect of phenyl substitution on the lifetime and product
distribution of cyclobutylidene: preference change in the
rearrangements via 1,2-carbon shift and 1,2-hydrogen shift
Yasutake Takahashi,a,* Takurou Sakakibara,a Makoto Inaba,a Hideo Tomioka,a
Shiro Koseki,b,* Kazuyoshi Fujimotoa and Hiroaki Umedab
aChemistry Department for Materials, Faculty of Engineering, Mie University, Tsu 514-8507, Japan
bDepartment of Chemistry, Graduate School of Science, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai,
Osaka 599-8531, Japan
Received 15 February 2006; revised 29 March 2006; accepted 7 April 2006
Abstract—Steady state and laser flash photolytic experiments with precursors 6 and 11 revealed that diphenyl substitution affects the
lifetime and reaction mode of cyclobutylidene. 2,2-Diphenylcyclobutylidene 3 (s <0.1 ns) produces methylenecyclopropane 1 via 1,2-
carbon in significant preference to the positional isomer 2 or cyclobutene 4. On the other hand, 3,3-diphenylcyclobutylidene 5
(s = ca. 4 ns) gives 1,2-hydrogen shift product 4 more favorably than 1,2-carbon shift product 2 together with formal carbene dimer
14. MRMP2//MP2 calculations afford useful results to understand the interrelationship among substitution, structure, and
reactivity.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Our recent studies1 on the photochemistry of methylene-
cyclopropane 1 suggested a mechanism involving 1,2-
e
carbon shift (Scheme 1, arrow a) for transformation
from 1 to isomer 2 and cyclobutene 4 in competition
with ordinary 1,3-carbon shift (Scheme 1, arrow b).
The 1,2-carbon shift process leading to generation of
cyclobutylidene 3 is shown to be important because
the intervention of 3 rationally explains the formation
of both methylenecyclopropane 2 and cyclobutene 4
(Scheme 1, arrows d and e). In relation to this is that
not only cyclobutylidene 3, but also isomer 5 can be gen-
erated (Scheme 1, arrows f and g) and postulated to play
a role in the photoreversal from isomer 2 to the starting
methylenecyclopropane 1 and the formation of cyclo-
butene 4. Although intermediacy of these isomeric
cyclobutylidenes 3 and 5 has been our current issue, pre-
vious experiments designed to trap these carbenes have
remained unsuccessful presumably because of their
short lifetimes and low quantum yields for generation.
Ph
b
Ph
a
••
Ph
Ph
a, h [1,2-C]
ν
H
d
c
c [1,2-C]
3
1
f
hν,
b [1,3-C]
e [1,2-H]
d [1,2-C]
h
••
H
g
g
f
Ph
Ph
h
Ph
i
i
Ph Ph
Ph
5
4
2
Scheme 1.
the reactivity.3 On the other hand, it is much less obvi-
ous in the three-center type of rearrangements of singlet
carbenes such as 3 and 5 whether substituents on the a-
or b-position to a carbene carbon play a significant role
in determination of the product distribution and
lifetime. Thus, we sought to generate carbenes 3 and 5
It is fairly common that substituents directly combined
to a carbene carbon play a decisive role in determining
*
Corresponding authors. Tel.: +81 59 231 9417; fax: +81 59 231
9418; e-mail: ytaka@chem.mie-u.ac.jp
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.04.007

3996
Y. Takahashi et al. / Tetrahedron Letters 47 (2006) 3995–3999
independently by photolysis of the appropriate precur-
sors. Oxadiazolines4,5 were chosen because of their syn-
thetic easiness and their general photochemical behavior
as straightforward carbene precursors. In fact, parent
singlet cyclobutylidene6,7 can be generated from the cor-
responding oxadiazoline.5 Accordingly, oxadiazolines 6
and 11 were synthesized and their steady-sate photo-
chemistry and laser flash photochemical behavior were
investigated in order to gain insight into the reactivity
of carbenes 3 and 5. Also carried out were computa-
tional investigations concerning the effects of phenyl
groups on the rearrangement pathways of 3 and 4.
In order to gain insight and evidence for the intervention
of cyclobutylidenes 3 and 5, nano-second laser flash
photolysis (LFP, XeCl, 308 nm) of oxadiazoline precur-
sors 6 and 11 was examined. Since cyclobutylidenes 3
and 5 are expected to be spectroscopically invisible, a
probe technique12 with pyridine13 was used. Many alkyl
carbenes are known to form ylides with pyridine13d,e in
competition with their uni- and bimolecular reactions
and the resulting ylide absorptions appear in UV–vis
region (typically between 300 and 400 nm).14 Cyclobutyl-
idenes 3 and 5 may also be trapped with pyridine if
they have trappable lifetime. Despite our effort, LFP
of a solution of oxadiazoline 6 in cyclohexane or aceto-
nitrile containing pyridine (1.2–10 M) resulted in no
transient absorptions ascribable to pyridine ylide. It is
likely that the lifetime of carbene 3 is too short. If we
assume a rate constant for pyridine ylide formation to
be 1 · 109 MÀ1 sÀ1, 10 M pyridine could easily trap the
carbene with its lifetime of ca. 0.1 ns.15
Table 1 summarizes the results from steady-state photo-
lyses with oxadiazoline 6.8 One can find that methylene-
cyclopropane 1 is formed as a major rearrangement
product in significant preference to the positional isomer
2 or cyclobutene 4. Also produced were cyclobutanone
8, diphenylethylene 9, and azine 10. Formation of diphen-
ylethylene 9 can be explained in terms of photochemi-
cal [2+2]cycloreversion of oxadiazoline 6. Azine 10 is
most likely due to a bimolecular reaction of diazocyclo-
butane 7 as is often the case in photoreaction of diazo-
alkanes. In fact, lowering the concentration of
oxadiazoline 6 resulted in significant suppression in the
yield of azine 10 and led to almost exclusive formation
of 1.
In contrast, LFP of oxadiazoline 11 in cyclohexane con-
taining pyridine (1.2 M) resulted in transient absorp-
tions as shown in Figure 1. Fairly intense absorption
with kmax 380 nm can be ascribed to pyridine ylide
because it was not observed in the absence of pyridine.
Broad and weaker absorption around 470 nm is due to
at least two species: one is triplet excited state of 11
(kmax ꢀ 450 nm) and the other is much longer-lived di-
azocyclobutane 12 (kmax 510 nm). These assignments
are based on their kinetic behavior and quenching
experiments.16
Photolysis of isomeric oxadiazoline 11 in solution
proceeded more slowly compared to 6 and afforded a
mixture of 2 and 4 in reversed distribution ratio, cyclo-
butene 4 being formed in preference to 2 (entry 1). Also
interesting in this case is that formal carbene dimer 14
was obtained9 in 43% yield together with azine 15
(36%) and a trace amount of cyclobutanone 13. Photo-
lysis of precursor 11 at somewhat lower concentration in
benzene resulted in a similar product distribution, but
no practical suppression was observed in the formation
of 14 or 15 (entry 2). In acetonitrile, the photolysis pro-
ceeded more efficiently to give higher yield of dimer 14
but hydrocarbons 2 and 4 were formed in somewhat
lower yields (entry 3). It was found that irradiation at
254 nm afforded cyclobutene 4 (30%) in much higher
yield and in preference to 2 (2%), with the total material
balance being reduced to some extent (entry 4).11
It is desirable to directly measure the growth rate (kobs
)
for pyridine ylide absorption so that the bimolecular
rate constant for pyridine ylide formation (kpyr) and
the lifetime of carbene 5 (s = 1/k0) can be determined
from the slope and the intercept of a plot of kobs versus
concentration of pyridine according to the equation:
kobs = k0 + kpyr [pyridine].13 Unfortunately, kobs in this
case turned out to be too fast to measure directly with
our instruments, presumably because the lifetime of
cyclobutylidene 5 is very short. In such a case, Stern–
Volmer type analysis is useful.13 Indeed, a double reci-
procal plot of ylide absorbance (Aylide) versus concentra-
tion of pyridine gives a straight line as shown in the inset
Table 1. Steady-state photolysis of oxadiazoline 6
OMe
MeO
N
)
2
O
N
N2
N
Ph
Ph
hν
Ph hν
Ph
Ph
O
1 + 2 + 4 +
3
+
+
Ph
Ph
Ph
Ph
Ph
7
8
6
9
10
Entry
Solvent
[6] (mM)
Time (h)
Products (yield (%))
1
2
3
4
c-Hexane
c-Hexane
MeCN
67.0
6.70
67.0
6.70
8
2
8
2
1 (34), 2 (2), 4 (1), 8 (<1), 9 (6), 10 (23)
1 (70), 2 (4), 4 (1), 8 (<1), 9 (17), 10 (8)
1 (25), 2 (2), 4 (1), 8 (<1), 9 (12), 10 (29)
1 (72), 2 (3), 4 (1), 8 (<1), 9 (14), 10 (7)
MeCN
A nitrogen-saturated solution of 6 (0.20 mmol) was irradiated by a 300 W medium pressure mercury lamp (k >300) and the reaction mixture was
analyzed by 1H NMR (300 MHz).

Y. Takahashi et al. / Tetrahedron Letters 47 (2006) 3995–3999
3997
Figure 2, cyclobutylidene 3 is calculated to have dis-
torted bicyclobutane-like structure with substantial
bonding between C1 and C3. The type of non-classical
structure was already reported for parent singlet cyclo-
butylidene.7 In the present case, the C1–C3 bond length
0.10
0.08
0.06
0.04
0.02
0.00
50
40
30
20
10
–1
380
˚
A
(1.677 A) is even shorter than that of parent cyclobutyl-
21
˚
idene (1.715 A) calculated by the present method.
Notably, the energy difference between cyclobutylidene
3 and transition state TS3–1 leading to 1 by 1,2-C shift
is only 0.9 kcal molÀ1 (Fig. 2), which is much lower than
the corresponding energy difference of 8.4 kcal molÀ1
between parent cyclobutylidene and its transition state
to methylenecyclopropane.17,21 Phenyl groups on C2
play an important role in stabilization of TS3–1 by elec-
tronic delocalization. It is noteworthy that two phenyl
groups also lower the energy level of transition state
for the other mode of 1,2-C shift (TS3–2, 5.7 kcal molÀ1).
On the other hand, little effect is found for 1,2-H shift
(TS3–4) since the energy barrier of 10.1 kcal molÀ1 is vir-
tually the same as calculated for 1,2-H shift for parent
cyclobutylidene (10.9 kcal molÀ1). These results are in
line with the observations that formation of 1 is much
more favored than the formation of 2 or 4 and that
2,2-diphenylcyclobutylidene 3 has eluded both trapping
and spectroscopic observations.
0
2
4
6
8
10 12
[Py]–1 / M–1
300
400
500
600
Wavelength / nm
Figure 1. Transient absorption spectrum observed 200 ns after LFP
(XeCl, 308 nm) of a cyclohexane solution containing oxadiazoline 11
(2 · 10À2 M) and pyridine (1.2 M). Inset shows a double reciprocal
plot for ylide absorbance at 380 nm and the concentration of pyridine,
which affords the intercept/slope ratio of 4.4.
In contrast to 3, isomeric cyclobutylidene 5 is calculated
of Figure 1 and the intercept/slope ratio is obtained to
be 4.4 MÀ1, which corresponds to kpyrÆs. If we assume
that kpyr = 1 · 109 MÀ1 sÀ1, then the lifetime (s) would
be ca. 4 ns.15 This corresponds to a shorter limit of the
estimated lifetime of parent cyclobutylidene (4–20 ns in
cyclohexane).5
to be destabilized by 6.3 kcal molÀ1 relative to 3 (Fig. 2).
A major difference between their structures is the C1–C3
˚
˚
bond length: 1.836 A (5) versus 1.667 A (3). It is evident
that diphenyl substitution on C3-position of a cyclo-
butylidene ring would result in considerable congestion
around C3, because C3 is a pentacoordinate carbon.
Thus, elongation of the C1–C3 bond is an obvious struc-
tural consequence to diminish the steric energy. On the
other hand, calculations for parent cyclobutylidene
predict that departure from the optimum C1–C3 bond
Product analyses in the steady-state photolytic experi-
ments and the results of LFP experiments suggest that
the phenyl groups on C2 of cyclobutylidene ring would
lead to particular acceleration in the rate of rearrange-
ments from 3 to 1. Indeed, MRMP2//MP2 based theo-
retical calculations17 support this view. As shown in
˚
length of 1.715 A results in significant molecular
destabilization.17,21 Eventually, cyclobutylidene 5 adjusts
1.369 C1
••
1.385
C4
C1 1.414
1.447
H
C1
C1
1.456
Ph
Ph
1.475
C4
1.586
1.230
1.567
C2
C2
Ph
Ph
C2
Ph
Ph
C4
C4
Ph
C2
2.142
1.932
1.636
1.692
Ph
1.583
1.531
1.430
2.234
1.572
1.451
C3
C3
C3
1.751
C3
1.677
3 [0.0]
ϕ = 127.2°
TS3-1 [0.9]
ϕ = 122.9°
TS3-2 [5.7]
ϕ = 124.9°
TS3-4 [10.1]
ϕ = 172.2°
C1 1.349
••
1.393
C1
1.551
C1
H
1.467
1.516
C2
1.235
C4
C2
C2
C4
C4
2.136
1.824
1.430
1.526
1.612
1.577
2.256
1.491
C3
C3
1.836
Ph
Ph
Ph
Ph
Ph
Ph
5 [6.3]
TS5-2 [9.4]
(3.1)
ϕ = 122.0°
TS5-4 [11.1]
(4.8)
ϕ = 178.6°
(0.0)
ϕ = 126.8°
Figure 2. Illustrative structures of singlet cyclobutylidenes (3 and 5) and the transition states for the corresponding 1,2-C shifts (TS3–1, TS3–2, and
TS5–2) and 1,2-H shifts (TS3–4 and TS5–4), respectively. Bond lengths are in A. Numbers in square brackets are energies (kcal molÀ1) relative to 3.
˚
Numbers in parentheses are energies relative to 5. u represents dihedral angle C2–C1–C3–C4.

3998
Y. Takahashi et al. / Tetrahedron Letters 47 (2006) 3995–3999
Table 2. Steady-state photolysis of 11
Ph Ph
OMe
O
)
N
MeO
2
N
N2
N
O
hν
hν
2 + 4 + 9 +
5
+
+
Ph Ph
Ph Ph
Ph
Ph
Ph Ph
Ph Ph
11
12
13
14
15
Entry
Solvent
[11] (mM)
Time (h)
Products (yield (%))
1
2
3
4a
Benzene
Benzene
MeCN
MeCN
6.70
3.35
6.70
6.70
13
13
8
2 (4), 4 (10), 9 (2), 13 (<1), 14 (43), 15 (36)
2 (5), 4 (11), 9 (2), 13 (2), 14 (39), 15 (38)
2 (1), 4 (8), 9 (0), 13 (1), 14 (52), 15 (9)
2 (2), 4 (30), 9 (5), 13 (1), 14 (5), 15 (1)
13
A nitrogen-saturated solution of 11 (0.20 mmol) was irradiated by a 300 W medium pressure mercury lamp (k >300) and the reaction mixture was
analyzed by 1H NMR (300 MHz).
a Irradiated by four low pressure mercury lamps (254 nm, Toshiba GL-20, 15 W).
itself to have the optimum C1–C3 bond length of
1.836 A, maintaining the C1–C3 bonding and avoiding
References and notes
˚
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overelongation.
Two transition states, TS5–2 and TS5–4 for 1,2-C and
1,2-H shift of 5, respectively, also have longer C1–C3
distances and thereby are not stabilized compared to
TS3–2 or TS3-4. However, energy barriers for both 1,2-
C and 1,2-H shifts leading to 2 and 4, respectively,
become somewhat low (3.1 and 4.8 kcal molÀ1) because
the energy level of the starting structure 5 is already
high. While these lower energy barriers of 3.1 and
4.8 kcal molÀ1 (relative to 5) appear to be in line with
the LFP results showing that the lifetime of 5 is compa-
rable to or a little shorter than that of parent cyclobutyl-
idene, the predicted preference based on the calculated
barrier heights disagrees with the observation that 1,2-
H migration is more feasible to give 4 than 1,2-C migra-
tion to give 2. However, it is important to note that the
energy difference between the barrier heights is fairly
small (only 1.7 kcal molÀ1) and that such a small differ-
ence in calculated energies can be upset in solution even-
tually by enthalpic factors like solvation and/or entropic
factors. In fact, 4/2 ratios observed in the steady-state
photolyses of oxadiazoline 11 appear to depend on sol-
vent polarity (Table 2). The values are almost one order
of magnitude higher in polar acetonitrile than those in
non-polar benzene. These observations are reminiscent
of a LFP study of 1-phenyldiazoethane,22 which demon-
strated that the transition state of the 1,2-H shift in 1-
phenylethylidene is polar and the rate can be accelerated
by more than a factor of 30 in polar acetonitrile.
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8. Progress of the reaction can be monitored by IR
spectroscopy with particular regard to diazocyclobutane
7 (2027 cmÀ1) and methyl carbonate (1755 cmÀ1).
9. Considering from the lifetime of carbene 5, it seems more
likely that dimer 14 is produced by the reaction of carbene
5 with diazocyclobutane 12 rather than dimerization of
Acknowledgments
Support of this work by a Grant-in-Aid for Scientific
Research from Ministry of Education, Culture, Sports,
Science and Technology of Japan is greatly acknowledged.

Y. Takahashi et al. / Tetrahedron Letters 47 (2006) 3995–3999
3999
carbene 5 itself. See Ref. 10 for alkene formation by the
reaction of carbene with diazocompounds. Reaction of 5
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for the formation of dimer 14 is also a possibility, for
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17. All calculations were carried out by using the GAMESS
suite of program codes.18 Multi-reference second-order
Møller–Plesset perturbation calculations (MRMP2)19 on
the molecules were performed at geometries optimized by
using single-reference Møller–Plesset perturbation (MP2)
method, where the 6-31G(d) basis set was employed.20
Since rearrangements of carbenes are known to proceed
through the singlet state, our calculations were limited to
the singlet surfaces.
18. Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S.
T.; Gordon, M. S.; Jensen, J. H.; Koseki, S.; Matsunaga,
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Phys. 1971, 54, 724–728; (b) Hehre, W. J.; Ditchfield, R.;
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11. IR spectroscopic observations indicated that initially
formed diazocyclobutane 12 can be photolyzed more
effectively with 255 nm light.
12. Scaiano, J. C. Acc. Chem. Res. 1982, 15, 252–258.
13. (a) Jackson, J. E.; Soundararajan, N.; Platz, M. S.; Liu,
M. T. H. J. Am. Chem. Soc. 1988, 110, 5595–5596; (b)
Jackson, J. E.; Platz, M. S. In Advances in Carbene
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14. Absorption of pyridine ylide of parent cyclobutylidene
appears at 350–360 nm.5
21. Our results for parent cyclobutylidene are in line with the
previous report by Sulzbach et al.7a and Stracener et al.7b
They already investigated the structure, bonding, rear-
rangements,7a and singlet–triplet energy gap7b of parent
cyclobutylidene using reliable computational methods.
They also reported that singlet cyclobutylidene is pre-
dicted to have a non-classical bicyclobutane-like structure
15. The assumed rate constant for pyridine ylide formation
may be an upper limit because cyclobutylidenes 3 and 5
are sterically hindered by two phenyl groups.
16. T–T absorption due to 311* decayed with rate constant of
8 · 105 sÀ1. It can be quenched by typical triplet quenchers
like oxygen (kq = 3 · 108 MÀ1 sÀ1
)
or ferrocene
3
(kq = 2.5 · 109 MÀ1 sÀ1). After complete decay of 11*,
much weaker absorption due to 12 persisted for a few
seconds.
with the C1–C3 bond lengths between 1.72 and 1.85 A.
˚
22. Sugiyama, M. H.; Celebi, S.; Platz, M. S. J. Am. Chem.
Soc. 1992, 114, 966–973.