Photochemistry of 2,2,4,4-tetramethyl-3-thietan-1-ylidene: A heterocyclic carbene with an unusually short lifetime and evidence for a nonclassical structure

Article abstract of DOI:10.1021/jp0016840

Laser flash (308 nm) and steady-state photolysis (300 nm) of Δ³-1,3,4-oxadiazoline (3) produces a transient that can be trapped with pyridine to give an adduct with λ_max = 350 nm, attributed to ylide 10. Laser flash photolysis (LFP) studies of the ylide indicate that the value of k_pyrτ (where k_pyr, is the absolute rate constant of carbene reaction with pyridine and τ is the carbene lifetime) of the reactive intermediate is 25 times smaller than that deduced for cyclobutylidene. The sulfur-containing transient may have a much shorter lifetime than cyclobutylidene or a much smaller value of k_pyr. These experimental results, along with ab initio calculations, suggest that the structure of 2,2,4,4-tetramethyl-3-thietan-1-ylidene is best represented as nonclassical, bisected structure 2 and not classical structure 1.

Full text of DOI:10.1021/jp0016840

2106
J. Phys. Chem. A 2001, 105, 2106-2111
Photochemistry of 2,2,4,4-Tetramethyl-3-thietan-1-ylidene: A Heterocyclic Carbene with an
Unusually Short Lifetime and Evidence for a Nonclassical Structure
John R. Snoonian and Matthew S. Platz*
Department of Chemistry, The Ohio State UniVersity, Columbus, Ohio 43210
ReceiVed: May 3, 2000; In Final Form: NoVember 30, 2000
Laser flash (308 nm) and steady-state photolysis (300 nm) of ∆3-1,3,4-oxadiazoline (3) produces a transient
that can be trapped with pyridine to give an adduct with λmax ) 350 nm, attributed to ylide 10. Laser flash
photolysis (LFP) studies of the ylide indicate that the value of kpyrτ (where kpyr is the absolute rate constant
of carbene reaction with pyridine and τ is the carbene lifetime) of the reactive intermediate is 25 times smaller
than that deduced for cyclobutylidene. The sulfur-containing transient may have a much shorter lifetime than
cyclobutylidene or a much smaller value of kpyr. These experimental results, along with ab initio calculations,
suggest that the structure of 2,2,4,4-tetramethyl-3-thietan-1-ylidene is best represented as nonclassical, bisected
structure 2 and not classical structure 1.
Introduction
SCHEME 1a
Cycloalkylidenes are of interest because of their ability to
undergo a variety of reactions such as ring contraction1 and
intramolecular bond insertion.2 Furthermore, the ground-state
multiplicity of cycloalkylidenes is of fundamental interest
because theory predicts that a singlet ground state is favored
with decreasing ring size.3
Modern techniques such as time-resolved laser spectroscopy
and computational methods have cast new light on the under-
standing of carbene chemistry, yet the effects of remote
substituents such as heteroatoms on the properties and reactions
of carbenes have received little attention. Our interest in the
effects of heteroatom perturbation on the properties and reactions
of cycloalkylidenes motivated this study, and herein we report
the study of 2,2,4,4-tetramethyl-3-thietan-1-ylidene (1).
a Conditions: (a) H2S, NaOMe/MeOH; (b) H2NNH2, HOAc, MeOH,
100 °C; (c) Ac2O, DMAP, imidazole, CH2Cl2; (d) Pb(OAc)4, MeOH/
CH2Cl2, -10 °C.
Solvents were dried prior to use by standard techniques.
Reagents purchased from Aldrich were used as received. For
the preparation of 5, a cylinder of H2S (Aldrich, 99.5+%) was
used as received. Proton and carbon NMR spectra were recorded
at 300 and 75 MHz, respectively, in CDCl3 using δ 7.26 and
77.0 ppm (center of triplet) as the reference peak for proton
and carbon NMR, respectively. FT IR spectra were recorded
between NaCl plates as CCl4 solutions. GC/MS studies were
recorded at an ionizing voltage of 70 eV. Where mass spectral
data are given, the peak abundance immediately follows in
parentheses. Elemental analysis of 7 was performed by Atlantic
Microlab, Norcross, GA. Caution!6 Because this preparation
uses H2S (toxic), it should be performed only in an efficient
fume hood. Furthermore, since H2S can spontaneously ignite
as a 4-45% mixture with air, argon should be used as a bath
gas to purge the system of air until the system is aboVe the
ignition Volume of H2S, and this purging should also be done
when closing the flow of H2S.
Carbene 1 is a sulfur analogue of cyclobutylidene.1 Unlike
cyclobutylidene, however, 1 may exist as an ylide, 2, which
represents a “nonclassical” form of the carbene. A similar type
of structure has been proposed to describe the 3-thietan-1-yl
cation,4 and recent calculations suggest that even cyclobutylidene
itself may have a “nonclassical” geometry.5
We are pleased to report that photolysis of oxadiazoline 3
(Scheme 1) provides entry into this system and that kinetic and
computational evidence favors structure 2, the nonclassical form
of the carbene.
The synthetic sequence is outlined in Scheme 1.
2,2,4,4-Tetramethyl-3-thietanone (5). To 16.0 g (0.314 mol)
of NaOMe in 200 mL of MeOH was passed H2S while
maintaining the temperature of the solution below 15 °C. To
this was added 50.0 g (0.184 mol) of 2,4-dimethyl-2,4-dibromo-
3-pentanone (4) dropwise with continuous passage of H2S while
maintaining the solution temperature below 20 °C. After 3 h
Experimental Section
General Procedures: Synthesis. All reactions were con-
ducted in flame-dried glassware under an argon atmosphere.
10.1021/jp0016840 CCC: $20.00 © 2001 American Chemical Society
Published on Web 02/13/2001

Photochemistry of a 3-Thietan-1-ylidene
J. Phys. Chem. A, Vol. 105, No. 10, 2001 2107
under H2S, the solution was poured into 800 mL of water and
extracted with Et2O. The total Et2O solution was dried over
Na2SO4 and concentrated by rotary evaporation. Recrystalliza-
tion of the white residue from MeOH gave 15.3 g of 5 as long,
white needles. Spectral data was consistent with that previously
reported. Mp ) 104-106 °C (lit. mp ) 106 °C).7
2,2,4,4-Tetramethyl-3-thietanone Hydrazone (6). In 75 mL
of MeOH was placed 10.0 g (69.4 mmol) of 5, 11.6 g (232
mmol) of hydrazine hydrate, and 0.80 g (13.3 mmol) of glacial
acetic acid. The solution was refluxed at 100 °C for 5 days
after which time the MeOH was removed by rotary evaporation.
Recrystallization from 20% aqueous MeOH gave 9.2 g of 6.
Spectral data were consistent with that previously reported. Mp
) 66-67 °C (lit. mp ) 66 °C).8
septum, and then the solution was purged with argon for 5 min,
followed by irradiation with 254 nm light (Rayonet reactor) at
5 °C for 3 h. Analysis of the mixture by GC/MS showed that
thietanone 5 had been formed, but neither 9 nor 11 nor 12 could
be detected in the product mixture.
LFP Studies: General Procedures. The cyclohexane used
in the LFP studies was distilled from sodium and stored over 4
Å molecular sieves. Cyclohexane-d12 (CIL, 99.7%) was used
as received. Pyridine was distilled and stored over 4 Å molecular
sieves. Freon-113 (Aldrich, HPLC grade) was passed through
a column of alumina immediately prior to use. For Stern-
Volmer kinetics, 1-hexene, 2,3-dimethyl-2-butene, cyclohexene,
and acrylonitrile were passed through a column of alumina
immediately prior to use.
LFP Studies: Kinetics. A stock solution of 3 was prepared
to an optical density of 0.55 at 308 nm in the appropriate solvent,
and 1.0 mL of this solution was added to quartz cuvettes. To
each cuvette a specific amount of pyridine was added, and then
an appropriate amount of solvent such that the total volume in
each cuvette was 2.0 mL. Each cuvette was then capped with
a rubber septum, wrapped in foil, and degassed by purging
with dry, oxygen-free argon for 5 min immediately prior to
LFP at 308 nm. Three transient spectra were recorded for
each cuvette, and averaged transient A350 values were used in
the data analysis. In the lifetime experiments, the pyridine
concentration was typically varied from 0.1 to 3.0 M for a series
of 12-15 cuvettes.
Stern-Volmer quenching experiments were performed by
adding 1.0 mL of the Freon-113 stock solution of 3 used in the
lifetime experiments containing 3.0 M in pyridine to quartz
cuvettes. To each cuvette was then added a specific amount of
olefin, and then an appropriate amount of Freon-113 was added
such that the total volume in each cuvette was 2.0 mL. Each
cuvette was then capped with a rubber septum, wrapped in foil,
and degassed by purging with dry, oxygen-free argon for 5 min
immediately prior to LFP at 308 nm. Three transient spectra
were recorded for each cuvette, and average A350 values were
used in the data analysis. For a given olefin, the range of
quencher concentration was typically varied between 0.00 and
1.0 M for a series of 8-10 cuvettes. The spectrometer has been
described previously.10
2,2,4,4-Tetramethyl-3-thietanone Acetylhydrazone (7). To
9.2 g (58.2 mmol) of 6, 0.28 g (2.3 mmol) of p-(dimethylamino)-
pyridine (DMAP), and 6.5 g (96.0 mmol) of imidazole in 290
mL of CH2Cl2 was added dropwise 8.9 g (87.3 mmol) of acetic
anhydride in 40 mL of CH2Cl2 over 30 min. After 4 h the
mixture was washed with two 80 mL portions of water. The
aqueous layer was extracted with 40 mL of CH2Cl2, and the
total organic layer was dried over Na2SO4 and concentrated by
rotary evaporation to give a white solid. Three recrystallizations
from MeOH gave 11.1 g of analytically pure 7 as fine white
needles which begin to sublime at 192° C, mp (sealed capillary)
) 197-199 °C. IR: 1692, 1667 cm-1. H NMR: δ 1.65 (s, 6
H), 1.79 (s, 6 H), 2.21 (s, 3 H), 8.39 (broad s, NH) ppm. C
NMR: δ 19.79, 29.27, 31.04, 52.95, 54.70, 159.91, 173.08 ppm.
GC/MS: 202.00 (0.4) 126.05 (100), 125.05 (39), 11.05 (28),
43.00 (21). Anal.Calcd for 7: C, 54.00; H, 8.00; N, 14.00; S,
16.00. Found: C, 54.06; H, 8.01; N, 13.95; S, 16.08.
7-Methoxy-1,1,3,3,7-pentamethyl-8-oxa-2-thia-5,6-diaza-
spiro[3.4]oct-5-ene (3). To 16.0 g (36.1 mmol) of Pb(OAc)4
in 150 mL of MeOH cooled to -8 °C was added dropwise 6.0
g (30.0 mmol) of 7 in 40 mL MeOH and 60 mL CH2Cl2 over
3 h while the solution temperature was maintained between -8
and -6 °C. After 4.5 h with warming of the solution to between
10 and 15 °C, followed by 2 h at 20 °C, the solution was cooled
to -8 °C and 2.3 g KOH in 25 mL MeOH was added dropwise
such that the solution temperature was maintained between -8
and -7 °C. After 8 h, the MeOH was removed by rotary
evaporation, the white residue was taken up in 300 mL CH2Cl2
and washed with three 100 mL portions of water, and the organic
layer was dried over Na2SO4. Following removal of the CH2-
Cl2 by rotary evaporation, silica gel column chromatography
using 5:1 pentane-Et2O (Rf ) 0.62) as the eluant gave 3.1 g of
General Procedures: Calculations. All molecular geom-
etries were fully optimized using GAUSSIAN 9411 and the
frozen-core approximation.12
Results and Discussion
3 as a clear, colorless liquid. IR: 3083, 3004, 2941, 2858 cm-1
.
Synthesis of the Precursor
H NMR: δ 1.52 (s, 6 H), 1.54 (s, 6 H), 2.10 (s, 3 H, CH3),
3.13 (s, 3H, -OCH3) ppm. GC/MS: 202 [M+ (-N2) (3)], 128
(100), 113 (69), 95 (44), 85 (77), 59 (36), 43 (75). UV
(pentane): λmax ) 232, 336 nm.
Steady-State Photolysis (300 nm) of 3 in C6H12. Into a
quartz cuvette was placed 10.0 mg of 3 (0.043 mmol) in 1.0
mL of cyclohexane. The cuvette was capped with a rubber
septum, and then the solution was purged with argon for 5 min,
followed by irradiation with 300 nm light (Ray-o-Net reactor)
at 5 °C for 3 h. Analysis of the mixture by GC/MS showed
that azine 9 and sulfide 11 were formed. There was no evidence
for the formation of an adduct with cyclohexane (e.g., 12) The
mass spectrum of 11 was consistent with that previously
reported.9
The first task at hand was the preparation of a precursor that
cleanly yields a thiocyclobutylidene upon photolysis. It has been
shown that diazirines are frequently inefficient precursors to
carbenes, as products are often derived from excited-state
reactions of the diazirine or from a biradical resulting from C-N
bond cleavage.13 Also, small-ring spiro diazirines are sometimes
difficult to synthesize in good yield and to handle in a pure
state.
Oxadiazolines are quite often efficient carbene precursors and
are easy to prepare and handle.14 We initially directed our
attention toward the preparation of an oxadiazoline precursor
(13) to the unsubstituted sulfur analogue of cyclobutylidene,
but treatment of acetylhydrazone 14 with Pb(OAc)4 did not lead
to the desired 13 but rather to incorporation of an acetoxy group
adjacent to sulfur. The replacement of hydrogen R to sulfur by
OAc with Pb(OAc)4 has been reported.15 To block this process,
we next turned our attention to the preparation of tetramethyl
Steady-State Photolysis (254 nm) of 3 in C6H12. Into a
quartz cuvette was placed 10.0 mg of 3 (0.043 mmol) in 1.0
mL of cyclohexane. The cuvette was capped with a rubber

2108 J. Phys. Chem. A, Vol. 105, No. 10, 2001
Snoonian and Platz
SCHEME 2
Figure 1. Transient spectrum obtained following LFP of oxadiazoline
3 in Freon-113 containing the following: A, 0.00 M pyridine; B, 2.48
M pyridine. The spectra were recorded over a time window of 250 ns,
50 ns following the laser flash.
derivative 3, as described in Scheme 1. Owing to the poor yields
of 7 obtained from reaction of 5 with acetic hydrazide, even
under stringent conditions such as in glacial acetic acid at 100
°C, 7 was prepared stepwise from 5 in good yield.
Figure 2. Formation and decay of 10 in Freon-113 containing the
following pyridine concentrations: a, 2.48 M; b, 1.12 M; c, 0.62 M; d,
0.37 M. The spectra were recorded over a time window of 2000 ns.
transient with λmax ) 350 nm, attributed to that of ylide 10 was
observed (Figure 1). As postulated (vide supra), 1 may exist as
ylide-type structure 2. This ylide was not detected as shown in
Figure 1 (curve A). Although this is a negative result, the failure
to detect 2 may reflect the fact that 2 has a poor chromophore.
Evidence will be presented (vide infra) that supports 2 as the
structure of the transient intermediate.
Ylide 10 was formed faster than the time resolution (≈20
ns) of the spectrometer (Figure 2); thus, it was not possible to
measure the lifetime of 1 by measurement of the growth of ylide
10. Measurement of the absorption at 350 nm as a function of
pyridine concentration, however (Figures 2 and 3), revealed that
when [pyridine] > 2.5 M, the optical yield of 10 did not
increase.
This implies that every intermediate generated by the laser
pulse is trapped by pyridine to form 10 when [pyridine] > 2.5
M. The lifetime of the intermediate generated from the pho-
tolysis of 3 may be deduced from the data in Figure 3. Double
reciprocal treatment of the data in Figure 3 is predicted and
found to be linear (Figure 4).17 Division of the intercept by the
slope of Figure 4 yields kpyrτ, and using kpyr ) 1 × 109-5 ×
109 M-1 s-1, which is a typical range for reaction of singlet
carbenes with pyridine,18 the deduced lifetime of the intermedi-
ate in Freon-113 is found to be 0.2-0.8 ns (Table 1). It is
interesting to note that the lifetime of 1/2 is considerably shorter
than that deduced for cyclobutylidene (4-20 ns) in Freon-113,
Photochemistry
Steady-state photolysis (Ray-o-Net) of 3 in degassed cyclo-
hexane at 300 nm followed by GC/MS analysis of the resulting
mixture showed that one product had a cracking pattern
consistent with that of 11 (Scheme 2), which has been reported
as a product from the pyrolysis of the tosylhydrazone salt
derived from 5.9 It is interesting to note that 11 is only one of
two allene episulfide derivatives known, the other being a
perfluorinated derivative.16 The second product observed from
the photolysis of 3 had strong peaks in the mass spectrum at
m/z of 284 and 142, which are attributed to the azine, 9. The
azine is most likely derived from the diazo intermediate 8, which
is initially formed by fragmentation of 3. None of the other
possible photoproducts from 3, such as that from insertion into
the C-H bonds of cyclohexane solvent, were detected.
Laser flash photolysis (LFP, 308 nm, XeCl) of 3 in 1,1,2-
trifluorotrichloroethane (Freon-113) did not produce detectable
transient absorption. In the presence of pyridine, however, a

Photochemistry of a 3-Thietan-1-ylidene
J. Phys. Chem. A, Vol. 105, No. 10, 2001 2109
Figure 3. Optical yield of ylide 10 produced by LFP of 3 measured
at 350 nm (A350) vs [pyridine] in Freon-113.
Figure 5. Double reciprocal plot for calculation of the lifetime of 1
in (O, - - -) C6D12 and (+, s) C6H12.
Figure 4. Double reciprocal treatment of the data in Figure 3.
Figure 6. Stern-Volmer plot for quenching of 10 with [1-hexene] at
[pyridine] ) 3.0 M in Freon-113.
TABLE 1: Deduced Lifetimes of Singlet 1/2
solvent
kpyr
τ
τ (ns)
(methyl)carbene is a relatively long-lived carbene.21 The fast
(τ < 20 ns) formation of 10 and the short deduced lifetimes of
the transient intermediate derived from 3 (Table 1) do not
support the generation of methoxy(methyl)carbene. It is interest-
ing to note, however, that when the steady-state photolysis was
performed at 254 nm (Ray-o-Net), GC/MS analysis revealed
that 5 was indeed formed. Thus, the mode of fragmentation of
3 is wavelength dependent. Wavelength-dependent photochem-
istry of oxadiazolines has been reported previously.22
CF2ClCFCl2
cyclohexane
cyclohexane-d12
0.8
0.7
0.7
0.2-0.8
0.1-0.7
0.1-0.7
C6H12, and C6D12 by the same method.19 However, 1/2 may
not be a typical carbene and may have a value of kPYR lower
than 1 × 10-9 - 5 × 109 M-1 s-1. This would increase the
deduced lifetime of 1/2.
It is also noteworthy that the lifetime of 1/2 in cyclohexane
is essentially the same as in cyclohexane-d12 (Figure 5). Thus,
there is no solvent isotope effect on the lifetime of 1/2 in C6H12
or C6D12. This suggests that 1/2 does not appreciably form
products from C-H(D) insertion with solvent, which is
consistent with the results obtained from steady-state photolysis
of 3 in cyclohexane solvent in which no products resulting from
C-H insertion of 1/2 were detected by GC/MS. The lifetime
of 1/2 in cyclohexane is controlled by intramolecular rearrange-
ment. Thamattoor et al. have shown that a methyl substituent
will accelerate carbon-bond migration in cyclopropylcarbene.20
It was of interest to determine the absolute (and therefore
relative) rate constants for reaction of 1/2 with olefins. In the
case of 1/2, eq 1 is valid when [pyridine] > 2.5 M.17
0
Ay
Ay
1 + kolefin[olefin]
kpyr[pyridine]
)
(1)
Thus, LFP of a Freon-113 solution of 3 containing [pyridine]
) 3.0 M in the presence of an olefin quencher is expected to
Thus, it is possible that the gem-dimethyl substitution in 1/2
accelerates bond migration relative to cyclobutylidene.
o
reduce the absorption of ylide, Ay , at infinite pyridine concen-
Of particular issue in the use of oxadiazolines as carbene
precursors is their ability to fragment by an alternative mode
to yield methoxy(methyl)carbene. In the case of precursor 3,
this would yield the ketone 5. This product was not detected
upon 300 nm (Ray-o-Net) steady-state photolysis of 3. Methoxy-
tration17 as the concentration of olefin increases. This is exactly
what is observed as shown in Figure 6 for 1-hexene. As may
be expected for a short-lived intermediate, the spread of
reactivity of 1/2 for reaction with the olefins of this study is
not large. Nonetheless a trend in reactivity exists. The most

2110 J. Phys. Chem. A, Vol. 105, No. 10, 2001
Snoonian and Platz
TABLE 2: Stern-Volmer Analysis for Rate of Reaction of
1/2 with Olefins
TABLE 3: Relative Energies (kcal/mol) of Structures
1 and 2
Figure 7. Geometry of diazo compound 8 optimized at the MP2/6-
311+G** level of theory.
method
E2 - E1
HF/6-31+G*
-7.4
-15.6
-25.3
-60.6
B3LYP/6-311+G**
MP2/6-311+G**
CCSD(T)/DZP//MP2/6-311+G**
Computational Chemistry
It was of interest to examine the structures 1 and 2 by ab
initio molecular orbital theory. To simplify the calculations, the
unsubstituted parent structures of 1 and 2 were optimized using
HF,12 B3LYP,24 and MP225 levels of theory. Since the 300 nm
steady-state photolysis suggests that diazo compound 8 is the
immediate precursor to 1/2, diazo compound 8 was optimized
at the MP2/6-311+G** level of theory, and the ring was found
to have a puckering or flap angle of 10°.
Removal of the N2 group from the optimized structure 8
served as the starting point to determine what type of geometry
this initial structure would adopt. Geometry optimization at the
HF, B3LYP, and MP2 levels of theory revealed that the
stationary point was that of nonclassical structure 2, which was
lower in energy than 1 at all levels of theory examined (Table
3). Frequency calculations at the B3LYP/6-311+G** level of
theory on the structures 1 and 2 optimized at this same level
showed that 2 is a minimum, with 1 being characterized by an
imaginary frequency. Thus, the calculations suggest that 1 and
2 are not in equilibrium with each other and that 1 is a transition
state. Furthermore, positive and negative charge in 2 is
concentrated at S1 (+0.183) and C2 (-0.145), and nonclassical
structure 2 has a dipole moment of 2.8 D.
Figure 8. Geometry of 1 optimized at the MP2/6-311 + G** level of
theory.
Although at the expense of increasing angle strain, four-
membered rings are known to adopt “puckered” conformations
in order to alleviate eclipsing strain between adjacent bonds.26
The puckering angle would be zero degrees if both cyclopropane
rings of a [1.1.0] ring system were coplanar. It was of interest
to compare the “puckered” geometry of 2 with bicyclo[1.1.0]-
butane,27 which is an experimentally known all-carbon analogue
of 2. The geometry of bicyclo[1.1.0]butane was optimized at
the same levels of theory employed in the study of 2. Although
the presence of sulfur in the ring of 2 prohibits any comparison
of bond lengths and angles between 2 and bicyclo[1.1.0]butane,
Table 4 shows that 2 has a puckered geometry that very much
resembles bicyclo[1.1.0]butane. The puckering angles of these
two ring systems are very different from that of cyclobutane,
which is 22°.28
Figure 9. Geometry of 2 optimized at the MP2/6-311+G** level of
theory.
electron-poor olefin, acrylonitrile, reacts nearly twice as rapidly
with 1/2 as does the most electron-rich olefin, 2,3-dimethyl-2-
butene. One may attribute this to a steric effect, as the methyl
groups of 2,3-dimethyl-2-butene may make addition of 1/2 to
the double bond more difficult than in either 1-hexene or
acrylonitrile, in which the double bond is relatively unhindered.
However, if the krel data are solely due to a steric effect, then
1-hexene would be expected to react with 1/2 at nearly the same
rate as it does with acrylonitrile instead of 26% more slowly.
One might also expect that 1 would react more slowly with
acrylonitrile as compared to 1-hexene, as the classical structure
of 1 would resemble a typical electrophilic carbene.23 This
suggests that the structure of the transient may be nonclassical
structure 2, which appears nucleophilic as written.
Conclusions
The LFP (308 nm) and steady-state (300 nm) studies of this
work are consistent with the generation of an intermediate from
oxadiazoline precursor 3. This intermediate can be trapped with
pyridine to give a transient with λmax ) 350 nm, attributed to
that of ylide 10. Ab initio calculations at different levels of

Photochemistry of a 3-Thietan-1-ylidene
J. Phys. Chem. A, Vol. 105, No. 10, 2001 2111
References and Notes
TABLE 4: Calculated Puckering Angles of 2 and
Bicyclo[1.1.0]butane
(1) Friedman, L.; Schecter, H. J. Am. Chem. Soc. 1960, 82, 1002.
(2) Friedman, L.; Schecter, H. J. Am. Chem. Soc. 1961, 83, 3159. Cope,
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(4) Morita, H.; Oae, S. Heterocycles 1977, 6, 1593.
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J. Am. Chem. Soc. 1997, 119, 5682 and references therein.
(6) Manufacturing Chemical Association Guide for Safety in the
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1972; p 184.
(7) Claeson, G.; Thalen, A.; Schotte, L. Ark. Kemi 1963, 21, 295.
(8) Quast, H.; Fub, A.; Jakobi, H. Chem. Ber. 1991, 124, 1747.
(9) Hortman, A. G.; Bhattacharjya, A. J. Am. Chem. Soc. 1976, 98,
7081.
(10) Gritsan, N. P.; Zhai, H. B.; Yuzawa, T.; Karweik, D.; Brooke, J.;
Platz, M. S. J. Phys. Chem. A 1997, 101, 2833.
theory indicate that 2 is lower in energy than 1 and that the
latter structure is characterized by an imaginary frequency. The
classical structure of carbene 1 is a transition state. Thus, theory
suggests that 1 and 2 are not in equilibrium and that the
intermediate generated by the photolysis of 3 is 2. The lifetime
of 2 in C6H12 is essentially the same in C6D12 and indicates
that 2 does not undergo C-H(D) insertion with solvent. This
is supported by the 300 nm steady-state photolysis results in
which insertion products were not detected. The lifetime of 2
in Freon-113, C6H12, and C6D12 is about 25 times shorter than
that deduced for cyclobutylidene. The shorter lifetime deduced
for intermediate 2 as compared to cyclobutylidene may be an
artifact. If 2 is the true structure of the intermediate, then the
lone pair of electrons on the nitrogen of pyridine would have
to attack a carbon with a formal negative charge to give adduct
10; thus the actual kpyr value may be considerably smaller than
the typical value of 1 × 109 - 5 × 109 M-1 s-1 used here to
deduce the lifetime of cyclobutylidene and a reduction of kpyr
will lead to an increase in τ.17
Due to the rigid nature of the four-membered ring, the large
sulfur atom in 1 is ideally situated to donate an electron pair to
the carbene center to give 2. Thus, the photoproduct 11 may
result from rearrangement of 2. Furthermore, Stern-Volmer
analysis showed that acrylonitrile and 1-hexene react most
rapidly with 2 and this is likely due to a steric effect. It was
found, however, that acrylonitrile reacts 26% more rapidly than
does 1-hexene with 2. This is not what one may predict for
classical carbene structure 1 which is expected to be electro-
philic. The data are consistent with nonclassical structure 2,
which is expected to be nucleophilic.
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Acknowledgment. Support of this work by the NSF (Grant
CHE-9613861) is gratefully acknowledged. J.R.S. gratefully
acknowledges support by an Ohio State University Postdoctoral
Fellowship administered by the Graduate School of The Ohio
State University.
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