First direct detection of 2,3-dimethyl-2,3-diphenylcyclopropanone

Article abstract of DOI:10.1021/jo062259r

(Chemical Equation Presented) 3,5-Dihydro-3,5-dialkyl-3,5-diaryl-4H- pyrazol-4-ones stimulate interest as potential precursors for 2,3-diarylcyclopropanones. Photoreactions of trans-3,5-dihydro-3,5-dimethyl-3,5- diphenyl-4H-pyrazol-4-one were studied by c

Full text of DOI:10.1021/jo062259r

First Direct Detection of 2,3-Dimethyl-2,3-diphenylcyclopropanone
Andrey G. Moiseev, Manabu Abe,† Evgeny O. Danilov, and Douglas C. Neckers*
Center for Photochemical Sciences,‡ Bowling Green State UniVersity, Bowling Green, Ohio, 43403, and
Department of Applied Chemistry, Graduate School of Engineering, Osaka UniVersity (HANDAI),
Suita 565-0871, Osaka, Japan
neckers@photo.bgsu.edu
ReceiVed NoVember 1, 2006
3,5-Dihydro-3,5-dialkyl-3,5-diaryl-4H-pyrazol-4-ones stimulate interest as potential precursors for 2,3-
diarylcyclopropanones. Photoreactions of trans-3,5-dihydro-3,5-dimethyl-3,5-diphenyl-4H-pyrazol-4-one
were studied by continuous-wave (CW) and pulsed laser UV photolysis revealing an intermediate that
undergoes rearrangement to form cis- and trans-1,3-dimethyl-1-phenyl-2-indanones with the yield of ca.
60%. Steady-state photolysis (254 and 350 nm excitation) in different solvents produced an intermediate
1
cyclohexadiene as evidenced by UV/vis, IR, and H NMR spectra. In contrast, the nanosecond laser
pulsed photolysis at 355 nm produced 2,3-dimethyl-2,3-diphenylcyclopropanone along with two products
of retro-1,3-dipolar addition phenylmethylketene and 1-phenyldiazoethane. These can be observed by
time-resolved IR (TRIR) spectroscopy as characteristic absorption bands at 1814, 2101, and 2038 cm-1,
respectively. Similar retro-1,3-dipolar addition showed 1-phenyldiazoethane formed following flash
photolysis of 1-pyrazoline (trans-4,5-dihydro-3,5-dimethyl-3,5-diphenyl-3H-pyrazol-4-ol). The formation
of the corresponding cyclopropanone as well the products of retro-1,3-dipolar addition during photoreaction
of starting pyrazol-4-one is directly confirmed by the nanosecond TRIR spectroscopy for the first time.
On the basis of the CW and pulsed laser UV photolysis, a dynamic equilibrium between cyclopropanone
and intermediate 2,4-diphenyl-3-pentanone-2,4-diyl (dimethyldiphenyloxyallyl) was proposed.
SCHEME 1
Introduction
Cyclopropanones are key intermediates in a number of
organic reactions.1 One unique feature of cyclopropanones is
that they exist in equilibrium with oxyallyls (Scheme 1).2
Oxyallyls have been proposed as transient species in classical
organic reactions such as the Favorskii rearrangement,1 [3+2]
as well as in [4+3] cycloadditions,3 Nazarov cyclization,4
photochemical rearrangement of cyclohexadienones,5 and also
the biosynthesis of prostaglandins.6 Though various dialkyl- and
tetralkylcyclopropanones have been isolated and characterized
spectroscopically,7 there is no spectroscopic evidence for the
existence of diarylcyclopropanones including diphenylcyclo-
propanone.8 3,5-Dialkyl-3,5-diaryl-3,5-dihydro-4H-pyrazol-4-
ones9 (1, R ) Ar, R′ ) alkyl) are potential photochemical
precursors for 2,3-dialkyl-2,3-diarylcyclopropanones (2, R )
Ar, R′ ) alkyl), because photodecomposition of a similar
derivative, 3,5-dihydro-3,3,5,5-tetramethyl-4H-pyrazol-4-one (1,
† Osaka University.
‡ Contribution No. 608 from the Center for Photochemical Sciences.
(1) (a) Chenier, P. J. Chem. Educ. 1978, 55, 286. (b) Lee, E.; Yoon, C.
H. J. Chem. Soc., Chem. Commun. 1994, 479.
10.1021/jo062259r CCC: $37.00 © 2007 American Chemical Society
Published on Web 03/16/2007
J. Org. Chem. 2007, 72, 2777-2784
2777

Moiseev et al.
SCHEME 2
R ) R′ ) CH3), is a well-established route to 2,2,3,3-
tetramethylcyclopropanone (2, R ) R′ ) CH3)10 (Scheme 1).
Results and Discussion
We propose the mechanism in Scheme 2. Evidence substan-
tiating the proposed mechanism follows. In general, we expect
our photoreaction to be a reverse of that presented in
Scheme 1.
Recently we have optimized a five-step stereoselective
synthesis of the trans isomer of 1 (R ) Ph, R′ ) CH3) and
developed the synthesis of two new derivatives of 1 (R ) Ph,
R′ ) alkyl).9a In this paper, we report the first observation of
2,3-dimethyl-2,3-diphenylcyclopropanone (2) photochemically
generated from trans-1 by 10-ns laser pulses at 355 nm. Another
pathway of photodecomposition of trans-1 is found to be a retro-
1,3-dipolar addition.
Steady-State Photolysis of trans-1. CW photodecomposition
of trans-1 at 350 nm in acetonitrile and carbon tetrachloride
produced a mixture of cis- and trans-1,3-dimethyl-1-phenyl-2-
indanones (7) (Scheme 2) in ca. 60% along with minor products
of retro-1,3-dipolar addition (ca. 10% yield). The same photo-
chemical products were observed during the CW irradiation of
trans-1 at 254 nm in acetonitrile or CW irradiation at 350 nm
in benzene and methanol. One of the products of retro-1,3-
dipolar addition is phenylmethylketene (4) found during the CW
photolysis. This was trapped with methanol producing the
methyl ester of 2-phenylpropanoic acid. Another product of
retro-1,3-dipolar addition, styrene, was observed in the GC/MS
during the CW irradiation of trans-1 at 350 nm in argon-
saturated benzene. The retention times and mass spectra of 4
and styrene were compared to those of authentic samples.
1H NMR, IR, and UV spectroscopies were used to follow
the course of the steady-state photolysis of trans-1 in order to
observe cyclopropanone 2.
(2) (a) Green, F. D.; Sclove, D. B.; Pazos, J. F.; Camp, R. L. J. Am.
Chem. Soc. 1970, 92, 7488. (b) Sorensen, T. S.; Sun, F. Can. J. Chem.
1997, 75, 1030. (c) Ichimura, A. S.; Lahti, P. M.; Martlin, A. R. J. Am.
Chem. Soc. 1990, 112, 2868. (d) Hirano, T.; Kumagai, T.; Miyashi, T.;
Akiyama, K.; Ikegami, Y. J. Org. Chem. 1991, 56, 1907. (e) Rauk, A.;
Sorensen, T. S.; Sun, F. J. Am. Chem. Soc. 1995, 117, 4506. (f) Hrovat, D.
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Soc. 1996, 118, 6241.
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Hammond, W. B. J. Am. Chem. Soc. 1969, 91, 2283. (b) Takaya, H.;
Makino, S.; Hayakawa, Y.; Noyori, R. J. Am. Chem. Soc. 1978, 100, 1765.
(c) Takaya, H.; Hayakawa, Y.; Makino, S.; Noyori, R. J. Am. Chem. Soc.
1978, 100, 1778. (d) Hayakawa, Y.; Yokoyama, K.; Noyori, R. J. Am. Chem.
Soc. 1978, 100, 1791. (e) Hayakawa, Y.; Baba, Y.; Makino, S.; Noyori, R.
J. Am. Chem. Soc. 1978, 100, 1786. (f) Lee, K.; Cha, J. K. Org. Lett. 1999,
1, 523. (g) Harmata, M.; Sharma, U. Org. Lett. 2000, 2, 2703. (h) Lee, J.
C.; Jin, S. J.; Cha, J. K. J. Org. Chem. 1998, 63, 2804. (i) Lee, J.; Oh, J.;
Jin, S. J.; Choi, J. R.; Atwood, J.; Cha, J. K. J. Org. Chem. 1994, 59, 6955.
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55, 4403. (b) Miesch, M.; Miesh-Gross, L.; Frank-Neumann, M. Tetrahedron
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Trans. 1 1996, 1213. (d) Liang, G.; Trauner, D. J. Am. Chem. Soc. 2004,
126, 9544.
1H NMR Spectroscopy during Steady-State Photolysis of
trans-1. CW irradiation of a 0.05 M solution of trans-1 in argon-
saturated acetonitrile-d3 at 350 nm for a time that resulted in
20% consumption of the initial material produced a mixture of
two diastereoisomers 7 as seen in the 1H NMR spectrum.
However, 254 nm irradiation of trans-1 (to 20% conversion)
in argon-saturated acetonitrile-d3 for 10 min revealed, along with
the signals from 7, additional peaks due to an intermediate
(Figure 1). The change in the NMR spectrum was accompanied
by a color change of the solution from colorless to bright yellow.
(5) (a) Patel, D. J.; Schuster, D. I. J. Am. Chem. Soc. 1968, 90, 5137.
(b) Schuster, D. I.; Patel, D. J. J. Am. Chem. Soc. 1968, 90, 5145.
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121, 5603. (e) Baertschi, S. W.; Brash, A. R.; Harris, T. M. J. Am. Chem.
Soc. 1989, 111, 5003.
1
The H NMR signals of this intermediate disappeared after
several minutes in the dark at 25 °C with a concomitant growth
of signals corresponding to 7, along with a change in the color
of the solution to light-yellow. The intermediate was assigned
the structure of the cyclohexadiene 6 based on the characteristic
1H NMR signals of the cyclohexadiene ring and integration:
1.45 (s, 3H), 1.78 (d, 3H), 4.17 (broad s, 1H), 6.20 (m, 1H),
6.29 (dd, 1H), 6.40 (dd, 1H), 6.80 (dd, 1H), 7.26 (m, 2H),
(7) Turro, N. J. Acc. Chem. Res. 1969, 2, 25.
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Hayakawa, Y.; Takaya, H.; Murai, S.; Kobayashi, R.; Sonoda, N. J. Am.
Chem. Soc. 1978, 100, 1759. (c) Hayakawa, Y.; Yokoyama, K.; Noyori, R.
J. Am. Chem. 1978, 100, 1799.
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Pirkle, W. H.; Hoover, D. J. J. Org. Chem. 1980, 45, 3407.
(10) (a) Adam, W.; Fuss, A.; Mazenod, F. P.; Quast, H. J. Am. Chem.
Soc. 1981, 103, 998. (b) Sander, W.; Wrobel, R.; Komnick, P.; Rademacher,
P.; Muchall, H. M.; Quast, H. Eur. J. Org. Chem. 2000, 1, 91.
2778 J. Org. Chem., Vol. 72, No. 8, 2007

2,3-Dimethyl-2,3-diphenylcyclopropanone
CHART 1
Starting trans-1 has a distinct carbonyl absorption peak at
1764 cm-1. Cyclopropanones can be easily identified by their
characteristic carbonyl absorptions in the 1813-1843 cm-1
region.7 To assist with assignments, Gaussian 98 quantum
chemical calculations11 at the B3LYP/6-31G(d) level of theory
were used to calculate carbonyl vibrational frequencies of 2.
Chart 1 shows intermediates and photochemical products
observed during the photodecomposition of trans-1. Cyclopro-
panone 2 can exist as both the cis and trans isomer and,
according to the calculations, cis-2 (1841 cm-1 [this value, as
all other calculated frequencies in this paper, was obtained and
reported in the text by scaling the frequency resulting from the
quantum chemical calculations by the factor of 0.96; we used
the same scaling factor for all calculated frequencies in this
paper]) and trans-2 (1843 cm-1) have very similar carbonyl
vibrational frequencies.
FIGURE 1. 1H NMR spectra of trans-1 in the argon-saturated
acetonitrile-d3 at 300 MHz after 254 nm irradiation for 10 min at 25
°C: cyclohexadiene 6 (b), cis- and trans-7 (/), and starting trans-1
([).
CW irradiation of trans-1 at 350 nm in carbon tetrachloride
or acetonitrile leads to formation of three distinct absorption
peaks at 2103, 1755, and 1691 cm-1 (Figure 2) accompanied
by a decrease of carbonyl absorption of trans-1 at 1764 cm-1
.
However, no carbonyl absorption for product 2 expected in
accordance with the proposed mechanism (Scheme 2) was found
in the spectra. The strong absorption at 2103 cm-1 can be readily
assigned to ketene 4, based on the comparison with the IR
spectrum of an authentic sample (2099 cm-1).
7.35 ppm (m, 3H). These data indicate that 6 is a precursor of
7. Signals in Figure 1 have been assigned as follows: the doublet
centered at 1.362-1.387 ppm and the singlet centered at
1.726 ppm correspond to CH3 groups of cis-7. The doublet
centered at 1.405-1.430 ppm and the singlet centered at
1.714 ppm correspond to CH3 groups of trans-7. The singlet
centered at 1.663 ppm corresponds to a CH3 group of starting
trans-1.
The band at 1755 cm-1, which overlaps with carbonyl
absorption of trans-1 at 1764 cm-1, corresponds to the carbonyl
absorption of 7. Isolated trans-7 (Chart 1) showed carbonyl
absorption at 1749 cm-1. In addition, quantum chemical
calculations11 predict 1765 cm-1 for carbonyl absorption of
trans-7 in good agreement with the observed value of
It should be noted that the top and bottom spectra in
1
Figure 1 are parts of the same H NMR spectrum. However,
we magnified the 3-6.9 ppm region to obtain a better view of
the cyclohexadiene pattern. The intensity of signals in the bottom
panel is lower, because each signal corresponds to a single
hydrogen, while in the top spectrum each signal corresponds
to three hydrogens of methyl groups. [We also performed 600
MHz 1H NMR experiments to record a better spectrum of
cyclohexadiene 6. The signals between 4.0 and 6.9 ppm
appeared to be much less noisier than the ones recorded at
300 MHz (see the Supporting Information).]
IR Spectroscopy during Steady-State Photolysis of trans-
1. Due to the characteristics of our nanosecond TRIR spec-
trometer, carbonyl vibrations with higher IR oscillator strength
(IR intensity) were used for identification as most reliable.
1755 cm-1
.
(11) Gaussian 98 program was used for the calculations: Frisch, M. J.;
Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman,
J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant,
J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M.
C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci,
B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.;
Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov,
J. V.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson,
B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon,
M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A 11.3; Gaussian,
Inc.: Pittsburg, PA, 2002.
J. Org. Chem, Vol. 72, No. 8, 2007 2779

Moiseev et al.
FIGURE 2. Time evolution of infrared spectra of trans-1 in
carbon tetrachloride acquired after 350 nm CW irradiation
(0-60 min) at 25 °C.
FIGURE 4. Time evolution of UV/vis absorption spectra of trans-1
in argon-saturated carbon tetrachloride acquired after CW irradiation
at 350 nm (0-5 min) at 25 °C.
FIGURE 5. UV/vis absorption spectra of trans-1 and trans-7 in carbon
tetrachloride at 25 °C.
FIGURE 3. IR spectra of 4, trans-6, and trans-7 calculated with the
Gaussian 98 package11 at the B3LYP/6-31G(d) level of theory and
scaled by 0.96.
trans-1 in argon-saturated carbon tetrachloride or acetonitrile
leads to the formation of an intermediate having a broad
absorption in the 300-400 nm region and a much higher
extinction coefficient than the starting trans-1 (Figure 4). This
300-400 nm band cannot be attributed to the absorption of
formed products such as indanones 7 because trans-7 following
isolation has no absorption at wavelengths above 350 nm
(Figure 5). The band cannot be attributed to fragmentation
products either since they are formed in low yield and have
different absorption characteristics. This broad absorption can,
however, be assigned to highly conjugated 6 as observed by
1H NMR and IR spectroscopies. A related derivative of
1,3-cyclohexadiene 6, 5-methylene-1,3-cyclohexadiene13 (MCH,
Chart 1), has a similar UV/vis absorption spectrum. The
extinction coefficient of 6 is expected to be higher than that of
MCH (ꢀ ) 52013 at 350 nm in benzene) due to the extended
conjugation of the MCH moiety with the carbonyl group in
enone 6.
Cyclohexadiene 6, observed in the 1H NMR spectra
(Figure 1), is a precursor of 7. The quantum chemical calcula-
tions11 predict strong carbonyl absorption for trans-6 at
1703 cm-1, which is expected to appear in the IR spectra
(Figure 2). The s-trans and s-cis conformations of R,â-
unsaturated ketones may have different carbonyl vibrational
frequencies. For example, benzalacetone in carbon disulfide at
25 °C shows carbonyl absorption at 1699 cm-1 for s-cis and
1674 cm-1 for s-trans conformation.12 Since cyclohexadiene 6
has an s-cis configuration, its carbonyl absorption is expected
to lie close to 1699 cm-1, similar to the s-cis conformation of
benzalacetone. This experimental evidence and the quantum
chemical calculations compel the assignment of the 1691 cm-1
peak to the carbonyl absorption of 6.
The quantum chemical calculations11 were also carried out
to compare IR band intensities in compounds 4, 6, and 7. The
results show that the ketene absorption of 4 (2115 cm-1) has
much higher intensity than the carbonyl absorptions of trans-6
(1703 cm-1) and trans-7 (1765 cm-1) (Figure 3). This explains
the high intensity of the ketene band of 4 in Figure 2 compared
to the carbonyl intensities of 6 and 7 even though 6 and 7 are
produced in much higher yield (ca. 60%) than 4 (ca. 10%).
UV/Vis Spectroscopy during Steady-State Photolysis of
trans-1. CW irradiation at 350 nm of a 0.002 M solution of
(13) (a) Graham, W. D.; Green, J. G.; Pryor, W. A. J. Org. Chem. 1979,
44, 907. (b) Gajewski, J. J.; Gortva, A. M. J. Org. Chem. 1989, 54, 373.
(14) McMahon, R. J.; Chapman, O. L. J. Am. Chem. Soc. 1987, 109,
683.
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(b) Schneider, M. P.; Bippi, H. J. Am. Chem. Soc. 1980, 102, 7363. (c)
Adam, W.; Gillaspey, W. D.; Peters, E. M.; Peters, K.; Rosenthal, R. J.;
Schnering, H. G. J. Org. Chem. 1985, 50, 580. (d) Adam, W.; Doerr, M.;
Peters, E. M.; Peters, K.; Schnering, H. G. J. Am. Chem. Soc. 1985, 50,
587. (e) Adam, W.; Oppenlander, T.; Zang, T. G. J. Org. Chem. 1985, 50,
3303. (f) Begley, M. J.; Dean, F. M.; Houghton, L. E.; Johnson, R. S.;
Park, K. B. J. Chem. Soc., Chem. Commun. 1978, 461.
(12) Silverstein, R. M.; Webster, F. X. Spectrometric Identification of
Organic Compounds; Wiley: New York, 1998.
2780 J. Org. Chem., Vol. 72, No. 8, 2007

2,3-Dimethyl-2,3-diphenylcyclopropanone
TABLE 1. Experimental and Calculated Carbonyl Frequencies of
Photochemical Products
species no.
label
obsd freq (cm-1
)
calcd freqa (cm-1
1843 (238)
)
trans-2
1
1814
cis-2
trans-6
trans-7
1841(238)
1703 (250)
1765 (162)
2
3
1691
1755
a Gaussian 98 results at the B3LYP/6-31G(d) level of theory and scaled
by 0.96.11 The IR intensities are given in parentheses.
SCHEME 3
FIGURE 6. TRIR difference spectra recorded 2 µs after 355 nm pulsed
laser excitation of argon-saturated (blue) and oxygen-saturated (red)
0.06 M solutions of trans-1 in carbon tetrachloride at 25 °C.
On the basis of the structures of the photochemical products
and 1H NMR, IR, and UV/vis data obtained during the steady-
state photolysis of trans-1, two photodecomposition pathways
are proposed. The first is the expulsion of nitrogen to form
2,4-diphenyl-3-pentanone-2,4-diyl (5), Scheme 2. Oxyallyl 5
could be of diradical or zwitterionic nature. Since calculations
show the zwitterionic character of oxyallyls16 predominates upon
R-substitution, oxyallyl 5 should have zwitterionic nature.
Electrocyclic conrotatory ring closure of zwitterionic oxyallyl
5 (Nazarov-type intermediate4) results in cyclohexadiene 6
followed by the aromatization to cis- and trans-indanones 7. A
second pathway of photodecomposition of trans-1 is a retro-
1,3-dipolar addition affording 1-phenyldiazoethane (3) and 4.
Photochemically labile 3 easily decomposes with nitrogen
expulsion to give styrene. Previously Chapman and McMahon14
showed that photodecomposition of matrix-isolated 3 leads to
phenylmethylcarbene that rearranges to styrene.
Similar retro-1,3-dipolar addition was proposed as a photo-
reaction for the related 1-pyrazolines.15 For example, the
photodecomposition of 3,5-diphenyl-1-pyrazoline led mainly to
1,2-diphenylcyclopropane (70%) along with minor retro-1,3-
dipolar addition (10%).15a It was assumed that retro-1,3-dipolar
addition leads to alkene and a labile diazo compound followed
by decomposition of the latter. However, the formation of diazo
compounds has not been confirmed following the photodecom-
position of 1-pyrazolines.
and to measure lifetimes of short-lived intermediates in solution.
Absorption peaks in TRIR spectra are more pronounced and
preferred for structure characterization compared to featureless
and broad absorption bands in transient UV/vis spectra.
Therefore, nanosecond TRIR spectroscopy was the technique
of choice for observing cyclopropanone 2 by its characteristic
carbonyl absorption in the IR spectrum.
Nanosecond TRIR Studies of trans-1. The TRIR spectrum
of trans-1 in argon-saturated carbon tetrachloride was obtained
with a 30-ns time resolution (Figure 6). Three positive transients
at 1814, 2038, and 2101 cm-1 were observed.
The transients formed within 30 ns after the 355 nm laser
pulse remained unchanged throughout the experimental time
window of ca. 10 µs (see the kinetic traces in the Supporting
Information for details). The position of the absorption peak at
1814 cm-1 is in good agreement with calculated carbonyl
vibrational frequencies of cis and trans isomers of 2 (Table 1,
row 1) and with reported IR absorption of cyclopropanones.7
Therefore, the 1814 cm-1 band was assigned to the carbonyl
absorption of 2. No decrease in IR intensity or lifetime of 2
was observed in the presence of oxygen, implying that 2 is not
reactive toward oxygen. The absence of carbonyl absorption
The observation of cyclohexadiene 6 during the steady-state
photolysis of trans-1 gives credence to the existence of oxyallyl
5, which can close to cyclopropanone 2. The absence of 2 during
the steady-state photolysis of trans-1 indicates that 2 is a
transient species and therefore should be amenable to observa-
tion by transient techniques. TRIR17 and time-resolved UV/vis
(TR UV)18 spectroscopies were used to determine the structure
bands of steady-state products, cyclohexadiene 6 (1691 cm-1
)
and indanones 7 (1755 cm-1), in the TRIR spectra (Figure 6)
suggests that cyclopropanone 2 forms cyclohexadiene 6 on a
longer time scale.
(16) (a) Magnetic Properties of Organic Materials; Lahti, P. M., Ed.;
Marcel Dekker, Inc.: New York, 1999. (b) Hrovat, D. A.; Murko, M. A.;
Lahti, P. M.; Borden, W. T. J. Chem. Soc., Perkin Trans. 2 1998, 1037. (c)
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C. M.; Toscano, J. P. J. Phys. Org. Chem. 2000, 13, 309.
The transient absorption peak at 2101 cm-1 (Figure 6) looks
similar to the strong peak at 2103 cm-1 in Figure 2. Accordingly,
the 2101 cm-1 transient was assigned to the ketene 4. As
mentioned, the presence of styrene during the steady-state
photolysis of trans-1 and the literature data on photoreactions
for other 1-pyrazolines15 suggest the presence of labile diazo
compound 3 in the reaction. The diazo group of matrix-isolated
314 has characteristic strong absorption at 2050 cm-1, which is
(18) (a) Johnston, L. J. Chem. ReV. 1993, 93, 251. (b) Johnston, L. J.;
Scaiano, J. C. Chem. ReV. 1989, 89, 521.
J. Org. Chem, Vol. 72, No. 8, 2007 2781

Moiseev et al.
TABLE 2. Calculated Free Energy Differences and Equilibrium
Constants for Ring Opening of Cyclopropanone 2 and
Cyclohexadiene 6
entry
reaction
∆G (kcal/mol)
Keq
1
2
3
4
cis-2 f oxyallyl 5
trans-2 f oxyallyl 5
cis-6 f oxyallyl 5
trans-6 f oxyallyl 5
5.3
7.2
17.0
17.7
1.3 × 10-4
5.2 × 10-6
3.1 × 10-13
1.0 × 10-13
Parallel to the calculations done for compounds 4, 6, and 7,
the IR spectra of cis- and trans-2, 3, and 4 were simulated at
the B3LYP/6-31G(d) level of theory (Figure 8). The results
show that the ketene absorption in 4 and diazo absorption in 3
have much higher IR intensities relative to the carbonyl peak
of cyclopropanone 2. This explains the strong signals from 3
and 4 (Figure 6) compared to the carbonyl absorption of 2, even
though 3 and 4 are produced in lower yield than 2 (ca. 10%
according to the steady-state photolysis data).
FIGURE 7. TRIR difference spectra recorded 2 µs after 355 nm pulsed
laser excitation of an argon-saturated 0.06 M solution of trans-8 in
carbon tetrachloride at 25 °C.
Labile diazo compounds have not been observed or isolated
during the steady-state photolysis of 1-pyrazolines.15 The reason
for this is that they easily decompose forming carbenes with
subsequent rearrangement to alkenes following the absorption
of an additional photon.14,15 Whether the photoreaction of 3,5-
diphenyl-1-pyrazoline is a direct decomposition or retro-1,3-
dipolar addition remains unclear.15a,b The direct decomposition
should form styrene, phenylcarbene, and nitrogen in one step
while the retro-1,3-dipolar addition should produce phenyldia-
zomethane and styrene followed by the decomposition of the
diazo compound. Decisive evidence in favor of the retro-1,3-
dipolar addition would be the presence of the diazo compound
among the photodecomposition products. Previously,15a irradia-
tion of 3,5-diphenyl-1-pyrazoline at 5.5 K revealed phenylcar-
bene. Apparently, the photodecomposition rate of the diazo
compounds is too high to observe them under the steady-state
conditions. In contrast, we observed characteristic IR peaks of
diazo compound 3 in the time-resolved spectra during the
nanosecond laser photolysis of trans-1 and trans-8 (Figures 6
and 7). Therefore, the evidence collected suggests that the
photodecomposition of 1-pyrazolines proceeds by retro-1,3-
dipolar addition.
FIGURE 8. IR spectra of cis- and trans-2, 3, and 4 calculated with
the Gaussian 98 package11 at the B3LYP/6-31G(d) level of theory and
scaled by 0.96.
Calculations by the UDFT Method at the UB3LYP/6-31G-
(d) Level of Theory. To rationalize the interrelation between
oxyallyl 5 and cyclopropanone 2 and thermodynamical reasons
for rearrangement of cyclopropanone 2 to cyclohexadiene 6,
spin-unrestricted quantum chemical calculations (UDFT) were
performed to estimate free energy differences and equilibrium
constants. The results are summarized in Table 2.
There are several reports of the free energy difference between
cyclopropanone and the corresponding oxyallyl for several stable
cyclopropanones.2 The free energy difference between cis-2,3-
(di-tert-butyl)cyclopropanone and the corresponding oxyallyl
is 11.3 kcal/mol and that for trans-2,3-(di-tert-butyl)cyclopro-
panone is 28 kcal/mol.2i Berson and Cordes found that the free
energy difference between spiro(bicycle[2.2.1]heptane-2,1-cy-
clopropane)-2-one and corresponding oxyallyl is 16-19 kcal/
mol.2j We found that the calculated free energy difference
between oxyallyl 5 and cyclopropanone 2 (5.3-7.2 kcal/mol)
is two times less than that for the reported cyclopropanones.
Therefore, cyclopropanone 2 should be more likely to be in
equilibrium with oxyallyl 5 than the cyclopropanones reported.
The calculated equilibrium constant (0.052-1.3 × 10-4) shows
that the equilibrium between 2 and 5 should be shifted toward
cyclopropanone 2. Therefore, cyclopropanone 2 should be
similar to the transient observed at 2038 cm-1 in Figure 6;
therefore, this absorption was tentatively assigned to 3.
One of the goals of the current study was to confirm the
assignment of the 2038 cm-1 transient in the TRIR spectra of
trans-1. trans-4,5-Dihydro-3,5-dimethyl-3,5-diphenyl-3H-pyra-
zol-4-ol (trans-8) has a structure similar to that of trans-1 and,
therefore, should also generate products of retro-1,3-dipolar
addition upon photodecomposition. The steady-state photolysis
of trans-8 at 350 nm in different solvents mainly gave a product
of nitrogen expulsion: trans-2,3-dimethyl-2,3-diphenylcyclo-
propanol (trans-9, ca. 80%) along with ca. 10% of products of
retro-1,3-dipolar addition (styrene and 2-phenylpropionaldehyde)
(Scheme 3).
The expected retro-1,3-dipolar addition of trans-8 in argon-
saturated carbon tetrachloride was observed and persisted within
the experimental time window. The immediate formation of a
transient absorption at 2041 cm-1 was found in the TRIR
spectrum of trans-8 (Figure 7) and this transient did not change
within ca. 10 µs, similar to that of trans-1. Thus, the absorptions
at 2038 (trans-1) and 2041 cm-1 (trans-8) in the TRIR spectra
were assigned to the same transient species, namely diazo
compound 3.
2782 J. Org. Chem., Vol. 72, No. 8, 2007

2,3-Dimethyl-2,3-diphenylcyclopropanone
present in higher concentration, while a lower concentration of
oxyallyl 5 should be seen. Indeed, we observed only cyclopro-
panone 2 in the TRIR spectra (Figure 6) and no signal
corresponding to oxyallyl 5 was found. [The absence of the
carbonyl band of oxyallyl 5 in the TRIR spetra can also be
explained by two additional reasons: (a) The UDFT calculations
predict a low IR intensity (ca. 150) for the 1536 cm-1 band. As
a result, the corresponding transient signal will be weak and
masked by the baseline noise in the TRIR spectra (Figure 6).
(b) The formation time of cyclopropanone 2 within the time
resolution of our TRIR setup (30 ns) suggests that the time scale
of interconversion between 5 and 2 is less than 30 ns. Therefore,
oxyallyl 5 is too short-lived to be observed in the TRIR spectra.]
Experimental Section
trans-1 and trans-8 were synthesized according to literature
procedures.9a Phenylmethylketene (4) was synthesized according
to the method described by Dehmlow.19
The detailed description of our time-resolved FTIR (TRIR) setup
is available elsewhere.20 Briefly, the third harmonic of a YAG:
Nd3+ laser (354.7 nm, 10 ns, 1.5-5 mJ per pulse, 10 Hz repetition
rate) was used as an excitation source in all experiments. The sample
solutions were pumped through a 1 mm thick CaF2 flow cell with
a flow rate up to 150 mL per minute. All spectra were recorded in
the 1500-2800 cm-1 spectral window with 8 cm-1 resolution every
20 ns. The raw data were processed and visualized with custom
written LabView based software.
Isolation of cis- and trans-7. Starting trans-1 (0.128 g,
0.76 mmol) was dissolved in acetonitrile (50 mL) in a 125 mL
Erlenmeyer flask. The flask was fitted with a rubber septum, argon
inlet, and magnetic stir bar. The solution of trans-1 was purged
with argon for 15 min and irradiated in a photochemical reactor at
350 nm for 12 h. After irradiation was complete, the solution
became bright yellow. Solvent was removed under reduced pressure
to afford crude product (0.100 g) as light-yellow oil. Column
chromatography on silica gel with hexanes-ethyl acetate (98:2)
as eluant afforded cis- and trans-7 (0.061 g, 57%) as a light-yellow
oil.
Because it is in dynamic equilibrium with cyclopropanone 2
oxyallyl 5 also forms cyclohexadiene 6. The comparison of
calculated free energy difference and equilibrium constant for
oxyallyl 5-cyclohexadiene 6 equilibrium (Table 2, entries 3 and
4) with free energy difference and equilibrium constant for
oxyallyl 5-cyclopropanone 2 equilibrium (Table 2, entries 1 and
2) indicates that cyclohexadiene 6 should be much more stable
than cyclopropanone 2 and the overall equilibrium should be
shifted toward cyclohexadiene 6. Another driving force toward
the formation of cyclohexadiene 6 is aromatization to form
indanone 7, which is an irreversible process. The calculations
agree with our experimental observations. Fast ring closure of
oxyallyl 5 leads to cyclopropanone 2 observed in TRIR spectra
on a shorter time scale, while on a longer time scale oxyallyl 5
forms cyclohexadiene 6.
1
trans-7: H NMR (300 MHz, CDCl3) δ 7.34-7.41 (m, 3H),
7.28-7.34 (m, 2H), 7.20-7.28 (m, 1H), 7.13-7.20 (m, 3H), 3.57
(q, J ) 7.2 Hz, 1H), 1.73 (s, 3H), 1.43 (d, J ) 7.2 Hz, 3H); 13C
NMR (75.5 MHz, CDCl3) δ 14.2, 24.2, 45.1, 57.8, 123.9, 125.0,
127.0, 127.3, 128.0, 128.1, 128.6, 141.6, 142.3, 144.9, 220.0; MS,
m/z (I) 236 (44) (M+), 221 (68), 208 (7), 193 (77), 178 (100), 165
(33), 152 (13), 128 (14), 103 (41), 89 (23), 77 (32), 63 (14), 51
(19); IR (neat) 3063, 2968, 2926, 2869, 1749, 1597, 1447, 1373,
1290, 1151, 1075, 1026, 964, 909, 731, 697 cm-1; HRMS (EI)
m/z calcd for C17H16O 236.1201, found 236.1204.
Conclusion
cis-7: 1H NMR (300 MHz, CDCl3) δ 7.34-7.41 (m, 3H), 7.28-
7.34 (m, 2H), 7.20-7.28 (m, 1H), 7.13-7.20 (m, 3H), 3.59 (q, J
) 7.8 Hz, 1H), 1.74 (s, 3H), 1.36 (d, J ) 7.8 Hz, 3H); 13C NMR
(75.5 MHz, CDCl3) δ 17.6, 24.9, 46.4, 57.9, 124.2, 125.2, 126.6,
126.7, 126.9, 128.0, 128.5, 141.8, 143.0, 145.8, 220.0.
We have found distinct differences in the products of the UV
photolysis of trans-1 depending on the regime of irradiation
(steady state vs pulsed). From the analysis of 1H NMR, IR, and
UV/vis spectra, the steady-state photolysis of trans-1 at 254
A total of 15 rather than the expected 17 signals in the 13C NMR
spectrum of indanone trans-7 were observed. The reason for that
is an overlap of two signals of ortho-carbons of the aromatic ring
(at the C1 carbon of the indanone ring) at 126.7 ppm as well as
two signals of meta-carbons at 128.7 ppm (see the Supporting
Information).
We believe that this apparent mismatch is a common phenom-
enon for 1-phenyl-2-indanone aromatic systems (see, for example,
the previously reported 13C NMR spectrum of 1-phenyl-1,3,3-
trimethyl-2-indanone21).
and 350 nm produces intermediate cyclohexadiene
6
(Scheme 2). The cyclohexadiene undergoes rearrangement to
form cis- and trans-1,3-dimethyl-1-phenyl-2-indanones 7 with
a yield of ca. 60%. On the other hand, the pulsed laser UV
photolysis of trans-1 at 355 nm results in a characteristic
carbonyl absorption peak attributed to 2,3-dimethyl-2,3-diphe-
nylcyclopropanone 2 at 1814 cm-1 in the TRIR spectra.
Cyclopropanone 2 was observed for the first time in our time-
resolved experiments. The other two products, 1-phenyldiazo-
ethane 3 and phenylmethylketene 4, were detected by their
characteristic IR bands in the TRIR spectra as characteristic
absorption bands at 2101 and 2038 cm-1, respectively. Similarly,
the formation of 1-phenyldiazoethane was observed after the
pulsed laser photolysis of another 1-pyrazoline (trans-4,5-
dihydro-3,5-dimethyl-3,5-diphenyl-3H-pyrazol-4-ol, trans-8).
Steady-State Photolysis of trans-8. A 0.05 M solution of trans-1
in deuterated solvent (C6D6, CD3CN, or CD3OD) was placed in a
NMR tube and the tube was capped with a rubber septum. The
solution was purged with argon for 15 min. Then the appropriate
volume of toluene (0.05 M) was added. The solution was photolyzed
for 30 min in a photochemical reactor at 350 nm monitoring by
1
300 MHz H NMR spectroscopy in 2 min intervals (conversion
1
40%). According to H NMR spectra trans-9 was formed with
∼77% yield. The solvent was evaporated under reduced pressure
trans-9: 1H NMR (300 MHz, C6D6) δ 7.43-7.49 (m, 2H), 7.38-
7.42 (m, 2H), 7.32-7.37 (m, 4H), 7.23-7.30 (m, 2H), 3.74 (d, J
We propose that the irradiation of trans-1 leads to zwitterionic
oxyallyl 5 by nitrogen expulsion. The intermediate cyclopro-
panone 2 exists in a dynamic equilibrium with 5, and the latter
closes to cyclohexadiene 6 on a longer time scale. Aromatization
of 6 produces indanones 7. Another decomposition pathway of
trans-1 is retro-1,3-dipolar addition that yields the diazo
compound 3 and ketene 4. Accordingly, our TRIR experiments
conclusively prove, for the first time, that retro-1,3-dipolar
addition is indeed an actual fragmentation pathway of 1-pyra-
zolines (trans-1 and trans-8).
to afford trans-9 as a light-yellow oil (∼0.010 g).
(19) (a) Dehmlow, E. V.; Slopianka, M. Liebigs. Ann. Chem. 1979, 572-
593. (b) Baigrie, L. M.; Seiklay, H. R.; Tidwell, T. T. J. Am. Chem. Soc.
1985, 107, 5391.
(20) Fedorov, A. V.; Danilov, E. O.; Merzlikine, A. G.; Rodgers, M. A.
J.; Neckers, D. C. J. Phys. Chem. A 2003, 107, 3208.
(21) Korth, H. G.; Sustmann, R.; Lommes, P.; Paul, T.; Ernst, A.; de
Groot, H.; Hughes, L.; Ingold, K. U. J. Am. Chem. Soc. 1994, 116, 2767.
J. Org. Chem, Vol. 72, No. 8, 2007 2783

Moiseev et al.
) 3.0 Hz, 1H), 1,36 (d, J ) 3.0 Hz, 1H), 1.27 (s, 3H), 1.03 (s,
3H); 13C NMR (75.5 MHz, C6D6) δ 20.3, 26.7, 33.4, 35.6, 63.2,
126.4, 128.7, 128.8, 129.2, 129.4, 131.0, 141.6, 144.6; MS, m/z (I)
238 (17) (M+), 220 (7), 205 (9), 178 (10), 145 (8), 129 (24), 117
(18), 105 (100), 91 (65), 77 (38), 65 (8), 51 (14); HRMS (EI) m/z
calcd for C17H18O 238.1358, found 238.1359.
Acknowledgment. We gratefully acknowledge Dr. Dmitry
E. Polyansky, Prof. William J. Leigh, and Prof. Thomas H.
Kinstle for fruitful discussions. The authors thank the Ohio
Laboratory for Kinetic Spectrometry for providing equipment
for transient spectroscopy experiments, the Wright Photoscience
Laboratory (ODOD TECH03-054) for providing analytical
equipment, and the National Science Foundation for support of
the MALDI-TOF spectrometer, CHE-0234796. A.M. is grateful
to the McMaster Endowment for a Fellowship.
Formation of 2-phenylpropionaldehyde (fragmentation product,
ca. 10%) was observed in the 1H NMR spectrum of irradiated
solution. The 1H NMR spectrum of 2-phenylpropionaldehyde was
identified by comparison of its 1H NMR spectrum to the previously
reported spectrum.22 Formation of styrene was confirmed by GC/
MS with use of an authentic sample.
Supporting Information Available: Kinetic traces, calculated
1
IR spectra of photochemical products and intermediates, H and
13C NMR spectra of photochemical products. This material is
available free of charge via the Internet at http://pubs.acs.org.
(22) Blankespoor, R. L.; Smart, R. P.; Batts, E. D.; Kiste, A. A.; Lew,
R. E.; Van der Vliet, M. E. J. Org. Chem. 1995, 60, 6852.
JO062259R
2784 J. Org. Chem., Vol. 72, No. 8, 2007