Photolysis dynamics of cyclopropyl and oxiranyl aryl ketones: Drastic ring-activation effect of oxygen

Article abstract of DOI:10.1021/jp004061v

Although 1-(o-tolyl)-1-benzoylcyclopropane (I) and 2-(o-tolyl)-2-benzoyloxirane (II) are structurally analogous, they go through dramatically different reaction pathways upon absorption of photons. I yields a single photoproduct, while ²H₂-substituted II gives birth to three different photoproducts. The structural and kinetic features of the entire reaction intermediates have been measured with laser flash techniques. I at T₁ undergoes intramolecular hydrogen transfer at 2.3 × 10⁸ s^-1 from the methyl group to the carbonyl group to form a biradical intermediate, which transforms into a cyclized photoproduct (φ 0.14; 5 × 10⁷ s^-1). However, the carbonyl group of II at T₁ can abstract a hydrogen atom from the oxiranyl ring (6 × 10⁷ s^-1) as well as from the methyl group (2.3 × 10⁸ s^-1). The biradical intermediate from the former process immediately rearranges into a photoproduct (φ 0.025), while the one from the latter transforms into a new oxiranyl ring-opened intermediate (1.1 × 10⁸ s^-1) or a cyclized photoproduct (φ 0.075; 3.3 × 10⁸ s^-1). The new intermediate rearranges into another photoproduct (φ 0.025; 3.7 × 10⁷ s^-1). The dramatic changes of II are theoretically attributed to the drastic increase in the acidity and instability of the three-membered ring with oxygen substitution.

Full text of DOI:10.1021/jp004061v

J. Phys. Chem. A 2001, 105, 3555-3563
3555
Photolysis Dynamics of Cyclopropyl and Oxiranyl Aryl Ketones: Drastic Ring-Activation
Effect of Oxygen
Hahkjoon Kim, Taeg Gyum Kim, Joeoong Hahn, and Du-Jeon Jang*,†
School of Chemistry and Molecular Engineering, Seoul National UniVersity, Seoul 151-742, Korea
Dong Jo Chang and Bong Ser Park*
Department of Chemistry, Dongguk UniVersity, Seoul 100-715, Korea
ReceiVed: NoVember 6, 2000; In Final Form: January 25, 2001
Although 1-(o-tolyl)-1-benzoylcyclopropane (I) and 2-(o-tolyl)-2-benzoyloxirane (II) are structurally analogous,
they go through dramatically different reaction pathways upon absorption of photons. I yields a single
photoproduct, while 2H2-substituted II gives birth to three different photoproducts. The structural and kinetic
features of the entire reaction intermediates have been measured with laser flash techniques. I at T1 undergoes
intramolecular hydrogen transfer at 2.3 × 108 s-1 from the methyl group to the carbonyl group to form a
biradical intermediate, which transforms into a cyclized photoproduct (Φ 0.14; 5 × 107 s-1). However, the
carbonyl group of II at T1 can abstract a hydrogen atom from the oxiranyl ring (6 × 107 s-1) as well as from
the methyl group (2.3 × 108 s-1). The biradical intermediate from the former process immediately rearranges
into a photoproduct (Φ 0.025), while the one from the latter transforms into a new oxiranyl ring-opened
intermediate (1.1 × 108 s-1) or a cyclized photoproduct (Φ 0.075; 3.3 × 108 s-1). The new intermediate
rearranges into another photoproduct (Φ 0.025; 3.7 × 107 s-1). The dramatic changes of II are theoretically
attributed to the drastic increase in the acidity and instability of the three-membered ring with oxygen
substitution.
Introduction
above examples. Key reactions of the radical intermediates are
the rearrangements of cyclopropylcarbinyl and oxiranylcarbinyl
radicals. Kinetic studies of the rearrangements are vital to
understand and predict their reaction pathways. The rate
constants of the radical rearrangements have been indirectly
determined with competition-kinetic methods in most cases. The
rate constant for the ring-opening of the cyclopropylcarbinyl
radical is about 108 s-1 at ambient temperature,20 and that of
the corresponding oxiranyl radical is at least 2 orders of
magnitude faster than this.21-23 Recently, more direct methods
such as laser flash photolysis have been applied to measure rate
constants for fast radical reactions.24,25 All of these studies utilize
the radical reactions that start from the required precursors of
the corresponding radicals. Molecules should contain proper
chromophores to be probed with laser flash photolysis.
Photochemistry of ketones containing a three-membered ring
such as a cyclopropane or an oxirane adjacent to a carbonyl
group has been extensively studied for over three decades.1-14
Photochemical properties of these ketones are differentiated from
those of isolated ketones without such rings. These are attributed
to the conjugative nature between the rings and the carbonyl
group. Even though many examples have been reported, most
studies have been focused on product analysis with classical
steady-state kinetics, and the detailed analyses on reaction
dynamics have been lacking. Getting the complete pictures of
the reaction mechanism with more direct kinetic measurements
would be certainly helpful in understanding and predicting the
photochemical behaviors of compounds containing such natu-
rally abundant functional groups.
In the course of our studies on the photochemistry of R-(o-
tolyl)acetophenones with a small ring conjugated with the
carbonyl group, we have envisioned that the photochemical
reaction routes of these molecules could provide useful kinetic
information on cyclopropylcarbinyl and oxiranylcarbinyl radical
rearrangements.26 Here we would like to describe mechanistic
dichotomy between 1-(o-tolyl)-1-benzoylcyclopropane (I) and
2-(o-tolyl)-2-benzoyloxirane (II), which enables us to compare
the ring-opening rates of cyclopropylcarbinyl and oxiranyl-
carbinyl radicals. The spectral and kinetic features of the
intermediates occurring during their photolysis reactions in
cyclohexane, as well as the photoproduct analyses, have been
studied with laser flash photolysis methods. The product
formation kinetics of II is compared with that of 2-phenyl-2-
benzoyloxirane (III) as well, to understand the roles of the
methyl group during the reaction. We have also carried out
Among several types of photochemical processes available
to cyclopropyl and oxiranyl ketones, ring-opening has been most
frequently encountered. For example, trans-1,2-dibenzoyl-
cyclopropane and trans-1-benzoyl-2-phenylcyclopropane pho-
toisomerize via cyclopropyl ring rupture to yield cis-1,2-
dibenzoylcyclopropane and cis-1-benzoyl-2-phenylcyclopro-
pane, respectively.15-18 Also, 1-benzoyl-2-methyl-2-phenyloxirane
photorearranges into 1,3-diphenyl-3-buten-2-ol-1-one with ox-
iranyl ring-opening during the reaction pathway.19 The ring-
opening reaction can occur either directly in the excited reactants
or in radical intermediates formed from the hydrogen-abstraction
reaction of the excited carbonyl group, as illustrated with the
* Authors to whom correspondence should be addressed.
† Telephone: +82-2-875-6624. Fax: +82-2-889-1568. E-mail:
djjang@plaza.snu.ac.kr.
10.1021/jp004061v CCC: $20.00 © 2001 American Chemical Society
Published on Web 03/15/2001

3556 J. Phys. Chem. A, Vol. 105, No. 14, 2001
Kim et al.
semiempirical calculations to explain the mechanistic dichotomy
of the two molecular systems.
77(33.3). Z-3-hydroxy-1-phenyl-2-o-tolylpropenone (IIp2): 1H
NMR(CDCl3) δ 16.2(d, 1H, J ) 5.0 Hz), 8.59(d, 1H, J ) 5.0
Hz), 7.39-7.16(m, 9H), 2.03(s, 3H); 13C NMR(CDCl3) δ 185.7,
183.6, 137.9, 135.7, 134.9, 131.7, 131.5, 130.6, 128.7, 128.3,
128.1, 126.6, 114.7, 20.2; IR(KBr) 3460, 3063, 2927, 1727,
1681 cm-1; EI MS 238(M+, 0.7), 210(9.9), 105(100), 77(34.6).
Materials. Cyclohexane, ethanol, 2,5-dimethyl-2,4-hexadiene
(DH), and methyl viologen dichloride (MVCl2), purchased from
the Sigma, were used as received. Sample concentration was 5
× 10-3 M if not specified otherwise, and solvent was cyclo-
hexane in kinetic measurements except that ethanol was used
as solvent with the presence of MVCl2. Samples were contained
in 10-mm cells for static measurements, while they were in
2-mm cells for time-resolved measurements.
Static Measurements. Absorption-spectra changes with
irradiation were monitored by measuring absorption spectra at
scheduled intervals with a UV/vis spectrophotometer (Scinco,
S-2040) while directing 2.5-mW white beam from 300-W Xe
lamp (Schoeffel, LPS 255) to a sample with the spot diameter
of 5 mm. Fluorescence spectra were obtained using a home-
built fluorometer, which consists of a 75-W Xe lamp (Acton
Research, XS 432), 0.15-m and 0.30-m monochromators (Acton
Research, SpectroPro-150 and 300), and a photomultiplier tube
(Acton Research, PD 438).
Experimental Section
Preparation of I. R-Methylidenyl-R-(o-tolyl)acetophenone
was obtained with 85% yield by alpha methylenating R-(o-tolyl)-
acetophenone with the aldol condensation using potassium
carbonate and paraformaldehyde.27 The resulting R,â-unsatur-
ated ketone was treated with a mixture of NaH and trimethyl-
sulfoxonium iodide in DMSO until all of the starting materials
disappeared.28 After regular workup and column chromatogra-
phy using n-hexane and diethyl ether (30:1) as eluents, we
obtained colorless liquid of I with 76% yield. 1H NMR(CDCl3)
δ 7.62-6.97(m, 9H), 2.14(s, 3H), 1.86(m, 2H), 1.33(m, 2H);
13C NMR(CDCl3) δ 202.7, 139.9, 139.8, 138.7, 132.0, 131.0,
129.7, 129.2, 128.2, 127.8, 126.5, 35.7, 20.3, 18.6; IR(CCl4)
3072, 3025, 1665 cm-1; EI MS 236(M+, 2.7), 208(10.4), 131-
(16.5), 105(100), 77(47.7).
Preparation of II. R-Methylidenyl R-(o-tolyl)acetophenone,
described in the preparation of I, was added to a mixture of
30% hydrogen peroxide solution in methanol and 6 M NaOH.
The mixed solution was refluxed for 3 h. After regular workup
and column chromatography using n-hexane and ethyl acetate
(20:1) as eluents, colorless liquid was obtained with 94% yield.
1H NMR(CDCl3) δ 8.10(distorted d, 2H, J ) 7.5 Hz), 7.63-
7.01(m, 7H), 3.43(d, 1H, J ) 5.8 Hz), 3.19(d, 1H, J ) 5.8 Hz),
2.37(s, 3H); 13C NMR(CDCl3) δ 196.6, 138.3, 135.5, 135.0,
133.7, 131.1, 129.9, 129.3, 128.9, 128.3, 126.5, 64.4, 53.3, 20.3;
IR(CCl4) 3067, 2925, 1690, 1678 cm-1; EI MS 238(M+, 4.9),
133(90.1), 105(100), 77(66.3).
Picosecond Kinetic Measurements. An actively/passively
mode-locked Nd:YAG laser (Quantel, YG 701) with the pulse
duration of 25 ps was employed for picosecond fluorescence
and transient-absorption kinetic measurements. Samples were
excited with 266-nm laser pulses or 320-nm pulses generated
from a Raman shifter filled with 20-atm methane gas. Fluores-
cence was collected from the front surface of the sample
excitation for all static and time-resolved fluorescence measure-
ments. Fluorescence kinetic profiles were obtained with a 10-
ps streak camera (Hamamatsu, C2830) attached with a CCD
(Princeton Instruments, RTE-128-H). Transient absorption of
a sample was probed using fluorescence from an organic dye
excited with the pulses split from sample-excitation pulses. The
comparison of dye-emission kinetic profiles measured with
streak camera without and with sample excitation yields
picosecond transient-absorption kinetic profiles.30 Fluorescence
and transient-absorption kinetic constants were extracted by
fitting measured kinetic profiles to computer-simulated kinetic
curves convoluted with the temporal response functions.
Nanosecond Spectral Measurements. Transient-absorption
spectra were obtained by monitoring the transmittance changes
of samples excited made with 266-nm pulses from a 6-ns
Q-switched Nd:YAG laser (Quanta System, HYL-101). Probe
pulses emitted from an organic dye excited with the pulses split
from sample-excitation pulses were detected with a CCD
(Princeton Instruments, ICCD-576-G) attached to a 0.5-m
spectrometer (Acton Research, SpectroPro-500).
2
Preparation of H2-Substituted II (II(d2)). The synthetic
procedures were the same as those of II except that paraform-
aldehyde-d6 was used instead. 1H NMR(CDCl3) δ 8.10(distorted
d, 2H, J ) 7.5 Hz), 7.63-7.01(m, 7H), 2.37(s, 3H); 13C NMR-
(CDCl3) δ 196.5, 138.3, 135.6, 135.0, 133.6, 131.0, 129.9, 129.2,
128.8, 128.3, 126.5, 64.4, 53.3, 20.3; IR(CCl4) 3064, 3025, 2916,
1678 cm-1; EI MS 240(M+, 4.9), 135(90.1), 105(100), 77(66.3).
Preparation of III. The synthetic procedures were the same
as that of II except that R-phenylacetophenone was used instead
1
of R-(o-tolyl)acetophenone. H NMR(CDCl3) δ 8.03(distorted
d, 2H, J ) 7.5 Hz), 7.59-7.25(m, 8H), 3.40(d, 1H, J ) 5.4
Hz); 3.09(d, 1H, J ) 5.4 Hz). 13C NMR(CDCl3) δ 194.8, 135.7,
134.4, 133.9, 130.1, 128.9, 128.7, 128.6, 125.5, 63.3, 55.1; IR-
(CCl4) 3064, 2945, 1688 cm-1; EI MS 224(M+, 5.4), 105(100),
77(68.5).
Photoproducts. Photoproducts generated from the irradiation
of I and II were isolated with either column- or preparative
thin-layer chromatography. The white beam from a medium-
pressure mercury arc lamp (Hanovia, 450 W) was filtered with
an aqueous solution of 2 mM K2CrO4 and 1% K2CO3 contained
in a Pyrex cell of 1-cm path length to select the sample
irradiation light of 313 nm. Quantum efficiencies were measured
using 0.1 M valerophenone in benzene as an actinometer.29
Theoretical Calculation. Deprotonation enthalpies of some
selected groups in the molecules I and II, as well as the ring-
opening enthalpies of their intermediates, were calculated
semiempirically using the Parameterized Model 3 of MOPAC
97 (Fujitsu).
Results
1
Spiro[cyclopropane-1,1′-(2′-phenyl-2′-hydroxy)indane] (Ip): H
Photoproduct of I. The ketone I is shown to yield only one
isolated product, Ip with IR, NMR, and mass spectroscopy.
Observation of the clean conversion is somewhat surprising,
since R-(o-tolyl)isobutyrophenone, which structurally resembles
I, is known to give only R-cleavage products.31 We postulate
that the conjugation between carbonyl and cyclopropyl strength-
NMR(CDCl3) δ 7.65-6.75(m, 9H), 3.83, 3.43(AB quartet, 2H,
J ) 16.8 Hz), 2.41(s, 1H, -OH), 1.20-0.59(m, 4H); 13C NMR-
(CDCl3) δ 146.5, 144.1, 140.6, 128.4, 127.8, 127.4, 126.9, 126.6,
125.2, 119.6, 84.2, 50.2, 39.8, 18.9, 11.0; IR(KBr) 3553, 1485
cm-1; EI MS 236(M+, 29.3), 218(14.7), 208(42.0), 105(100),

Photolysis of Aryl Ketones with a Small Ring
J. Phys. Chem. A, Vol. 105, No. 14, 2001 3557
ens the connecting bond and retards the R-cleavage process.1
Formation of Ip from I, with the quantum yield (Φ) of 0.14,
can be explained with hydrogen abstraction of the excited
carbonyl group from the o-tolyl group followed by coupling of
two radical sites of the biradical intermediate (eq 1), as proposed
in the photochemistry of R-(o-tolyl)acetophenone.32
hydrogen abstraction, ring-opening, and disproportionation such
as eq 3.
Photoproducts of II(d2). To see if this mechanism is really
working, II(d2) was prepared and irradiated under the same
1
conditions. The H NMR spectrum of the major photoproduct
isolated with column chromatography was essentially the same
as that of IIp2 except that the peak at 16.02 ppm became singlet
and that the peak at 8.59 ppm was absent. A careful examination
of the spectrum has revealed that a singlet at 2.02 ppm overlaps
with a broad triplet with an equal intensity. The 13C NMR
spectrum of the sample also showed a similar pattern at 20 ppm.
1
The triplet patterns of both H- and 13C NMR spectra clearly
Photoproducts of II. Judging from TLC analysis, preparative
scale irradiation of II under the same condition results in a
complex mixture of photoproducts. However, after column
chromatography using silica gel, only one product has been
isolated purely enough to identify the structure. Quantitative
analysis has shown that more than 70% of whole products are
this product. Spectroscopic data have led us to assign it as IIp2.
To examine the reaction more closely, we took the 1H NMR of
0.02 M II in benzene-d6, contained in an NMR tube under
irradiation, at scheduled intervals. In addition to IIp2, formation
of another major product was apparent from the 1H NMR spectra
(eq 2). It has been assigned to a spiro epoxyindanol (IIp1) from
the fact that it shows an AB quartet at 3.3-3.5 ppm and two
doublets at 2.66 and 2.57 ppm. While only one set of AB quartet
was observed in the 1H NMR spectrum of the reaction mixture,
the stereochemistry of the product could not be determined due
to its instability. The AB quartet is typical of 2-phenyl-2-
indanols, as seen in many photolysis examples of R-(o-
alkylphenyl)acetophenones.31,32 The assignment is reasonable
not only because the NMR spectrum fits into the structure but
also because I, that is almost identical to II geometrically, shows
analogous reactivity. The yield ratio of IIp1/IIp2 was 3/2, and
the quantum yield of IIp2 was 0.05. If the sample with a small
amount of the starting ketone remaining was kept in dark for
several days, the peaks corresponding to IIp1 diminished
considerably, and the growth of peaks for IIp2 was noticeable
in reference to the peaks of starting ketones. If a crystal of
p-toluenesulfonic acid was added to the sample, disappearance
of IIp1 was accelerated considerably. It may be the reason IIp1
escaped our efforts to isolate using column chromatography in
preparative scale photolysis. The rearrangement from IIp1 to
IIp2 is not common in organic chemistry, and we have no
definite clue to the mechanism of the transformation at this
moment.
2
suggest that a H atom is incorporated at the benzylic methyl
group of IIp2, and the overlap of the singlet and the triplet tells
us that the sample contains two types of IIp2(d2) as shown
below. The yield ratio of IIp2(d2A)/IIp2(d2B), which was
obtained integrating the 1H NMR peaks at 2.02 and 16.02 ppm,
was 1.
Formation of IIp2(d2A) from II(d2) could be explained by
the mechanism of eq 3. However, the reaction pathway to
IIp2(d2B) was not obvious at this stage. One possible route to
IIp2(d2B) would be hydrogen abstraction from the oxiranyl ring
followed by ring-opening (eq 4). Even though this type of
reaction sequence has been ignored in the photochemistry of
R,â-oxiranyl ketones, it is still probable especially in the case
of phenyl ketones with acidic hydrogen atoms nearby. Our
theoretical results also support this idea (vide infra). To obtain
experimental evidences for this mechanism, III was prepared
and irradiated under the same reaction condition. As expected,
a â-hydroxy enone (IIIp) was isolated as a major product.
Involvement of singlet-excited states also could be ruled out
by the fact that the reaction was quenched by triplet quenchers
such as DH.
Picosecond Kinetics. The cyclohexane solutions of I and II
show fluorescence decaying in 70 and 25 ps, respectively (Figure
1). Because of these short lifetimes the fluorescence quantum
efficiencies are too low to observe the static fluorescence of
the reactants buried under long-lived luminescence from impuri-
ties such as photoproducts. The photolyzed cyclohexane solu-
tions of I and II expose product fluorescence spectra with the
peaks at 390 and 370 nm, respectively. The short fluorescence
lifetimes of both reactants suggest that, following unresolvably
fast internal conversion from the S2 states, the population of
the S1(n,π*) states undergoes a rapid intersystem crossing to
the T2(π,π*) states that happen to lie near the S1 states in energy.
The El-Sayed rule,34 that the intersystem crossing transition of
S1(n,π*)-to-T(π,π*) is allowed, also supports that the fast
lifetimes of the S1 states are resulting from rapid intersystem
crossing. The shorter fluorescence lifetime (25 ps) of II,
compared with that (70 ps) of I, implies that II undergoes
intersystem crossing more rapidly, probably owing to its more
nonbonding character at the S1 state introduced with the
additional oxygen atom in the small ring.
There could be several reaction pathways possible for the
formation of IIp2 from II. Singlet excited states are reported to
be responsible for analogous rearrangements of alkyl ketones
with substituted oxirane conjugation.33 However, intersystem
crossing from excited singlet states to triplet states is extremely
efficient for phenyl ketones; therefore, the reaction pathway from
the first excited singlet state would be negligible. At any rate,
no experimental evidences have been observed on the singlet-
state reaction (vide infra). More likely mechanisms would be

3558 J. Phys. Chem. A, Vol. 105, No. 14, 2001
Kim et al.
Figure 3. Transient-absorption kinetic profiles, excited at 266 nm and
monitored at 350 nm, of I in cyclohexane without (solid) and with
MVCl2 (dotted). The solid profile has a decay time of 20 ns, while the
dotted profile decays in 2.5 ns.
Figure 1. Fluorescence kinetic profiles at 450 nm of I (solid) and II
(dotted) in cyclohexane. I and II were excited at 320 and 266 nm,
respectively. The convoluted decay time constants of I and II are 70
and 25 ps, respectively.
Figure 4. Transient-absorption kinetic profiles of II in cyclohexane
with 266-nm excitation. The kinetics at 320 (solid) and 350 nm (dotted)
have decay times of 27 and 2.3 ns, respectively.
Figure 2. Transient-absorption kinetic profile, excited at 266 nm and
collected at 400 nm, of I in cyclohexane. A biexponential decay of 0.6
ns (92%) and 20 ns (8%) fits to the profile.
absorbance imply that the intermediate Ii is a radical, more
specifically a biradical in nature. We will show the effect of
MVCl2 on the spectrum of Ii in the later part of this paper. The
decay time of 20 ns is ascribed to the cyclization time of
intermediate Ii to transform into the photoproduct Ip.
The faster decay component (0.6 ns) of transient absorption
at 400 nm, shown in Figure 2, is assigned to the T1-state decay
of I, while the slower one with small amplitude is assigned to
the decay of the succeeding intermediate Ii as shown in Figure
3. The decay kinetics of the T1 state could not be measured for
II. The transient absorption of quencher-free I at 350 nm decays
with the time constant of 20 ns, and this is attributed to the
transient Ii (Figure 3). To understand the nature of the
intermediate Ii, we have also measured the effects of quenchers
on the transient-absorption kinetic profile. The drastic quenching
of the transient-absorption signal with 1.0 M DH suggests that
the precursor of Ii is the lowest triplet state of I. The reaction
at the T1 state is intramolecular hydrogen transfer from the
methyl group to the carbonyl group. The time of 0.6 ns is quite
short for hydrogen abstraction to occur at a triplet state. The
fast abstraction becomes possible with the increased acidity of
the methyl group in the tolyl group and the increased basicity
of the carbonyl group at the first excited triplet state.35-38 The
decay time of 2.5 ns observed in the presence of MVCl2, a type
of saturated radical quencher, is much shorter than that without
MVCl2. However, the presence of MVCl2 does not reduce the
absorption signal at zero delay time at all. The significant
reduction in the decay time and the same signal in the initial
Figure 4 shows that the transient absorption at 350 nm of II
decays much faster (with the time of 2.3 ns) than that of I given
in Figure 3. Furthermore, the transient absorption at 320 nm
with the decay time of 27 ns suggests that there is an additional
photolyzed transient (IIi2) besides the IIi1. The decay time of
2.3 ns at 350 nm is assigned to the transformation time of IIi1
into IIi2 and IIp1, while the decay time of 27 ns at 320 nm is
ascribed to the transformation time of IIi2 into IIp2. The transient
absorption at 320 nm rises more slowly than that at 350 nm.
The slow formation of IIi2 transient absorption is contributing
to the relatively slow rising kinetics at 320 nm. The quenching
effects of DH and MVCl2 on the transient absorption of II are
similar to those on the transient absorption of I.
Nanosecond Transient Absorption Spectra. For better
understanding of photochemical reaction intermediates, we have
also measured time-resolved transient-absorption spectra besides
kinetic profiles. The transient-absorption spectra in Figure 5
expose that the intermediate Ii contains the 350-nm absorption
band of hydroxybenzyl radical chromophore as well as the 320-
nm band of benzyl radical chromophore.39-41 This again
supports that the intermediate Ii is a biradical.

Photolysis of Aryl Ketones with a Small Ring
J. Phys. Chem. A, Vol. 105, No. 14, 2001 3559
Figure 5. Transient-absorption spectra of I in cyclohexane at the shown
delay times after 266-nm excitation.
Figure 7. Transient-absorption spectra, at 10-ns delay after 266-nm
excitation, of I (a) and II (b) in cyclohexane (solid) and in the 19:1
(v:v) mixture of cyclohexane and MVCl2-saturated ethanol (dotted).
Figure 6. Transient-absorption spectra of II in cyclohexane at 2- (a)
and 20-ns (b) delay times after 266-nm excitation and a difference
spectrum (c) simulated from 5 (a - 1.45b).
However, the transient-absorption spectra of a and b in Figure
6 are much stronger and shifted to the blue with the peak at
330 nm, compared with the 350-nm absorption band in Figure
5. We ascribe the transient absorption in the spectra a and b
mainly to the biradical of IIi2. The 330-nm absorption band of
IIi2 is mainly attributable to the olefin conjugated with phenyl
and benzyl radical chromophores because the maximum-
absorption wavelength and the large extinction coefficient
resemble those of stilbenes. The absorption band of the benzyl
chromophore of IIi1 is buried under the strong absorption band
with the peak at 330 nm, as the transient absorption resulting
from IIi1 decays too fast to measure directly with our spec-
trometer of 6-ns temporal resolution. However, we can simulate
its spectrum as c in Figure 6, on the basis of the assumption
that the transient-absorption spectrum at 2 ns contains more
contribution of the fast decaying IIi1 than that at 20 ns.
Nevertheless, the spectrum c displays the buried absorption band
of hydroxybenzyl radical chromophore. This implies that II at
T1 state can undergo intramolecular hydrogen transfer to yield
the biradical IIi1. IIi1 then undergoes ring-opening to form the
biradical of IIi2 as well as cyclization to produce the product
IIp1.
Figure 8. Transient-absorption spectra of MVCl2 (1 × 10-3 M) ethanol
solutions with I (a) and II (b) at two given delay times after 266-nm
excitation.
amplitudes of the signals are reduced with the concentration
increase of DH as discussed already. These facts also support
that all of the reaction intermediates of Ii, IIi1, and IIi2 have a
T1 state as a precursor at a time and that they are biradicals
indeed.
Static Absorption Changes with Irradiation. The absorp-
tion spectrum of II in cyclohexane changes significantly with
irradiation, while that of I in cyclohexane does not (Figure 9).
A significant increase in the extinction coefficient of the lowest
absorption band with irradiation suggests that the transition of
S1rS0 in a product has more conjugation with more (π,π*)
character than that in the reactant II. A negligible change with
irradiation in the absorption spectrum of sample I agrees with
the fact that the absorption chromophores of the product Ip in
the low energy region are very similar to those of the reactant
I. The observations in Figure 9 agree with the photoproduct
Transient-absorption bands arising from Ii and IIi2 are
quenched by MVCl2 (Figure 7) to give birth to a new absorption
band of reduced methyl viologen cation with the peak at 620
nm (Figure 8). The transient-absorption decay times of Ii and
IIi2 are not reduced by the presence of DH. However, the

3560 J. Phys. Chem. A, Vol. 105, No. 14, 2001
Kim et al.
TABLE 1: Calculated Deprotonation Enthalpies of Some
Selected Groups in I and II
molecule group distancea ∆Hb(S0) ∆H(T1) ∆H(T1) - ∆H(S0)
I
C
C
C
(1)H3
(2)H2
(3)H
2.9
2.7
2.8
351
359
375
186
347
355
374
184
348
358
358
212
313
329
360
243
-3
-1
-17
26
O
(1)H+
II
C
C
C
(1)H3
(2)H2
(3)H
2.7
3.0
2.5
-34
-26
-14
59
O(1)H+
a From the closest hydrogen atom of each group to O(1) in Å.
b Deprotonation enthalpy in kcal/mol.
transition between S1(n,π*) and T2(π,π*) is allowed by the El-
Sayed rule,34 the quantum efficiency of the T1 state population
is assumed to be about 1. Then the quantum efficiency (0.14)
of the only product Ip should be the fraction of Ii formation
rate constant to the observed decay rate constant, (0.6 ns)-1, of
T1 state. The resulting rate constants of hydrogen abstraction
and intersystem crossing to the S0 state are 2.3 × 108 and 1.4
× 109 s-1, respectively. The intersystem crossing is very fast
as expected again with the El-Sayed rule30 that the transition
between T1(n,π*) and S0(n2) is allowed. The other rate constants
can be automatically calculated as the inverse of the observed
time constants.
Figure 9. Absorption spectra of cyclohexane solutions of I (a) and II
(b) without (solid) and with irradiation (dashed).
It has been shown that the quantum yield of IIp2 is 0.05, while
the product yield ratios of IIp1/IIp2 and IIp2(d2B)/IIp2(d2A) are
1.5 and 1, respectively. Since we could measure neither the
decay time of the T1 state nor any formation times of IIi1 and
IIi3, we cannot calculate hydrogen-transfer rate constants without
assuming that the formation rate constants of Ii and IIi1 are the
same. The assumption is reasonable as the substitution of an
oxygen atom in the three-membered ring, which is distant from
the methyl group, would not affect the acidity of the methyl
group significantly. With the assumption, the formation rate
constant of IIi3 is estimated to be 6.7 × 107 s-1, as the quantum
yield of IIp2(d2B) produced from IIi3 is one-fourth of the total
quantum yield of IIp1 and IIp2(d2A) produced from IIi1. The
relative quantum yields of IIp1 and IIp2(d2A) with the decay
time of IIi1 allow us to calculate the rate constants of the ring-
opening and cyclization processes of IIi1 as well. The other rate
constants can be trivially calculated.
Figure 10. Absorption changes with time at 320 nm for the 2.5 ×
10-3 M cyclohexane solutions of II (b), III (×), and III with 1.0 M
DH (9) under the same irradiation conditions. The profile of II
converges to 1.7 with a time constant of 75 min, while that of III (×)
to 0.7 with a time constant of 200 min.
Theoretical Consideration. Thus far we have experimentally
shown that the photolysis reactions of II are dramatically
different from those of I. The only structural change in II is
the substitution of an oxygen atom for the methylene group of
the three-membered ring in I. A simple semiempirical calcula-
tion reveals that the drastic changes in the reactions are a result
of the significantly increased acidity and instability of the small
ring with the oxygen substitution. Table 1 shows that a proton
of C(2)H2 becomes very acidic at the T1 state of II, while that
at the T1 state of I is not. Because of this acidity increment, the
new reaction pathway of IIp2(d2B) product formation can occur
within the short T1 lifetime of II. It should be mentioned that
deprotonation enthalpies instead of dehydrogenation enthalpies
were calculated to avoid spin contamination in our low-level
calculation. We can also note from Table 1 that both the acidity
of the methyl group and the basicity of the carbonyl group
increase very significantly in the excited triplet state, explaining
that all of the hydrogen-transfer reactions at the T1 states of I
and II are plausible. Table 2 reveals the reason that II undergoes
the ring-opening while I does not. The ring rupture is energeti-
cally very favorable for IIi1, but it is not at all for Ii. We have
also noticed that a C-H bond length of the oxiranyl ring of
IIi1 is to extend as long as 1.9 Å after the ring-opening,
analyses16 that II forms more conjugated photoproduct IIp2,
while I produces the cyclized photoproduct Ip.
As shown in product studies, II(d2) produces three different
photoproduct isomers. As the methylene group of the oxiranyl
ring was speculated to be acidic for the carbonyl group to
abstract a hydrogen atom from the oxiranyl ring, the absorption
changes of the II and III samples with irradiation time were
compared at 320 nm. Although the amplitude and rate of the
absorption rise are smaller for III than for II, Figure 10 clarifies
that III produces the photoproduct corresponding to IIp2 without
having the methyl group. This confirms the earlier suggestion
that intramolecular hydrogen transfer from the oxiranyl ring to
the carbonyl group can also occur to produce an intermediate
(IIi3) at the T1 state of II, together with the transfer from the
methyl group to the carbonyl group. IIi3(d2) is suggested to
rearrange into the product IIp2(d2B) immediately.
Discussion
Rate Constants. Observed time constants were used with
quantum efficiencies and product yield ratios to calculate the
rate constants of individual elementary reactions. As the

Photolysis of Aryl Ketones with a Small Ring
J. Phys. Chem. A, Vol. 105, No. 14, 2001 3561
TABLE 2: Calculated Enthalpy Changes with
Three-Membered Ring Ruptures
a result of the small hydrogen-abstraction rate compared with
the intersystem crossing rate of the T1 state. On the other hand,
the carbonyl group of II at T1 can abstract a hydrogen atom
from the oxiranyl ring to form another biradical intermediate
IIi3 at a rate constant of 6 × 107 s-1, or from the methyl group
to form IIi1 at 2.3 × 108 s-1. The additional reaction channel
of IIi3 formation is opened as a result of the acidity increase of
the oxiranyl ring with oxygen substitution. While IIi3(d2)
produces immediately a rearranged product of IIp2(d2B) with a
quantum efficiency of 0.025, IIil transforms into a cyclized
product of IIp1 with a quantum efficiency of 0.075 at a rate
constant of 3.3 × 108 s-1. (The product IIp1 undergoes
rearrangement to form IIp2 in a very slow time scale.) However,
the opening process of the oxiranyl ring also competes with
the cyclization process at a rate constant of 1.1 × 108 s-1 to
form a new biradical intermediate of IIi2. The ring-opening
becomes fast enough to compete with the cyclization, as the
three-membered ring of the biradical intermediate becomes
extremely unstable with oxygen presence. IIi2(d2) then under-
goes intramolecular hydrogen transfer at a rate constant of 3.7
× 107 s-1 to form another product of IIp2(d2A) with a quantum
yield of 0.025. The acidity increase of a C(2)-H bond in the
oxiranyl ring with the ring-opening expedites the formation rate
of IIp2 from IIi2. The photolysis reactions of II are extremely
different from those of I. They are much more complicated and
faster overall than reactions of I. All of these changes are driven
by the drastic ring-activation effect of oxygen substitution in
the three-membered ring.
molecule
closed intermediate
opened intermediate
∆Ha
b
I
Ii
Ii′
3
-30
c
II
IIi1
IIi2
a Enthalpy change in kcal/mol. b The biradical intermediate of I
corresponding to IIi2 of II. c A C(2)-H bond of IIi1 is to extend as long
as 1.9 Å after the ring-opening, indicating that the bond is very acidic.
Figure 11. Schematic photolysis mechanism of 1-(o-tolyl)-1-benzoyl-
cyclopropane in cyclohexane.
indicating that the bond becomes very acidic. Although the
C(2)-H bond is acidic from the reactant form of II, its acidity
increases further with the ring rupture. The acidity increase
expedites the hydrogen-transfer reaction of IIi2 obviously.
Photolysis Mechanism. Photoexcitation of I and II in
cyclohexane leads to the photolysis reactions through the
pathways with the rate constants of the individual elementary
reactions shown in Figures 11 and 12, respectively. Following
rapid internal conversion from the UV-excited S2(π,π*) states,
the S1(n,π*) states of I and II molecules undergo intersystem
crossing in 70 and 25 ps, respectively, to populate the T1 states.
I at T1 goes through intramolecular hydrogen transfer from the
methyl group to the carbonyl group to produce a biradical
intermediate with a rate constant of 2.3 × 108 s-1. The
intermediate transforms into a cyclized photoproduct Ip at 5 ×
107 s-1. The low quantum efficiency of Ip formation (0.14) is
Physical Meanings. Bowry et al. have shown that the ring-
opening process of R-phenyl cyclopropylcarbinyl radicals set
an equilibrium favoring the cyclic forms.42 More recently, the
cyclization rate constant at 20 °C is reported to be 5.4 × 106
s-1 25
These results fit nicely into our observation that in the
.
photochemical reaction of I the biradical intermediate Ii cyclizes
to Ip at a rate of 5 × 107 s-1 without opening the cyclopropyl
ring. The oxiranylcarbinyl radical ring-opening were known to
be much faster than the cyclopropylcarbinyl radical rearrange-
ment.21-23 Our kinetic study of II also shows that IIi1, a biradical
intermediate from II, opens up the oxiranyl ring at a rate of 1.1
× 108 s-1, which is now able to compete with the biradical
closure. This number is smaller by 2 orders of magnitude than
3.2 × 1010 s-1 of an alkyl-substituted oxiranylcarbinyl radical
reported by Rawal.22 Both phenyl and hydroxy substituents of
2
Figure 12. Schematic photolysis reaction mechanism of H2-substituted 2-(o-tolyl)-2-benzoyloxirane in cyclohexane.

3562 J. Phys. Chem. A, Vol. 105, No. 14, 2001
Kim et al.
IIi1 are considered to be responsible for the reduced rate.43-46
The reaction dichotomy between I and II is interesting in
contrast to the photochemistry of 1-benzoyl-2,2-dimethylcyclo-
propane1 and 2-benzoyl-3,3-dimethyloxirane.19 The latter ke-
tones are known to give similar photochemical rearrangements
(eqs 5 and 6), where both cyclopropyl and oxiranyl rings open
up following hydrogen abstraction from the γ-position. In these
cases, the ring closure of the biradical intermediates could not
compete with the ring-opening due to energetically unfavorable
bicyclic products.
into a new oxiranyl ring-opened intermediate IIi2 (1.1 × 108
s-1) or a cyclized photoproduct IIp1 (Φ 0.075; 3.3 × 108 s-1).
(IIp1 rearranges into IIp2 at a slow rate.) IIi2 transforms into
another photoproduct IIp2(A) (Φ 0.025; 3.7 × 107 s-1). The
dramatic changes of II are the result of the increase in the acidity
and instability of the three-membered ring with oxygen substitu-
tion.
Acknowledgment. We thank the Center for Molecular
Catalysis and the Korea Research Foundation (KRF-2000-015-
DP0913) for financial support. B.S.P. acknowledges the support
from the Korea Science and Engineering Foundation (96-0501-
09-01-3). H.K. thanks the Brain Korea 21 Program for a
fellowship. We also acknowledge the Equipment Joint Use
Program of the Korea Basic Science Institute.
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There are three different routes leading to IIp2 from II, where
IIp2(d2A) becomes identical to IIp2(d2B) with nondeuterated II
(Figure 12). All of the three routes are unique features of II,
which have not been discussed until now in the photoinduced
rearrangement of the conjugated oxiranyl ketones to 1,3-
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study shows that the same skeletal rearrangement can occur via
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Regarding the differences in the behaviors of Ii and IIi1, it is
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than that of Ii by a factor of 7 despite the similarity of two
structures. Caldwell et al. have shown from their study on the
photochemistry of valerophenone and R-ethoxyacetophenone
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intermediates.48 It would be interesting to find out how important
the epoxy-oxygen bridge in IIi1 is in the determination of the
1,5-biradical lifetime. Obviously, more studies have to be done
to evaluate the oxygen effect in this regard. We are currently
investigating into this matter with other substrates.
Conclusions
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