Photochemistry of 1,2-dihydronaphthaiene oxide: Concurrent triplet and singlet processes via singlet excitation

Article abstract of DOI:10.1021/jo0614184

The photochemistry of 1,2-dihydronaphthalene oxide (254 nm) was reexamined and indan was found to be a primary photoproduct, as well as the traditionally assumed secondary photoproduct. Quenching studies demonstrated that indan, as a primary photoproduct, is derived from a triplet pathway, competing with a singlet route, back to the ground state surface. CASSCF calculations strongly suggest that the triplet pathway consists of a dissociation of the oxirane moiety to give a triplet carbene and aldehyde, which via hydrogen abstraction-decarbonylation-ISC recloses to give indan. Conical intersections corresponding to the presumed 1,2-hydrogen shift and 1,2-alkyl shift to give 2-tetralone and 1-indancarbaldehyde, respectively, were located computationally.

Full text of DOI:10.1021/jo0614184

Photochemistry of 1,2-Dihydronaphthalene Oxide: Concurrent
Triplet and Singlet Processes via Singlet Excitation
Rick C. White,* Benny E. Arney Jr.,* and Katherine M. White
Department of Chemistry, Box 2117, Sam Houston State UniVersity, HuntsVille, Texas 77341
chm_bea@shsu.edu; chm_rcw@shsu.edu
ReceiVed July 7, 2006
The photochemistry of 1,2-dihydronaphthalene oxide (254 nm) was reexamined and indan was found to
be a primary photoproduct, as well as the traditionally assumed secondary photoproduct. Quenching
studies demonstrated that indan, as a primary photoproduct, is derived from a triplet pathway, competing
with a singlet route, back to the ground state surface. CASSCF calculations strongly suggest that the
triplet pathway consists of a dissociation of the oxirane moiety to give a triplet carbene and aldehyde,
which via hydrogen abstraction-decarbonylation-ISC recloses to give indan. Conical intersections
corresponding to the presumed 1,2-hydrogen shift and 1,2-alkyl shift to give 2-tetralone and 1-indan-
carbaldehyde, respectively, were located computationally.
Introduction
SCHEME 1. Accepted Photochemical Pathways for
Aryloxiranes
Early work on the photochemistry of aryloxiranes led to the
adoption of two distinct major photochemical mechanisms for
these systems. In the first, vicinal diaryloxiranes underwent
photochemical scission of the benzylic carbon-carbon bond of
1
2
the oxirane unit to produce carbenes or carbonyl ylids which
1a
were characterized by trapping experiments with alcohols and
with double bonds.1 In the second, monoaryloxiranes under-
b,3
4
went carbon-oxygen bond cleavage to generate 1,3-diradicals
Scheme 1). In general, is has been accepted that each
(
SCHEME 2. Hitherto Accepted Product Formation
Sequence for the Photolysis of 1,2-Dihydronaphthalene
Oxide
aryloxirane only follows one path or the other depending on
structure.
It has been reported that photolysis of 1,2-dihydronaphthalene
5
oxide (1) yielded â-tetralone (2) and indan-1-carbaldehyde (3)
*
To whom correspondence should be addressed. Tel: 1-936-294-1531.
Fax: 1-936-294-4996. E-mail: chm_bea@shsu.edu.
1) (a) Griffin, G. W.; Smith, R. L.; Manmade, A. J. Org. Chem. 1976,
(
as the primary photoproducts (Scheme 2). The assumed pathway
for this reaction has been the initial homolysis of the benzylic
oxirane C-O bond giving the corresponding 1,3-diradical which
subsequently underwent 1,2-hydrogen migration or 1,2-alkyl
migration to form 2 or 3, respectively. Indan (4) was also formed
early in the photolysis and was presumed to arise as a secondary
photoproduct from the photoinduced decarbonylation of 3, but
its reported rate of formation far exceeded what would be
4
1, 338. (b) Smith, R. L.; Manmade, A.; Griffin, G. W. J. Heterocycl. Chem.
1969, 6, 443. (c) Trozzolo, A. M.; Yager, W. A.; Griffin, G. W.; Kristinnson,
H.; Sarkar, I. J. Am. Chem. Soc. 1967, 89, 3357.
2) (a) Becker, R. S.; Bost, R.; Bertioniere, N. R.; Smoth, R. L.; Griffin,
(
G. W. J. Am. Chem. Soc. 1970, 92, 1302. (b) Do-Minh, T.; Trozzolo, A.
M.; Griffin, G. W. J. Am. Chem. Soc. 1970, 92, 1402. (c) Griffin, G. W.;
Ishikawa, K.; Lev, I. J. J. Am. Chem. Soc. 1976, 98, 5697.
(
3) (a) Brokatzky-Geiger, J.; Eberbach, W. Tetrahedron Lett. 1984, 25,
1
1
137. (b) Kristinnson, H.; Griffin, G. W. Angew. Chem., Int. Ed. Engl.
965, 4, 868.
(
4) Arney, B. E., Jr.; White, R. C.; Ramanathan, A.; Barham, L.; Sherrod,
S.; McCall, P.; Livanec, P.; Mangus, K.; White, K. Photochem. Photobiol.
Sci. 2004, 3, 851 and references contained therein.
(5) Padwa, A.; Griffin, G. W. In Organic Photochemstry; Marcel Dekker
Inc.: New York, 1976; Vol. 3, pp 41-121.
1
0.1021/jo0614184 CCC: $33.50 © 2006 American Chemical Society
Published on Web 09/23/2006
J. Org. Chem. 2006, 71, 8173-8177
8173

White et al.
FIGURE 2. Yield of 2 and 4 versus solvent dielectric constant.
unaffected. Early primary formation of 4 must arise from a triplet
3
FIGURE 1. Indan formation (mg) versus time (min) (0.06 M in CH -
CN, 254 nm).
pathway encountered after initial singlet excitation. Such
9
pathways have been proposed for other unrelated systems. A
consistent with this model.6 Curiously, cis-1,2-dihydroxy-
triplet-singlet surface crossing (ISC) near a low barrier to a
(S1/S0 or S2/S1) conical intersection could provide a highly
efficient conduit to populating the triplet surface near a
molecular geometry inaccessible to an initially triplet sensitized
ground state structure. Quenching of the triplet state could return
the molecule to the ground state surface prior to significant
changes in the molecular geometry along the pathway towards
triplet photoproducts. In this case, we suppose that this would
imply that the geometry would still resemble either the starting
epoxide or the presumed 1,3-diradical that would return to 1 or
continue along the singlet pathway, respectively.
In general, ground state radical reactions are not sensitive to
solvent polarity; however, we have found that as the solvent
polarity increased the ratio of indan to 2-tetralone also increased,
as illustrated in the plot in Figure 2. Two implications were
suggested by the plot. Enhanced indan formation, with increased
polarity, indicated an increase in molecular or localized polarity
in the early stages of indan formation. 2-Tetralone (2) formation
also exhibited enhancement with solvent polarity but showed a
limitation as its formation plateaus at higher dielectric constants.
Extremely noteworthy in this regard is the virtually complete
suppression of the formation of 2 when photolysis is carried
out in protic solvents (MeOH, EtOH, or t-BuOH). This
suppression is accompanied by an increase in the formation of
aldehyde 3. That 2 and 3 are probably derived from the same
initially formed species (an excited state 1,3-diradical) is
supported by this behavior, which suggests that the solvent can
play a pivotal role in the evolution of the excited species on
the excited state surface. In this instance, we suspect that
extensive solvation involving the oxirane oxygen in the excited
singlet ring-opened species effectively increases the potential
barrier to the formation of 2 on the excited state surface or
alternatively may decrease the barrier to form 3.
7
1,2,3,4-tetrahydronaphthalene carbonate yielded only 1, 2, and
3
under photolysis with no reported observation of the early
formation of 4 despite presumably proceeding via the same 1,3-
diradical.
Previously, we reported on the anomalously high yield of
6
indan (4) early in the irradiation of 1 (245 nm) requiring us to
reexamine the presumption of 4 being solely a secondary
photoproduct. Measurement of the quantum yield for indan
formation from 1 (Φ ) 0.51) showed it to be several times
6
greater than its quantum yield from 2 (Φ ) 0.10). The
possibility of a photoinduced bimolecular radical process acting
on 1 was examined as a possible cause but was not supported
6
by experiment. Sensitization of the photolysis of 3 to 4 by the
more abundant starting epoxide, 1, was also examined and found
to be probable but could not account for the production of 4 in
excess6 of the production of 3. This behavior reasonably
precluded 3 from being the dominant/sole photochemical
precursor to 4 and strongly suggested the existence of an
additional photochemical pathway to 4 from 1. Notably this type
of behavior has also been reported in the photolysis of styrene
oxide to give bibenzyl (as a secondary photoproduct), where
bibenzyl formation was observed to be much faster from styrene
oxide than from its presumed immediate precursor phenylac-
8
etaldehyde. These inconsistencies have prompted us to inves-
tigate the formation of 4 in the photolysis of 1. Our experimental
and computational findings are reported herein and compared
with the accepted model for the photochemistry of 1.
Results and Discussion
Photolysis Studies: Examination of the formation of 4 (mg)
versus time, using short time intervals of 0-60 min, displayed
2
linear growth (R ) 0.9971), showing it to be a primary
Solutions consisted of 30 mg of 1 in solvents consisting of
mixtures of hexane (ꢀ ) 2.2) and acetonitrile (ꢀ ) 39.2) in ratios
photoproduct in the photolysis of 1. No upward curvature,
expected of a biphotonic product, was noted until after a
significant time interval such that the concentration of 3 was
significant (Figure 1).
(mL:mL) of 1:4, 2:3, 3:2, and 4:1, respectively. Dielectric
constant of mixtures was calculated as the weighted average of
the two solvents.
Although 1 failed to react under triplet sensitized photolysis6
Finally, a subtle observation in the photolysis of 3 is the
(no observable 2, 3, or 4), early formation of 4 was completely
10
formation of indene, 5, in addition to 4. In every photolysis
quenched when direct irradiation was carried out in the presence
of isoprene, while the rates of formation of 2 and 3 were
of 3, we noted that 5 was always produced in a small fraction
(
∼5-10%) of the indan formation. We speculate that the
deformylation of 3 proceeded via three paths (Scheme 3):
(
6) White, R. C.; Wedyck, E. J.; Lynch, K. O. J. Photochem. Photobiol.
A: Chem. 1993, 71, 127.
7) White, R. C.; Drew, P.; Moorman, R. J. Heterocycl. Chem. 1988,
5, 1781.
(
(9) Merch a´ n, M.; Serrano-Andr e´ s, L.; Robb, M. A.; Blancafort, L. J.
Am. Chem. Soc. 2005, 127, 1820.
2
(8) Kristinnson, H.; Griffin, G. W. J. Am. Chem. Soc. 1966, 88, 1579.
(10) White, R. C.; Arney, B. E. Unpublished results.
8174 J. Org. Chem., Vol. 71, No. 21, 2006

Photochemistry of 1,2-Dihydronaphthalene Oxide
SCHEME 3. Photoinduced Decarbonylation of
1-Indancarbaldehyde
FIGURE 3. Active spaces utilized for CASSCF calculations.
hydrogen transfer from formyl to indanyl giving 4, R-hydrogen
abstraction from indanyl by formyl giving 5 and formaldehyde,
or biindanyl (6) formation11 analogous to bibenzyl formation
8
in the photolysis of phenylacetaldehyde. However, 5 is never
observed in the photolysis of 1 until very long reaction times
are used, at which time traces of two components displaying
the m/z of 6 also appear in the GC/MS trace. We have tentatively
assigned these species to the DL- and meso-isomers of 6.
Computational Studies: In light of the experimental results
obtained, we chose to employ computational methods to
examine possibilities for this system that are not directly
observable experimentally. One of the more powerful compu-
tational methods for exploring photochemical reactions has been
the use of MC-SCF computational techniques to locate and
characterize molecular geometries of excited state species.
Complete active space SCF (CASSCF) calculations provide
configuration interaction of the ground state and excited state
surfaces via utilization of a matrix of matrices, each one of
which represents one of the possible electron excitation states
or the ground state based on the selection of occupied and virtual
orbitals with which to generate the possible states. Use of the
CASSCF method also allows for the location of those molecular
geometries where the ground state surface and the first excited
state surface are degenerate, a point known as a conical
intersection.12 Conical intersections can serve as barrier-free,
highly efficient passage points to the ground state surface where
available relaxation modes determine the geometry changes
FIGURE 4. Views of conical intersection CCCI.
photochemistry, and they corresponded well with those com-
monly utilized in the location of hydrogen-allyl and alkyl-
allyl-type conical intersections, respectively. All three active
spaces (I, II, and III) successfully led to the location of conical
intersections.
Active space I converged to a (S /S ) conical intersection
1
0
(CCCI, Figure 4) with a geometry that corresponded to the
complete dissociation of both benzylic σ-bonds of the arylox-
irane ring, giving an aryl carbene and a formyl group as clearly
depicted in the two views of CCCI. Additionally, a structurally
similar (T /S ) surface crossing (TSC, Figure 5; also known as
1
0
an intersystem crossing, ISC) was also located using an active
space comprised of the two unshared valence orbitals of the
carbenic carbon. Thus this carbenic-aldehyde structure is
potentially accessible via a singlet or triplet pathway from the
initial excited state structure. Triplet state arylcarbenes are well-
known for hydrogen atom abstraction and if formed could
16
readily abstract the aldehydic hydrogen leading to decarbo-
13
probable along its pathway to a minima. CASSCF calculations
CAS(4,4)/6-31G(d)) were performed (opt)conical algorithm)
in our labs using the appropriate active spaces shown in Figure
. Aromatic π-system orbitals were not included14 in the utilized
(13) 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, B. B.; 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. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon,
M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.6; Gaussian,
Inc.: Pittsburgh, PA, 1998.
3
active spaces as accurate reaction energetics, excitation energies,
and energy barriers were not within the scope of this work,
which focused on the examination of potential photogenerated
intermediates in the photolysis of the parent 3. The aromatic
π-system was viewed as a “photon antenna”15 to gather light
and supply the excitation to the oxirane moiety. Active space I
included the two benzylic σ/σ* orbital pairs of the epoxide
moiety, which have been known to dissociate in some arylox-
iranes under photolytic conditions. Active spaces II and III were
modeled on the ubiquitously proposed singlet 1,3-diradical
generally proposed as the primary intermediate in aryloxirane
(14) The requisite CASSCF(10,10)/6-31G(d) calculations on this system
are beyond the capacity of our current computational facilities.
(15) Celani, P.; Bernardi, F.; Olivucci, M.; Robb, M. A. J. Am. Chem.
Soc. 1997, 119, 10815.
(16) (a) A similar ring formation from a triplet carbene can be seen in:
Tomioka, H.; Okada, H.; Watanabe, T.; Banno, K.; Komatsu, K.; Hirai, K.
J. Am. Chem. Soc. 1997, 119, 1582. (b) Aldehydic hydrogen dissociation
energy (∼86.0 kcal/mol) is comparable to that of a benzylic hydrogen (88.0
kcal/mol): McMillen, D. F.; Golden, D. M. Annu. ReV. Phys. Chem. 1982,
33, 493.
(17) (a) Wilsey, S.; Houk, K. N. J. Am. Chem. Soc. 2000, 122, 2651.
(b) Wilsey, S.; Houk, K. N. J. Am. Chem. Soc. 2002, 124, 11182.
(11) Formation of indanyl dimers is strongly suggested by the consistent
presence of two high boiling components in the GC/MS analysis of the
photolysis of 1, both with base peaks of m/e ) 145 corresponding to indanyl.
(
12) Robb, M. A.; Bernardi, F.; Olivucci, M. Pure Appl. Chem. 1995,
6
7, 783 and references contained therein.
J. Org. Chem, Vol. 71, No. 21, 2006 8175

White et al.
FIGURE 6. Energy profile for the triplet carbene-aldehyde hydrogen
abstraction mechanism. Calculations were performed at the UB3LYP/
6
-311G(d) level. TS1 is the transition state for H transfer; TS2 is the
transition state for decarbonylation of 8, and 9&CO stands for 9 and
CO. Energy for 9&CO was obtained by optimizing the dissociation
from TS2.
FIGURE 5. Views of conical intersection TSC.
SCHEME 4. Carbenic Pathway to Indan Formation
FIGURE 7. Conical intersection HACI.
nylation followed by intersystem crossing (ISC) and ring closure
to give indan 4 (Scheme 4). We speculate that, being intramo-
lecular, these processes would proceed rapidly and relatively
unimpeded providing a pathway to the formation of 4 that is
independent of the formation of 3.
Examination of the energetics of the triplet mechanism
outlined in Scheme 4 was performed computationally at the
UB3LYP/6-311G(d) level (Figure 6). The transition state (TS1)
for the abstraction of the aldehydic hydrogen by the triplet
carbene lies only ∼4.5 kcal/mol above 7, strongly supporting a
facile path to 8 which is ∼22.1 kcal/mol lower in energy than
7
. If these results are representative, then the path from 7 to 8
is virtually irreversible due to the difference in barriers to TS1
4.5 vs 26.6 kcal/mol). Transition state (TS2), for the decar-
bonylation of 8, is much more accessible than TS1, ∼15.9 versus
6.6 kcal/mol, providing an efficient avenue to diradical 9,
(
2
FIGURE 8. Structure of conical intersection AACI1.
which on intersystem crossing will close to give 4. Single-point
singlet energy calculation for the geometry obtained optimizing
triplet 9&CO gives an energy that differs by less than 0.05 kcal/
mol, suggesting a highly efficient ISC then ring closure.
The conical intersection located using active space II was a
hydrogen-allyl-type17 conical (HACI, Figure 7) intersection that
displayed extreme lengthening of the C1-H1 bond (rC-H )
order saddle-point by two imaginary frequencies corresponding
to homolytic dissociation of the hydrogen and migration of the
hydrogen between O and C1. Geometrically, this conical
intersection appears as a functional equivalent for a transition
state for the ubiquitous 1,2-hydrogen migration observed in the
chemistry of 1,3-diradicals and is consistent with the observed
regiochemistry for such migrations. As such, if it arises from
the supposed ring-opened 1,3-diradical, then the ring opening
leads to the 1,3-diradical as a species on an excited state surface
(probably S1) with conical intersection HACI providing a path
to the ground state surface.
Two distinct conical intersections, corresponding to active
space III, were located and both were clearly alkyl-allyl-type17
conical intersections (AACI1 and AACI2, Figures 8 and 9,
respectively) displaying lengthening of the C3-C4 bonds
(rC2-C3 ) 2.24 and 2.23 Å, respectively) consistent with reported
values. Normal mode analysis performed on the geometries,
AACI1 and AACI2, showed both to be second-order saddle-
1
1
.732 Å) and lateral displacement toward the oxygen (rO-H )
.756 Å) consistent with examples from previously calculated
17
hydrogen-allyl-type conical intersections. However, the posi-
tion of the hydrogen in HACI was virtually centered over the
C-O bond with the benzylic center apparently interacting with
the aromatic π-system and not with the hydrogen. The asym-
metry of the position of the hydrogen in HACI is rational due
to the greater contribution of oxygen to the bonding MO’s. The
predicted shortening of the O-C1, C1-C2, and C2-C3
σ-bonds denotes extensive π-interaction with indications of a
developing double bond between C1 and C2. Normal mode
analysis of the geometry at HACI indicates that it is a second-
8176 J. Org. Chem., Vol. 71, No. 21, 2006

Photochemistry of 1,2-Dihydronaphthalene Oxide
SCHEME 5. Proposed Photochemical Pathway for the
Dihydronaphthalene Oxide System
FIGURE 9. Structure of conical intersection AACI2.
points. The two imaginary modes correspond to homolytic
dissociation of the C3-C4 bond and to migration of C4 between
C2-C3 for both AACI1 and AACI2. Geometrically, these CI’s
provide an efficient pathway to the formation of aldehyde 3.
Since aldehyde 3 is never formed in more than small amounts
and is apparently a minor reaction pathway, we would speculate
that there exists a barrier to accessing these pathways, at least
relative to the others.
aldehyde, which subsequently undergoes hydrogen abstraction
by the triplet carbene, decarbonylation, then ring closure
following ISC. This triplet process appears to derive from an
ISC (intersystem crossing) by the excited 1 prior to significant
geometric modification or very quickly after ring opening to a
1
,3-diradical. Since quenching showed no signs of increased
quantum yields for 2 or 3, we speculate that the ISC occurs
prior to the ring opening. Further studies into the photochemistry
of this and related systems continue in our laboratories as we
suspect that this type of behavior is much more ubiquitous than
previously thought for aryloxiranes.
Conclusion
The classic view of the aryl oxirane photochemistry has
characterized these systems as an exclusive dichotomy, follow-
ing either a dissociative path, as in the case of the diaryloxiranes,
or a path of ring-opening cleavage of the benzylic C-O bond,
as accepted for styrene oxide. We believe that this view must
be amended for dihydronaphthalene oxide, 1. In this system, at
least, both types of pathways appear to be operating to the
production of photoproducts. Our experimental and supporting
computational results speak strongly that the formative events
of the reaction occur on excited state surfaces or at the points
of return, via conical intersections, to the ground state surface.
The initial ring opening or dissociation does not deliver the
excited species to the ground state as many believe. At least in
this system it appears that these initial structural changes occur
on the initial excited state surface or on passage to another
excited state surface. Passage to a triplet surface by some of
the molecules must occur early in the excited state as quenching
precludes formation of 4 without increasing the yield of 2 or 3.
Irradiation at 254 nm may excite 1 (Scheme 5) to a higher singlet
state surface (S2 or S3) from which it may pass to the T1 surface
or pass to the S1 surface, possibly via C-O bond cleavage,
where it could pass through distinct singlet conical intersections
Experimental Section
Indan (4), 2-tetralone (2), and indene (5) were commercially
available and used as received. 1,2-Dihydronaphthalene oxide (1)
was prepared via epoxidation of 1,2-dihydronaphthalene as de-
scribed earlier,6 and indane-1-carbaldehyde was prepared by a
published procedure.18
Photolyses were carried out using a Rayonet photochemical
reactor using 8 RPR 2537 lamps and a carousal apparatus. Direct
-
2
irradiations were carried out using 6 × 10 M samples of the
epoxide in quartz tubes, capped with a septum, and purged with
argon for 10 min prior to irradiation. Products were analyzed with
HPLC on a C-18 column using 60:40 acetonitrile/water as the
solvent and a 0.75 mL/min flow rate. MS data were obtained using
a ion-trap GC/MS with a temperature ramp from 50 to 250 °C using
a ramp set at 8 °C/min.
Quenching studies were performed on solutions of 1,2-dihy-
dronaphthalene oxide in acetonitrile (0.02 M) containing isoprene
(
0.02-1.0 M). The amount of indan formed was determined using
HPLC and with toluene as an internal standard.
(
HACI and AACI’s), leading to the formation of 2 and 3.
Acknowledgment. We thank the Robert A. Welch Founda-
tion for support of this study. The support of the Gibbs-
Farrington Research Chair in Chemistry (SHSU) and a Faculty
Research Grant from the SHSU Research Council is also
acknowledged and our gratitude expressed.
Superficially, the singlet component of this proposed mech-
5
anism is very similar to Griffin’s original mechanism, except
that the “migrations” result from relaxation of the molecular
geometry corresponding to conical intersection transitions to
the ground state surface. Preliminary computational results
support the advent of a hydrogen-allyl-type conical intersection
as the mechanistic pathway for the 1,2-hydrogen migrations
ubiquitously observed in these systems. Similar computational
results support an alkyl-allyl-type conical intersection as the
pathway for the ring contraction to form 3, which we speculate
is probably the usual pathway for 1,2-alkyl shifts in 1,3-diradical
systems. The triplet pathway that we have proposed proceeds
by dissociation of the oxirane moiety to a triplet arylcarbenic
Supporting Information Available: Optimized atomic coor-
dinates and energies for the computed conical intersections (CCCI,
TSC, HACI, AACI1, and AACI2) and structures 7, TS1, 8, TS2,
and 9&CO. This material is available free of charge via the Internet
at http://pubs.acs.org.
JO0614184
(18) Kavadias, G.; Velkof, S. Can. J. Chem. 1978, 56, 730.
J. Org. Chem, Vol. 71, No. 21, 2006 8177