Geometric requirements for the photolysis of aryloxiranes and cyclic carbonate esters

Article abstract of DOI:10.1016/S0040-4039(01)02392-9

The photolysis of trans-β-methylstyrene oxide proceeds smoothly, but neither the corresponding cis-isomer nor β,β-dimethylstyrene oxide are photochemically active. Moreover, although styrene glycol carbonate undergoes photolysis to extrude carbon dioxide, the β,β-dimethylstyrene glycol carbonate is photochemically unreactive. These results are explained by looking at the energies required to obtain a conformation in which the plane of the aromatic ring is orthogonal to the orbitals of the epoxide moiety.

Full text of DOI:10.1016/S0040-4039(01)02392-9

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 1169–1170
Geometric requirements for the photolysis of aryloxiranes and
cyclic carbonate esters
Sorana Linder, Katherine White, Mandelin Palmer, Benny Arney and Rick White*
Department of Chemistry, Sam Houston State University, Huntsville, TX 77341, USA
Received 12 December 2001; accepted 19 December 2001
Abstract—The photolysis of trans-b-methylstyrene oxide proceeds smoothly, but neither the corresponding cis-isomer nor
b,b-dimethylstyrene oxide are photochemically active. Moreover, although styrene glycol carbonate undergoes photolysis to
extrude carbon dioxide, the b,b-dimethylstyrene glycol carbonate is photochemically unreactive. These results are explained by
looking at the energies required to obtain a conformation in which the plane of the aromatic ring is orthogonal to the orbitals
of the epoxide moiety. © 2002 Elsevier Science Ltd. All rights reserved.
The photochemistry of monoaryloxiranes has been gen-
erally described as occurring via cleavage of the ben-
zylic carbonꢀoxygen bond to generate a 1,3-diradical
such as seen in the photolysis of styrene oxide (1)1 and
tetrahydronaphalene oxide (3).2 The same intermediate
has been formed via extrusion of carbon dioxide from
the corresponding cyclic carbonate esters such as sty-
rene glycol carbonate (2)3 and 1,2,3,4-tetrahydro-
naphalene carbonate (4).4 These are summarized in
Scheme 1.
styrene glycol carbonate (6) for 1 h resulted in the
formation of the 1,3-diradical leading to phenylacetone
(7) in 20–25% conversion with none of the methyl
migration product 8 being detected and is shown in
Scheme 2.
However, the photolysis of the corresponding cis-b-
methylstyrene oxide (9) resulted in no reaction as
shown in Scheme 3. This puzzled us as to why the trans
isomer was reactive, yet the cis isomer was not, even
though their UV spectra were virtually identical. More-
over, the irradiation of either b,b-dimethyl styrene
oxide 10 or b,b-dimethyl styrene glycol carbonate 11
resulted in no detectable reaction even after 2 h. We
were puzzled as to why 5 and 6 were photoreactive, yet
9, 10 and carbonate 11 were relatively inactive.
The intermediate 1,3-diradicals can undergo a 1,2-
hydrogen migration, or a 1,2-aryl migration.5 We were
interested in 1,2-alkyl migrations in these systems since
alkyl migrations in radical systems are rare, although
they have been reported.6 The photolysis7 of either
trans-b-methylstyrene oxide (5) or trans-b-methyl-
Photochemical reaction at the benzylic carbonꢀoxygen
bond in aryl oxiranes requires the interaction of the
electronically excited p-system with the bonding/anti-
binding orbitals of the carbonꢀoxygen bond. This inter-
action is a symmetry-controlled function of the dihedral
angle (q) between the aryl plane and the car-
Scheme 1.
* Corresponding author. Tel.: 936-294-1060; fax: 936-294-4996;
e-mail: chm rcw@shsu.edu
Scheme 2.
–
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)02392-9

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S. Linder et al. / Tetrahedron Letters 43 (2002) 1169–1170
Scheme 3.
bonꢀoxygen bond. The observed positional dependence
of reactivity caused us to suspect that van der Waals
repulsions between the substituents on the oxirane moi-
ety with the ortho aryl hydrogens were preventing the
necessary orbital alignment for charge density transfer
into the antibonding orbitals of the benzylic car-
bonꢀoxygen bond. We examined the calculated energy
profile compounds 1, 5, 9, and 10 as a function of
rotation about the phenylꢀoxirane bond using HF/3-
21G(*) optimized geometries with the dihedral (q) to
five (5) degree increments. This is outlined in Fig. 1.
Figure 1. Energy profile HF/3-21G(*).
Compounds 16 and 56 display virtually identical profiles
with moderately restricted rotation (barrier:5.7 and
6.0 kcal/mol, respectively) as expected because of the
geometric isolation of the trans-beta position from the
aryl group. This finding agrees well with the similar
photochemical reactivity observed for 1
6 and 56 . Com-
pounds 9 and 10 exhibited significantly greater barriers
6
to rotation (9.8 and 10.2 kcal/mol, respectively) reflect-
ing the greater interaction of the cis-beta substituents
with the ortho aryl hydrogens. Just as important as the
increased height of the barrier is its position from
q:50–120°, which is the most critical range for effec-
tive electronic interaction with the excited aryl p sys-
tem. Conformations in this range are approaching a
potential energy maximum and should expectedly pos-
sess low residence times.
Figure 2. Normalized population distribution.
Acknowledgements
Support for this work by the Robert A. Welch Founda-
tion and the SHSU Faculty Research Enhancement
Council is gratefully acknowledged.
Calculation of a population distribution graph of the
fraction of molecules (logarithmic scale) with sufficient
internal energy versus dihedral angle clearly illustrates
the drastic reduction (as much as 10−3) in the fraction
of energetically viable molecules in the region of great-
est probability of reaction. These calculated results
strongly suggest a dramatic decrease in the expected
photochemical reactivity of the benzylic oxirane ring in
the presence of a cis-beta substituent, which is clearly
expressed in the observed photo-inactivity of com-
pound 10 (Fig. 2).
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These results explain the variation of photoreactivity of
aryloxiranes and aryl carbonate esters and also match
well with a recent investigation into the structure of
phenylcyclopropane8 which showed, that the lowest
energy conformation of phenylcyclopropane was one in
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