Page 1147 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

1132 Ph
Ph
Ar
CHAPTER 12 Ar
Photochemistry – O – O

1,4 shift with 1,4 shift with
inversion retention
allowed forbidden
4 electrons 4 electrons
Mobius system Hückel system

As is clear from the preceding examples, there are a variety of overall reactions
that can be initiated by photolysis of ketones. The course of photochemical reactions of
ketones is very dependent on the structure of the reactant. Despite the variety of overall
processes that can be observed, the number of individual steps involved is limited.
For ketones, the most important are inter- and intramolecular hydrogen abstraction,
-cleavage at the carbonyl group, and substituent migration to the ß-carbon atom of
,ß-unsaturated ketones. Reexamination of the mechanisms illustrated in this section
will reveal that most of the reactions of carbonyl compounds that have been described
involve combinations of these fundamental processes. The final products usually result
from re-bonding of reactive intermediates generated by these steps.

12.3.3. Cycloaddition of Carbonyl Compounds and Alkenes

Ketones and aldehydes can undergo photochemical [2+2] cycloaddition reactions
with alkenes to give oxetanes. This is called the Paterno-Buchi reaction. For alkyl
carbonyl compounds both singlet and triplet excited states seem to be involved, but for
aromatic compounds the reaction occurs through the triplet state. 151 The regiochemistry
can usually be accounted for on the basis of formation of the most stable 2-oxa-1,4-
diradical. For example, styrene and benzaldehyde give 2,3- not 2,4-diphenyloxetane. 152

Ph
CHPh
O
PhCH O* + CH 2 CHPh O
Ph
Ph
3:1 trans:cis

The same generalization can also account for the reversal of orientation between furan
and dihydrofuran. 153

151 R. A. Caldwell, G. W. Sovocool, and R. P. Gajewski, J. Am. Chem. Soc., 95, 2549 (1973).
152 S. A. Fleming and J. J. Gao, Tetrahedron Lett., 38, 5407 (1997).
153
A. G. Griesbeck and S. Stadtmuller, Chem. Ber., 123, 357 (1990).