Hydrogen abstraction, ketones conformational effects

With some cyclic ketones, the a-cleavage can also be followed by intramolecular hydrogen abstraction that eventually leads to an unsaturated ring-opened aldehyde. An alternative reaction path involves formation of a ketene. The competition between these two reactions is determined by the effect of substituents on the conformation and reactivity of the diradical intermediate. ... 

Studies of deuterium isotope effects have proved useful in the determination of the details of excited chemical and physical processes. However, some of the observed isotope effects pose as many questions as they answer. Such is the case for the formation of unsaturated ketones from certain cyclic ketones, e.g. (l).ei The observed primary isotope effect is of the same order as that observed earlier by these authors for similar systems, but is much smaller than literature values for radical disproportionations. One explanation is that the rate of hydrogen abstraction is controlled by the conformation at the starred carbon ( ). 

The type II reaction requires close approach of a y-hydrogen to carbonyl oxygen and effective overlap of the hydrogen s-orbital with the oxygen n-orbital responsible for abstraction. Flexible ketone molecules change their conformation freely and achieve conformational equilibrium before y-hydrogen abstrac-tion. Conformational restriction caused by environment decreases the type II reactivity and may allow appearance of other reactivities. 

Descotes et al. reported an approach to the synthesis of crombenine that involves 8-hydrogen abstraction from an acetal to generate the required spiroketal structure. Unfortunately the required 2,4-dioxy substituents lower the yield from 87% when R = H to 20 and 0% when R = OAc and OMe. Because p-methoxy groups deactivate triplet phenyl alkyl ketones by causing the lowest triplet to become n,n, electronic and conformational effects apparently combine to lower values so much that they barely compete with phosphorescence. Moreover, the bulk of the cyclic ether very likely worsens the conformational problem common to o-alkoxyphenyl ketones. 

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