Hydrogen abstraction, ketones substituent 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. ... 

During the mechanistic studies of estrogen biosynthesis, selective oxidation of androstene-3,17,19-trione (n) to the corresponding carboxylic acid was found to proceed by iron porphyrin complexes (Scheme 14A) [253]. On the basis of substituent effect on the benzaldehyde oxidation and kinetic isotope effect, direct hydrogen abstraction mechanism has been proposed [254]. The relative reactivity of aldehydes and alkenes is as follows cyclooctene, styrene > aldehyde, terminal alkene > a, 3-unsaturated ketone. 

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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