Conformation hydrogen abstraction

In crystal structures in which there is appreciable intermolecular overlap of the C-2=C-3 double bonds, irradiation of the solid leads only to (2 + 2) cycloaddition. This is true of reactants 113, which yield in all cases file centro-symmetric dimers 114, even when the reactant conformation is suited also to intramolecular hydrogen abstraction. 

The acrolein coordinated with catalyst then gives an acyl radical by abstracting its aldehyde hydrogen. In the general oxidation of aldehydes, the acyl radical is considered to be discontinuing coordination with catalyst, as was described by Bawn (3) and Hoare and Waters (17). However, in the acrolein oxidation, the acyl radical formed by hydrogen abstraction may not conform to this proposal, as described below. 

Bredt s rule. In this way, 1-adamantyl p-methoxyacetophenone 86a was forced to yield only cyclobutanols 87a and 88a as photoproducts [281]. Whereas (benzene) solution phase irradiations of 86a yielded a 2.6 ratio of 87a/88a, the solid state photoproduct ratio, 0.5, favors the more sterically hindered cyclobutanol. X-Ray diffraction studies predict a chair-like y-hydrogen abstraction pathway for 86a (in contrast to the boat-like transition states of 82) in which the C=0-Ha distance is 2.67 A. Other abstractable hydrogens (Hb) are at least 0.3 A farther from the carbonyl oxygen (Scheme 44). If i-BR has a conformation which mimics that of the ketone, its least motion pathway favors formation of the more sterically hindered cyclobutanol 88a. 

Rather phase-insensitive Norrish II photoproduct ratios are reported from irradiation of p-chloroacetophenones with a-cyclobutyl, a-cyclopentyl, a-cycloheptyl, a-cyclooctyl, and a-norbonyl groups [282], In each case, the E/C and cyclobutanol photoproduct ratios are nearly the same in neat crystals as measured in benzene or acetonitrile solutions. On this basis, we conclude that the reaction cavity plays a passive role in directing the shape changes of these hydroxy-1,4-biradicals. As long as the initial ketone conformation within the cavity permits -/-hydrogen abstraction (and these ketones may be able to explore many conformations even within their triplet excited state lifetime), the cavity free volume and flexibility allow intramolecular constraints to mandate product yields. 

It was then found that a tertiary center is not required at C-5, and that tetra-0-acetyl-/ -D-xylopyranose (27), treated with radicals derived from N-bromosuccinimide, affords a mixture of (55)- and (5R)-tetra-O-acety 1-5-bromo-/ -D-xylopyranose (28 and 29 see Scheme 6), which is in accord with, but does not necessarily follow from, the finding that 27 exists in solution in both chair conformations, each of which is subject to axial hydrogen abstraction and subsequent axial bromination27 35 (see Section III,2). In this... 

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