Hydrogen abstraction distances, solid

Table I. Reactants, Hydrogen Abstraction Distances and Steric Compressions in Solid State Photorearrangements...

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. 

Scheffer, Trotter, and co-workers have provided elegant demonstrations of the distance and angular dependence between a carbonyl oxygen atom and a gamma hydrogen atom on the ease of initial abstraction [276], Their singlecrystal X-ray structural information allows the various courses of Norrish II reactions of neat solids to be understood in the context of topochemical control [11,13]. For instance, they have noted that the cyclic diones 77 (n = 7,8,10, and 12) follow different Norrish II pathways depending upon the conformations of the individual molecules in their crystals (Eq. 9) [277]. 

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