Solid-state hydrogen abstraction

Fig. 3. Simplified scheme for solid-state hydrogen abstraction reactions.

Hydrogen Abstraction by Thiocarbonyl Groups in the Solid State... 

Sakamoto et al. provided the first example of hydrogen abstraction by thiocarbonyl sulfur in the solid state [37,38]. Irradiation of AT-diphenylacetyl-AT-iso-propylthioaroylamides 6 in the solid state gave thioketones 7 (P-hydrogen abstraction products by thiocarbonyl sulfur), mercaptoazetidin-2-ones (y-hy-drogen abstraction products by thiocarbonyl sulfur), and aroylthioamides (y-hydrogen abstraction products by thiocarbonyl sulfur followed by cleavage). 

Irradiation of the powdered crystals of 6a-d with a high-pressure mercury lamp under argon at 0 °C showed a different photochemical behavior from that in benzene solution. Contrary to the photochemical results in solution media in which azetidin-2-ones 8 (y-hydrogen abstraction products) were formed as major products, thioketones 7 (P-hydrogen abstraction products) were obtained as major components in the solid-state photoreactions (Scheme 7). The... 

Scheffer et al. also reported hydrogen abstraction of thioketones in the solid state [42]. The nTi" and 7171" excited states differ in the spatial properties of the orbitals involved in abstraction, and it follows that ketones and thiones should exhibit significantly different hydrogen atom abstraction geometries. 

Sakamoto et al. reported the X-ray crystallographic data and the solid-state photoreaction of eight Ar,AT-disubstituted a,p-unsaturated thioamides 23a-h, which involves hydrogen abstraction by the alkenyl carbon atom conjugated with thiocarbonyls (Fig. 9) [44-46]. 

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. 

What is as noteworthy as the excess of 88a from the solid-state irradiation of 86a is the presence of any 87a at all. Its formation requires either that one of the more distant -/-hydrogen atoms be abstracted also or that the reaction cavity suffer some dramatic changes in shape and size while the hydroxy-1,4-biradical is present. Based upon the lattice properties of 86a, the latter seems the more probable scenario without the movement of groups on adjacent... 

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