摘要：

The solution phase photochemical transformations of several substituted cyclobutanones and cyclopentanones are described with emphasis placed on the mechanism and scope of the photochemical ring expansion reaction. For cyclobutanone, ring expansion appears to be a singlet state reaction, and the extent to which it occurs is found to depend upon the nature and position of substituents on the four-membered ring (Table I). For example, the following yields of ring-expanded acetals are obtained from the designated ketones in methanol : 2,2-bis(trifluoromethyl)-3-phenylcyclobutanone (0%); cyclobutanone (8%); 2,2-dimethyl-3-phenylcyclobutanone (82%); 2-isopropylidenecyclobutanone (100%). The results can be interpreted in terms of an intermediary oxacarbene in equilibrium with a singlet biradical (Scheme II). For monocyclic cyclopentanones, ring expansion is only observed for spiro[2.4]heptan-4-one (56). Quantum yield and kinetic and product analyses demonstrate that ring expansion for 56 is a triplet state reaction. These results suggest that substituents capable of stabilizing the oxacarbene intermediate favor ring expansion.

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