Photochemical oxetan formation: the Paterno–Büchi reaction of aliphatic aldehydes and ketones with alkenes and dienes

Abstract

The photochemistry of four aldehydes (acetaldehyde, pivalaldehyde, and cyclopropane- and cyclobutane-carbaldehyde) and two ketones (isopropyl and cyclopropyl methyl ketone) in the presence of various alkenes, viz.(Z) and (E)-penta-1,3-diene, 2,3-dimethylbut-2-ene, (Z)-but-2-ene, (E)-1,2-dicyanoethylene, styrene, cyclopropylethylene, 1,1-dicyclopropylethylene, and dicyclopropylmethylenecyclopropane upon irradiation at 310 nm has been studied. With the aldehydes, oxetan formation dominates over other reactions, such as alkene (Z)–(E) isomerization alkene dimerization, aldehyde dimerization with formation of an acyloin, and abstraction of allylic hydrogens from the alkene by the aldehyde with formation of a hydroxy-compound. Twenty-nine oxetans have been identified. The mechanism of oxetan formation has been discussed with special emphasis on (i) the nature of the reacting excited state (the reacting excited state is the ¹n–π* state of the carbonyl compound), (ii) the intermediacy of exciplexes and 1,4-diradicals, (iii) the stereospecificity, (iv) the regiospecificity, and (v) the absence of cyclopropyl ring opening.