Electronic excited states of small ring compounds. VII. Bicyclo[2.1.0]pentanes by the photocycloaddition of 1,2,3-triphenylcyclopropene to fumaro- and maleonitrile

Abstract

Irradiation of the charge-transfer complex between 1,2,3-triphenylcyclopropene (1) and fumaronitrile (7) and maleonitrile (8) leads to formation of bicyclo[2.1.0]pentane cycloaddition products (9, 10, 11) and the photoene product (12). These products were also formed when the reaction was photosensitized (triplet–triplet transfer). The structure of the adducts was established by analysis of 13C and 1Hmr spectra. Nuclear Overhauser effect studies were also useful. The triplet energies of 7 and 8 (ET ca. 50 kcal mol−1) were determined by studying the photosensitized isomerization as a function of sensitizer triplet energy. The thermal stability of the new bicyclo [2.1.0]pentanes has been studied, particularly with regard to the isomerization resulting from cleavage of the central [0] bond in these strained systems. The mechanism of the photocycloaddition reactions is discussed.