Photolytic decomposition of some polycyclic α-diazoketones: generation and reactivity of 1,2-oxocarbenes in benzene with and without tert-butylamine

Abstract

Irradiation in benzene of the two isomeric 2,3-diazofluoranthenones 11 and 14 results in the formation of only products due to the formal insertion of the intermediate oxocarbenes into the C–H bonds of the solvent, unless the reactions are carried out in the presence of tert-butylamine, whereupon products of an apparent [3 + 2]-cycloaddition of the oxocarbene to the solvent are also obtained, viz. dihydrofurans 13 and 16, respectively. Wolff rearrangement of the generated oxocarbenes to give products derived from direct trapping of the ketene intermediates with tert-butylamine are not observed. The formal insertion product, 2-phenylfluoroanthen-3-ol 12 (from 11) undergoes partial oxidation during chromatographic purification on silica to generate a persistent free radical species 17 which has been detected and characterised by EPR spectroscopy. A reinvestigation of the photolytic decomposition of the related polycyclic α-diazoketone 5 under the same conditions has established that a more likely structure for the isolated dihydrofuran is 8, and not the reported isomeric dihydrofuran 20, which is formed from 8 by hydrogen migration during isolation by repeated thick-layer chromatography.