Semiconductor‐Catalysed Photoaddition of Olefins and Enol Ethers to 1,2‐Diazenes: A New Route to Allylhydrazines

Abstract

Suspensions of zinc or cadmium sulfide powders in a protic solvent catalyse the linear addition of enol ethers and olefins to 1,2‐diaryl‐ and 1‐aryl‐2‐alkyl‐1,2‐diazenes, producing allylhydrazine derivatives. Relative quantum yields decrease sharply when the 1,2‐diazene is more difficult to reduce, while their relationship to the oxidation potential of the enol ether/olefin is complicated. Reduction to 1,2‐diarylhydrazine and concomitant dehydrodimerization of the enol ether occurs as a side reaction. It is favoured by increasing light intensity and becomes the major reaction path when platinized (5 mol%) photocatalysts are employed. It is proposed that the photogenerated electron‐hole pair in a proton‐coupled electron transfer reduces the diazene to a hydrazyl radical and oxidizes the olefin/enol ether to a radical cation. The allylic radical obtained from the latter by deprotonation then undergoes CN coupling with the hydrazyl radical to afford the allylhydrazine. Diarylhydrazine formation occurs by disproportionation of the hydrazyl radical or by a successive proton‐coupled reduction. Thus photoaddition can be classified as a 1 e−/1 h+ process while 2e−/2h+ are necessary for the reduction. Copyright © 1995 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim