SEMICONDUCTOR-CATALYZED PHOTOADDITION OF OLEFINS AND ENOL ETHERS TO 1,2-DIAZENES - A NEW ROUTE TO ALLYLHYDRAZINES

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 1e(-)/1h(+) process while 2e(-)/2h(+) are necessary for the reduction.