Radical-chain reactions of sulfonyl azides and of ethyl azidoformate with allylstannanes: homolytic allylation at nitrogen

Abstract

4-Methylbenzenesulfonyl azide reacts with allyltriphenylstannane (ATPS) in refluxing benzene, in the presence of 2,2′-azobis(2-methylpropionitrile) as initiator, to give N-allyl-4-methylbenzenesulfonamide in good yield after hydrolytic work-up. Small amounts of allyl 4-methylphenyl sulfone were also formed. The reaction follows a free-radical chain mechanism which involves competitive addition of Ph₃Sn˙ to N^a and to N^c of the azido group in ArSO₂N^aN^bN^c. Addition to N^a followed by loss of nitrogen gives ArSO₂ṄSnPh₃, the precursor of the N-allylarenesulfonamide, while addition to N^c leads to the formation of ArṠO₂ and thence to the allyl aryl sulfone. Allyltrimethylstannane behaves in a similar way to ATPS, but allyltributylstannane gives only a low yield of N-allylarenesulfonamide and the major product is the unsubstituted sulfonamide MeC₆H₄SO₂NH₂, which results because the radical ArSO₂ṄSnBu₃ undergoes intramolecular 1,5-hydrogen-atom transfer in preference to adding to the allylstannane. 2-Methylallyltriphenylstannane reacts in an analogous way to ATPS, but allylstannanes containing nonterminal double bonds do not react successfully. The arenesulfonyl azides 4-XC₆H₄SO₂N₃(X = H, MeO, F) react in a similar way to tosyl azide, but the reaction is very sluggish when X = NO₂. With 1-octanesulfonyl azide, reaction with Ph₃Sn· is much less selective and products arising from attack at N^a and N^c are formed in comparable yields. Ethyl azidoformate reacts with allylstannanes in a similar manner to, although more slowly than, tosyl azide and gives good yields of the corresponding allylic carbamates.