Ring expansions of N-methyl-1,2,5-oxadiazolium and 1,2,3-triazolium perchlorate salts with bases to six-membered azines: direct detection of an addition intermediate in an addition–elimination mechanism and a degradation of 1,2,5-oxadiazolium salts to α-cyano nitrones

Abstract

Reactions of 2-methyl-1,2,5-oxadiazolium perchlorate salts and 1-methyl-2-aryl-1,2,3-triazolium perchlorate salts with the bases KCN, NaOEt, KOBu^t, LiNPrⁱ₂ give ring expansions to substituted 1,2,5-oxadiazines and 1,2,4-triazines. With cyanide as base the reactions follow an addition–elimination pathway and in two cases the addition intermediate has been isolated or directly detected by low-temperature NMR spectroscopy. The oxadiazolium system with cyanide also gives a useful new route to α-cyano nitrones via a ring degradation which competes with ring expansion. The reactions with KOBu^t and LiNPrⁱ₂ do not involve an addition of the base to the azolium salt. Reactions have been monitored by ¹³C NMR spectroscopy by using ¹³CN^–. An X-ray crystal structure is reported for (E)-N-(α-cyanobenzylidene)methylamine N-oxide.