Triazenes by Acid‐Mediated Opening of the Dihydro‐1,2,3‐triazole Ring of1,3‐Dipolar Cycloadducts of Organic Azides to Cyclic Ketene N,N‐Acetals

Spirocyclic 1,3‐dipolar cycloadducts 1 and 4 of azides to heterocyclic ketene N,N‐acetals open their dihydro‐1,2,3‐triazole ring in the presence of weak Brønsted acids to afford novel 1,3‐substituted triazenes 2 X and 5 X, respectively, which form colorless, crystallized tetrafluoroborates (X = BF 4 ) and hexafluorophosphates (X = PF 6 ). Ring‐opening is reversed in alkaline solutions. Methyl triflate methylates the dihydro‐1,2,3‐triazole ring of 1a and 4 at N ‐3 and thus induces ring‐cleavage to 1,3,3‐trialkyltriazenes 3 and 6 , respectively. The 1,3‐substituted triazenes 2a X, 2b BF 4 , 2d X and 5 X exist as tautomers that have the larger substituents, which include the heterocyclic rings, connected with the azo group ( N ‐1). By contrast, triazene 2c PF 6 has a bulky alkyl group at each terminal nitrogen and hence forms two rapidly equilibrating tautomers of similar stability. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)