The head-to-tail dimerization of N-heterocyclic diazoolefins is described. The products of these formal (3+3) cycloaddition reactions are strongly reducing quinoidal tetrazines. Oxidation of the tetrazines occurs in a stepwise fashion, and we were able to isolate a stable radical cation and diamagnetic dications. The latter are also accessible by oxidative dimerization of diazoolefins.Diazoalkanes undergo (2+3) cycloaddition reactions with a large variety of dipolarophiles (Scheme 1a). [1] First evidence for such a transformation was provided in 1888 by Buchner. [2] He noted that the reaction of methyl diazoacetate with dimethyl fumaric acid gives a well-defined addition product. Soon after, von Pechmann investigated similar reactions with diazomethane. [3] Today, 1,3-dipolar cycloaddition reactions (Huisgen reactions) [4] between diazoalkanes and dipolarophiles represent an indispensable tool in heterocyclic chemistry. [1] In contrast to the rich chemistry of diazoalkanes, there are only limited studies with diazoolefins. [5][6][7][8][9] A key problem for experimental work is the low stability of most diazoolefins, as they tend to lose dinitrogen, even at low temperatures. However, it could be established that diazoolefins are also able to undergo 1,3-dipolar cycloaddition reactions (Scheme 1b). [6,7]

The head-to-tail dimerization of N-heterocyclic diazoolefins is described. The products of these formal (3+3) cycloaddition reactions are strongly reducing quinoidal tetrazines. Oxidation of the tetrazines occurs in a stepwise fashion, and we were able to isolate a stable radical cation and diamagnetic dications. The latter are also accessible by oxidative dimerization of diazoolefins.Diazoalkanes undergo (2+3) cycloaddition reactions with a large variety of dipolarophiles (Scheme 1a). [1] First evidence for such a transformation was provided in 1888 by Buchner. [2] He noted that the reaction of methyl diazoacetate with dimethyl fumaric acid gives a well-defined addition product. Soon after, von Pechmann investigated similar reactions with diazomethane. [3] Today, 1,3-dipolar cycloaddition reactions (Huisgen reactions) [4] between diazoalkanes and dipolarophiles represent an indispensable tool in heterocyclic chemistry. [1] In contrast to the rich chemistry of diazoalkanes, there are only limited studies with diazoolefins. [5][6][7][8][9] A key problem for experimental work is the low stability of most diazoolefins, as they tend to lose dinitrogen, even at low temperatures. However, it could be established that diazoolefins are also able to undergo 1,3-dipolar cycloaddition reactions (Scheme 1b). [6,7]