Head‐to‐Tail Dimerization of N‐Heterocyclic Diazoolefins

Abstract

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]

Synthetic Strategies of N‐Heterocyclic Olefin (NHOs) and Their Recent Application of Organocatalytic Reactions and Beyond

“…In 2023, the group of Severin showed studies with N‐heterocyclic diazo olefins, NOBF 4 mediated a head‐to‐tail dimerization can also be accomplished via oxidation. For example, the diazo olefin 172 with Dipp wingtip aryl groups in the presence of NOBF 4 in CH 3 CN or Me‐I/DCM transformed into the tetrazine heterocycles with BF 4 or I salts 173 a – b in good yield up to 90 % as cited in Scheme 57 [86] …”

Synthesis of sterically congested unsymmetrical 1,2-dicarbonyl radicals through a stepwise approach

“…1a. 24–39 This approach can result in the synthesis of highly stable organic radicals that are resistant to air and heat. In our previous research, we synthesized 1,2-dicarbonyl radical cations derived from two identical NHCs or CAACs (Fig.…”

Synthesis and Reactivity of an Anionic Diazoolefin

“…For the neutral diazoolefin 1, we had observed that methanol mediates a head-to-tail-dimerization to give a quinoidal tetrazine. [20,21] No dimerization was observed for the anionic diazoolefin 3. Instead, the addition of methanol gave the diazo compound 4 in 92 % yield (Scheme 2).…”

“…Methylation of the neutral diazoolefin 1 results in a dimerization. [20] In contrast, when MeI was added to a solution of 3 in benzene, the alkylation product 5 was formed in high yield (Scheme 2). The structure of 5 could be established by an XRD analysis, [18] and the structural parameters are similar to what was found for 4.…”

“…In the case of 1, these reactions had resulted in the formation of dimers. [20] The ligand properties of 3 were evaluated in reactions with Au I , Rh I , Ir I , and Ge II complexes. Salt metathesis reactions gave C-bound complexes for Au I , Rh I , and Ge II , whereas an unusual N-bound coordination mode was found for the Ir I complex 9.…”