The Bromodiazirinyl Anion: A Weakly Bound Complex of Diazirinylidene and a Bromide Ion

Abstract

Experimental and computational evidence suggests that the cleavage of n-butyl and tert-butyl 3-bromodiazirine-3-carboxylates by sodium n-butoxide in DMF, affording high yields of dibutyl carbonates, may proceed by nucleophilic acyl displacement of the bromodiazirinyl anion, a weakly bound complex of a cyclic carbene c-CN 2 (diazirinylidene) and a bromide ion. We explain the formation of substantial

Evidence for the Cyclic CN2 Carbene in Solution

“…Although c ‑ CN 2 should also benefit from aromatic stabilization, it is expected to behave quite differently from typical NHCs, as its σ-symmetry nitrogen lone pairs cannot be delocalized into the carbene center. Thus c -CN 2 is calculated to exhibit nucleophilicity as low as that of CF 2 and electrophilicity comparable to that of CFCl . The c -CN 2 still should be capable of binding to some transition metals, although less strongly than the regular nucleophilic NHCs…”

“…The only suitable conditions available thus far for the fragmentation of esters 1 , the strongly nucleophilic environment of an alkoxide solution in DMF, have hindered our attempts at the direct trapping of c - CN 2 by cycloaddition to a CC bond of alkenes, expected to form the unprecedented and highly strained 1,2-diaza­spiro­[2.2]­pent-1-enes . The electrophilic c - CN 2 may rather prefer to react with the excess alkoxide ions, likely resulting in a 3-alkoxy­diazirinyl anion 5 , and this alternative reactivity is supported by experimental results (Scheme ).…”

“… a Values in kcal/mol at 298 K calculated at the PBE/6-311+g(d,p) level in DMF (CPCM solvation model). b Values from ref . …”

“…Other possible precursors of products 2 and 7 (but not 9 ) obtained upon fragmentation of 1 are DMF itself (the solvent used) and dialkoxyesters 19 potentially formed from 1 by a halogen exchange/denitrogenation/O–H insertion sequence. Both alternatives can be excluded, as the cleavage of 1 also proceeds in DMSO, no deuterium incorporates to 7 when the reaction is performed in DMF- d 7 , and an authentic sample of 19 (R = Bu) does not afford any 2 or 7 when subjected to the cleavage reaction conditions …”

“…Thus c-CN 2 is calculated to exhibit nucleophilicity as low as that of CF 2 and electrophilicity comparable to that of CFCl. 4 The c-CN 2 still should be capable of binding to some transition metals, 5 although less strongly than the regular nucleophilic NHCs. 6 We have recently shown that c-CN 2 is a plausible reactive intermediate in the facile fragmentation of 3-bromodiazirine-3carboxylic esters 1 with alkoxide ions in DMF, affording, upon liberation of molecular nitrogen, high yields of dialkyl carbonates 2 (Scheme 1).…”