Chemical Oxidation of 1,3,2‐Diazaboroles and 1,3,2‐Diazaborolidines

Oxidation of the 1,3,2‐diazaborole t BuN a CH=CHN b ( t Bu)BBr( N a –B ) ( 1h ) with NO + PF 6 – afforded the diazaborolium salts [ t BuN a =CH–CH=N b ( t Bu)BF 2 ( N a –B )] + X – (X = Br – , Br 3 – , PF 6 – ) ( 3h ) as the result of different oxidation processes. The same product was obtained when the 1,3,2‐diazaborolidine t BuN a CH 2 CH 2 N b ( t Bu)BBr( N a –B ) ( 2h ) was subjected to reaction with NO + PF 6 – . In contrast to this, oxidation of 1h and 2h with NO + BF 4 – cleanly furnished [ t BuN a CH–CH=N b ( t Bu)BFBr( N a –B )] + BF 4 – ( 6h ). Treatment of the 1,3,2‐diazaboroles t BuN a CH=CHN b ( t Bu)BR( N a –B ) [R = H ( 1e ), CN ( 1i ), C≡CH ( 1j )] with NO + PF 6 – under comparable conditions led to the borolium salts [ t BuN a =CH–CH=N b ( t Bu)BRF( N a –B )] + PF 6 – [R = H ( 3e ), CN ( 3i ), C≡CH ( 3j )]. All the borolium salts investigated in this study were reversibly reduced by cyclic voltammetry. The novel products were characterized by elemental analyses and NMR spectra ( 1 H, 11 B, 13 C, 19 F, 31 P). The X‐ray structure analysis of 3h reveals two independent planar cations in the asymmetric unit, accompanied by one bromide and one tribromide ion.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)