1,2‐Diaza‐3,5‐diborolidines CH 2 B R 1 ‐(N R 2 ) 2 ‐B R 1 : Potential Precursors for Cyclic Diborylcarbene Ligands

The reaction of the aminohaloboranes (Me 2 N)( R )B X ( 1 ) ( a : R = Ph, X = Cl; b : R = Me, X = Br; c : R = i Pr, X = Cl) with the di‐Grignard reagent (BrMg) 2 CH 2 in ethyl ether/benzene mixtures afforded the diborylmethanes [(Me 2 N)( R )B] 2 CH 2 ( 2 ) ( a : R = Ph; b : R = Me; c : R = i Pr), which were subsequently converted into the corresponding bis(chloroboryl)methanes [(Cl)( R )B] 2 CH 2 ( 3 ) ( a : R = Ph; b : R = Me; c : R = i Pr). Base‐assisted cyclocondensation of 3a – c with 1,2‐diisopropylhydrazine afforded 1,2‐diisopropyl‐1,2‐diaza‐3,5‐diorgano‐3,5‐diborolidines ( 4 ) ( a : R = Ph; b : R = Me; c : R = i Pr). Derivative 4a was deprotonated by potassium bis(trimethylsilyl)amide to give (1,2‐diisopropyl‐1,2‐diaza‐3,5‐diphenyl‐3,5‐diborolidinyl)potassium ( 5 ). Attempts to introduce a halogen atom to the carbon bridge between the two boron atom by treating 5 with either iodine or 1,2‐dibromoethane failed. Instead, bis(1,2‐diisopropyl‐1,2‐diaza‐3,5‐diphenyl‐3,5‐diborolidinyl) (C‐C) ( 6 ) was formed. In contrast to this, C‐silylation of the heterocycle to give 7 was effected by reaction of salt 5 with trimethylsilyl chloride. Similarly, treatment of HgCl 2 with two molar equivalents of 5 gave rise to the formation of bis(1,2‐diisopropyl‐1,2‐diaza‐3,5‐diphenyl‐3,5‐diborolidinyl)mercury ( 8 ). The novel compounds were characterized by elemental analyses and various spectroscopic methods ( 1 H‐, 11 B{ 1 H}‐, 13 C{ 1 H} NMR, MS). The molecular structures of 4a and 6 – 8 were elucidated by X‐ray diffraction analyses.