Contribution to the Reactivity of N , N′ ‐Diaryl‐1,4‐diazabutadienes Aryl–N=CH–CH=N–Aryl (Aryl = 2,6‐Dimethylphenyl; 2,4,6‐Trimethylphenyl) Towards Boron Trichloride

The reaction of Aryl–N=CH–CH=N–Aryl ( 3a : Aryl = 2,6‐Me 2 C 6 H 3 ; 3b : Aryl = 2,4,6‐Me 3 C 6 H 2 ) with 2 equiv. BCl 3 in a toluene/hexane mixture at –50 °C led to an inseparable mixture of borolium salts [Aryl–N a =CH–CH=N(Aryl)BCl 2 ( N a – B )] + BCl 4 – ( 4a : Aryl = 2,6‐Me 2 C 6 H 3 ; 9a : Aryl = 2,4,6‐Me 3 C 6 H 2 ) and [Aryl–N a =CH–C(Cl)=N(Aryl)BCl 2 ( N a – B )] + BCl 4 – ( 4b : Aryl = 2,6‐Me 2 C 6 H 3 ; 9b : Aryl = 2,4,6‐Me 3 C 6 H 2 ) and the bicyclic species [HC a =N(Aryl)BCl 2 N(Aryl)–C b =( C a – C b )] 2 ( 5 : Aryl = 2,6‐Me 2 C 6 H 3 ; 10 : Aryl = 2,4,6‐Me 3 C 6 H 2 ). Sodium amalgam reduction of borolium salts 4a , b and 9a , b afforded inseparable mixtures of the diazaboroles 2,6‐Me 2 C 6 H 3 N a CH=CR–N b (2,6‐Me 2 C 6 H 3 )BCl( N a – B ) ( 1 : R = H; 6 : R = Cl) and 2,4,6‐Me 3 C 6 H 2 N a CH=CR–N b (2,4,6‐Me 3 C 6 H 2 )BCl( N a – B ) ( 2 : R = H; 11 : R = Cl). In order to obtain pure 1 and 2 , diazabutadienes 3a and 3b were slowly combined with 2 equiv. BCl 3 in the same solvents at –78 °C. The acyclic adducts Cl 3 BN(Aryl)=CH–CH=N(Aryl)BCl 3 ( 7a : Aryl = 2,6‐Me 2 C 6 H 3 ; 7b : Aryl = 2,4,6‐Me 3 C 6 H 2 ) were reduced with sodium amalgam to furnish the aminoboranes Cl 2 BN(Aryl)CH=CH–N(Aryl)BCl 2 ( 8 : Aryl = 2,6‐Me 2 C 6 H 3 ; 12 : Aryl = 2,4,6‐Me 3 C 6 H 2 ). Stirring solutions of 8 and 12 in the presence of CaH 2 cleanly gave the diazaboroles 1 and 2 , respectively. The novel compounds were characterized by elemental analyses and 1 H‐, 11 B‐, 13 C NMR and mass spectra. The molecular structures of 1 , 4a , 5 and 8 were elucidated by single X‐ray diffraction analyses. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)