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

Abstract

The reaction of Aryl-N=CH-CH=N-Aryl (3a: Aryl = 2,6-Me2C 6H3; 3b: Aryl = 2,4,6-Me3C6H 2) with 2 equiv. BCl3 in a toluene/hexane mixture at -50°C led to an inseparable mixture of borolium salts [Aryl-N a=CH-CH=N(Aryl)BCl2(Na-B)]+BCl 4- (4a: Aryl = 2,6-Me2C6H3; 9a: Aryl = 2,4,6-Me3C6H2) and [Aryl-N a=CH-C(Cl)=N(Aryl)BCl2(Na-B)] +BCl4- (4b: Aryl = 2,6-Me2C 6H3; 9b: Aryl = 2,4,6-Me3C6H 2) and the bicyclic species [HCa=N(Aryl)BCl 2N(Aryl)-Cb=(Ca-Cb)]2 (5: Aryl = 2,6-Me2C6H3; 10: Aryl = 2,4,6-Me 3C6H2). Sodium amalgam reduction of borolium salts 4a,b and 9a,b afforded inseparable mixtures of the diazaboroles 2,6-Me2C6H3NaCH=CR-N b(2,6-Me2C6H3)BCl(Na-B) (1: R = H; 6: R = Cl) and 2,4,6-Me3C6H2N aCH=CR-Nb(2,4,6-Me3C6H 2)BCl(Na-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. BCl3 in the same solvents at -78°C. The acyclic adducts Cl 3BN(Aryl)= CH-CH=N(Aryl)BCl3 (7a: Aryl = 2,6-Me 2C6H3; 7b: Aryl = 2,4,6-Me3C 6H2) were reduced with sodium amalgam to furnish the aminoboranes Cl2BN(Aryl)CH=CH-N(Aryl)BCl2 (8: Aryl = 2,6-Me2C6H3; 12: Aryl = 2,4,6-Me 3C6H2). Stirring solutions of 8 and 12 in the presence of CaH2 cleanly gave the diazaboroles 1 and 2, respectively. The novel compounds were characterized by elemental analyses and 1H-, 11B-, 13C 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, 2006.