Synthesis, structure, and reactions of (acylimino)triaryl-lambda(5)-bismuthanes: First comparative study of the (acylimino)pnictorane series

The synthesis, structure, and reactions of (acylimino)triaryl-lambda (5)-bi smuthanes and a comparative study of the structure and reactivity of a seri es of (acylimino)pnictoranes are reported. Treatment of ortho-substituted t riarylbismuth dichlorides 1 (Ar3BiCl2; Ar = 2-MeC6H4, 2-MeOC6H4, 2,4,6-MC3C 6H2) with amides 2 (H2NCOR; R = CF3, CCl3, 3,5-(CF3)(2)C6H3) in the presenc e of 2.2 equiv of KO-t-Bu in dichloromethane afforded (acylimino)triaryl-la mbda (5)-bismuthanes 3 (Ar3Bi=NCCR) in yields of 77-96%. The ortho-substitu ted aryl ligands and the electron-withdrawing N-substituents afford kinetic and thermodynamic stabilization, respectively, to the reactive Bi=N bond. The structures and properties of a series of (acylimino)pnictoranes (Ar3M=N COR and H3M=NCOCF3; M = P, As, Sb, Bi) are compared by IR and C-13 and N-15 NMR, X-ray crystallography, and ab initio molecular orbital calculations. It was found that the contribution of the M+-N=C-O- canonical form becomes more prominent and the single-bond character of the M=N bond increases prog ressively as the pnictogen atom becomes heavier. The Bi=N bond of (acylimin o)-lambda (5)-bismuthanes 3 possesses a highly polarized single-bond charac ter, probably due to the differences in orbital size and electronegativity between the bismuth and nitrogen atoms. Thermal decomposition of (aroylimin o)triaryl-lambda (5)-bismuthane 3f (o-Tol(3)Bi=NCOAr; Ar = 3,5-(CF3)(2)C6H3 ) produces a gel in dry conditions or aniline 12 (ArNH2) in slightly wet co nditions with a good recovery of tris(2-methylphenyl)bismuthane (4a). It is likely that the aryl isocyanate 13 (ArNCO) is produced during the thermoly sis via a concerted C --> N migration of the Ar group with an elimination o f the triarylbismuthonio group as bismuthane 4a. (Acylimino)triaryl-lambda (5)-bismuthanes 3 oxidize 1,1,2,2-tetraphenylethanediol, benzenethiol, meth anol, and ethanol to benzophenone, diphenyl disulfide, methyl formate, and acetaldehyde, respectively, in two different reaction pathways depending on the structure of the substrates. Compound 3d (o-Tol(3)Bi=NCOCCl3) transfer s the nitrenoid moiety to triphenylphosphane, triphenylarsane, and tris(2-m ethylphenyl)stibane to give the corresponding (acylimino)pnictoranes (Ar3M= NCOCl3; M = P, As, Sb) and 4a, suggesting that 3d is thermodynamically much less stable than their lighter pnictogen counterparts. The copper-catalyze d decomposition of 3 (o-Tol(3)Bi=NCOR) afforded N-acyl-o-toluidines 18 (o-T olNHCOR) via a Bi --> N migration of the tolyl group. The observed reactivi ties of (acylimino)triaryl-lambda (5)-bismuthanes 3 demonstrate a good leav ing ability of the bismuthonio group.