Abstract
The synthesis, structure, and reactions of (acylimino)triaryl-λ⁵-bismuthanes and a comparative study of the structure and reactivity of a series of (acylimino)pnictoranes are reported. Treatment of ortho-substituted triarylbismuth dichlorides 1 (Ar₃BiCl₂; Ar = 2-MeC₆H₄, 2-MeOC₆H₄, 2,4,6-Me₃C₆H₂) with amides 2 (H₂NCOR; R = CF₃, CCl₃, 3,5-(CF₃)₂C₆H₃) in the presence of 2.2 equiv of KO-t-Bu in dichloromethane afforded (acylimino)triaryl-λ⁵-bismuthanes 3 (Ar₃Bi=NCOR) in yields of 77–96%. The ortho-substituted 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=NCOR and H₃M=NCOCF₃; M = P, As, Sb, Bi) are compared by IR and ¹³C and ¹⁵N 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 progressively as the pnictogen atom becomes heavier. The Bi=N bond of (acylimino)-λ⁵-bismuthanes 3 possesses a highly polarized single-bond character, probably due to the differences in orbital size and electronegativity between the bismuth and nitrogen atoms. Thermal decomposition of (aroylimino)triaryl-λ⁵-bismuthane 3f (o-Tol₃Bi=NCOAr; Ar = 3,5-(CF₃)₂C₆H₃) produces a gel in dry conditions or aniline 12 (ArNH₂) in slightly wet conditions with a good recovery of tris(2-methylphenyl)bismuthane (4a). It is likely that the aryl isocyanate 13 (ArNCO) is produced during the thermolysis via a concerted C → N migration of the Ar group with an elimination of the triarylbismuthonio group as bismuthane 4a. (Acylimino)triaryl-λ⁵-bismuthanes 3 oxidize 1,1,2,2-tetraphenylethanediol, benzenethiol, methanol, 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-Tol₃Bi=NCOCCl₃) transfers the nitrenoid moiety to triphenylphosphane, triphenylarsane, and tris(2-methylphenyl)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-catalyzed decomposition of 3 (o-Tol₃Bi=NCOR) afforded N-acyl-o-toluidines 18 (o-TolNHCOR) via a Bi → N migration of the tolyl group. The observed reactivities of (acylimino)triaryl-λ⁵-bismuthanes 3 demonstrate a good leaving ability of the bismuthonio group.