Oxidizing Ability of a Series of (Tropon-2-ylimino)pnictoranes (Pnictogen = P, As, Sb, and Bi) toward Some Alcohols

Abstract

Abstract In order to gain a better understanding of the oxidizing ability of a series of (tropon-2-ylimino)pnictoranes of the general structure Ph3M=NR (R = tropon-2-yl; M = P, As, Sb, and Bi), reactions were run with some alcohols such as benzopinacol (1,1,2,2-tetraphenyl-1,2-ethanediol), benzoin, cinnamyl alcohol, 1-phenylethanol, a mixture of cis- and trans-4-t-butylcyclohexanol, 2-phenylethanol, and 1-phenyl-1,3-propandiol. Iminophosphorane oxidized only benzopinacol to give benzophenone, while both arsorane and stiborane oxidized benzopinacol and benzoin to give benzophenone and benzil, respectively. On the other hand, iminobismuthorane has appreciable oxidizing ability, and reacted with the alcohols mentioned above, except the primary alcohol, to give the corresponding carbonyl compounds. Iminobismuthorane reacted with 1-phenyl-1,3-propandiol selectively to oxidize benzyl alcohol moiety, but not a primary alcohol moiety, to give 3-hydroxy-1-phenyl-1-propanone. Thus, the oxidizing ability of a series of (tropon-2-ylimino)pnictoranes is demonstrated to be in the order of iminophosphorane < iminoarsorane < iminostiborane < iminobismuthorane: the dipolar (degree of contribution of the Ph3M+–−NR canonical structure) and electrophilic character of pnictogen elements of a series of iminopnictoranes appear to increase their oxidizing ability when the pnictogen stands lower in the periodic table.