Diverse Bonding Activations in the Reactivity of a Pentaphenylborole toward Sodium Phosphaethynolate: Heterocycle Synthesis and Mechanistic Studies.

The reaction of the pentaphenylborole [(PhC)BPh] (1) with sodium phosphaethynolate·1,4-dioxane (NaOCP(1,4-dioxane)) afforded the novel sodium salt of phosphaboraheterocycle 2. It comprises anionic fused tetracyclic P/B-heterocycles that arise from multiple bond activation between the borole backbone and [OCP]anion. Density functional theory calculations indicate that the [OCP] anion prefers the form of phosphaethynolate O-C≡P over phosphaketenide O═C═P to interact with two molecules of 1, along with various B-C, C-P, and C-C bond activations to form 2. The calculations were verified by experimental studies: (i) the reaction of 1 with NaOCP(1,4-dioxane) and a Lewis base such as the N-heterocyclic carbene I [:C{N(Ar)CH}] (Ar = 2,6-iPrCH) and amidinato amidosilylene [{PhC(NtBu)}(MeN)Si:] afforded the Lewis base-pentaphenylborole adducts [(PhC)B(Ph)(LB)] (LB = I (3), :Si(NMe){(NtBu)CPh} (4)), respectively; (ii) the reaction of 1 with the carbodiimide ArN═C═NAr afforded the seven-membered B/N heterocycle [B(Ph) (CPh)C(═NAr)N(Ar)] (5). Compounds 2-5 were fully characterized by NMR spectroscopy and X-ray crystallography.