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

The reaction of the pentaphenylborole [(PhC)4BPh] (1) with sodium phosphaethynolate·1,4-dioxane (NaOCP­(1,4-dioxane)1.7) 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)1.7 and a Lewis base such as the N-heterocyclic carbene IAr [:C­{N­(Ar)­CH}2] (Ar = 2,6-iPr2C6H3) and amidinato amidosilylene [{PhC­(NtBu)2}­(Me2N)­Si:] afforded the Lewis base-pentaphenylborole adducts [(PhC)4B­(Ph)­(LB)] (LB = IAr (3), :Si­(NMe2)­{(NtBu)2CPh} (4)), respectively; (ii) the reaction of 1 with the carbodiimide ArNCNAr afforded the seven-membered B/N heterocycle [B­(Ph) (CPh)4C­(NAr)­N­(Ar)] (5). Compounds 2–5 were fully characterized by NMR spectroscopy and X-ray crystallography.