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 I_Ar [: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 = I_Ar (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.