Why do N-heterocyclic carbenes and silylenes activate white phosphorus differently?

Abstract

We explored the possible reaction of three model carbenes and two model silylenes with white phosphorus leading to insertion, bis-adduct, and tris-adduct products. The energetics of the calculated mechanisms are in accordance with the experimental observations explaining key features like carbene–P4 ratio-dependent reactivity of carbonyl decorated bisamidocarbene and cyclic alkyl-amino carbene. The analysis of the bonding motif of C–P and Si–P bonds in the key intermediate revealed the importance of double-bond and donor–acceptor resonance structures. We found correlation between the σ-donor and π-acceptor strength of investigated carbenes and silylenes and their reactivity with white phosphorus. The combination of low σ-donor and high π-acceptor strength in N-heterocyclic silylene (NHSi) indicated the instability of the key intermediate, bis-adduct, and tris-adduct products and the stability of the insertion product, while high σ-donor and low π-acceptor strength of N-heterocyclic carbene (NHC) resulted in reversed stability order of the previous compounds explaining the different reaction of NHC and NHSi with white phosphorus. Donor-stabilized silylene (DSSi) has high σ-donor and low π-acceptor strength similar to NHC supporting their similar reactivity and the difference between NHSi and DSSi.