Mechanism, catalysis and predictions of 1,3,2-diazaphospholenes: theoretical insight into highly polarized P–X bonds

Abstract

Density functional theory (DFT) calculations were carried out to investigate the hydridic character of several main group hydrides. A P-hydrido-1,3,2-diazaphospholene 1f with two π-electron donor amino groups on the heterocyclic skeleton framework performs as a strong hydride donor owing to the significant n(N)–σ*(P–H) hyperconjugation. The natural bond orbital analysis reveals that high π-electron delocalization exists in both 1f and the corresponding stable phosphenium E_f⁺. In addition, 1f is calculated to have a similar catalytic ability for the hydroboration of acetone with pinacolborane, compared to 1e. Thus, a variety of organic substrates activated by 1f are explored, including ketone, imine, isocyanate, CO₂, diazene, alkene, alkyne and epoxide. The results show that the highly polarized and electron-deficient bonds such as CO π bonds are readily activated, whereas 1f seems difficult to react with electron-rich unsaturated bonds of propene and propyne. More importantly, 1,3,2-diazaphospholene-based compounds, featuring an extremely polarized P–X bond (X = CCMe, NMe₂, PMe₂ and SMe), are predicted to have a useful catalytic ability. The preliminary computational results suggest that these P–X compounds could catalyze the silylamination, silylphosphination and silylsulfenylation of acetone with TMSNMe₂, TMSPMe₂ and TMSSMe, respectively. The products are silylethers, which are equivalent to the corresponding alcohols since they easily undergo hydrolysis. Our computational study opens a new avenue to the design of novel main group organocatalysts.