Since the discovery of the first stable carbene in 1988 in our group (phosphinosilylcarbene), carbene chemistry has been largely developed allowing to get more insight into their stabilization mode. Stable singlet carbenes can be classified in two main types: Push-Pull (phosphinosilylcarbene) or Push-Push (NHC).
Surprisingly, probably the most representative push-pull carbene was unknown, ie phosphino(boryl)carbene 1. We have recently described its preparation by deprotonation of the corresponding carbocation (J. Am. Chem. Soc. 2010, 132, 8864). The reactivity study has shown a pronounced ambiphilic character, typical of transient carbenes, illustrated by reaction with electron deficient olefins and aldehydes Chem. Eur. J. 2014, 20, 297). More interestingly, carbene 1 also exhibits a very original reactivity towards small molecules such as CO2 and SO2 (Angew. Chem. Int. Ed. 2012, 51, 2489).
Cyclic Stable Carbenes with a Pronounced Ambiphilic Character
The ambiphilic character of carbenes (s-donor and p-acceptor, small energetic HOMO-LUMO gap) is a key point of stable carbenes technology that governs activities in organocatalysis or coordination with transition metal. The development of new family of carbene allowing to tune easily the singlet-triplet gap is therefore extremely important. In this vein, we have developed a new synthetic pathway to prepare cyclic push pull carbenes, namely azavinylidenephosphoranes 2 that exhibit an important ambiphilic character (Gap S/T: 24.3 kcal/mol). These compounds keep the transient carbenes reactivity (C-H insertion reaction, cyclopropanation) but they are also excellent ligands for transition metals (Chem. Eur. J. 2014, 20, 12528).