Stable singlet carbenes, electronically active

Up to 2000, the number and variety of stable carbenes have been limited by the perceived necessity for two strongly interacting substituents. The preparation of stable or persistent (aryl)- or (alkyl)-(phosphino)carbenes as well as (aryl)(amino)-carbenes demonstrates that a single electron-active substituent allows the spectroscopic characterization of singlet carbenes under standard laboratory conditions. It has been shown that an amino substituent is more efficient than a phosphino substituent to stabilize a carbene center and that the steric bulk of the spectator substituent can be decreased even to the size of a methyl group in the phosphino series, so that a broad range of observable carbenes will be readily available. 

In the last ten years, it has been shown that although bis(amino)carbenes such as 25 do not behave as classical transient singlet carbenes (no cyclopropanation reaction, no 1,2-shift, no C-H insertion...), they are wonderful ligands for transition metals which have very high catalytic activities. [27] Their three center four k electron system makes them strong a-donor and poor Ji-attractor. In other words, they constitute a new class of Lewis base. Despites these facts, in the last two highlights of Angew. Chem. [28] devoted to stable carbenes, only bis(amino)carbenes were considered, without any mention of our work This prompted us, to work harder and harder to demonstrate that our carbene 17 was a true carbene, and that it was even possible to obtain other types of stable carbenes, which are closer to the transient species. 

---