Diaminocarbenes stable singlet carbene

Since this discovery, a few types of other stable singlet carbenes have been described and reviewed5 imidazol-2-ylidenes,6 l,2,4-triazol-3-yli-denes,7 imidazolidin-2-ylidenes,8 acyclic diaminocarbenes,9 thiazol-2-yli-denes,10 and acyclic aminooxy- and aminothiocarbenes.11... 

Most carbenes R-C-R, in which R and R are H, alkyl, vinyl, or aryl, are too reactive to be observable inside a hemicarcerand. " For example, incarcerated arylcarbenes rapidly insert into C-H or C-0 bonds of the hemi-carcerand or add to one of the cavitand s aryl units even at very low temperature. " However, stability and reactivity of carbenes can be tailored especially with heteroatom substituents that stabilize the carbene s singlet state through electron donation (push effect). " " In fact, many diaminocarbenes are stable and isolable at room temperature. " " In cases where intrinsic stabilization (e-donation) is not sufficient, extrinsic effects (incarceration) may render an otherwise fleeting singlet carbene stable under normal conditions. 

Reviews have appeared on the preparation, structure and typical singlet carbene reactivity of phosphinocarbenes (1). Their properties as ligands for metal cations have been covered. The shorter history of another class of stable carbenes, the diaminocarbenes (2), has also been reviewed. ... 

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