Amino-phosphinocarbene

So far, all stable phosphinocarbenes feature two amino groups at phosphorus and a trimethylsilyl group at carbon with only two exceptions (i) the [2,2, 6, 6 - tetramethylpiperidino) (phenyl) phosphino] (trimethylsilyl) -carbene 2c,27 which is stable for 1 day in solution at room temperature, but for weeks at -20°C and (ii) the [bis(diisopropylamino)phosphino]-[bis(diisopropylamino)phosphonio]carbene 2d,28 which is stable for years in the solid state (mp 88°C). 

The (phosphino)(silyl)carbene 2a has been shown to exhibit this diagnostic reactivity. Under rather drastic conditions (300°C, 10 2 mm Hg), 2a has been converted into the azaphospholidine 37, in high yield, as a mixture of four diastereomers, which result from the presence of three asymmetric centers.4 The high regioselectivity of the carbene insertion (no formation of a four-membered ring was detected) is intriguing, especially since the same regioselectivity is observed upon thermolysis of bis(diisopropyl-amino)phosphinodiazomethane li.4 Yet, in contrast, four-membered heterocycles were obtained with other phosphinocarbenes (Sections V,C,1 and 3). 

In the past few years, a few other types of stable carbene have been discovered. All of them bear two 77-donor substituents, of which at least one is an amino group. In contrast to the phosphinocarbenes, they do not feature much of the reactivity of their transient cousins. However, they are excellent ligands for metal centers, and some of the complexes prepared show promising catalytic activity.53... 

From these data, it can be stated that an amino group is much more efficient for stabilizing a carbene center than a phosphino group. These results also resolve a controversy If an aminocarbene is considered a carbene despite the interaction between the amino group and the carbene center, a phosphinocarbene should also be considered a carbene. 

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