Phosphinocarbenes Phosphino carbenes

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). 

Table II lists all pertinent chemical shifts and coupling constants for the known phosphinocarbenes and their respective diazo precursors. The (phosphino)(silyl)carbenes are all characterized by high field chemical shifts for phosphorus ( 24 to 50 ppm) and silicon (-3 to -21 ppm), and low field chemical shifts for carbon (120 to 143 ppm) with large couplings to phosphorus (147 to 203 Hz).

The great advantage of the (phosphino)(phosphonio)carbene 2d over its silyl analogues is that it can be crystallized. As we have seen, these two types of phosphinocarbenes are very similar thus, conclusions from the X-ray analysis of 2d28 can probably be extended to the silylcarbenes. Ball and stick views of the molecule are shown in Fig. 2, and the pertinent geometric parameters are in the legend. 

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). 

Among the very rare phosphinocarbenes that undergo dimerization are the (phosphino)(phosphoryl)carbenes 2r. Two examples of direct head-to-tail dimerization leading to the corresponding 1 A5,3A5-disphosphetes 73i2 have been reported and occur as a result of the multiple bond character of transient carbenes 21 2.77... 

This is the second type of stable phosphinocarbene to have been reported. In contrast to (phosphino)(silyl)carbenes, very little is known concerning their reactivity. No simple reactions typical of a carbene-like behavior have been reported. However, the reactivity at the periphery of the carbene center makes these derivatives powerful building blocks for novel compounds. 

The exact nature of the bonding system in phosphinocarbenes was clarified to some extent by the X-ray analyses performed on the (phosphino)(phosphonio) carbene (Ila), and more recently on the (phosphino)(silyl)carbene Ib. ... 

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. 

In marked contrast, direct complexation of (phosphino)(silyl)carbenes I has not yet been reported, and only a few phosphinocarbene complexes are known. ° The reluctance of carbenes I to act as ligands has recently been rationalized theoretically. However, in 2002, the hrst complexes featuring the stable (aryl)... 

FIGURE 5. Possible structures of phosphinocarbenes and phosphino(silyl)carbenes and the calculated structures for singlet and triplet species bond lengths (A) and bond angles (°) are given... 

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