Phosphinocarbene

Striking feature of 25 is the slight pyramidalization around the N1 and N2 centers, which are generally flat in other diaminocarbenes. Finally, carbene 25 revealed a very deshielded carbene center with a C-NMR signal at 5 285 ppm, which is 100 ppm downfield from that of phosphinocarbene 24. 

In this review, we present successively the theoretical aspect, the synthesis, the structural features, the reactivity, and lastly, the ligand properties of phosphinocarbenes. 

Several theoretical investigations have been carried out for the phosphinocarbenes (Fig. 1).13-16... 

Fig. 1. Calculated geometric parameters for singlet and triplet phosphinocarbenes (bond lengths in A bond angles in degrees).

In summary, independently of the second carbene substituent, phosphinocarbenes have a singlet ground state, with a small singlet-triplet gap. They have a planar geometry at phosphorus and a short phosphorus-carbon bond, indicating an interaction between the phosphorus lone pair and the carbene vacant orbital. In the case of silyl- and phosphoniophosphinocar-benes, there is an additional interaction between the carbene lone pair and lowlying [Pg.179]

Most of the transient and all the stable phosphinocarbenes 2 have been prepared from the corresponding diazo derivatives 1. However, three other... 

A number of phosphinocarbenes bearing various substituents at phospho-... 

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). 

In 1985, Fluck et al. reported that treatment of the P-fluoromethylene-phosphorane 3f with 2 equiv of butyllithium at -95°C gave the 1,1,3,3-tetrakis(dimethylamino)-lA5,3A5-diphosphete 5f in 34% yield.30 The formation of this four-membered ring could result from a [2 + 2] head-to-tail dimerization of the transient phosphinocarbene 2f. However, an alternative... 

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).

Pertinent Chemical Shifts (in ppm) and Coupling Constants (in Hz) for the Phosphinocarbenes and their Diazo Precursors... 

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. 

Not much is known about the reactivity of the phosphinocarbene 2i. Problems arise, at least in part, from the high 1,3-dipolar reactivity of the diazo precursor li, which hides any carbene reactivity. Indeed, although li is stable in a toluene solution at 60°C for hours, the addition of an electron-poor olefin, such as a perfluoroalkyl-monosubstituted alkene, induces the exclusive formation of the thermodynamically more stable anti-isomer of the cyclopropane 14 (see Section V,B,3,a).36 This clearly demonstrates that the cyclopropanation reaction does not involve the carbene 2i, but that an initial [2 + 3]-cycloaddition occurs leading to the pyrazoline 13, which subsequently undergoes a classical N2 elimination.37... 

The formation of the phosphoranyl alkyne 19, in the spontaneous room temperature decomposition of the phosphinodiazoketone lm, has been explained in terms of an intramolecular Wittig-type reaction involving the phosphorus-vinyl-ylide form of the phosphinocarbene 2m.39... 

These are by far the most studied phosphinocarbenes, mainly because of their stability. As often as possible, their reactivity will be compared with that of standard transient carbenes. 

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). 

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