Phosphinocarbenes reactivity

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

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... 

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). 

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. 

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... 

These considerations clearly suggest that kinetic stabilization is a far better way to stabilize the triplet states of carbenes than thermodynamic stabilization. It is also important to note that thermodynamic stabilization usually results in the perturbation of electronic integrity of the reactive center, as has been seen in the case of phosphinocarbene and imidazol-2-ylidene (112). " On the other hand,... 

However, the carbene character of Bertrand s phosphinocarbene 89 was initially doubted, as its chemical reactivity appeared to resemble moreaphosphaacetylene90 (Dixon etal. 1991 Soleihavoupetal. 1992 Bourissou and Bertrand 1999), but recent studies seem to support its carbene character (Scheme 20) (Despagnet et al. 2002). 

When compared to nitrogen, phosphorus is much more reluctant to achieve a planar configuration with sp hybridization. [31,32] The ensuing smaller stabilizing effect of phosphorus compared to nitrogen is illustrated by the small singlet-triplet gap predicted for the (phosphino)(silyl)earbenes (5.6-13.9 kcaPmol) [29] compared to that calculated for acyclic as well as cyclic diaminocarbenes (58.5-84.5 kcal/mol). [33] This means that the commitment of the lone pair to donation into the vacant orbital on the divalent carbon atom is less definitive for phosphorus than for nitrogen and thus, the phosphinocarbenes retain more of a divalent-carbon behavior. This is well illustrated by their reactivity. 

The great reactivity of isonitriles towards phosphinocarbenes, while no carbene dimerization occurs, can easily be explained in terms of steric factors the reactive site of RN=C is comparatively unhindered. 

Carbenes are chemical species that possess a bivalent carbon atom with two nonbonding electrons. The divalent carbenes were considered as highly reactive intermediates, and the isolation of stable carbenes has been a challenge for a long time. Based on Wanzlick et al. s work in 1960, Arduengo et al. [1] reported the synthesis of stable imidazolium carbenes in 1991. Independently, Bertrand and coworkers [2] reported the synthesis of phosphinocarbene in 1988. In 1995, Enders et al. [3] reported the synthesis of triazolium carbenes. 

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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