Carbodiphosphoranes

Phenyl[(trimethylsilyl)methylene]phosphinous chloride is a yellow oil that solidifies at---10° and boils at 46° (10 torr). The proton-decoupled phosphorus 

In analogy to the chlorophosphines, chloro(methylene)phosphine undergoes substitution reactions via HCl elimination that result in P-N, P-O, P-S, and P-P linkages. Additionally, a scrambling reaction takes place with aminophos-phines similar to the scrambling reactions of the chlorophosphines in which the phosphorus has coordination number 3.  

Twenty years after the discovery of the methanetetraylbis(phosphoranes) (carbodiphosphoranes), preparative chemists have an interest in this class of compounds because of its potential as a precursor to other materials. The first of 

This molecule, in contrast to other carbodiphosphoranes for which X-ray structural measurements have been made, has a PCP angle of exactly 180°.  

To a solution of 8.2 g (22 mmol) of the phosphonium salt dissolved in 100 mL of THF contained in a 250-mL flask equipped as above is added 2.4 g (100 mmol) of sodium hydride. 

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Carbodiphosphoranes (R3P = C = PR3) are known,79 but ylides with a P-H bond are rare.80 Therefore, the spectroscopic characterization of 77 was unexpected. Even more surprising was the characterization of the carbodiphosphorane 79 featuring two P-H bonds.31 This compound, prepared by treatment of 2d with tert-butyllithium, rearranged in solution at room temperature over a period of 16 h to afford the phosphorus ylide 80 with one remaining P-H bond. This compound was also unstable and transformed completely into the diphosphinomethane 81 overnight. Note that calculations for the model compounds where R = NH2 predicted 79 to be 28 kcal/mol less stable than 80, which is also 34 kcal/mol above 81.16 The surprising stability of 79 and 80 is probably due to the presence of bulky substituents, since tetracoordinate phosphorus atoms can more readily accommodate the increased steric constraints than can their tricoordinate counterparts. 

In a less highlighted, but - perhaps - not less promising, manner, the story proceeded with the first insights into the organometallic chemistry of yUde and related carbodiphosphorane ligands, involving a pure simple metal-carbon bond (M-CR jP lls) [8-13]. 

A. The ylidic alkyls or simply ylides and related carbodiphosphoranes... 

Class A. Neutral -Carbon Ligands Ylides and Carbodiphosphoranes... 

Scheme 14 Structures found for the carbodiphosphorane Ph3P=C=PPh3 (type II)...

After this brief discussion of the reactivity and applications of ylides, we return to the K C bonding mode, introducing new ligands carbodiphosphoranes and cyclopentadienyl ylides. 

Abstract The theoretical and experimental research on carbodiphosphoranes C(PR3)2 and related compounds CL2, both as free molecules and as ligands in transition metal complexes, is reviewed. Carbodiphosphoranes are examples of divalent carbon(O) compounds CL2 which have peculiar donor properties that are due to the fact that the central carbon atom has two lone electron pairs. The bonding situation is best described in terms of L C L donor acceptor interactions which distinguishes CL2 compounds (carbones) from divalent carbon(ll) compounds (carbenes) through the number of lone electron pairs. The stmctures and stabilities of transition metal complexes with ligands CL2 can be understood and predictions can be made considering the double donor ability of the carbone compounds. 

Keywords Carbodicarbene Carbodiphosphorane calculations Transition metal complex... 

Transition Metal Complexes with Cyclic Carbodiphosphoranes. 82... 

Carbodiphosphoranes C(PR3)2 and related ligands CL2 which are also termed double ylides differ from the other carbon ligands which are discussed in this volume in the number of lone electron pairs at the carbon donor atom. Ylides, carbenes, allenylidenes, and cumulenylidenes have one lone electron pair but carbodiphosphoranes have two lone electron pairs with a and n symmetry. The bonding situation in a carbodiphosphorane (a special class of CL2 compounds, abbreviated as CDP) is best described in terms of donor-acceptor bonding between the phosphane ligands which serve as a donor and a naked carbon atom in an... 

Fig. 1 (a) Schematic representation of the bonding situation in carbodiphosphoranes. (b) Schematic representation of the donor-acceptor interaction in a divalent C(0) compound CL2 between a carbon atom in the electronic D state with the electron configuration 2pJ 2s°... 

Fig. 2 Schematic representation of the bonding situation in carbodiphosphoranes using Lewis...

The theoretical studies of carbodiphosphoranes and related divalent carbon(O) compounds suggest that the CL2 species should be very strong c donors but they should also possess a decent n donor strength. The theoretical and experimental investigations which are discussed below agree with the classification. 

Fig. 6 Various substituted carbodiphosphoranes which have been synthesized...

A series of heteroatom substituted carbodiphosphoranes C PR2ECH(CF3)2 2 have been prepared in the last 10 years by various groups as shown in Fig. 6. The main synthetic approach consists of the reaction of hexafluoroacetone or thioacetone with the related diphosphines R2P-CH2-PR2 [25, 26]. The bent structure with a P-C-P angle (140°) confirms the double ylidic nature [27] and a related chemistry to C(PPh3)2 is expected however, no reports about coordination activities were reported so far. Theoretically, double alkylation at the heteroatoms of the dianion in Fig. 9 would lead to the substituted carbodiphosphoranes. The amino derivative C(P NMe2 3)2 has a linear structure and was not investigated further [28]. 

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