Carbodiphosphoranes, structure

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

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]. 

The number of theoretical investigations of transition metal complexes with carbodiphosphoranes and related divalent carbon(O) ligands is rather small. Quantum chemical calculations of the nickel complexes (CO) Ni-C(PPh3)2 with n = 2, 3 have been pubhshed together with experimental work which describes the S3mthesis and X-ray structure analyses of the compounds [107]. The first systematic... 

Carbodiphosphoranes of type 89 [209] have been known for many years. The bonding parameters and the reactivity indicate that the electronic situation in these compounds is reasonably described by the polar structure 89 with two electron pairs at the central carbon atom, although alternative representations (89" and 89 ") are also possible (Fig. 28) [210]. 

The intermediate of a Wittig reaction has been isolated (26) and structurally characterized (60) from the reaction of HFA with a carbodiphosphorane. 

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(II) compounds (carbenes) through the number of lone electron pairs. The structures 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. 

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

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

Unstabilised a-iodoalkyl ylides (1) have been prepared and used in Wittig reactions to give 1-iodoalkenes with (Z)-stereoselectivity. The P-fluoro-substituted ylides (3), containing a chiral alkoxy ligand, are formed as 1 1 mixtures of diastereoisomers from the corresponding alkoxydifluorophosphoranes (2). However, (3) can be epimerised to predominately (>95% in one case) one diastereomer by treatment with lithium fluoride. The first carbodiphosphoranes (4) and (6) containing a P-H bond have been reported. Compound (4) is thermally stable in the solid state but in solution at room temperature both (4) and (6) slowly isomerise to the ylides (5) and (7), respectively. The structures of all these products are supported by n.m.r. data. 

Persistent carbenes were introduced into synthetic chemistry by Bertrand, who isolated the first carbene Rj P-C-SiR 3 in 1988 [13], and by Arduengo [14], who synthesized the N-heterocyclic carbenes (NHCs) in 1991. Stable carbones were actually isolated much earlier than carbenes. In 1961, Ramirez et al. [15] reported on the synthesis of the compound hexaphenyl-carbodiphosphorane ClPPhjlj- The authors assumed that the molecule has a linear structure PhjP=C=PPh3, which they described with resonance structures in terms of double bonds between phosphorous and carbon and as a double ylide Ph3P=C=PPhj Phj P(-l-)-C(2—[-(-l-jPPhj. 

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