Pentacoordinate phosphorus compounds

Pentacoordinate phosphorus compounds are generally referred to as phosphoranes (Figure 1.7) or oxyphosphomnes depending on the... 

It is generally known that phosphorus is capable not only of forming species with five formal bonds to the phosphorus atom, but also that such compounds can be stable under ordinary conditions. We are concerned here with those pentacoordinate phosphorus compounds that have at least one formal carbon-phosphorus bond. We will focus primarily on the processes that generate such a bond. 

TABLE 1. Synthesis of some pentacoordinated phosphorus compounds starting from tricoordinated phosphorus derivatives No. P(III) Reagent P(V)... 

Most pentacoordinate phosphorus compounds prefer TBP structures. 

A Model for Calculating Conformational Energies in Pentacoordinate Phosphorus Compounds. R. R. Holmes, J. Am. Chem. Soc. 100, 433 (1978). 

The phosphoranes are derivatives of the pentahydride of phosphorus, PH6, in which the five ligands are covalently bonded to phosphorus. The stereochemistry of pentavalent phosphorus relates to the trigonal bipyramid, just as that of tetravalent carbon relates to the tetrahedron. There are significant differences in the stereochemistry of the compounds of these two elements, other than isomer numbers. Some tetracoordi-nated phosphorus compounds become pentacoordinated rapidly and reversibly. Most pentacoordinated phosphorus compounds change their ligand distribution on the trigonal bipyramidal skeleton very easily by mechanisms that involve the simple deformation of bonds, rather than the rupture and re-formation of bonds. 

For nomenclatural classification, a pentacoordinated phosphorus compound, P(L],L2,L3,L4,L5), with five different ligands, Lx. . . L5, attached to a skeleton of five positions identified by the indices s =1... 5, and of trigonal bipyramidal symmetry, D3h, can exist as 5 /6=20 isomers, or 10 pairs of enantiomers. To represent these isomers one defines a Reference Molecule ME in which the indices of all the ligands and the indices of all the skeletal positions coincide. Those indices will not coincide in the isomers of ME. The skeletal indices are assigned to the Reference Molecule according to some definitions,65 for example, as shown in ME. The ligand indices are assigned to the Reference Molecule... 

G. Robinet, M. Barthelat, J. Devillers, V. Gasmi, D. Houalla, and R. Wolf, Struct. Chem., 2, 11 (1991). Molecular Mechanics Extension of the MM2 Program to Pentacoordinate Phosphorus Compounds. 1. Conformational Study of the Bicyclic Phosphoranes of the Type R-PiHlNfCHjCH O) (R = H, Me, Ph). 

In a discussion of the role of pentacoordinate phosphorus compounds in biochemistry, Zhao et al. reported the isolation of a silicon-protected pentacoordinate phosphorus compound (148) by the cyclisation of (147) formed from (145) and (146). Phosphorane (148) was then diseussed as a model for the involvement of pentacoordinate phosphorus in activating the formation of peptides from amino acids such as histidine, serine, threonine and oc-alanine but not (3- alanine. 

Dynamic processes causing ligand permutation in pentacoordinate phosphorus compounds constitute a well-studied area (148). As illustrated in Sect. V-A, various authenticated penta- or hexacoordinate organosilanes have been reported. However studies of the isomerization processes of these compounds are limited. Such processes are of importance, since penta- or hexacoordinate intermediates are implicated in substitution reactions or in racemization of tetra-coordinate silicon compounds. The stereochemical outcome of such reactions is therefore related to the configurational stability of penta- or hexacoordinate intermediates. 

The stability of pentacoordinate phosphorus compounds has been discussed in terms of apicophilicity of the ligands. The isomerization of these compounds arises from intramolecular ligand permutation via Berry pseudo-rotation or turnstile rotation. It is of interest to know whether the concepts developed in the field of phosphorus chemistry are applicable or not to related organosilicon compounds. It is the aim of this Section to critically review recent studies of the dynamic stereochemistry of pentacoordinate organosilanes. 

There follows a series of four papers by Houalla et al. on further extensions of the Atherton-Todd reaction to the synthesis of pentacoordinated phosphorus compounds with the phosphorus as a heteratom within a wide variety of macrocyclic structures. The first paper22 describes the reaction of 1-hydridobicyclophosphoranes containing a secondary amine group (e.g. 56) with (57) which leads to a cyclic / /s(bicyclophosphorane)-(58), as well as the symmetrical bicyclophosphorane (59). The opportunity to exploit the reaction in the synthesis of novel macrocyclic structures was too good to miss and compounds such as (62) were prepared from the corresponding 6/s-phosphorane (60) and the Ws-phenol (61).23... 

One particularly fascinating class of pentacoordinate phosphorus compounds is the class of chiral spirophosphoranes with amino acid residues as chiral chelate ligands since they can serve as important structural features relevant to chiral phosphoryl transfer pathways. Therefore, their synthesis and stereochemistry were recently widely explored." ... 

Based on the above observation, a reaction mechanism has been proposed, involving pentacoordinate phosphorus intermediates (48a) and (48b). Due to the strong affinity between fluoride and phosphorus, the nucleophilic attack of a fluoride anion to give cyclic phosphite (41a) or thiophosphite (41b) generates pentacoordinated phosphorus compounds... 

Interestingly, trigonal bipyramidal structures shown as chiral intermediates in Scheme 10.55 and common in pentacoordinate phosphorus compounds enjoy a small energy difference between the trigonal bipyramid and tetragonal pyramid structures. The consequence is that axial and equatorial groups (unless linked as in... 

O-Apical stereoisomers of pentacoordinated phosphoranes can be obtained by different routes, including the Kawashima s synthesis of the pentacoordinated phosphorus compound bearing a pentacoordinate silicon atom, both possessing Martin ligands in the structure or by the reaetion of 8-chloro-2-cyclohexyl-4-phenylbenzo[e]-1,2-oxaphosphinine-2-oxide with tetramethylenebis(magnesium bromide) as reported by Konovalov et al ... 

The intermediate pentacoordinate phosphorus compound exists probably in the trigonal bipyramid structure (60), facilitating the -A1(0R)2 group transfer. 

As in previous years, little new work has appeared relating to the synthesis of pentacoordinated phosphorus compounds. For example, a novel... 

Pentacoordinated and hexacoordinated compounds Romana Pajkert and Gerd-Volker Rdschenthaler 1 Introduction 2 Pentacoordinated phosphorus compounds 3 Hexacoordinated compounds References 354 354 354 366 374... 

Appropriate structural and kinetic studies of pentacoordinated phosphorus compounds in solution and solid state have been further investigated. Some... 

The synthetic routes to pentacoordinated phosphorus compounds have continued to generate considerable interest, however little new works has... 

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