Pentacoordinated phosphorus

Fig. 1. Berry pseudorotation about pentacoordinate ( ) phosphorus, where (Q) represent fluorine atoms, (a) Original trigonal bipyramid (b) square...

This would explain the fact that groups which form stable radicals, e.g., benzyl, allyl, or phenacyl, are easiest to remove electrolytically, and that tertiary alkyl groups are removed more readily than secondary or primary. This mechanism is more likely than one previously suggested, which involves a pentacoordinate phosphorus hydride ( 70) 61 > ... 

Hypervalent Structures with a Pentacoordinated Phosphorus and Two Five-Membered Rings... 

Later on, an intermediate with a pentavalent pentacoordinated phosphorus atom (103) was suggested on the basis of kinetic examinations [73],... 

The cyclic phosphoranes so obtained demonstrate properties typical for the compounds with pentacoordinated phosphorus atom (Scheme 30) [78, 83],... 

Pentacoordinate phosphorus offers an example of the application of EHT to covalent compounds that do not contain carbon (34). There are two possible high-symmetry structures for PHS, namely, a D3h trigonal bipyramid and a C4v square pyramid. The energies and shapes of tire MO s for each of the two are given in Fig. 26. For the latter, the optimal value of a was found to be 99.8°. Still another structure was considered ... 

Orbital symmetry arguments and EHT calculations have also provided a way of discriminating between axial and apical substitution in the above mentioned case of pentacoordinate phosphorus. This analysis leads the way to more complex problems of coordination around transition metal atoms. 

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

Intramolecular alkyl transfer is a fundamental problem with this reaction this problem can be addressed with modification in structure of the reagents. Neutral trivalent phosphorus reagents do react with carbonyl compounds at much lower temperatures, but lead to several types of pentacoordinated phosphorus products.190-198 More will be noted about the use of such pentacoordinated phosphorus species for carbon-phosphorus bond formation in Chapter 5. 

In this chapter, we consider two aspects of carbon-phosphorus bond formation as they relate to pentacoordinated phosphorus species. The first aspect is the preparation of stable phosphorane species — compounds bearing five bonds to phosphorus with at least one of them being a C-P linkage. At present, this is an area of rather specialized interest, but one that has potential for broader applications. 

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. 

Holmes, R.R., Reaction mechanisms, in Pentacoordinated Phosphorus—Reaction Mechanisms, ACS Monograph 176, American Chemical Society, Washington, D.C., Volume II, Chapter 2, 1980, p. 87. 

Stepwise mechanism . The possibility exists that the metal-catalyzed reaction really proceeds as described in Equations (18) plus (20), where the actual kinetic steps involve formation of a pentacoordinate phosphorus intermediate, suggested to be akin to 241. When viewed within the context of the More-O Ferrall Jencks... 

The traditional limitation of the stereoisomer concept to isomeric compounds with different configurations stems from the idea that configurations are not interconvertible under the observations conditions. ) A definition of stereoisomers on this basis is questionable as its application is then dependent upon observation conditions. It also fails to account for the fact that many mobile interconversions of configurations (e.g. configurations involving tricoordinate Nitrogen or pentacoordinate Phosphorus) are known, as well as thermally stable conformations (e.g. the atropisomers). 

In 2006 Terada and Kouchi reported the investigation of phosphonium salts in catalysis [112]. A pentacoordinated phosphorus atom is a hypervalent [113] atom, which has a formal valence shell of more than eight electrons. As shown in Scheme 40, it is possible for the lower lying o orbital of a P -EWG (Electron Withdrawing Group) bond to take up a free electron pair of a Lewis base, in order to form a new bond. If the new formed bond is trans to the EWG, the formed complex is more stable. 

The recent review literature on pentacoordinate phosphorus is dominated by two books by Holmes240,241. These compounds have occupied an important place in the development of molecular chemistry, having generated much discussion in the past concerning their structure, dynamic stereochemistry and their non-octet bonding. All of these important issues have been treated in detail by Holmes240,241 and not much significant... 

R. R. Holmes, Pentacoordinated Phosphorus, Vol. 1, Structure and Spectroscopy, ACS Monograph 175, American Chemical Society, Washington, DC, 1980. 

R. Luckenbach, Dynamic Stereochemistry of Pentacoordinated Phosphorus and Related Elements, Georg Thieme, Stuttgart, 1973 (a) p. 109-124 (b) 141-179. 

The equilibria P(III) P(V), P(IV) P(V) and P(VI) P(V) show that phosphoranes constitute pivots , allowing pentacoordinated phosphorus to be reached from any other coordination number, as probably postulated for the first time in 1929 by Fenton and Ingold6, and later observed many times experimentally. 

The importance of equilibria in the chemistry of pentacoordinated phosphorus arises from the possibility of passing from a given structure to another, even if the latter has a totally different molecular geometry and distribution of electrons, by a mere shift of hybridization. This has important consequences on the chemical properties of phosphoranes and on the possibilities of interconversion of isomers by bond rupture and recombination processes akin to regular true stereomutations, i.e. without bond cleavage. 

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

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