Phosphorus pentacoordination

The lack of direct evidence for TBP intermediates has allowed the proposal of alternative pentacoordinate intermediates and of hexacoordinate intermediates in nucleophilic substitution reactions at tetrahedral phosphorus. Pentacoordinate phosphorus species (spirocyclic phosphoranes) have been observed in which the most stable form adopts the square pyramidal (SP) geometry (Howard et ai, 1973). Such SP species have been postulated as intermediates in substitution reactions at tetrahedral phosphorus (Boudreau et ai, 1975). 

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

ATP is activated by coordination to magnesium ion, and nucleophilic addition of a fatty acid caiboxylate to phosphorus then yields a pentacoordinate intermediate. . ... 

Three- and pentacoordinate organic phosphorus compounds can be oxidized through a free radical Arbuzov reaction, i.e., formation and p-scission of a phosphoranyl radical (Scheme 24). The P-scission is regioselective homolysis occurs on a ligand located in an equatorial site. Both a- and P-scissions are strongly dependent on the strength (bond dissociation energy) of the cleaved... 

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 > ... 

Phosphorus at the core can be tri-, tetra- or pentacoordinated and therefore a number of reactions on the core can be envisaged. 

The chlorine atom of 43 may be displaced readily by carbon-based nucleophiles with predominant overall retention of configuration at the phosphorus atom, similar to that observed in pentacoordinated analogues [37, 38], In some reactions, unselective substitution was observed [39], This approach was successfully applied for the... 

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. 

A review of the chemistry of compounds containing a phosphorus-chalcogen bond includes mention of dithiadiphosphetanes <00YGK208>. The first stable pentacoordinate 1,2-thiaphosphetene 47 has been described <99PS119> and l,2-thiaphosphole-2-sulfides 48 have... 

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. 

Common "parent" phosphorus-halogen species include particularly the P(III) system PC13 and the P(V) species P(0)C13. Of course, the pentacoordinated species PC15 must also be considered for its availability as reagent precursor. These are species that often serve not only in a direct manner in carbon-phosphorus bond formation but also in the construction of partially or fully esterified species prior to carbon-phosphorus bond incorporation. 

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). 

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