Valency expansion of phosphorus

The reaction between trialkyl phosphites and a,P-unsaturated nitriles has been less thoroughly investigated. Whereas the hydrophosphonylation of unsaturated nitriles with diakyl phosphites proceeds smoothly, the addition of trialkyl phosphites requires more severe conditions. It involves the nucleophilic attack by the trivalent phosphorus reagent at the tenninal carbon atom of the conjugated system followed by valency expansion of phosphorus in agreement with the Michaelis-Arbuzov mechanism (Scheme 6.27). - ... 

This process, which we will refer to as valency expansion of phosphorus, appears to be an important and unique aspect of phosphorus chemistry. It represents a valence shell expansion of phosphorus from 8 to 10 electrons made possible by the availability of low-energy... 

Since it is reasonably well established that the Michaelis-Arbuzov reaction occurs in two stages, via an ionic phosphonium intermediate which decomposes by a valency expansion of phosphorus, the overall process may be formulated more generally as ... 

The more detailed recent studies of homologous reactions by Denney and DiLeone (90) support the concept of initial formation of [ClP(OR)3] R 0 followed by exchange to furnish [R OP(OR)3]+ Cl , which decomposes by valency expansion of phosphorus. 

The clearest evidence for characterization of the second step as bi-molecular nucleophilic substitution with valency expansion of phosphorus, symbolized as SN2 (VE), or more briefly as VE2, is provided by the stereochemical studies of Gerrard and Green (107). Although less studied, the apparent unimolecular valency expansion (VEl) of the alkoxyphosphonium intermediate deriving from tertiary alcohols, touched upon in a previous section (cf. Section II-A-1), represents the other extreme of this mechanistic type. 

Despite some controversy regarding the precise reaction pathway (210), it is generally accepted that its nature is ionic and that at some stage in the overall mechanism there occurs the process of valency expansion of phosphorus. 

Nucleophilic attack of phosphite on the easily polarizable disulfide linkage displaces a mercaptide ion which is alkylated in a valency expansion of phosphorus to produce a thioether (50,125,143,218,257) ... 

The oxime from (4-chloro-l-oxobutyl)phosphonic acid has been cyclized to Pro ". An unusual rearrangement based on valence expansion of phosphorus is of interest the treatment of an (oxoalkyl)phosphonic oxime with Ph2PCl initially yields the phosphorus(III) derivative, but this rearranges spontaneously to give a phosphinic amide derivative 300, reduction of which then affords the [(V-diphenylphosphinoylamino)alkyl]phosphonic acid, readily hydrolysable under acid conditions to the free (aminoalkyl)phosphonic diester (Scheme 34)" ". 

With phthalic anhydride and trialkyl phosphites, the reaction takes V an entirely different course, the main products being biphthalyl (70%) and trialkyl phosphate. It was suggested that the phosphorus atom of a phosphite ester attacks the oxygen of the anhydride carbonyl, forming an intermediate which undergoes valency expansion to generate in this unique instance a carbene and a phosphate ester (276). Dimerization of this resonance-stabilized carbene w ould furnish the product. 

On the basis of such evidence, it now seems to be widely accepted that the intermediates in valence expansion reactions of the Michaelis-Arbuzov type can have either an ionic, or a non-ionic, pentacoordinate structure, or both can be involved, possibly sequentially, or through equilibration, the choice being dependent on the ligands surrounding the central phosphorus atom, i.e. on the nature of the reactants. Thus reaction 1 might well be written as reaction 4. 

Yet another example of valence expansion through rearrangement occurs after an initial reaction between a phosphorus(III) chloride and a thiocarboxamide in the presence of Et3N the relative proportions of the reaction, products, 37 and 38(R = EtO or Et2N R = Me or Ph), may be altered when the mixtures are heated because of the isomerization of the phosphorus(III) compounds 37 to the quinquevalent esters 38 when heated at 80 °C (R = EtO) or at 150 °C (R = E2N) ... 

Fused-ring bicyclic compounds have been formed by transannular attack of an amino group at phosphorus either to displace a phosphorus substituent, for example, formation of the salt (53), or by valence expansion to produce phosphorane (68). In the latter case the five-coordinate metalloid atom needs to be stabilized by the presence of electronegative substituents and the formation of small rings—preferably, five-membered rings. 

The octet rule does not apply to elements of higher periods. Oxidation levels of +3 and +4 stand for normal oxidation levels of phosphines or phosphonium salts, but the valence shell can be expanded beyond that. Tetraphenylphosphonium iodide readily combines with phenyllithium to afford the colorless and crystalline but ether soluble pentaphenylphosphorane (62). Would it be possible to produce a lithium hexaphenylphosphate (63) by merely using phenyllithium in excess (Scheme 1-44) Such an expansion of the valence shell of pentaphenylstiborane to the electron dodecet of lithium hexaphenylantimonate is possible indeed.However, the smaller phosphorus atom tolerates steric congestion not as well. When a series of pentaarylphosphoranes was incubated with the corresponding aryllithiums, no high-field P nmr signal testifying the presence of an ate complex was detected. ... 

There are 12 valence electrons (five from phosphorus, six from the fluorines and one negative charge). These are distributed to form six single bonds to fluorine in a regular octahedral shape. Note the expansion of the octet, which is possible for third- and later-row elements. 

Reaction 17 resembles the displacements on phosphorus discussed earlier, except that since trialkyl borons are electron deficient with only six electrons around boron, no valence-shell expansion is required. It appears to be a general reaction for boron and has been suggested by Matteson (15) to explain the conversion of trimethyl boroxine by tert-buty hypo-... 

Arsenic.—Evidence of the much smaller tendency of arsenic than phosphorus to undergo valence-shell expansion to form a quinquecovalent species has been presented. However, the rapid exchange of alcohol and trialkyl arsenates is ascribed to a five-co-ordinate transition state. The reduction of arsenic(in) by... 

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