Apicophilicity, scale

Substituent site preferences in a tbp are based on (what is commonly referred to as) an apicophilicity scale which takes into account several factors, so the ordering of axial/equatorial site preferences may not always strictly follow electronegativity expectations (Section 5.1). For a sp geometry, the more electronegative groups occupy the basal positions. 

Fig. 2 Trippett s relative apicophilicity scale (kcal mol ) for ligands attached to trigonal bipyratnidal pentacoordinate phosphorus.

Both Trippett s and Holmes s approaches to apicophilicity are entirely empirical. Recently, Streitweiser has initiated a reasonably high level [3-21G( ) and 3-21G-E ] ab initio study of apicophilicity, using PH X as a model (McDowell and Streitweiser, 1985). The apicophilicity scale derived from these calculations, differs enormously from empirically derived scales. A comparision for selected ligands is given below ... 

The differences between the apicophilicity scales summarized above results from the overemphasis of d-orbital contributions in the ab initio study. Thus strong (p- d) Jt-donor ligands, NHj and BHj, have artificially high equatophilicity. Nevertheless, important conclusions may be drawn as to the role of d-orbitals. 

Hydrolysis of the cyclic thiophosphate system [14] would be predicted to proceed by initial attack apical to endocyclic oxygen both by Trippett s apicophilicity scale and Holmes s model. In base, exclusive P—S bond... 

Both of these results suggest reactivity patterns in five-membered ring systems that are not in accordance with Westheimer s guidelines and the relative apicophilicity scale. However, definitive evidence can only be obtained from stereochemical studies, since retention necessitates a pseudorotation and inversion implies in-line direct displacement. 

The conclusion from these extensive elegant stereochemical studies is that in five-membered ring systems the apicophilicity of endocyclic ligands does not conform to Trippett s relative apicophilicity scale nor to Holmes s preference rules. Clearly, apicophilicity is a redundant term in such species. Relative apicophilicity is an empirically derived parameter formulated for universal application to phosphorus TBP species. The apicophilicity of a particular ligand is independent of the other phosphorus substituents and the nature of the TBP and is a transferable parameter. Since this definition of apicophilicity breaks down in five-membered TBP intermediates. Hall and Inch defined the alternative term apical potentiality . Thus, in the... 

The phosphorylation of alcohols by CEP-imidazole (41 X=N) with CEP-ring retention is already well-established. Following from the observation that CEP-pyrrole (41 X=CH) phosphorylates alcohols with CEP-ring opening, an explanation has been advanced based upon the differences in apicophilicities of the pyrrole and imidazole moieties in pentaco-ordinate intermediates (Scheme 10). A scale of relative reactivities based upon the reactions in the equations... 

The trifluoromethylphosphoranes (CF3)2PMe3 and (CF3)3PMe2 have been obtained as stable, unreactive, white solids from the reactions of tetramethyl-lead with the corresponding chlorophosphoranes.23 Pseudorotation of (CF3)3PMe2 is slow on the n.m.r. time-scale at 100 °C, indicating a very considerable difference in apicophilicity between methyl and trifluoromethyl groups. Among other acyclic phosphoranes prepared are (23),24 (24),25 and (25).26... 

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