Apicophilicity

A theoretical study of model apically-substituted and equatorial ly-substituted acyclic phosphoranes, H PX, revealed a ligand apicophilicity in the order, Ct > CN > F > CeCH > H > CHg > OH >... 

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... 

Tetracoordinate sulfur compounds containing a lone pair of electrons at sulfur possess a more or less distorted trigonal-bipyramidal structure, in common with the vast majority of other pentacoordinated molecules of the main group elements (189,191,199). A common name, sulfurane, is generally accepted for this type of compound. In principle, sulfuranes are chiral. However, both the number of optically active isomers and their optical stability depend on the nature of substituents bonded to the central sulfur atom, the apicophilicity of the substituents, and the energy required for permutational isomerization processes. In this context it is interesting to note that acyclic sulfuranes with four different ligands should exist in 20 isomeric forms. 

In addition to the foregoing explanation of retention of configuration, two possibilities may be taken into account. The first is that apical entry is followed by basal departure, and vice versa. Second, the sulfurane intermediate formed may have the structure of a square basal pyramid. In both cases nucleophilic substitution reactions may occur with retention of configuration without ligand reorganization. It appears that the apicophilicity of substituents in sulfurane species... 

Between 236 and 237, the two other sulfurane intermediates that may be responsible for inversion at sulfur, 237 appears to be the more probable candidate for two reasons. The first is that the ethoxy and hydroxy groups occupy the proper apical positions in accord with their Idgh apicophilicities, and the lone electron pair is placed equatorially, consistent with all the available structural data on sulfuranes. Second, the four-membered ring in a stable cyclic difluorosulfurane 238 was found by Denney et al. (287) to occupy a diequatorial position. 

Recent detailed discussions229,283,284,286,289,290 of the bonding in phosphoranes has usually taken for granted the three-centre, four-electron bond model described above and have concentrated on other related issues of importance in these systems, viz. apicophilicity, pseudorotation and the oxaphosphetanes involved in the Wittig reaction. 

Apicophilicity is usually correlateld with high electronegativity240,251 but it is also affected by other factors. Thus, simple qualitative MO considerations suggest that n donors should prefer the equatorial rather than the apical position241,251,294,295 and the introduction of a ring introduces extra strain effects250. Some of these issues have been addressed in recent theoretical studies. 

B. Electronic Factors (Electronegativity, Apicophilicity, Polarity Rules). . 188... 

The presence of steric strains implies, a priori, at least the presence of a ring in which two out of the five bonds to the P atom are involved. This immediately raises a number of questions what is the resulting geometry of the TBP, the TP (or of any other structure involved) as a function of the nature and of the number of atoms directly bonded to the P atom, and how will the presence of steric constraints affect the apicophilicity of the ligands ... 

This observation, that the aptitude of X to be displaced with inversion, is parallel to the tendency of the Si—X bond to be stretched under the influence of an attacking nucleophile, does not agree with the apicophilic-ity order stated from dynamic NMR studies on stable pentacoordinate phosphoranes (42), which is as follows ... 

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