Nucleophilic attack trigonal bipyramids

Nucleophilic substitution at RSO2X is similar to attack at RCOX. Many of the reactions are essentially the same, though sulfonyl halides are less reactive than halides of carboxylic acids. The mechanisms are not identical, because a tetrahedral intermediate in this case (148) would have five groups on the central atom. Though this is possible (since sulfur can accommodate up to 12 electrons in its valence shell) it seems more likely that these mechanisms more closely resemble the Sn2 mechanism, with a trigonal bipyramidal transition state (148). There are two major experimental results leading to this conclusion. 

The mechanism of phosphate ester hydrolysis by hydroxide is shown in Figure 1 for a phosphodiester substrate. A SN2 mechanism with a trigonal-bipyramidal transition state is generally accepted for the uncatalyzed cleavage of phosphodiesters and phosphotriesters by nucleophilic attack at phosphorus. In uncatalyzed phosphate monoester hydrolysis, a SN1 mechanism with formation of a (POj) intermediate competes with the SN2 mechanism. For alkyl phosphates, nucleophilic attack at the carbon atom is also relevant. In contrast, all enzymatic cleavage reactions of mono-, di-, and triesters seem to follow an SN2... 

Trigonal-bipyramidal species and nucleophilic displacement reactivity The 3c/4e cu-bonding motif can also be achieved in nonlinear polyatomics by backside attack of a nucleophile X - on a polar Y—Z bond of a conventional Lewis-structure molecule,... 

Archer et al.206 surveyed all phosphonium salts known in 1981 in a correlation analysis of the positioning of cation and anion. Exclusive face orientation of the anion with respect to the cation centre was found, i.e. the anion was positioned along the direction of approach leading to the apical position in a trigonal bipyramidal intermediate. The positioning of the anion did not correlate with nucleophilic attack at the phosphorus centre but did correlate with a-hydrogen abstraction (ylide formation)206. 

The initial trigonal bipyramid (TBP) can be formed in two ways from the tetrahedral phosphorus atom either by nucleophilic attack of the hydroxide ion at any of the six edges of the tetrahedron, leaving the nucleophile 1 in an equatorial position (equatorial attack), or by attack at any of the four faces of the tetrahedron, which would set the nucleophile in an apical position (apical attack). Because apical bonds are longer, and therefore weaker, than equatorial bonds, apical attack is favoured since the formation energy of the initial TBP is then lower. In the same way, since apical bonds are weaker they should be more easily broken than their equatorial counterparts, hence the apical departure of the leaving group 2 is favoured. 

Positional isotope exchange experiments conducted on creatine kinase,45 hex-okinase,46 and pyruvate kinase47 have provided no evidence in favor of a dissociative mechanism. In each case, the data are consistent with a straightforward nucleophilic attack on phosphorus to generate a trigonal bipyramidal intermediate.47... 

The chemistry and stereochemistry of the reactions were extensively discussed in Chapter 8, sections El and E3. There is an in-line mechanism that generates a pentacovalent intermediate or transition state, with the attacking nucleophile and leaving group occupying the apical positions of the trigonal bipyramid. 

A possible interpretation generally invoked in phosphorus chemistry assumes that the four- or five-membered rings are unable to occupy the diequatorial position of a trigonal bipyramidal intermediate and must occupy the apical-equatorial position (48). Such a geometry implies an attack of the nucleophile at 90° to the leaving group and so leads to retention (Scheme 23). 

Retention requires the nucleophile to attack from the same side as the leaving group in 9 and this can give rise to two different trigonal bipyramidal structures, 10 and 11. 

Stannyl anions with a highly coordinated tin center are also known. A hydridostannyl anion in the shape of a trigonal bipyramid in which two iodine atoms occupy the apical positions was obtained by oxidative addition of lithium iodide to the corresponding tin hydride (equation 58) . It was characterized by Sn NMR. Since apical iodines are more nucleophilic than the hydrogen, in its reactivity with a-ethylenic carbonyl compounds, attack by iodine precedes reduction by hydrogen, achieving regioselective 1,4 reductions. 

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