Nucleophilic substitution at tetrahedral

For nucleophilic substitution at tetrahedral carbon, the relationships between stereochemistry and mechanism are well understood. Direct displacement on a carbon turns the molecule inside out (inversion of configuration) if the site of reaction is an asymmetric carbon, a molecule in the D series will be converted to one in the L series (or vice versa). Substitution by the dissociation mechanism at an asymmetric carbon... 

The study of pseudorotation in five-co-ordinate phosphorus(v) compounds is undertaken not only out of intrinsic interest but also for its relevance to rearrangements in transition states for nucleophilic substitution at tetrahedral phosphorus(v) compounds (c/. pp. 118—120). Pseudorotation in the stable compound MeaN PF4 takes place via a square-pyramidal transition state. The variation of line widths with temperature for various lines in the n.m.r. spectrum permits a distinction to be drawn between possible processes involving simultaneous interchange of two pairs of fluorine atoms, as required for the Berry mechanism for pseudorotation, or of only one pair. Inversion or pseudorotation in... 

Some nucleophilic substitutions at a carbonyl carbon are catalyzed by nucleophiles.There occur, in effect, two tetrahedral mechanisms ... 

Nucleophilic substitution at a vinylic carbon is difficult (see p. 433), but many examples are known. The most common mechanisms are the tetrahedral mechanism and the closely related addition-elimination mechanism. Both of these mechanisms are impossible at a saturated substrate. The addition-elimination mechanism has... 

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. 

In these reactions (12-41-12-44), a carbonyl group is attacked by a hydroxide ion (or amide ion) giving an intermediate that undergoes cleavage to a carboxylic acid (or an amide). With respect to the leaving group, this is nucleophilic substitution at a carbonyl group and the mechanism is the tetrahedral one discussed in Chapter 10. 

Nucleophilic Substitution at an Aliphatic Trigonal Carbon. The Tetrahedral Mechanism... 

The electrophile shown in step 2 is the proton. In almost all the reactions considered in this chapter the electrophilic attacking atom is either hydrogen or carbon. It may be noted that step 1 is exactly the same as step 1 of the tetrahedral mechanism of nucleophilic substitution at a carbonyl carbon (p. 331), and it might be expected that substitution would compete with addition. However, this is seldom the case. When A and B are H, R, or Ar, the substrate is an aldehyde or ketone and these almost never undergo substitution, owing to the extremely poor nature of H, R, and Ar as leaving groups. For carboxylic acids and their... 

Scheme 53 Products 162-165 of nucleophilic substitution at the tetrahedral phosphorus atom in 159b...

This type of map can be used to discuss the different types of nucleophilic displacement reaction. Using the simplified version shown in Fig. 2 we have already seen that SN1 reactions, for instance the solvolysis of triarylmethyl halides, go through the separated ions in the top right-hand corner (Swain et al., 1953 Ritchie, 1971). At the opposite extreme, nucleophilic substitution at centres where the number of ligands can be increased may proceed over the bottom left-hand corner of the diagram. Examples are acyl transfer reactions and substitution at tetrahedral phosphorus centres (Alder et al., 1971) as well as substitution at square planar transition metal compounds (Wilkins, 1974). The nucleophilic reactions studied by Ritchie (1976), for which the rate... 

You might have been surprised that the intermediate in the aldol step of this reaction did not decompose. This intermediate could be described as a tetrahedral intermediate in a nucleophilic substitution at a carbonyl group (Chapter 12). Why then does it not break down in the usual... 

Only now does something different happen, The aldehyde dimer simply captures a proton from the solvent to give an aldol product. The aldof from the ester (not, in fact, an aldol at all) has a leaving group, EtO-, instead of a hydrogen atom and is actually the tetrahedral intermediate in a nucleophilic substitution at the carbonyl group. Compare the two different steps again. 

This process is sometimes abbreviated to S f2 at silicon to save space. The intermediate is a trigonal bipyramid with negatively charged pentacovalent silicon. It is often omitted in drawings because it is formed slowly and decomposes quickly. This mechanism is similar to nucleophilic substitution at boron except that the intermediate is pentacovalent (Si) rather than tetrahedral (B). The hydrolysis of a boron ester at the end of a hydroboration-oxidation sequence would be an example of an analogous boron reaction. 

Fox, J. M., Dmitrenko, O., Liao, L.-A., Bach, R. D. Computational Studies of Nucleophilic Substitution at Carbonyl Carbon the Sn2 Mechanism versus the Tetrahedral Intermediate in Organic Synthesis. J. Org. Chem. 2004, 69, 7317-7328. 

---