Nucleophilic substitution tetrahedral center

The intrinsic stability of the aromatic n system has two major consequences for the course of reactions involving it directly. First, the aromatic ring is less susceptible to electrophilic, nucleophilic, and free-radical attack compared to molecules containing acyclic conjugated n systems. Thus, reaction conditions are usually more severe than would normally be required for parallel reactions of simple olefins. Second, there is a propensity to eject a substituent from the tetrahedral center of the intermediate in such a way as to reestablish the neutral (An + 2)-electron system. Thus, the reaction is two step, an endothermic first step resulting in a four-coordinate carbon atom and an exothermic second step, mechanistically the reverse of the first, in which a group is ejected. The dominant course is therefore a substitution reaction rather than an addition. 

The oxygen derivatives of three centers, carbon, phosphorus, sulfur, can be compared with respect to the AO s available for nucleophilic substitution. Unlike 45, participation of the 77-system in the process is possible for 53 and 54. In 49, we have sp3 bonds and the tetrahedral intermediate in 53 and 54, which may be transition states or intermediates, we can have pd overlap between the orbital covering the three collinear atoms and sp2d orbitals to oxygen. 

Let us now examine how substituent effects in reactants influence the rates of nucleophilic additions to carbonyl groups. The most common mechanism for substitution reactions at carbon centers is by an addition-elimination mechanism. The adduct formed by the nucleophilic addition step is tetrahedral and has sp hybridization. This adduct may be the product (as in hydride reduction) or an intermediate (as in nucleophilic substitution). For carboxylic acid derivatives, all of the steps can be reversible, but often one direction will be strongly favored by product stability. The addition step can be acid-catalyzed or base-catalyzed or can occur without specific catalysis. In protic solvents, proton transfer reactions can be an integral part of the mechanism. Solvent molecules, the nucleophile, and the carbonyl compound can interact in a concerted addition reaction that includes proton transfer. The overall rate of reaction depends on the reactivity of the nucleophile and the position of the equilibria involving intermediates. We therefore have to consider how the substituent might affect the energy of the tetrahedral intermediate. 

Also the prediction has proved correct of a molecular configuration, in which intramolecular alkyl shifts are realized, i.e., an intramolecular nucleophilic substitution at the tetrahedral carbon atom. It has been found [6] that a fast (10 s at 25°C) bond-switching process associated with the rupture-formation of the C—S bonds of the anchored center does occur in the degenerate rearrangement of the l,8-fcis-(arylthio)-anthracene-9-carbinyl cation XIV in solution ... 

Several other recent reviews contain material relevant to this section. An article by Blandamer and Burgess on the thermodynamics, kinetics, and mechanisms of solvation, solvolysis, and substitution in nonaqueous solvents contains a contribution on the controversial dissociative mechanism for isomerization of square-planar molecules. This is outlined in Section 5.5. A review of ligand substitution reactions at low-valency transition-metal centers contains sections on five-coordinate metal carbonyl complexes and on ML4 complexes (mainly tetrahedral configurations with L being a tertiary phosphine), as well as on acid- and base-catalyzed reactions. A review by Constable " surveying the reactions of nucleophiles with complexes of chelating heterocyclic imines contains a sizable section on square-planar palladium and platinum derivatives. Most discussion centers on [Pt(bipy)2] and [Pt(phen)2] (bipy = 2,2 -bipyridine phen = 1,10-phenanthroline). The metal center, ligand, or both are susceptible to nucleophilic attack and the mechanisms involved are critically assessed. 

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