Stereochemical changes at a saturated carbon centre

In general, the most convincing evidence that the reaction of an alkyl halide or tosylate with a metal complex adopts a nucleophilic displacement (8 2) mechanism is the demonstration that inversion of the configuration of the saturated carbon centre has occurred as a consequence of this process. However, as will be seen, inversion of the configuration of the carbon atom is not an essential consequence in the reactions of coordinatively-unsatur-ated. transition-metal nucleophiles. 

Examples are known, however, in which halogen cleavage of metal-alkyl bonds occurs with retention (Whitesides and Boschetto, 1969 Calderazzo and Noack, 1966 Johnson, 1970 Slack and Baird, 1974, 1976) and others with inversion (Whitesides and Boschetto, 1971 Jensen and Davis, 1971 Jensen et aL, 1971 Anderson et al., 1972). Without firm evidence about the stereochemistry of the cleavage of metal-alkyl bonds, little can be said about the stereochemistry of the reaction between metal nucleophiles with alkyl halides. 

A further complication that can occur in these stereochemical studies was demonstrated in the reaction between [Mn(CO)5] and optically active MeCHBrCOjEt, where subsequent bromine cleavage was used to establish the stereochemistry of the reaction. The stereospecificity observed in this 

Subsequent studies, occasionally using the trifluoromethane sulphonate derivative, have extended this work to less potent nucleophiles such as 

Another area where H nmr spectroscopy can be used to determine the stereochemistry at the carbon atom is demonstrated in the reactions of substituted cyclohexanes with [Co(dmgH)2Py] , as shown in the selective examples (14) and (15). The stereochemistry of the displacement (inversion of the configuration of the carbon centre) and the strong preference for the metal residue to occupy the less congested equatorial site, defines the orientation of the substituent Y at the 4-position. 

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The scope of the review 2 Introduction to metal complexes as nucleophiles 2 Types of metal-based nucleophiles 5 The scale of nucleophilicity 6 The influence of ligands on the nucleophilicity 7 The nucleophilicity of metal complexes 12 The influence of the solvent on the nucleophilicity 14 Stereochemical changes at a saturated carbon centre 18 Reaction of coordinatively-saturated complexes 18 Reactions of coordinatively-unsaturated complexes 22 Reactions of MCjSn 27... 

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