Stereochemical changes at an unsaturated carbon centre

A detailed study of the reactions between alkynyl or vinyl halides and [Co(dmgH)2py] has demonstrated a complex pattern of behaviour (Cooksey et al., 1972). At low concentrations of nucleophile over short reaction times, the reaction with prop-2-ynyl halides yields the prop-2-ynylcobal-oxime, presumably by a nucleophilic displacement pathway. However, at longer reaction times in the presence of an excess of cobaloxime, allenyl-cobaloxime is observed. This is formed as a consequence of an 8 2 displacement of one cobaloxime by another attacking the initially formed prop-2-ynylcobaloxime as shown in 8cheme 9. The reactions of substituted 

Stereochemical investigation of the reaction between vinyl halides and [Co(dmgH)2L] (L = py, H2O or PhNHj) indicate an addition-elimination mechanism (Van Duong and Gandemer, 1970). The reactions of the coba-loximes with cis- or /ranj-2-phenylethenylbromide give the cis- and /rfl i-vinylcobaloximes respectively. 

A Study of the reaction between rron5-[IrCl(CO)(PMe2Ph)2] and alkyl halides has demonstrated different behaviour to that observed with [Co(dmgH)2py], but the details of the reaction are still unclear (Pearson and Poulos, 1979). The complications associated with the stereochemistry of the addition to the metal will be left until Section 8 and we will concentrate here on the stereochemistry observed at the carbon centre. 

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Reactions of binuclear complexes 29 Stereochemical changes at an unsaturated carbon centre 30 Reactions of coordinatively-saturated complexes 30 Reactions of coordinatively-unsaturated complexes 31 Stereochemical changes at the metal 32 The iodide catalysis effect 38 The reactions of binuclear complexes 39 The reactivity of the carbon centre 41 The reactions of a,o>-dihalogenoalkanes 44 Activation parameters 47... 

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