Stereochemistry oftheS ’reaction

Willadsen, P. Eggerer, H. (1975) Eur. J. Biochem. 54,247 252. Suhstrate stereochemistry ofthe enoyl-CoA hydratase reaction. 

Based on the work of Bruce Lipshutz [226,227], it was found that vinylstannanes undergo transmetallation with dilithiodimethyl cyanocuprate, the driving force being the irreversible formation of tributylmethylstannane. The involvement of free methyllithium was postulated, but this could not be confirmed in a corresponding NMR experiment. Thus, during the course ofthe reaction this species is only present in very low concentrations - if at aU. The transmetallation proceeds with retention of the stereochemistry, which is not lost in the subsequent Michael reaction either. 

Figure 3-23. The treatment ofthe stereochemistry ofa further S,y2 reaction by permutation group theory.

The stereochemistry of the neutral ligand also affects the properties ofthe catalyst. Although [RhCl P(2-C6H40Me)3 3] initially reduces alkenes rapidly, the reaction ceases before all the aUcene has been consumed because of the formation of the coordinatively saturated complex (3). 

Additional studies demonstrated that replacement ofthe ligand ethylene by CO or PPh j did not influence the enantioselectivity of the reaction, suggesting that the ancillary ligand dissociates before the C H insertion. A bisimido ruthenium(VI) complex SO operating through an H abstraction/radical rebound mechanism, established in ruthenium porphyrin systems by Che, is proposed to account for the observed stereochemistry (Figure 12.2). 

The mechanism ofthe Sn2 reaction. The reaction takes place in a single step when the incoming nucleophile approaches from a direction i8o° away from the leaving halide ion, thereby inverting the stereochemistry at carbon. 

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