Reductive coupling reactions nickel metallacycles

The dearest empirical evidence for the productive involvement of an 1] , O-bound nickel enolate comes from the intermolecular reductive coupling of alkynes and enals (Scheme 8.9) [27]. The extremely high levels of Z-isomer stereoselectivity (>98 2) can best be rationalized via the metallacycle intermediate 5 which undergoes o-bond metathesis to afford nickel hydride 6, followed by reductive eUmina-tion to yield the Z-selective enol silane product 7. A mechanism consisting of a nickel Jt-allyl species would not be expected to lead to high selectivities of Z-enol silanes, and has been imphcated in reactions leading to the selective production of T-enol silanes [28],... 

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