Silylmetallation cyclizations

Tamao and Ito proposed a mechanism for the nickel-catalyzed cyclization/hydrosilylation of 1,7-diynes initiated by oxidative addition of the silane to an Ni(0) species to form an Ni(ii) silyl hydride complex. Gomplexation of the diyne could then form the nickel(ii) diyne complex la (Scheme 1). Silylmetallation of the less-substituted G=C bond of la, followed by intramolecular / -migratory insertion of the coordinated G=G bond into the Ni-G bond of alkenyl alkyne intermediate Ila, could form dienylnickel hydride intermediate Ilia. Sequential G-H reductive elimination and Si-H oxidative addition would release the silylated dialkylidene cyclohexane and regenerate the silylnickel hydride catalyst (Scheme 1). 

Ojima has proposed a mechanism for the rhodium-catalyzed cyclization/hydrosilylation of enynes initiated by oxidative addition of the H-Si bond of the hydrosilane to form the Rh(iii) silyl hydride complex If (Scheme 7). Silylmetallation of the G=G bond of the enyne coupled with coordination of the pendant G=G bond could form... 

Detailed mechanistic study on these intramolecular hydrosilylation of allylic O-silyl ethers 59 and allylic A -silylamincs 63 using deuterium labeling techniques shows that 5-endo cyclization giving 60 or 64 proceeds via a Chalk-Harrod type hydrometalation catalytic cycle, while 4-exo cyclization process yielding 61 or 65 includes a Seitz-Wrighton type silylmetalation mechanism89. 

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