Trans-carbometallation reaction

Alkythioacetylenes also smoothly add stabilized lithio carbanions located on the same substrate to provide functionalized exocyclic thioenol ethers (Scheme 7-14) [17a]. It should be noted that only one alkene stereoisomer was obtained this results from an atypical trans-carbometallation reaction, or from a cw-carbometallation followed by an equilibration of the metallated intermediate [15]. 

Since most of the facile and general hydro- and carbometallation reactions involve syn-addition, the preparation of trans-a.jS-substituted alkenylmetals via 5yn-addition of alkynes would require carbometallation of terminal alkynes placing the metal in the internal position. Although such reactions exemplified by carbopalladationf are known, they are still more exceptional than normal. From the perspective of the current discussion, more commonly used are (i) some anfi-hydrometallation reactions of proximally heterofunctional internal alkynes and (ii) the hydroboration-migratory insertion tandem process of 1-haloaIkynes. Whereas the H migration produces (Z)-/3-substituted alkenylboranes (Sect. D.iii), the corresponding C migration provides trans-a,/3-substituted alkenyhnetals. (See Table 15.)... 

Lanthanide-catalyzed enyne cyclization/hydrosilylation was also applied to the synthesis of silylated alkylidene cyclohexane derivatives. For example, reaction of the 3-silyloxy-l,7-enyne 17 with methylphenylsilane catalyzed by Gp 2YMe(THF) at 50°G for 8h gave 18 in quantitative yield as a 4 1 mixture of trans cis isomers (Equation (11)). Employment of methylphenylsilane in place of phenylsilane was required to inhibit silylation of the initially formed yttrium alkenyl complex, prior to intramolecular carbometallation (see Scheme 8). 

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