Zirconium-catalyzed methylalumination reactions

Alkynes are carbometallated with several organometallic compounds under mild conditions [3] but carbocupration has probably the greatest synthetic possibilities [4], associated with the zirconium-catalyzed methylalumination [5] reaction both proceed with very high stereoselectivity and afford the syn addition product as described in Scheme 7-2. 

Negishi coupling reactions can be combined with asymmetric carboalumination, a process that was also discovered by the Negishi group (Scheme 5-87). The asymmetric induction of this zirconium-catalyzed enantioselective methylalumination process ( ZACA ) stems from a chiral zirconium catalyst precursor, namely dichlorobis(neomenthylindenyl)zirconium [(NM aZrCb]. The stereoinduction is very high. 

Despite the formal similarity of the reaction, the mechanism of Cp2ZrCl2-catalyzed ethylalumination [64] with AlEt3 is different from that of either methylalumination with AlMe3 or ethylalumination with Et2AlCl [62]. The involvement of dimetallic species was confirmed by NMR spectroscopy as well as deuterolysis (Scheme 8.31). The proposed mechanism features an interesting zwitterionic bimetallic species, in which the zirconium center is cationic. A highly instructive treatise on the mechanistic pathways of carbometalation is presented in [65],... 

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