Preparation and reactions of l,n-bismetallic reagents

The diastereoselectivity of the carbometallation reaction was also investigated and it was observed that the diastereoselection is highly dependent on the temperature and on the nature of the solvent.11 Thus, decreasing the Lewis 

= Alkyl, CH2NEt2 OtBu, OMOM, SPh R2 = H R, = H R2 = Alkyl, CH2NEt2 OtBu, OMOM, SPh 

By this way, from two prochiral vinylic carbon atoms, two stereogenic centres were created with good diastereoselectivity. Knowing that the chelation between zinc and heteroatoms in w-heterosubstituted dialkylzinc reagents has already been shown by NMR studies,14 the allylzincation of substituted y-heterosubstituted vinyl metals has been studied.13,15 In all cases, the chelation promotes a difference between the two prochiral faces of the vinyl moiety since one is shielded by the alkyl group, and then the allyl13,15,16 or the substituted allylmetal13,1517 (Equation 7.4 and Protocol 6) reacts diastereoselectively. 

The same trend is observed for the allylmetallation of 8-heterosubstituted vinyl metals,18 of a metallated pentadiene19 (see Protocol 7) or in the propar-gylmetallation of vinyl metals13 (see Protocol 8). 

The allylmetallation of vinyl metals, y-heterosubstituted with a methoxy-methyl ether as the chelating group, leads to the corresponding gembismetal-lic derivatives,20 but now, warming the reaction mixture to room temperature promotes an internal nucleophilic substitution, leading to a metallated cyclopropane which can react with different electrophiles21 (Equation 7.5 and Protocol 9). 

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