Gem-dimetallic species

Mechanism Two pathways are suggested for this reaction (Scheme 4.49). The titanium-carbene complex A is formed as a key intermediate, which reacts with carbonyl compound to form an alkene via the oxatitanacyclobutane B (Path A). Alternatively, the addition of gem-dimetallic species C to a carbonyl compound gives the adduct D, which eliminates (TiCp2 RS)20 to give an alkene (Path B). 

An alkoxy group at the -position induces a cyclopropanation reaction of the dimetallic species as shown in equation 46. The formation of the gem-dimetal species proceeded diastereoselectively as described above. The ring closure proceeds with inversion of configuration to form stereospecifically the cyclopropane ring. An alkoxy substituent at the -position of the allylzinc reagent also induces the cyclopropanation reaction (equation 47)61a 72. 

The related addition reaction of allylzuic bromide to alkynil zinc reagents in refluxing THF leads to a mixture of vinyhc 1,1-organo-ggw-dimetallic species and gem-trimetalhc species, which respectively result from single and double additions (equation 51). The presence of a Lewis basic (see Lewis Acids Bases) group suitably placed for intramolecular (see Intramolecular) chelation, and bearing a secondary substituent, avoids the formation of double addition products and allows the reaction to take place in mild conditions (equation 52). 

A related approach to 1,1-dimetallated compounds relies on the allylation of 1-alkenyl-magnesium, -lithium, and - uminum derivatives (23) with allylzinc bromides (22), which proceeds readily at 35 C (Scheme 7 Table 4). The resulting gem-dimetallic intermediates can be trapped by protonation (24 26,25 - 29), stannylation (24 - 27) or, more interestingly, with aldehydes in the presence of BFs OEt (e.g. 24a - 28). The dimetallated species (24) were also trapped with alkylidene malonates, l2/H30, ... 

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