Alkenes halometallation

In the previous section we have discussed that 1-halo-1-alkenes can be conveniently synthesized. However, with these methods it is not possible to synthesize 2-halo-l-alkenes. Although the halometallation reaction would be a powerful tool for the preparation of 2-halo-1-alkenes, the reaction has not been adequately developed for such purpose 183). Recently, it has been reported that B-bromo-9-borabicyclo[3.3.1]-nonane (B-Br-9-BBN) and B-iodo-9-borabicvclo[3.3.1] nonane (B-I-9-BBN)1841 react with 1-alkynes, stereo-, regio- and chemoselectively, and after protonolysis, 2-halo-1-alkenes are obtained in excellent yields (Eq. 118)1851. 

In a series of papers [30-35], Blum and co-workers reported the hydrogenation of alkenes, alkynes, and arenes in the presence of a hydrated ion-pair [0ct3NMe] [RhCl4(H20)n] . The quaternary ammonium tetrachlororhodate also catalyzes HTR of alkynes, alkenes and a,j6-unsaturated carbonyl compounds with polymethylhydrosiloxane [36, 37]. Recently, heterogenized and therefore recyclable quaternary ammonium halometallates, namely glass-encapsulated catalysts [38] and polymer-bound tetrachlororhodate [39], have been described. Both types of the insoluble catalysts effectively promote various processes including the hydrogenation of alkenes and the HTR of a,/9-unsaturated carbonyl compounds [40]. 

Halometallation is rare with the early transition metals. Stable solutions of TiCl4- (CR=CR Cl) are obtained from the reaction of TiCU with alkynes . Both and Z isomers are present in solution. More useful are the reactions in CH2CI2 of molybdenum chlorides, M0CI5, M0O2CI2 and M0OCI4, with alkenes (hexenes, cyclohexenes, etc.), to form exclusively cis-chlorination products in 92% yields. ... 

Halometallation reaction of alkenes and alkynes is believed to be involved in... 

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