Regioselectivity carbomagnesiation

Titanocene dichloride also catalyzes a regioselective carbomagnesiation of alkenes 187 (equation 115) and dienes 188 (equation 116). The reaction proceeds at 0°C in THF in the presence of Cp2TiCl2, an organic halide and n-BuMgCl which leads to the catalytic species, affording benzyl, allyl or a-silyl alkylmagnesium halides, which are trapped with electrophiles (equation 117) . ... 

Scheme 10.20 Copper-catalyzed regioselective carbomagnesiation of non-activated 1,3-dienes with branched alkyl Grignard reagents [22].

Scheme 10.23 Synthetic approach to highly substituted alkylidenecyclopropane (ACP) derivatives via a copper-catalyzed regioselective carbomagnesiation of cyclopropenyl carbinols [23].

Considerably milder conditions for the regioselective carbomagnesiation of propargylic and homopropargylic alcohols were developed by Zhang and Ready... 

Scheme 10.95 Synthesis of disubstituted alkenes by silver-catalyzed regioselective carbomagnesiation of arylalkynes [77].

Scheme 10.96 Proposed catalytic cycle for the silver-catalyzed regioselective carbomagnesi-ation of alkynes.

Although carbomagnesiations of disubstituted alkenes proceeds sluggishly, the reaction of endo-5-norbornen-2-ol (5) proceeds in high yield and regioselectivity. However, the reactivity and regioselectivity are decreased in the reaction of the methyl ether of 5. 

Scheme 10.31 Regioselectivity of the copper-catalyzed carbomagnesiation of 1,3,3-trisubstituted cyclopropene derivatives [27].

Somewhat related to cross-coupling reactions, the carbomagnesiation of terminal alkenes was shown to proceed efficiently in the presence of [(IMes)AgCl] and 1,2-dibromoethane, which was used as an oxidant. The hydroboration of terminal alkynes was also achieved using [(NHC)Ag] catalysts, and with higher regioselectivity compared to copper catalysis conditions. 

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