Transmetallation to other Metals for Addition and Coupling Reactions

6 Transmetallation to other Metals for Addition and Coupling Reactions 

The covalent B-G bond of organoboron compounds shows little reactivity toward representative electrophiles, but the presence of an empty low-lying orbital facilitates transmetallation to other metals. The transmetallation to zinc709 and other metal reagents has been extensively studied for carbon-carbon bond formation via addition and coupling reactions of trialkylboranes and alkylboronic acids. 

The synthesis of 9-alkenyl-9-BBN via hydroboration of terminal alkynes with 9-BBN suffered from the formation of 1,1-diborylalkanes via dihydroboration along with desired monohydroboration products. Alternatively, selective monohydroboration of terminal alkynes with dicyclohexylborane was followed by transmetallation with 9-MeO-9-BBN (Equation (193)).710 Treatment of the hydroboration intermediates with DIBAL-H in the presence of a borane-trapping reagent such as 1-hexene gave 1-alkenylaluminum compounds with complete retention of the (E)-stereochemistry (Equation (194)).711 

A transmetallation sequence from the products of hydrometallation of alkynes provides ac S °t estereodefined 

Transmetallation to other elements includes BF3-mediated arylation of Ar3BiF2 with arylboronic acids giving [Ar3Ar Bi][BF4],735 BF3-mediated alkenylation or arylation of PhI(OAc)2 providing [R(Ph)I][BF4] (R = aryl, alkenyl),736 and arylation of Pb(OAc)4 with arylboronic acids to give ArPb(OAc)3.737 

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