Arylcalcium halides as substrates in Kumada-type cross-coupling reactions

A precondition of a Kumada-type cross-coupling reaction with arylcalcium halides is the easy availability of these organometallics. Arylcalcium halides are accessible with high yields via reduction of arylhalides with activated calcium in ethers such as tetrahydrofuran. In order to demonstrate the generality of this Grignard-type reduction of haloarenes, [(4-BrC6H4)CaI(thf)4] (1) and [(β-naphthyl)CaBr(thf)4] (2) are prepared. First investigations regarding arylcalcium halides as substrates in cross-coupling reactions are undertaken choosing [(C6H5)CaI(thf)4] (3) and [(4-CH3C6H4)CaI(thf)4] (4) as the organometallic substrate in a cross-coupling with chlorobenzene and 4-chlorotoluene. The nickel-mediated conversion of arylcalcium iodides and chloroarenes to (substituted) biphenyls proceeds with moderate yields and significant amounts of homo-coupling products are observed.