New And Improved Cross-Couplings

A general method for conducting direct reductive cross-couplings of aryl and alkyl halides without an intermediate stoichiometric organometallic reagent has been achieved by Daniel J. Weix and coworkers of the University of Rochester (J. Am. Chem. Soc., DOI: 10.1021/ja9093956). These indispensible reactions to prepare alkylated aromatics typically require preforming an alkyl magnesium or alkyl zinc halide in a separate step. But last year, chemists simplified the method by devising procedures to generate the reagents as in situ intermediates in one-pot reactions (C&EN, Oct. 26, 2009, page 6). Weix’s group has now simplified the reaction further by creating conditions that avoid the formation of an organometallic reagent entirely. The researchers prepared a chemical soup made from a host of ingredients: aryl iodide and alkyl iodide substrates, an NiI₂ catalyst, bipyridine and bis(diphenylphosphino)-benzene ligands, a pyridine additive, manganese powder as a reducing agent, and a pyrimidinone solvent (DMPU). Instead of forming a discrete organometallic reagent, such as RMnI, the reaction appears to proceed via a sequence of oxidative additions of the aryl iodide and alkyl iodide to nickel, aided by a synergistic effect of the dual ligands. The coupled alkylated aromatic products possess a variety of functionality, Weix says, including an unprotected alcohol.