Control of stereochemistry in atom transfer radical addition and step-growth radical polymerization by chiral transition metal catalysts

Abstract

A series of chiral transition metal catalysts of ruthenium(II), iron(II), copper(I), and rhodium(I) were used to control the stereochemistry in the step-growth radical polymerization of 3-butenyl 2-chloropropionate (BCP) with both unconjugated CC and reactive C–Cl bonds in a single molecule and atom transfer radical addition (ATRA) as a model reaction for the polymerization. Among the various metal catalysts with chiral ligands, Ru2Cl4[(−)-DIOP]3 [DIOP=2,3-(isopropylidenedioxy)-2,3-dihydroxy-1,4-bis(diphenylphosphanyl)butane] induced asymmetric ATRA with 10% ee between CCl4 and 3-butenyl acetate (BAc), which has an unconjugated CC bond as a model olefin of the monomer. However, this ruthenium catalyst was not sufficiently active to induce ATRA between the C–Cl bond in methyl 2-chloropropionate (MCP), which is a model chloride for the monomer, and the CC bond in BAc, as well as the step-growth radical polymerization of BCP. Whereas chiral iron and copper catalysts, such as FeCl2/(−)-DIOP and CuCl/(−)-sparteine, efficiently allowed the ATRAs and the step-growth radical polymerization to result in the desired 1:1 adducts and oligomers, respectively, almost no optical activity was observed for the products. However, the diastereoselectivity was changed in both the ATRA and the polymerization by the metal catalysts with chiral or achiral ligands.