Evidence for in situ catalyst modification in atom transfer radical reactions with ruthenium benzylidene complexes

Abstract

Since the first report of ruthenium- based catalysts in atom transfer radical addn. (ATRA, also called Kharasch addn.) and atom transfer radical polymn. (ATRP) , this area of research has attracted widespread interest. Well- defined ruthenium benzylidene complexes, commonly used as olefin metathesis catalysts, have also been reported to catalyze ATRA and ATRP. The ability of ruthenium benzylidene complexes to promote two reactions with such markedly different mechanisms has been utilized in various tandem reactions in which olefin metathesis and atom transfer radical (ATR) reactions take place in one pot. While the mechanism by which ruthenium benzylidenes initiate and catalyze olefin metathesis has been studied in great detail, little is known regarding the mechanism of ATR reactions promoted by these complexes. In the present study, H NMR kinetics revealed that ruthenium benzylidene complexes are rapidly converted into new metathesis inactive species under typical ATRA conditions. When ruthenium benzylidene complexes were pre- incubated prior to substrate addn., the resulting activated species exhibited enhanced kinetic reactivity in ATRA with no significant difference in overall product yield compared to the original complexes. Kinetic measurements along with addnl. mechanistic and computational investigations show that metathesis- inactive ruthenium species, generated in situ from ruthenium benzylidene complexes are active catalysts in ATR reactions including ATRA and ATRP.