Efficient Pincer‐Ruthenium Catalysts for Kharasch Addition of Carbon Tetrachloride to Styrene

Abstract

AbstractA series of NNN pincer‐ruthenium complexes (R2NNN)RuCl2(PPh3) (R=Cyclohexyl (Cy), t‐butyl (tBu), i‐propyl (iPr) and phenyl (Ph)) have been synthesized and characterized. These pincer‐ruthenium complexes have been used to catalyse the Kharasch addition or atom transfer radical addition (ATRA) of carbon tetrachloride to styrene. Among the pincer‐ruthenium catalysts screened for the Kharasch addition, the catalytic activity followed the order (Cy2NNN)RuCl2(PPh3)>(iPr2NNN)RuCl2(PPh3)≫(Ph2NNN)RuCl2(PPh3). The oxidation of Ru(II) is easier with (Cy2NNN)RuCl2(PPh3) and (iPr2NNN)RuCl2(PPh3) in comparison with RuCl2(PPh3) as indicated by cyclic voltammetry studies. The catalyst precursor (R2NNN)RuCl2(PPh3) itself is the resting state of the reaction. The rate determining step involves the generation of the five‐coordinate 16‐electron ruthenium(II) species (R2NNN)RuCl2. Owing to weaker binding of triphenyl phosphine to ruthenium, the generation of catalytically active 16‐electron species (Cy2NNN)RuCl2 and (iPr2NNN)RuCl2 are more favourable. The complex (Cy2NNN)RuCl2(PPh3) demonstrates very high productivity (5670 turnovers after 48 h at 140 °C) in the absence of any co‐catalyst radical initiator. To the best of our knowledge, our turnovers (ca. 5670) are much higher than that reported hitherto. Quantum mechanical calculations demonstrate that the path involving the activation of carbon tetrachloride by (Cy2NNN)RuCl2 is more favoured than the path where carbon tetrachloride is activated by (Cy2NNN)RuCl2(η2‐styrene). Density functional theory (DFT) and kinetic studies are in accord with the widely accepted mechanism involving the single electron transfer (SET) from ruthenium(II) to chloride radical with concomitant generation of a benzyl radical which is trapped by the resulting ruthenium(III) species.magnified image