Facile C(sp2)/O2CR bond cleavage by Ru or Os

Abstract

[RuHClL2]2, L = PiPr3, reacts with H2CCH(O2CR) (R = CH3, CF3, C6H5) during mixing at 20 °C, via two observable intermediates, to give RuCl(O2CR)(CHMe)L2; this carbene complex then redistributes the Cl and O2CR groups. Vinyl tosylate gives RuCl(OTs)(CHMe)L2 already at −60 °C. Vinyl chloroformate, H2CCH(O2CCl) reacts rapidly with [RuHClL2]2 to give the olefin metathesis catalyst RuCl2(CHMe)L2 and CO2. Os(H)3ClL2 (L = PiPr3 or PtBu2Me) reacts with vinyl esters H2CCHE (E = O2CR) to form first an η2-olefin adduct. This is followed by C/O bond cleavage, giving the carbyne OsHCl(O2CCF3)(CMe)L2. Vinyl chloroformate and Os(H)3ClL2 gives OsHCl2(CMe)L2 and CO2. RuHCl(PPh3)3 reacts with vinyl chloroformate, via RuCl(O2CCl)(CHMe)(PPh3)2, to give RuCl2(CHMe)(PPh3)2 while OsHCl(PPh3)3 reacts analogously, through observable OsCl2(CHMe)(PPh3)2, to form OsHCl2(CMe)(PPh3)2. Vinyl trifluoroacetate converts OsHCl(PPh3)3, to OsHCl(O2CCF3)(CMe)(PPh3)2. The less π-basic metal in OsH(CO)(PtBu2Me)2+ reacts with vinyl esters to give only an olefin adduct; detectable binding of the ester oxygen to Os in this adduct suggests a mechanism for carboxylate migration from carbene carbon to metal. The mechanisms of these reactions are explored, and the thermodynamic disparity between Ru and Os is discussed. DFT (B3PW91) calculations have been carried out to establish the energy pattern of possible products. The thermodynamic preference for cleaving the C–O2CR bond is shown to have a thermodynamic origin associated with the energy of the formed Ru–O2CR bond. The calculations also indicate the very large thermodynamic driving force for loss of CO2 in the case of H2CCH(O2CCl). The corresponding loss of CO2 is shown to be thermodynamically unfavorable in the case of H2CCH(O2CR). The energy of the Ru-R bond is a key factor.