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 P^tBu₂Me) 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)(P^tBu₂Me)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.