Ethane and Ethylidene Dicarboxylates from the Ruthenium(II)-Catalyzed “Disproportionative Addition” of Carboxylic Acids to Ethene

Abstract

Ethene is of paramount importance in chemistry and the addition reactions to the simplest olefin are paradigmatic. It is known that the acid-catalyzed addition of carboxylic acids yields esters. [1] Herein we report a Ru II -catalyzed ™disproportionative addition∫ (that is, addition to give both oxidized and reduced products simultaneously) of carboxylic acids to give ethane and ethylidene dicarboxylate [Eq. (1)].This novel reaction is carried out in anhydrous carboxylic acids. A number of ruthenium complexes can be employed as precursors, provided they generate [RuL n (CO(1) [2] is a suitable precursor for the reaction described in Equation (1), performed in propionic acid containing 10 % of propionic anhydride. In that reaction medium, ethane and ethylidene dipropionate are formed in equimolar amounts with a turnover frequency (TOF) of 3 h À1 at 1308 (P ethene ¼ 25 atm; [Ru] ¼ 0.05 m), with the TOF remaining almost unchanged after 10 h. With regard to the activation of precursor 1, it had already been shown that, in solution, the dinuclear complex is converted into the mononuclear [RuL(C 2 H 5 )(CO) 2 (H 2 O) 2 ] (2, L ¼ propionato or triflato) [2] . At 90 8C under an inert atmosphere, complex 2 (0.1m in propionic acid/anhydride) is dehydrated by the anhydride while, on acidolysis, quantitative ethane formation occurs. The IR absorptions of 1 in propionic acid/anhydride at 2049(s) and 1970(vs) cm À1 are shifted to 2090(s) and 2024(vs) cm À1 , which are attributable to cis-[RuL n (CO) 2 ] (2Àn)þ (L ¼ RC(O)O À or CF 3 SO 3 À ) species. When ethene (P ethene ¼ 25 atm, T ¼ 90 8C) is added, a reaction occurs that leaves the cis-(CO) 2 Ru group intact and gives rise to new absorptions at 2059(s), 1984(vs), and 1600(s) cm À1 . To elucidate this reaction, the propionic acid/anhydride solvent was removed under vacuum and the residue extracted with toluene. Upon addition of hexane, crystalline [{Ru(CF 3 SO 3 )(CHCH 3 -{OC(O)C 2 H 5 })(CO) 2 } 2 ] (3) was isolated in 37 % yield. The molecular structure of 3 is shown in Figure 1; two bridging