Preferential Bond Activation of sp3 C–H over sp2 C–H in α,β-Unsaturated Carboxylic Acids by Ruthenium Complex

Reactions of Ru(1,5-cod)(1,3,5-cot) (1)/Pme3 [cod = cyclooctadiene, cot = cyclooctatriene] with propenoic acids (CH2=CH(R)COOH) give unsaturated ruthenalactones Ru[OC(O)C(R)=CH-κ2O,C](Pme3)4 [R = Me (2a), Et (2b), Pr (2c), iPr (2d)]. In contrast, reactions of trans-2-methyl-2-butenoic acid and 2-methylcinnamic acid (R′CH=C(Me)COOH) give Ru[OC(O)C(CH2R′)=CH-κ2O,C](Pme3)4 [R′ = Me (2b), Ph (2e)] as major products, suggesting the preferential activation of the sp3 C–H over sp2 C–H bond on ruthenium(II) center.