One Pot Tandem Dual C=C and C=O Bond Reductions in β-Alkylation of Secondary Alcohols with Primary Alcohols by Ruthenium Complexes of Amido and Picolyl Functionalized N-Heterocyclic Carbenes

Two different class of ruthenium complexes, namely, [1-mesityl-3-(2,6-Me2-phenylacetamido)-imidazol-2-ylidene]Ru(p-cymene)Cl (1c) and {[1-(pyridin-2-ylmethyl)-3-(2,6-Me2-phenyl)-imidazol-2-ylidene]Ru(p-cymene)Cl}Cl (2c), successfully carried out the one-pot tandem alcohol-alcohol coupling reactions of a variety of secondary and primary alcohols, in moderate to good yield ca. 63-89 %. The mechanistic investigation performed on two representative catalysis substrates, 1-phenylethanol and benzyl alcohol using the neutral ruthenium (1c) complex, showed that the catalysis proceeded via a partially reduced C=C hydrogenated carbonyl species, [PhCOCH2CH2Ph] (3’) to the fully reduced C=O and C=C hydrogenated secondary alcohol, [PhCH(OH)CH2CH2Ph] (3). Further the time dependent study showed that the major product of the catalysis modulated between (3’) and (3) during the catalysis run performed over an extended period of 120 hours. Finally, the practical utility of the alcohol-alcohol coupling reaction was demonstrated by preparing five different flavan derivatives (13-17) related to various bioactive flavonoid natural products, in an one-pot tandem fashion.