Scope, mechanism and diene inhibition of isomerization of allylic alcohols to saturated ketones catalyzed by ruthenium(II)-cyclopentadienyl complexes

The isomerization of 3-buten-2-ol to butanone catalyzed by Ru(II)Cp-complexes (Cp = η5-cyclopentadienyl) with phosphine and amine ligands is described. The reaction catalyzed by [RuCp(MeCN)3](PF6) and two equivalents of triphenylphospine is first order in substrate with a kini of 0.43 h−1 and an initial TOF of 13,000 h−1. The catalyst precursor complex [RuClCp(dppb)] (dppb = bis(diphenylphosphino)butane) has been characterized by X-ray diffraction. This compound features a seven-membered ring incorporating the ruthenium centre and the dppb ligand. Combination of two equivalents of primary, secondary or tertiary amines and [RuCp(MeCN)3](PF6) results in active catalyst precursors. Within each group, increasing the bulk of the ligand gives lower isomerization rates. The combined effects of optimal pKa, nucleophilicity and steric bulk make RuCp-complexes with secondary amines the most active precursors. With di-n-butylamine, 745 turnovers can be reached after 1 h. 31P NMR spectra indicate that the resting state in the catalytic cycle is a complex in which 3-buten-2-ol is η2-coordinated through the alkene moiety. This implies that coordination of the oxygen moiety and concomitant β-hydrogen abstraction is the rate-limiting step. A counterintuitive result is that allylic alcohols bind stronger to RuCp complexes with phosphine ligands than dienes. Inhibition of the catalyst appears to be a result of interaction of the diene with a ruthenium–allyl alcohol complex, which is sufficiently strong to prevent coordination of the oxygen moiety of the allylic alcohol. This hinders orientation of the allylic alcohol substrate in a suitable way to undergo β-hydrogen abstraction, thereby blocking isomerization catalysis.