Two reactions of allylic alcohols catalysed by ruthenium cyclopentadienyl complexes with didentate phosphine ligands: isomerisation and ether formation

Abstract

The activity of chloro(η 5-cyclopentadienyl)(didentate phosphine)ruthenium(II) complexes in the catalysis of two types of reaction with allylic alcohols is described. The isomerisation of 3-buten-2-ol to butanone (MEK) and allyl alcohol to propanal proceeds at high rates. A trend in catalytic activity is observed upon variation of the carbon-chain length in the didentate phosphine ligand: dppm<dppe<dppp<dppb. Complexes with rigid didentate phosphine ligands like cis-dppv and dppph show no activity. The second type of reaction constitutes the first example of a ruthenium-catalysed ether formation directly from allylic alcohols. Homo-coupled ethers like di-allyl ether (DAE) are easily formed as well as ethers from heterocoupling of allyl alcohol with aromatic and aliphatic alcohols. In fact, the ruthenium complexes achieve much higher turnover frequencies and turnover numbers than reported before in palladium-catalysed ether formation. Complexes with dppe and dppp in the presence of a conjugated diene switch from isomerisation to ether formation, but the new complex [RuClCp( o-MeO-dppe)] (Cp=η 5-cyclopentadienyl) ( 3) has proven to be very active in ether formation, even in the absence of a diene. The mechanisms of the reactions have been studied by using both deuterium-labelled substrates and 31P nuclear magnetic resonance (NMR).