Unusual C–O bond cleavage of aromatic ethers in ruthenium complexes bearing a 2-alkoxypyridyl fragment

Abstract

Two tetradentate (NNOP) ruthenium products [{RO-C₅H₃N-C₅H₃N-CH(O)-C₆H₄-PPh₂}Ru(CO)(PPh₃)]Cl (1: R = Me; 3: R = Ph) were isolated by the reactions of the corresponding ligands with RuHCl(PPh₃)₃(CO). Complex 1 could also be transformed into 2 when heated in THF, through C–OMe bond cleavage. The mechanism of the unusual C–O cleavage in complex 1 was investigated, and the results indicate that it is an S_N2 mechanism through the attack of trace amounts of neutral H₂O. In contrast, in the presence of KOH, the reaction switches to an S_NAr chemistry. The bidentate ruthenium isomers [{CH₃O-C₅H₃N-C₅H₄N}RuH(CO)(PPh₃)₂]Cl (4a and 4b) could undergo similar C–O cleavage, affording product {O-C₅H₃N-C₅H₄N}RuH(CO)(PPh₃)₂ (5), which reacted with HCl to generate complex [{HO-C₅H₃N-C₅H₄N}RuH(CO)(PPh₃)₂]Cl (6). These complexes were tested as catalysts for β-alkylation of secondary alcohols with primary alcohols, and a series of β-alkylated 1-phenylethanol derivatives were isolated in high yields.