Conversion of propargylic alcohols to β-oxo esters catalyzed by novel ruthenium-phosphoramidite complexes

Article abstract of DOI:10.1002/adsc.200800355

A series of half-sandwich phosphoramidite complexes of ruthenium were synthesized and employed as catalysts in the atom-economical formation of β-oxo esters from carboxylic acids and propargylic alcohols. Reaction of the phosphoramidites (R)-BINOL-PNR2 (R=Me, 1a; i-Pr, 1b; benzyl, 1c) and (rac)-6,6′-dibromo-BINOL-PNMe₂ (1d) with the dimeric p-cymene-ruthenium dichloride complex, [RuCl₂(p-cymene)]₂, gave the complexes [RuCl₂(p-cymene)(L)] (L=1a, 7a; 1b, 7b; 1c, 7c; 1d, 7d) in 96-66% yield. Accordingly, reaction of (R)-BINOL(8H)-PNMe2 (2a) and (R)-BINOL(8H)-PN-(benzyl)₂ (2b) with [RuCl₂(p-cymene)] ₂ afforded the complexes [RuCl₂(p-cymene)(L)] (L=2a, 8a; 2b, 8b) in 82% and 86% yield. In a similar reaction, treatment of (R)-BIPHEN-PNMe₂ (9) with [RuCl₂(p-cymene)] ₂ gave the complex [RuCl₂(p-cymene)(9)] (11) in 60% yield. Finally, phosphoramidite 1b reacted with [RuCl₂(C₆Me ₆)]₂ to give [RuCl₂(C₆Me ₆)(1b)] (12) in 78% yield. All novel complexes are catalytically active in the formation of β-oxo esters from propargylic alcohols and carboxylic acids. Standard conditions involve cyclohexane solvent, propargylic alcohol (1.0 equiv.), carboxylic acid (1.0 equiv.), ruthenium catalyst (1.5 mol%), and 90°C for 5-18 h. Isolated yields of the β-oxo esters range from 87 to 16% and show broad substrate generality. The reaction proceeds without racemization if a chiral propargylic alcohol is employed. The method is practical as no additives are required and the exclusion of oxygen and moisture is not needed. Complex 7c turned out to be the most effective catalyst (5 h reaction time), showing that the ligand structure has a profound impact on the catalytic performance. The crystal structure of 7a was determined, confirming an octahedral coordination geometry about the ruthenium center.