Racemization of Olefinic Alcohols by a Carbonyl(cyclopentadienyl)ruthenium Complex: Inhibition by the Carbon-Carbon Double Bond

• Published in: European journal of organic chemistry 2015-04, Vol.2015 (11), p.2388-2393
• Main Authors: Warner, Madeleine C., Bäckvall, Jan-E.
• Format: Article
• Language: English
• Subjects: Alcohol; Alcohols; Alkenes; Kinetic resolution; NMR spectroscopy; Racemization; Ruthenium
• ISSN: 1434-193X; 1099-0690; 1099-0690
• DOI: 10.1002/ejoc.201500066

In this article, racemization of various olefinic sec‐alcohols by Ru(CO)2(η5‐C5Ph5)Cl was investigated. The racemization of three aliphatic sec‐alcohols with different chain lengths containing terminal double bonds was studied. A dramatic decrease of the racemization rate was found for these sec‐alcohols compared to that of the corresponding saturated substrates. The slow racemization rate of the former alcohols is ascribed to coordination of the double bond to the ruthenium centre, which blocks the free site needed for β‐hydride elimination. This mechanism was supported by a recent study, in which 5‐hexen‐2‐ol was found to form an alkoxycarbonyl complex having the double bond coordinated to the ruthenium atom. Aliphatic sec‐alcohol substrates with a di‐ or trisubstituted double bond were found to give a lower degree of inhibition of the racemization rate than the substrates with a monosubstituted double bond. Racemization of various sec‐alcohol substrates containing a carbon–carbon double bond catalyzed by Ru(CO)2(η5‐C5Ph5)Cl was investigated. A strong inhibition of the rate of racemization was found for substrates having a C3–C5 chain with a terminal double bond. Alcohol substrates with a di‐ or trisubstituted double bond were found to give less inhibition of the racemization rate. The racemization reactions were studied by NMR spectroscopy with the aim of detecting intermediates.