In situ PHIP-NMR studies during the stereoselective hydrogenation of sorbic acid with a [Cp*Ru](+) catalyst

We report mechanistic studies on the homogeneously catalyzed stereoselectiv e hydrogenation of sorbic acid (trans,trans-2,4-hexadienoic acid) using the catalyst [Cp*Ru(eta(4)-CH3CH =CHCH=CHCO2H)][CF3SO3]. The application of pa ra-hydrogen induced polarization (PHIP) NMR spectroscopy was found very use ful for this investigation, since it allows us to investigate the hydrogena tion mechanism of the reaction in situ, i.e. without disturbing the reactio n for product analysis, e.g, via gas chromatography. Additionally, the reac tion takes place under initial kinetic conditions without any product inhib ition. These in situ PHIP-NMR investigations proved the exclusive formation of cis-3-hexenoic acid as the primary hydrogenation product via a 1,4-addi tion of the dihydrogen to the sorbic acid's diene system. Any other mechani sm for the formation of cis-3-hexenoic: acid can be excluded, trans-3-Hexen oic acid is formed by a subsequent rearrangement reaction of the cis produc t, and all other reduced products are only obtained if the reaction is cont inued until total conversion of the starting material sorbic acid is reache d. The conclusions are supported by simulated PHIP-NMR spectra. Copyright ( C) 2000 John Wiley & Sons, Ltd.