Catalysis of Organic..

Musolino, Apa, Donato and Pietropaolo 251observed, practically suppressed. The E starting compound is, in fact,thermodynamically more stable than the corresponding Z one.ConclusionsThe main aim of the results concerns the comparison between E and Z isomers, bothtowards hydrogenation and/or isomerization processes. In particular, the less stable Zisomers favor the geometric isomerization products, whereas the most stable (E)-2-buten-1-ol mainly affords a hydrogenated compound. This agrees with earlierfindings, where Z isomers generally favor isomerized (both geometric and doublebond) derivatives (3).When 2-propen-1-ol reacts in the presence of NEt 3 and Rh(I), the reduced sterichindrance and the lack of a possible geometric isomerization are to be expected. So,we explain the large amount of the hydrogenated compound formed (Table 1). In theabsence of NEt 3 , as a rule, hydrogenation of organic substrates generally occurs.However, additionally in this specific case, the reduced reactivity of the systemsuggests that a different catalytic species is operating.Considering all these data, we conclude that the steric hindrance of coordinatedphosphines is the most important factor in determining the behavior of the reactionsstudied.Experimental SectionRhCl(PPh 3 ) 3 was prepared according to the literature (4). The hydrogenation ofdifferent α,β-unsaturated alcohols, 2-propen-1-ol, (E)-2-buten-1-ol, (Z)-2-penten-1-ol,(E)-2-penten-1-ol, (Z)–2-hexen-1-ol, (E)–2-hexen-1-ol, was carried out in liquidphase at 303 K and 0.1 MPa hydrogen pressure, using ethanol as solvent in a batchreactor. A stirring rate of 500 rpm was used. The experimental setup was thoroughlypurged with nitrogen before the beginning of the reaction. The rhodium complex(1.08 x 10 -5 moles) was dissolved, under stirring, in the solvent used (25 mL) inatmosphere of H 2 at 303 K; then, NEt 3 was added in the molar ratio RhCl(PPh 3 ) 3 /NEt 31:100, and the system was allowed to equilibrate for one hour. Finally 15 mL of a 0.6M solution of the α,β-unsaturated alcohol in ethanol, containing an internal standard,was added through one arm of the flask.The progress of the reaction was followed by analyzing a sufficient number ofsamples withdrawn from the reaction mixture. Product analysis was performed with agas chromatograph (Agilent Technologies model 6890N) equipped with a flameionization detector. The product separation was obtained by a capillary column (J&WDB-Waxetr, 50 m, i.d. = 0.32 mm). Quantitative analyses were carried out bycalculating the area of the chromatographic peaks with an electronic integrator.References1. M. G. Musolino, G. Apa, A. Donato and R. Pietropaolo, Catal. Tod; 100, 467(2005).