ABSTRACT
Maria G. Musolino, Giuseppe Apa, Andrea Donato and Rosario Pietropaolo
Department of Mechanics and Materials, Faculty of Engineering, University of Reggio Calabria, Loc. Feo di Vito, I-89060 Reggio Calabria, Italy
pietropaolo@ing.unirc.it
Abstract Homogeneous hydrogenation and isomerization reactions of α,β-unsaturated alcohols have been investigated in ethanol by using tris(triphenylphosphine) chlororhodium(I), RhCl(PPh3)3, and triethylamine at 303 K and 0.1 MPa hydrogen pressure. The results can be interpreted on the basis of a proposed mechanism in which RhH(PPh3)3 is the active species. A comparison is also reported with analogous reactions carried out in the absence of NEt3. Introduction In a previous paper we have investigated homogeneous hydrogenation and isomerization reactions of (Z)-2-butene-1,4-diol in ethanol at 303 K by using the Wilkinson catalyst (RhCl(PPh3)3) in the presence of triethylamine. Although RhCl(PPh3)3 was, so far, largely used for hydrogenation reactions and mainly affords fully hydrogenated compounds, it is worth noting that such a catalyst, in the presence of triethylamine and (Z)-2-butene-1,4-diol, is more selective towards the geometric isomerization product, (E)-2-butene-1,4-diol (1). In this work we extend our study to the hydrogenation and isomerization of a series of α,β-unsaturated alcohols, such as 2-propen-1-ol (A2), (E)-2-buten-1-ol (EB2), (Z)-2-penten-1-ol (ZP2), (E)-2-penten-1-ol (EP2), (Z)–2-hexen-1-ol (ZH2), (E)–2-hexen-1-ol (EH2), carried out in the presence of RhCl(PPh3)3, with and without triethylamine (NEt3), at 303 K, using ethanol as solvent. The major targets of our research are to investigate the influence of the unsaturated alcohol structure on the product distribution and to verify the possibility of extending the results, previously obtained with (Z)-2-butene-1,4-diol, to other analogous substrates. Results Reactions of α,β-unsaturated alcohols with hydrogen were carried out in the presence of RhCl(PPh3)3, with and without NEt3, at 303 K, 0.1 MPa hydrogen pressure and using ethanol as solvent. Under the experimental conditions adopted, the reaction, in
the presence of NEt3, proceeds according to the following scheme (in the case reported below we consider a Z geometric isomer as starting material):
Three main reaction routes are operating. In addition to the fully hydrogenated product, double bond and geometric isomerization derived compounds were also detected. Indeed, the double bond migration process from 2-3 carbons to the 2-1 (see the above scheme), involving, in this case, the carbon atom bearing the -OH group, affords the aldehydes through a vinyl alcohol intermediate. No hydrogenolysis products were observed and only a small amount of compounds, where the double bond moves from the 2-3 to the 3-4 carbons, was detected. The selectivity and the yield of the process depend mainly on three different factors: i) the catalytic species in solution; ii) the geometric isomer structure of the organic substrate; iii) the chain length of the reacting compound. Taking into account some of our previous results (1), showing the formation of an 1:1 electrolyte by conductometric measurements on the same rhodium(I) coordinated system, used in this paper, and including also an interesting observation by Schrock and Osborn (2), the following equilibria may be considered in our system:
RhCl(PPh3)3 + H2 + solv H2RhCl(PPh3)2(solv) + PPh3
H2RhCl(PPh3)2(solv) + NEt3 + PPh3 RhH(PPh3)3 + NEt3H+ + Cl-+ solv
Consequently, in the absence of NEt3, the main catalytic species should be H2RhCl(PPh3)2(solv), whereas, in the presence of NEt3, RhH(PPh3)3 should be formed. We are aware that such a conclusion is somewhat speculative; however, it seems the most likely if we look at all the experimental data reported on the subject. Table 1 reports experimental results concerning both activity and product distribution, determined in different conditions. Since the Rh(I) mono hydride complex is a catalytic species, it is reformed in every step of the reaction and its concentration remains constant. Therefore, rate data are calculated by the ratio of slopes of plots of organic substrate concentration, divided by the Rh(I) concentration, versus reaction time. The slope of these curves is obtained at about 70 % conversion of the substrate.
