Hydrogenation biphasic

The first successful hydrogenation reactions in ionic liquids were studied by the groups of de Souza [45] and Chauvin [46] in 1995. De Souza et al. investigated the Rh-catalyzed hydrogenation of cyclohexene in l-n-butyl-3-methylimidazolium ([BMIM]) tetrafluoroborate. Chauvin et al. dissolved the cationic Osborn complex [Rh(nbd)(PPh3)2][PFg] (nbd = norbornadiene) in ionic liquids with weakly coordinating anions (e.g., [PFg] , [BFJ , and [SbF ] ) and used the obtained ionic catalyst solutions for the biphasic hydrogenation of 1-pentene as seen in Scheme 5.2-7. 

Scheme 5.2-7 Biphasic hydrogenation of 1-pentene with the cationic Osborn complex ...

Monflier et al. (1997) have suggested Pd catalysed hydrocarboxylation of higher alpha olefins in which chemically modified P-cyclodextrin (especially dimethyl P-cyclodextrin) is u.sed in water in preference to a co-solvent like methanol, acetone, acetic acid, acetonitrile, etc. Here, quantitative recycling of the aqueous phase is possible due to easy phase separation without emulsions. A similar strategy has been adopted by Monflier et al. (1998) for biphasic hydrogenations for water-in.soluble aldehydes like undecenal using a water-soluble Ru/triphenylphosphine trisulphonate complex with a. suitably modified p-cyclodextrin. 

The aqueous biphase hydrogenation of dimethyl itaconate is accomplished with an Ir-(7 )-(7 )-3-benzyl(/>-sulfonate)-2,4-bis(diphenylphosphino)pentane complex.605... 

Dupont, J., Fonseca, G.S., Umpierre, A.P., Fichtner, P.F.P. and Teixeira, S.R. (2002) Transition-metal nanopartides in imidazolium ionic liquids recycable catalysts for biphasic hydrogenation reactions. Journal of the American Chemical Society, 124 (16), 4228—4229. 

Figure 6.2. Fluorous biphasic hydrogenation of methyl t/h-cinnamate catalysed by rhodium complexes.[23]...

The aqueous-biphase hydrogenation reactions of thiophenes to the corresponding cyclic thioethers have been shown to be mechanistically similar to those in truly homogenous phase. 

Several examples of achiral biphasic hydrogenations are shown in Table 38.1. It can be seen, that the activity of the various phosphine complexes of precious metals rarely exceeds 100 h-1 under mild conditions. In many cases hydrogenation is accompanied by isomerization of the olefinic substrates. 

Enantioselective hydrogenation of prochiral ketones has rarely been studied in aqueous biphasic media. In addition to the chiral bisphosphonic acid derivatives of 1,2-cyclohexanediamine [130], the protonated 4,4 -, 5,5 -, and 6,6 -amino-methyl-substituted BINAP (diamBINAP 2HBr) ligands (Scheme 38.7) served as constituents of the Ru(II)-based catalysts in the biphasic hydrogenations of ethyl acetoacetate [131, 132]. These catalysts were recovered in the aqueous phase and used in at least four cycles, with only a marginal loss of activity and enantio-selectivity. 

The d-lactone (Scheme 38.11) can be efficiently obtained by the telomerization of butadiene and C02. Its biphasic hydrogenation with an in-situ-prepared Rh/ mtppts catalyst yields 2-ethylidene-6-heptenoic acid (and its isomers) [136]. Note, that the catalyst is selective for the hydrogenolysis of the lactone in the presence of two olefmic double bonds this is probably due to the relatively large [P] [Rh] ratio (10 1) which is known to inhibit C = C hydrogenations with [RhCl(wtppms)3]. The mixture of heptenoic acids can further be hydrogenated on Pd/C and Mo/Rh catalysts to 2-ethylheptanol which finds several applications in lubricants, solvents, and plasticizers. This is one of the rare examples of using C02 as a Cl building block in a transition metal-catalyzed synthetic process. 

A cationic complex, formed in situ from 5 and [Rh(COD)2]OTf, was also active in biphasic hydrogenation [14]. No preference for the fluorous phase was found for ligands containing only one perfluoroalkyl tail, but neutral and cationic complexes, containing mono- and bidentate 4a or 5, respectively, were selectively dissolved in the fluorous phase. No leaching and recycling studies were performed. 

The immobilization of Pd(acac)2 as hydrogenation catalyst in the ionic liquids [BMIM][BF4] and [BMIM][PF6] was reported by Dupont et al. in 2000 [70]. These authors compared the biphasic hydrogenation of butadiene with the homogeneous system with all reactants being dissolved in CH2C12, with the reaction in neat butadiene and with a heterogeneous system using Pd on carbon as catalyst. 

Table 41.7 Comparative studies of the biphasic hydrogenation reactions of arenes in [BMIM][BF4] and water with [H4Ru4( 76-C6H6)4][BF4]2 as the catalyst precursor [88].

Hydrogenation reactions in water have been extensively studied and many of the water-solubilizing ligands described in Chapter 5 have been tested in aqueous-organic biphasic hydrogenation reactions. One of the earliest catalysts used was the water-soluble analogue of Wilkinson s catalyst, RhCl(tppms)3 (tppms = monosulfonated triphenylphosphine), but many other catalysts have since been used including [Rh(cod)(tppts)2]+, [Rh(cod)2]+ and [Rh(acac)(CO)2]+ (cod = cyclooctadiene). 

In the ideal biphasic hydrogenation process, the substrate will be more soluble or partially soluble in the immobilization solvent and the hydrogenation product will be insoluble as this facilitates both reaction and product separation. Mixing problems are sometimes encountered with biphasic processes and much work has been conducted to elucidate exactly where catalysis takes place (see Chapter 2). Clearly, if the substrates are soluble in the catalyst support phase, then mixing is not an issue. The hydrogenation of benzene to cyclohexane in tetrafluoroborate ionic liquids exploits the differing solubilities of the substrate and product. The solubility of benzene and cyclohexane has been measured in... 

It is often useful to keep some of the reactants or the products in separate phases (principle of chemical protection by phase separation [53]). For instance, when the reaction is inhibited by its own substrate having the latter in an other phase than the one in which the catalyst is dissolved helps to eliminate long induction periods or complete stop of the reaction. An example is the biphasic hydrogenation of aldehydes with the water-soluble... 

Even in an excess of ligands capable of stabilizing low oxidation state transition metal ions in aqueous systems, one may often observe the reduction of the central ion of a catalyst complex to the metallic state. In many cases this leads to a loss of catalytic activity, however, in certain systems an active and selective catalyst mixture is formed. Such is the case when a solution of RhCU in water methanol = 1 1 is refluxed in the presence of three equivalents of TPPTS. Evaporation to dryness gives a brown solid which is an active catalyst for the hydrogenation of a wide range of olefins in aqueous solution or in two-phase reaction systems. This solid contains a mixture of Rh(I)-phosphine complexes, TPPTS oxide and colloidal rhodium. Patin and co-workers developed a preparative scale method for biphasic hydrogenation of olefins [61], some of the substrates and products are shown on Scheme 3.3. The reaction is strongly influenced by steric effects. 

It is interesting to note that no specific study was devoted to the aqueous biphasic hydrogenation of aldehydes with water-soluble cobalt-phosphine complexes, although such a property has long been known from hydroformylation experiments [199,200]. 

Complexes of Rh, Pt, and Pd with the same ligands were active in the biphasic hydrogenation of chloro- and bromonitrobenzenes. At 80-100 °C and 20 bar H2 pressure the main products were the corresponding chloro-and bromoanilines, up to 99.8 % yield (Scheme 10.5) [12]. The selectivity of similar reactions catalyzed by a Rh/TPPTS was only about 90 %, i.e. the attached cyclodextrin moiety further decreased the extent of hydrodehalogenation, probably by complexation of the halonitroaromatic substrate. 

Table 4.2 Biphasic hydrogenation of aldehydes using sulfonated phosphines [20].

Table 15.2 Biphasic hydrogenation of arenes using soluble...

Table 15.3 Biphasic hydrogenation reactions by lr(0) nanoparticles in BMl PFs under 4atm of H2 (constant pressure) at 75°C and olefin/lr= 1200 molar ratio [24],...

The in situ reduction of the precursor [lr(COD)Cl]2 dispersed in BMI-PF at 75 °C and under 4atm of H2 provided a suitable medium for the synthesis of lr(0) nanoparticles, and represents an ideal system for the biphasic hydrogenation reactions of several olefins (Table 15.3) [24]. Of note, the TOP observed for this system (6000 h at 1200 rpm and 75 °C) was considerably higher than those obtained under biphasic conditions by classical transition-metal catalyst precursors in ILs under similar reaction conditions [46-48]. 

As expected, cyclohexanone hydrogenation performed in an IL has a longer reaction time than in solventless conditions. Where using iridium nanoparticles dispersed in an IL, the biphasic hydrogenation of cyclohexanone could be performed at least 15 times, without any considerable loss in catalytic activity this contrasted with the use of nanoparticles in solventless conditions, when the catalytic activity begins to decHne after the third cycle. The standard experimental conditions established for the hydrogenation of other carbonyl compounds were 75 °C, 4atm of H2 and a molar substrate Ir ratio of 250. 

Researchers performed the biphasic hydrogenation of cyclohexene with Rh(cod)2 BF4 (cod = cycloocta-1,5-diene) in ILs. They observed roughly equal reaction rates, reported as turnover frequencies of ca. 50 h in either [bmim][BF4] or [bmim][PF6]. The presumption here was that the [bmim][BF4] was free from chloride. In a separate report, the same group showed that RuCl2(Ph3P)3 in [bmim][BF4] was an effective catalyst for the biphasic hydrogenation of olefins, with turnover frequencies up to 540 h Similarly, (bmim)3-Co(CN)5 dissolved in [bmim][BF4] catalyzed the hydrogenation of butadiene to but-l-ene, with 100% selectivity at complete conversion. 

The PTA ligand has recently been employed as a water-soluble ligand in a variety of studies, including catalytic biphasic hydrogenation reactions,4,5 ligand-substitution reactions in metal clusters,6 and enzyme-mediated oxo-transfer processes.7... 

The water-soluble Ru(II) complex [Ru(i76-C6H6)(CH3CN)3](BF4)2 catalyzed the biphasic hydrogenation of alkenes and ketones with retention of the catalyst in the aqueous phase (87). However, the ruthenium complex moved to the organic phase when benzaldehyde was hydrogenated. In a benzene-D20 system, H-D exchange was observed between H2 and D20. Both monohydridic pathway and a dihydridic pathway are possible for hydrogen activation, and these two different catalytic cycles influence the yield and product distribution. 

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