Alkenes biphasic hydrogenation

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. 

The literature of biphasic hydrogenations contains plenty of substrates (al-kenes and cycloalkenes, arylaliphatic olefins, carbonyl compounds, etc.), mainly with TPPMS as water-soluble ligand (solubility approx. 200 g/1 [150] as compared with 1100 g/1 with TPPTS [37]). So far, no industrial process has been derived from these smdies. Besides the development of the basics of biphasic operation, the research concentrates on fundamental work concerning the question of where the reaction takes place phase boundary, organic phase, or aqueous phase. Wilkinson [29] concluded from his hydrogenation tests with hexenes or cyclohexenes in the presence of TPPMS that the somewhat lower rate of hydrogenation as compared with monophasic conversion should be due to the necessary diffusion of the hydrogen to the alkene/water interface. In this way the iso-... 

The anionic, water-soluble cobalt complex K3[Co(CN)5]H, under PTC conditions, effectively catalyzes the hydrogenation of dienes to monoenes [26-28]. In most cases 1,4-addition prevails, producing tran -alkenes in high yield. The catalytic pair Co /Q X" also accelerates the biphasic hydrogenation of styrene to ethylbenzene [29]. 

Whereas water-soluble rhodium complexes catalyze the biphasic hydrogenation of various unsaturated substrates but rapidly form colloids, the addition of yd-CD has been shown to lead to efficient homogeneous catalytic systems [40]. For instance, in the absence of transfer agent, as reported by Larpent et al. [41], maleic acid is completely transformed in 83 h instead of a reaction time of 17 h with yd-CD. Similarly, the hydrosilylation reaction of an alkene catalyzed by a platinum precursor can be notably accelerated by the presence of yd-CD the case of a Lamoreaux Pt Pl catalyst is more spectacular since the yield can reach 100% in 12 min, whereas no transformation of the substrate occurs in 24 h in the absence of yd-CD [42]. 

The preparation of (m-Bu4N)2W60i9 reported hoe is a sunplifieation of the procedure given in this series by Fournier and Fuchs. Peroxotungstic salts are catalysts for the epoxidation of alkenes with hydrogen pCTOxide. In addition, the quaternary phosphonium and ammonium salts arc of special interest due to their solubUity in nonchlorinated solvents, buturrfortunatcly these salts are difficult to isolate because their preparations involve the use of biphasic conditions. In contrast, the preparation of the cetylpyridinium cation requires only aqueous solutions. 

Ru3(CO)12(117)3] and [H4Ru4(CO)11(117)] as catalyst precursors in the hydrogenation of non-activated alkenes under biphasic conditions. Each cluster displays activity under moderate conditions, ca. 60 atm. H2 at 60 °C with catalytic turnovers up to ca. 500. The trinuclear clusters undergo transformations during reaction but can be used repeatedly without loss of activity.325... 

The monosulfonated PPh derivative, Ph2P(m-C6H4S03K) (DPM) and its rhodium complex, HRh(CO)(DPM)3 have been synthesized and characterized by IR and NMR spectroscopic techniques. The data showed that the structure was similar to [HRh(CO)(PPh3)3]. The catalytic activity and selectivity of [HRh(CO)(DPM)3] in styrene hydroformylation were studied in biphasic catalytic systems.420 421 Rh1 complexes [Rh(acac)(CO)(PR3)] with tpa (131), cyep (132), (126), ompp (133), pmpp (134), tmpp (135), PPh2(pyl), PPh(pyl)2, and P(pyl)3 were characterized with NMR and IR spectra. Complexes with (131), (132), and (126) were catalysts for hydrogenation of C—C and C—O bonds, isomerization of alkenes, and hydroformylation of alkenes.422 Asymmetric hydroformylation of styrene was performed using as catalyst precursor [Rh(//-0 Me)(COD)]2 associated with sodium salts of m-sulfonated diarylphosphines.423... 

Various other biphasic solutions to the separation problem are considered in other chapters of this book, but an especially attractive alternative was introduced by Horvath and co-workers in 1994.[1] He coined the term catalysis in the fluorous biphase and the process uses the temperature dependent miscibility of fluorinated solvents (organic solvents in which most or all of the hydrogen atoms have been replaced by fluorine atoms) with normal organic solvents, to provide a possible answer to the biphasic hydroformylation of long-chain alkenes. At temperatures close to the operating temperature of many catalytic reactions (60-120°C), the fluorous and organic solvents mix, but at temperatures near ambient they phase separate cleanly. Since that time, many other reactions have been demonstrated under fluorous biphasic conditions and these form the basis of this chapter. The subject has been comprehensively reviewed, [2-6] so this chapter gives an overview and finishes with some process considerations. 

The team of Crooks is involved in the synthesis and the use of dendrimers and, more particularly, poly(amidoamine) dendrimers (PAMAM), for the preparation of dendrimer-encapsulated mono- or bimetallic nanoparticles of various metals (Pt, Pd, Cu, Au, Ag, Ni, etc.) [55, 56]. The dendrimers were used as nanocatalysts for the hydrogenation of allyl alcohol and N-isopropylacrylamide or other alkenes under different reaction conditions (water, organic solvents, biphasic fluorous/or-ganic solvents or supercritical COz). The hydrogenation reaction rate is dependent on dendrimer generation, as higher-generation dendrimers are more sterically... 

Diphenylacetylene and 1-phenyl-1-propyne were hydrogenated to the corresponding 1,2-disubstituted alkenes in aqueous organic biphasic media using [ RuCl2(wtppms)2 2] and an excess of wtppms (80 °C, 1 bar H2, TOFs up to 25 h-1). The stereoselectivity of the reaction depended heavily on the pH of the catalyst-containing aqueous phase (Fig. 38.1) and, under acidic conditions, Z-al-kenes could be obtained with a selectivity close to 100% [71]. 

The first example of biphasic catalysis was actually described for an ionic liquid system. In 1972, one year before Manassen proposed aqueous-organic biphasic catalysis [1], Par shall reported that the hydrogenation and alkoxycarbonylation of alkenes could be catalysed by PtCh when dissolved in tetraalkylammonium chloride/tin dichloride at temperatures of less than 100 °C [2], It was even noted that the product could be separated by decantation or distillation. Since this nascent study, synthetic chemistry in ionic liquids has developed at an incredible rate. In this chapter, we explore the different types of ionic liquids available and assess the factors that give rise to their low melting points. This is followed by an evaluation of synthetic methods used to prepare ionic liquids and the problems associated with these methods. The physical properties of ionic liquids are then described and a summary of the properties of ionic liquids that are attractive to clean synthesis is then given. The techniques that have been developed to improve catalyst solubility in ionic liquids to prevent leaching into the organic phase are also covered. 

The hydrogenation of simple alkenes, such as hexene, cyclohexene, cyclo-hexadiene and benzene, has been extensively studied using biphasic, alternative solvent protocols. These hydrocarbon substrates are more difficult to hydrogenate compared to substrates with electron withdrawing groups. Benzene and alkyl substituted aromatic compounds are considerably more difficult to hydrogenate... 

The limitations of hydroformylation reactions in water are the same as those of hydrogenation reactions, i.e. the poor solubility of the substrates (see Section 8.2.1). While aqueous-organic biphasic hydroformylation works well for alkenes with chain lengths up to C7, the solubility of longer chain alkenes is too low for viable processes. Although simple alkenes are poorly soluble, many functional alkenes have solubilities in water that are sufficiently high to avoid mass transfer problems, but at the same time this can impede separation. 

Biphasic systems containing an ionic liquid and supercritical CO2 have been used effectively for catalytic hydrogenation of alkenes. The ionic liquid phase containing the catalyst could be reused (2/6). 

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