Hydrogenation, catalytic, alkene solvent effects

Syn-hydroxylation of alkenes is also effected by a catalytic amount of osmium tetroxide in the presence of hydrogen peroxide. Originally developed by Milas, the reaction can be performed with aqueous hydrogen peroxide in solvents such as acetone or diethyl ether.58 Allyl alcohol is quantitatively hydroxylated in water (Eq. 3.12).59... 

Titanium. Catalyses of hydrogenation of alkenes, alkynes, carbonyl-, and nitro-compounds have been described. The effect of the nature of the ligand L and of the alkene to be reduced on reactivity in catalytic hydrogenation by Ti(7r-C5H5)2L2 has been quantitatively studied. The dependence of rate constants on solvent for reduction of decene in the presence of Ti(7r-C5H5)Me+ is interpreted in terms of electrostatic interaction between the active ionic species and the solvent. There is also a thermochemical report relevant here, and that is of a determination of the heats of mixing of cyclohexene and of hex-l-ene with titanium tetrachloride. The heats of mixing are close to zero, which implies very small heats of complex formation between these alkenes and titanium. ... 

A comparative study has been made of DMAP, DABCO and imidazole as catalysts in the MBH reaction of methyl acrylate or acrylonitrile with aromatic aldehydes (Scheme 2.47). Using neat activated alkenes, where there is no hydrogen bonding or additives effects, DMAP and DABCO present similar catalytic activity at 76 °C in the reaction with i-nitrobenzaldehyde, and DMAP could be an option for DABCO. On the other hand, DABCO is better than DMAP when less reactive electrophiles are used. Imidazole, which exhibits catalytic activity in water media, does not show catalytic activity under solvent-free conditions. Rapid conversion using DMAP and DABCO at low temperature (at — 5 °C) was observed, presumably due to an entropy controlled... 

The TEAF system can be used to reduce ketones, certain alkenes and imines. With regard to the latter substrate, during our studies it was realized that 5 2 TEAF in some solvents was sufficiently acidic to protonate the imine (p K, ca. 6 in water). Iminium salts are much more reactive than imines due to inductive effects (cf. the Stacker reaction), and it was thus considered likely that an iminium salt was being reduced to an ammonium salt [54]. This explains why imines are not reduced in the IPA system which is neutral, and not acidic. When an iminium salt was pre-prepared by mixing equal amounts of an imine and acid, and used in the IPA system, the iminium was reduced, albeit with lower rate and moderate enantioselectivity. Quaternary iminium salts were also reduced to tertiary amines. Nevertheless, as other kinetic studies have indicated a pre-equilibrium with imine, it is possible that the proton formally sits on the catalyst and the iminium is formed during the catalytic cycle. It is, of course, possible that the mechanism of imine transfer hydrogenation is different to that of ketone reduction, and a metal-coordinated imine may be involved [55]. 

The electrochemical generation of hydrogen in aqueous acid or alkaline solutions reduces unactivated alkynes 46 a b). This process is similar to catalytic hydrogenation, however, and does not involve electron transfer to the substrate. The electrochemical generation of solvated electrons in amine solvents or HMPA has also been effective in reducing these compounds 29). The focus of this section, however, is the electrolysis of these difficult to reduce alkynes and alkenes at mercury cathodes with tetraalkyl-ammonium salts as electrolytes. Specific attention is also given to competitive reductions of benzenoid aromatics and alkynes or alkenes. 

The use of ionic hquids in asymmetric catalysis was reported even later, beginning with Chauvin s report on a catalytic asymmetric hydrogenation and hydroformylation of alkenes in 1995 [125]. Since then, enantioselective catalysis in ionic hquids has attracted remarkable interest as an approach to the facile recycling of expensive chiral ligands and catalysts, and a range of enantioselective catalyhc transformahons have been examined in ionic liquids [126]. In many cases, ionic hquids have a beneficial effect on the achvihes and enanhoselectivities, and demonstrate facile recovery and reusabihty of the ionic solvent-catalyst systems. The reader is referred to Chapter 7 for an excehent review on the development of enanhoselechve catalysis in ionic hquids. 

The cation of the metal /m.-butoxide strongly influences the rates but not the products of alkene isomerizations. For isomerization of 1-butene in dimethyl sulfoxide solutions at 55°C, the relative catalytic effectiveness of alkali / r/,-butoxides increase in the order NaOBu 1.0 KOBu 116 CsOBu 284, RbOBu, 447. This is probably attributable to the fact that large cations are more weakly bonded to the alkoxide ion than smaller cations . The anion of the alkoxide also strongly influences its catalytic effectiveness. Potassium tert.-buioxide is 126 times as effective a catalyst for 1-butene isomerization as potassium methoxide in dimethyl sulfoxide at 55°C . The rate of potassium / r/.-butoxide-catalyzed 1-butene isomerization in DMSO is strongly retarded by addition of / r/.-butyl alcohol to the solvent, probably due to hydrogen bonding between the alkoxide and the alcohoP . 

The syn-hydroxylation of alkenes can also be effected by hydrogen peroxide in presence of catalytic amount of OsO. This procedure was used earlier in solvents such as acetone or diethyl ether. By this method allyl alcohol is quantitatively hydroxylated in water (Scheme-70)." ... 

The catalysts applied to alkene epoxidation in fluorinated alcohol solvents can be subdivided into those which are metal/chalconide-based and those which are purely organic in nature (Scheme 4.5). The former comprise arsanes/arsane oxides [27,28], arsonic acids [29, 30], seleninic acids/diselenides ]31-35], and rhenium compounds such as Re207 and MTO (methylrhenium trioxide) ]36,37]. As shown in Scheme 4.5, their catalytic activity is ascribed to the intermediate formation of, for example, perseleninic/perarsonic adds or bisperoxorhenium complexes. In other words, their catalytic effect is due to the equilibrium transformation of hydrogen peroxide to kmetically more active peroxidic spedes. 

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