Rhodium-catalysed reactions hydrogenation

From these results it can be seen that in the hydrogenation of 1,2-dialkylcyclohexenes the expected cis-1,2-dialkylcyclohexane is not the sole product. Similarly, in the hydrogenation of the 1,4-dialky lcyclohexenes where both the cis- and trans-cyclohexanes are expected, the trans-isomer being the thermodynamically more stable, the stereoselectivity varies from metal to metal. Thus with palladium, the cis/trans ratio approaches the equilibrium composition, whereas with platinum and rhodium, the equilibrium composition is never approached. It is also instructive to note that in the palladium-catalysed reactions, hydrogenation is accompanied by extensive alkene isomerisation [220—223], whereas with rhodium and platinum, little or no isomerisation is observed [220,... 

There has been a review of asymmetric Friedel—Crafts reactions. It has been shown that the rhodium-catalysed reaction of potassium phenyltrifluoroborate with A-tosyl ketimines may lead to products such as (18) with high enantioselectivity. (g) The reaction of A-t-butanesuUinylimino esters with arenes to give products (19) is catalysed by Lewis acids, such as indium triflate, and may lead to enantio-metrically enriched a-glycines. The intramolecular alkylation of hydrogenated 0 tetralins, shown in Scheme 2, using iron(III) or aluminium(III) catalysts yields cis-hexahydrobenzophenanthridines. Formation of a stabilized carbocation intermediate is... 

Hamada s group has developed the rhodium-catalysed asymmetric hydrogenation of a-amino-)S-ester hydrochlorides via DKR. The reaction proceeded with the catalyst derived from a rhodium complex and a chiral ferrocenylphosphine under hydrogen in the presence of sodium acetate in acetic acid to alford the corresponding awti-jS-hydroxy-a-amino esters with 58-83% ee in a diastereomeric ratio of 92 8 to 97 3 (Scheme 2.83). 

Finally, the use of S/P ligands derived from (i )-binaphthol has been considered by Gladiali et al. in the asymmetric rhodium-catalysed hydrogen-transfer reduction of acetophenone performed in the presence of i-PrOH as the hydrogen donor.It was noted that racemisation occurred when the reaction time increased and consequently the corresponding alcohol was obtained in only low enantioselectivities (< 5% ee) as shown in Scheme 9.21. Similar results were more recently reported by these authors by using iridium combined with the same ligands. ... 

PEGs with average molecular weights above 1000 are waxy solids under ambient conditions, but they melt under C02 pressure to become liquids under typical conditions of scC02 catalysis [63], The approach was demonstrated for the rhodium catalysed hydrogenation of styrene as a test reaction using Wilkinson s complex [(PPhs RhCl] as the catalyst (Scheme 8.6) [61],... 

Ramies, O., Net, G., Ruiz, A. and Claver, C. (2000) Chiral diphosphites as ligands for the rhodium- and iridium-catalysed asymmetric hydrogenation Precatalyst complexes, intermediates and kinetics of the reaction. Eur. J. Inorg. Chem., 1287-1294. 

Over all the metals studied, except cobalt, nickel and copper, the selectivity and stereoselectivity decreased slightly as the reaction proceeded. In addition to the products shown in Table 20, in the rhodium- and iridium-catalysed reactions small yields (2—3%) of buta-1 2-diene were also observed. For all the catalysts, except rhodium, iridium and platinum, which were not investigated, the initial rate kinetic orders were unity in hydrogen and zero or slightly negative (Ni) in but-2-yne. 

In the rhodium- and platinum-catalysed reactions [167], it is of significant interest that the 7V-profiles calculated from the distributions of deuterium in the n-butane (Table 30) appear to bear no clear relationship to the 7V-profiles of the n-butenes formed simultaneously. This observation has been interpreted as indicating that either the butene which undergoes further hydrogenation never desorbs as butene, or that the sites responsi-... 

Chiral ionic liquids have also been used to aid enantioselective metal catalysed reactions. For example, homogeneous rhodium catalysed hydrogenations using tropoisomeric biphenylphosphine ligands have been reported using chiral ionic liquids derived from L-proline and L-valine. Enantioselectivities of up to 69% could be achieved and the catalytic system could be reused after extraction with SCCO2. 

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