Transfer Hydrogenation using Homogeneous Catalysts

A plethora of ligands has been reported in the literature but the most effective ones are 1,2-amino alcohols, monotosylated diamines and selected phosphine-oxazoline ligands. The active structures of the complexes reported are half-sand- 

Furthermore, Avecia has developed the Rh-cyclopentadienyl complexes in Fig. 3.36 for the large scale production of 1-tetralol and substituted 1-phenyl-ethanol [102]. The challenge in this area is to increase the activity of the catalyst. Recently, Anders son and co-workers reported an azanorbornane-based ligand which can reach a TOF up to 3680 h-1 for acetophenone hydrogenation (see 

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Besides solid transition metals, certain soluble transition-metal complexes are active hydrogenation catalysts.4. The most commonly used example is tris(triphenylphosphine)-chlororhodium, which is known as Wilkinson s catalyst.5 This and related homogeneous catalysts usually minimize exchange and isomerization processes. Hydrogenation by homogeneous catalysts is believed to take place by initial formation of a rc-complex, followed by transfer of hydrogen from rhodium to carbon. 

Several strategies have been developed to characterise the structural and functional role of particular molecular species of membrane lipid. The use of phospholipid exchange or transfer proteins have been described to exchange one molecular species of a lipid class for another or enrich membranes in specific molecular species, respectivly. Another method has been to hydrogenate the double bonds of polyunsaturated membrane lipids in situ using homogeneous catalysts and to study the consequences of lipid saturation on membrane functions [2]. With the isolation of mutant strains of Arabidopsis defective in desaturation of chloroplast fatty acids [3,4] the question can be addressed directly without the problems associated with the use of exchange or transfer proteins or introduction of transition metal catalysts. 

A new process developed by Institut Francais du Petrole produces butene-1 (1-butene) by dimerizing ethylene.A homogeneous catalyst system based on a titanium complex is used. The reaction is a concerted coupling of two molecules on a titanium atom, affording a titanium (IV) cyclic compound, which then decomposes to butene-1 by an intramolecular (3-hydrogen transfer reaction. ... 

Abstract The use of A-heterocyclic carbene (NHC) complexes as homogeneous catalysts in addition reactions across carbon-carbon double and triple bonds and carbon-heteroatom double bonds is described. The discussion is focused on the description of the catalytic systems, their current mechanistic understanding and occasionally the relevant organometallic chemistry. The reaction types covered include hydrogenation, transfer hydrogenation, hydrosilylation, hydroboration and diboration, hydroamination, hydrothiolation, hydration, hydroarylation, allylic substitution, addition, chloroesterification and chloroacylation. 

Establishment of a free radical mechanism via H-atom transfer for hydrogenation using HMn(CO)5 (see Section II,D), and possibly also HCo(CO)4 (see Section II,C), suggests that more serious consideration for such mechanisms should be given for other hydridocarbonyl catalyst systems, and indeed for other homogeneous catalysts systems in general. The pentacyanocobaltate(II) catalyst can certainly operate by such a mechanism (see Section II,D). 

In Figure 13.19 we have shown a route to L-699,392 published by Merck involving three steps based on homogeneous catalysts, viz. two Heck reactions and one asymmetric hydrogen transfer reaction, making first an alcohol and subsequently a sulphide [21], Stoichiometric reductions for the ketone function have been reported as well [22] and the Heck reaction on the left-hand side can be replaced by a classic condensation reaction. L-699,392 is used in the treatment of asthma and related diseases. 

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