Hydrogen transfer reactions catalyzed

Goldberg, K., Edegger, K., Kroutil, W. and Liese, A. (2006) Overcoming the thermodynamic limitation in asymmetric hydrogen transfer reactions catalyzed by whole cells. Biotechnology and Bioengineering, 95,192-198. 

Hydrogen Transfer Reactions Catalyzed by Ruthenium Complexes Linked to P-Cyclodextrin 43... 

A number of special oxidation methods have also been applied to partially protected aldoses. The Dess-Martin periodinane, l,l,l-triacetoxy-l,l-dihydro-l,2-benziodoxol-3(l//)-one [31], is a mild and efficient oxidant compatible with carbohydrate protecting groups [25]. However, it is also quite expensive and should only be used if the other procedures fail. The hydrogen transfer reactions catalyzed by 5% of RhH(PPh3)4 can also be applied to partially protected aldoses [8]. 

In our calculations [47], we treated the net hydrogen transfer reaction catalyzed by lipoxygenase as a PCET mechanism, as illustrated in Fig. 16.9. Quantum mechanical calculations indicate that the electron transfers from the n system of the linoleic acid to an orbital localized on the Fe(III) center, and the proton transfers... 

In an intermolecular hydrogen transfer reaction catalyzed by palladium on charcoal, 9-methyl-1,2,3,4-tetrahydro-l l//-dipyrido[l,2-n 4,3-rf]pyrimidin-ll-one in refluxing xylene is converted into the 6,7,8,9-tetrahydro isomer as the sole product, whereas the /V-benzyl derivative yields a 4 6 mixture of the 6,7,8,9-tetrahydro and the 1,2,3,4,6,7,8,9-octahydro compounds. With nitrobenzene as the hydrogen acceptor, the main product is the fully aromatic derivative.529,530... 

Fig. 12. Hydrogen transfer reactions catalyzed by Lactobacillus lekhmannii ribonucleotide reductase via deoxyadenosylcobalamin. (Co) indicates the cobalt-corrin complex. Hydrogen isotopes ( H) are exchanged between water and coenzyme, or vice versa, in presence of enzyme and a dithiol (i.e., reduced enzyme, E(SH)2) and an allosteric effector (not drawn) by reactions 1-3. Reaction 4 indicates degradation of the radical pair (center) to 5 -deoxyadenosine and cob(II)alamin. Reduction of a ribonucleotide substrate is described in step 5 the nucleotide radical (proposed in >) is a hypothetic intermediate...

Reduction of Schiff-bases has been performed also with hydrogen transfer reactions catalyzed by RhCl(PPh3)3, RhCl(CO)(PPh3)2, and... 

Hydrido(alkoxo) complexes of late transition metals are postulated as intermediates in the transition metal-catalyzed hydrogenation of ketones (Eq. 6.17), the hydrogenation of CO to MeOH, hydrogen transfer reactions and alcohol homologation. However, the successful isolation of such complexes from the catalytic systems was very rare [32-37]. 

The inhibitory function of the sulfur compounds then would appear to be to prevent the subsequent reaction of the initially formed ketyl radicals by catalyzing hydrogen transfer reactions. With this in mind, one can write the following mechanism for the inhibition of the photoreduction of benzo-phenone by sulfur compounds ... 

The generally accepted essential features for a catalyzed hydrogen transfer reaction are quite severe (a) the DH2 molecule must bind to a metal transfer hydrogen to it and must be released from the metal environment before back-transfer takes place (b) the A molecule must be stable to hydrogen abstraction under the reaction conditions employed. 

Considerable interest remains in catalyzed hydrogen-transfer reactions using as donor solvents alcohols, glycols, aldehydes, amides, acids, ethers, cyclic amines, and even aromatic hydrocarbons such as alkylben-... 

The scope of hydrogen transfer reactions is not limited to ketones. Imines, carbon-carbon double and triple bonds have also been reduced in this way, although homogeneous and heterogeneous catalyzed reductions using molecular hydrogen are generally preferred for the latter compounds. 

Since the first use of catalyzed hydrogen transfer, speculations about, and studies on, the mechanism(s) involved have been extensively published. Especially in recent years, several investigations have been conducted to elucidate the reaction pathways, and with better analytical methods and computational chemistry the catalytic cycles of many systems have now been clarified. The mechanism of transfer hydrogenations depends on the metal used and on the substrate. Here, attention is focused on the mechanisms of hydrogen transfer reactions with the most frequently used catalysts. Two main mechanisms can be distinguished (i) a direct transfer mechanism by which a hydride is transferred directly from the donor to the acceptor molecule and (ii) an indirect mechanism by which the hydride is transferred from the donor to the acceptor molecule via a metal hydride intermediate (Scheme 20.3). 

All hydrogen transfer reactions are equilibrium reactions. Consequently, both a reduction and an oxidation can be catalyzed under similar conditions. The balance of the reaction is determined by the thermodynamic stabilities of the spe-... 

Hydrogen transfer reactions are highly selective and usually no side products are formed. However, a major problem is that such reactions are in redox equilibrium and high TOFs can often only be reached when the equilibria involved are shifted towards the product side. As stated above, this can be achieved by adding an excess of the hydrogen donor. (For a comparison, see Table 20.2, entry 8 and Table 20.7, entry 3, in which a 10-fold increase in TOF, from 6 to 60, can be observed for the reaction catalyzed by neodymium isopropoxide upon changing the amount of hydrogen donor from an equimolar amount to a solvent. Removal of the oxidation product by distillation also increases the reaction rate. When formic acid (49) is employed, the reduction is a truly irreversible reaction [82]. This acid is mainly used for the reduction of C-C double bonds. As the proton and the hydride are removed from the acid, carbon dioxide is formed, which leaves the reaction mixture. Typically, the reaction is performed in an azeotropic mixture of formic acid and triethylamine in the molar ratio 5 2 [83],... 

Fig. 1. General scheme of the hydrogen transfer as catalyzed by dehydrogenase reactions. R represents the residue of the pyridine nucleotide molecule.

One of the systems was found to be very efficient catalyzing enantioface-selective hydrogen transfer reactions to aromatic and in particular to aliphatic ketones with up to 95% ee. Regarding the latter reaction these are unprecedented ee values. The reaction mechanism of these transformations is best described as a metal-ligand bifunctional catalysis passing through a pericyclic-like transition state. 

The mechanism for the iridium-catalyzed hydrogen transfer reaction between alcohols and ketones has been investigated, and there are three main reaction pathways that have been proposed (Scheme 4). Pathway (a) involves a direct hydrogen transfer where hydride transfer takes place between the alkoxide and ketone, which is simultaneously coordinated to the iridium center. Computational studies have given support to this mechanism for some iridium catalysts [18]. 

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