Alcohols via hydrogen transfer

The next step in the use of transfer hydrogenation catalysts for recycling of the unwanted enantiomer is the dynamic kinetic resolution. This is a combination of two reaction systems (i) the continuous racemization of the alcohol via hydrogen transfer and (ii) the enantioselective protection of the alcohol using a... 

Chemoselective reduction of conjugated enones to allylic alcohols via hydrogen transfer from propan-2-ol over metal oxides is investigated in vapour phase conditions. The unique ability of Mgo to reduce exclusively carbonyl group is observed. However, because of the high basicity of MgO side reactions are present. It is shown that by doping the Mgo catalyst with HC1 a significant decrease of its basicity occurs and consequently side reactions are minimized. 

Starting directly from unprotected amines and alcohols, N- and C(3)-dialkylation took place. This overall transformation resulted from dehydrogenation of the alcohol via hydrogen transfer followed by a mechanism similar to the previous one involving borrowing hydrogen processes (Scheme 41) [36]. 

Scheme 47 Alkylation of secondary alcohols with primary alcohols via hydrogen transfer reactions...

Finally, allene derivatives were also convenient unsaturated substrates allowing carbon-carbon bond formation from benzylic alcohol via hydrogen transfer processes. With these substrates, the best catalytic systems were based on RuHCl (CO)(PPh3)3 in the presence of an equimolar amount of phosphine ligand such as bis(diisopropylphosphino)ferrocene (dippf) [63], bis(dicyclohexyphosphino) ferrocene or PCyPh2 [64], Some examples of selective formation of homoallylic alcohols using this reaction are reported in Scheme 59. 

The racemization mechanism of sec-alcohols has been widely studied [16,17]. Metal complexes of the main groups of the periodic table react through a direct transfer of hydrogen (concerted process), such as aluminum complexes in Meerwein-Ponn-dorf-Verley-Oppenauer reaction. However, racemization catalyzed by transition metal complexes occurs via hydrogen transfer processes through metal hydrides or metal dihydrides intermediates (Figure 4.5) [18]. 

Primary alcohols 121 undergo an efficient oxidative dimerization by [IrCl(coe)2]2 under air, without any solvent, to form esters 122 in fair to good yields (Equation 10.30) [54]. The reaction is initiated by the in situ generation of an Ir-hydride complex via hydrogen transfer from alcohols to afford aldehydes, followed by the dehydrogenation of hemiacetals derived from alcohols and aldehydes by action of the Ir-complex to afford esters. 

Pinacol formation (equation 4) is considered to occur via coupling of two ketyl units, either within a higher aggregate as depicted or by direct coupling of two ketyls. Production of alcohols by hydrogen transfer (equation 5) probably occurs either within a ketyl aggregate or by the direct interaction of two metal ketyls. o... 

Ruthenium carbonyl catalyzes the oxidative coupling of alcohols to esters via hydrogen transfer to diphenylacetylene, chalcone, or maleic anhydride... 

Hydrogan Transfer- An extensive review of asymmetric transfo hydrogenation is of interest i as is an improved version for selective ester reduction via hydrogen transfer from HSi(OEt)3 in the presence of Ti(OPr )4. Addition to either the same or opposite enantiofaces of prochiral alkenes is catalysed by RuH (S)-BINAP species from either Pr>OH or H2 depending on the alkene functionality. A number of alcohol-ketone transfer hydrogenations catalysed by... 

Shvo s complex 1 catalyzes the racemization of stereocenters a to the OH group. Racemization proceeds via hydrogen transfer reactions. Dehydrogenation of the enantiomerically pure alcohol by complex (A) produces a ketone (Fig. 1). Subsequent re-addition of the hydride by 2 to either side of the prochiral ketone leads to racemization of the alcohol substrate (Fig. 1, cf. Scheme 1, X=0). 

C-N Bond Formation via Hydrogen Transfer Alcohol Preactivation... 

Mechanisms, exemplified by alcohol as donor (493, 496), usually invoke coordination of the substrate (olefins, saturated and unsaturated ketones, and aldehydes), then coordination of the alcohol and formation of a metal alkoxide, followed by /8-hydrogen transfer from the alkoxide and release of product via protonolysis ... 

Details of the various steps which will depend on the substrates and donors involved, are usually not well understood. Prior coordination of the donor followed by that of the substrate, equivalent to a hydride route (Section II,A), is also possible (494, 496). Formation of intermediate dihydrides from a donor (e.g., from an alcohol via oxidative addition to give a hydrido-alkoxide, and then /8-hydrogen transfer) has also been invoked (491, 492, 496, 499, 500) in mechanistic terms, the hydrogenations then become equivalent to using molecular hydrogen for the reductions. The /3-hydrogen transfer step is usually considered rate-determining (494, 496). 

A typical atom transfer sensitization process is shown in Figure 3.7 the alkylation of an unsaturated acid via a radical produced from an alcohol by hydrogen abstraction (in a green solvent, water). This principle has been applied to a large series of radical alkylation reactions, where the radical precursor is an alcohol, an ether, or even an alkane. ... 

A large number of reports have concerned transfer hydrogenation using isopropanol as donor, with imines, carbonyls-and occasionally alkenes-as substrate (Scheme 3.17). In some early studies conducted by Nolan and coworkers [36], NHC analogues of Crabtree catalysts, [Ir(cod)(py)(L)]PF,5 (L= Imes, Ipr, Icy) all proved to be active. The series of chelating iridium(III) carbene complexes shown in Scheme 3.5 (upper structure) proved to be accessible via a simple synthesis and catalytically active for hydrogen transfer from alcohols to ketones and imines. Unexpectedly, iridium was more active than the corresponding Rh complexes, but... 

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