Hydrogenation electrocatalytic

Hydrogenation of organic compounds is a very important reaction from the synthetic point of view. The electrocatalytic hydrogenation proceeds by the following steps (Eqs. 9-14 to 9-19)  

Fe) whereas direet eleetroreduetions and dimerizations would be most effieient at high-overpotential eleetrodes (Hg, Pb, Cd). Hydrogen overpotential values vary from 0.0 V for platinized platinum to 1.2 V for Cd relative to the normal lydrogen electrode. High- and low-hydrogen overvoltage eleetrodes are used on an industrial scale. 

Noble metals are eatalytieally very active, and many studies have been earried out on their surfaces, espeeially platinum, palladium, and rhodium. Noble metals have been used as polycrystalline metals or monocrystals, metal blacks, metals supported on graphite, microparticles ineorporated into redox active polymers, ete. The activity of these materials towards the electrocatalytieal hydrogenation depends mainly on the nature of the metal, pH, and supporting electrolyte, and the state of the surface. 

Platinum is among the most active catalysts. Ethylene is redueed to ethane on Pt, 

and Ru electrodes. Electrocatalytic hydrogenation of acetone, acetaldehyde, and acetophenone on platinized Pt in acidic solution produces the eorresponding hydrocarbons, whereas in alkaline media, alcohols or dimers are obtained. Phenol may be reduced to cyclohexanol on Pt in acidic solutions. Pt deposited or supported on ear-bon is more active than platinized Pt. However, the highest yield was observed on Rh/ 

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E. Lamy-Pitara, S.E. Mouahid, and J. Barbier, Effect of anions on catalytic and electrocatalytic hydrogenations and on the electrocatalytic oxidation and evolution of hydrogen on platinum, Electrochim. Acta 45, 4299-4308 (2000). 

Table 3. Electrocatalytic hydrogenations on carbon felt electrodes coated with polyviologen/Pd...

Scheme 13 Model complexes for [FeFe]-hydrogenase 4 (right) and proposed mechanisms for electrocatalytic hydrogen generation (left) (a) EECC mechanism, (b) ECCE mechanism PTA = phosphatriazaadamantane [33]...

Electrocatalytic Hydrogenation of Organic Compounds at Raney Metal Electrodes ... 

The electrocatalytic hydrogenation (ECH) of an unsaturated organic substrate (Y=Z) in aqueous or mixed aqueous-organic media (eqs [2] to [4] where M represents an adsorption site of the catalyst, and M(H) and M(Y=Z), chemisorbed hydrogen and the adsorbed substrate respectively) involves the same hydrogenation steps as those of classical catalytic hydrogenation (CH) (steps [2] to [4] stoichiometry for adsorbed species only, not for surface M) (1) ... 

Electrocatalytic hydrogenation has the advantage of milder reaction conditions compared to catalytic hydrogenation. The development of various electrode materials (e.g., massive electrodes, powder cathodes, polymer film electrodes) and the optimization of reaction conditions have led to highly selective electrocatalytic hydrogenations. These are very suitable for the conversion of aliphatic and aromatic nitro compounds to amines and a, fi-unsaturated ketones to saturated ketones. The field is reviewed with 173 references in [158]. While the reduction of conjugated enones does not always proceed chemoselectively at a Hg cathode, the use of a carbon felt electrode coated with polyviologen/Pd particles provided saturated ketones exclusively (Fig. 34) [159]. 

Electrocatalytic hydrogenation of carbonyl groups has been carried out by electrolysis of the ketone (336) in an aqueous Et0H-LiCl04-(C) system in the presence of Rh complexes to yield the alcohol (337) in 95 99% yields (Scheme 124) [459, 460]. 

The most effective cathode surface for the electrocatalytic hydrogenation of alkenes is based on Raney nickel [147], Preparation of the surface involves elec-... 

Reaction of 1,2 -dicarboxylic acids has been used for the formation of a number of strained alkenes and also applied to the Diels-Alder addition products from maleic anhydride (Table 9.5). Both cis- and tr s-diacids take part in the process. Aqueous pyridine containing, triethylamine as a strong base, is considered the best solvent and higher yields are obtained at temperatures of around 80 "C [130]. Use of a divided cell avoids a possibility of electrocatalytic hydrogenation of the product at the cathode. The addition of /a/-butylhydroquinone as a radical scavenger prevents polymerization of the product [127], An alternative chemical decarboxylation process is available which uses lead tetraacetate [131] but problems can arise because of reaction between the alkene and lead tetraacetate. 

Cathodic surfaces of finely divided platinum, palladium and nickel have a low hydrogen overvoltage and the dominant electrochemical reaction is the generation of a layer of hydrogen atoms. The electrocatalytic hydrogenation of aldehydes and ketones can be achieved at these surfaces. Cathodes of platinum or palladium black operate in both acid solution [203] and in methanol containing sodium methoxide [204], The carbonyl compound is converted to the alcohol. Reduction of 4-tert-butylcyclohexanone is not stereoselective, however, 1,2-diphenylpropan-l-one is converted to the / reo-alcohol. 

A Raney nickel surface is also suitable for electrocatalytic hydrogenation [205]. This surface is prepared by electrodepositing nickel from a solution containing suspended Raney nickel alloy (Ni 50% A1 50%). Some alloy particles stick to the surface, which is then activated by leaching the aluminium using hot aqueous sodium hydroxide. Cyclohexanone, acetophenone and benzil have been converted to the corresponding alcohol and there is no stereoselectivity for the formation of hydrobenzoin from benzil. 

Electrocatalytic hydrogenation is also achieved by reaction of carbonyl compounds with aluminium and nickel(ll) chloride in tetrahydrofuran. Nickel(li) is reduced to finely divided nickel(o) which is deposited on the aluminium.This se-tsup corrosion cells where aluminium dissolves, liberating electrons which are transferred to the nickel. Protons are then reduced to hydrogen at the nickel surface. Hydrogenation of benzaldehydes to the alcohol has been effected under these conditions [206]. 

Commercial catalytic hydrogenations of unsaturated compounds use Raney nickel or—less commonly—Pt catalyst supported on active carbon. Electrocatalytic hydrogenation can be performed at platinized platinum or other platinum-metal electrodes. Adsorbed hydrogen atoms are the active reactant in catalytic as well as in electrocatalytic hydrogenation. 

In general cathodic electrocatalytic hydrogenation and catalytic hydrogenations are not supposed to differ with respect to mechanism, yield, and selectivity (199, 200). 

Beck has shown (200) that electrocatalytic hydrogenations compare favorably with catalytic hydrogenation. The latter being H2 mass transfer hampered are by a factor of from 5 to 50 slower than electrocatalytic hydrogenations as the hydrogen is generated at the site where it is consumed, so that mass transfer for hydrogen is irrelevant with the electrocatalytic reaction. 

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