Electrocatalytic hydrogenation (part I)

[Vitus_Verdegast]

Electrocatalytic hydrogenation of organic compounds using current density gradient and sacrificial anode of nickel


Diogo S. Santana, Márcio V. F. Lima, Jorge R. R. Daniel and Marcelo Navarro, 

Departamento de Química Fundamental, C.C.E.N., Universidade Federal de Pernambuco, Cidade Universitária CEP, 50670-901, Recife PE, Brazil

Received 13 March 2003;  revised 22 April 2003;  accepted 22 April 2003. ; Available online 22 May 2003.

Tetrahedron Letters Vol 44, Iss 25 , 16 June 2003, Pages 4725-4727
DOI:

10.1016/S0040-4039(03)01035-9

Abstract
Preparative electrocatalytic hydrogenation (ECH) of some organic compounds were performed: cyclohexene, 2-cyclohexen-1-one, benzaldehyde, acetophenone, styrene, 1,3-cyclohexadiene, trans–trans-2,4-hexadien-1-ol, citral, linalool and geraniol. H₂O/MeOH (1:1), NH₄OAc or NH₄Cl (0.2 M) were used as solvent and supporting electrolyte. A sacrificial anode of nickel allowed the use of an undivided cell, with a cell voltage varying between 2.3 and 1.3 V, depending on the supporting electrolyte. A current density gradient was applied to diminish the time of reaction and obtain a good electrochemical efficiency. An in situ prepared cathode of nickel deposited on iron provided a highly efficient ECH process, and the constant deposition of nickel on the electrode surface avoided catalyst poisoning. The ECH system was somewhat selective, hydrogenating conjugated olefins in good yield.



Electrochemical generation of hydrogen provides an interesting means to hydrogenate organic compounds.¹ The technique was improved by the development of various specific electrodes, which allowed for selective hydrogenation of different classes of organic compounds.² The discoveries of influencing parameters such as: supporting electrolyte,³ solvent,⁴ surfactant,⁵ and the presence of inert gas,⁶ were important advances in electrocatalytical hydrogenation (ECH).

The success of the ECH is related to a conjugation of two mechanisms: the electrochemical generation of hydrogen and the catalytic hydrogenation.² The first one, also named hydrogen evolution reaction (HER),⁷ is classically based on a primary discharge step giving atomic hydrogen, which remains on the metal surface by chemical adsorption:^8-10

H⁺_(aq)  +  e^{-  __}>  H^._(ad)          (1) Volmer

the following step is the combination of either two adsorbed hydrogen atoms:

H^._(ad)  +  H^._(ad)  ^__>  H₂          (2) Tafel

or of a proton and an H atom (electrochemical desorption):

H^._(ad)  +  H⁺_(aq)  +  e^{-  __}>  H₂          (3) Heyrovsky

The ECH process involves three more steps,^1,2 the adsorption of the olefin on the metal surface (4), the hydrogenation of the unsaturated group (5), and the desorption of the hydrogenated product from the metal surface (6):

Y=Z  ^__M^__>  (Y=Z)_(ad.)          (4) Adsorption

(Y=Z)_(ad)  +  2H_(ad.)  ^__M^__>  (YH-ZH)_(ad.)          (5) Hydrogenation

(YH-ZH)_(ad.)  ^__M^__>  (YH-ZH)          (6)Desorption


Both processes, atomic hydrogen generation and catalytic hydrogenation, are cathode material dependent (M).² The ECH efficiency is determined by competition among hydrogenation of the unsaturated substrate, H₂ evolution, and, in some cases, the direct reduction of the substrate on the electrode surface.² The relative rates of these processes are not only affected by chemisorbed hydrogen activity, but also by the current density and the presence of any molecule (solvent, supporting electrode, surfactants) or reagent adsorbed on the cathode (catalyst) surface.^{6, 11} The synergetic effect of a metal deposited on a matrix has been recently studied, and the behavior of a deposit is different from a pure solid material.¹²

In this work we describe an efficient hydrogenation method for conjugated organic systems including olefins, aldehydes and ketones, using an electrochemical device made of an undivided cell fitted with a sacrificial anode of nickel, a cathode of iron and ammonium salt as supporting electrolyte . A current density gradient was implemented to increase the time/efficiency relation during electrolysis.

ECHs were carried out with 0.1 M substrate concentration , dissolved in 50 mL H₂O/MeOH (1:1). A sacrificial anode of nickel simplifies the procedure making possible the use of an undivided cell (fig. 1),¹³ at the same time nickel is responsible by activation of the cathode (catalyst).¹⁴ Increasing cell voltage was observed during electrolyses, using 0.2M NH₄OAc (delta-E = 2.3-2.5 V) or NH₄Cl (delta-E = 1.3-1.5 V) as supporting electrolyte. A current density gradient was applied by using initially 350 mA.dm^-2. ¹⁴  After passage of the first half charge necessary to a total theoretical hydrogenation, the current density was decreased to 306 mA.dm^-2, and successively to 262, 219 and 175 mA.dm^-2, at this point the current was maintained until total hydrogenation of the substrate was achieved. In Table 1 are shown ECHs of organic substrates using NH₄Oac as a supporting electrolyte in a first group, and NH₄Cl in a second one. Interesting results, like the unreactivity of non-conjugated substrates, were observed in the NH₄OAc group reactions. Cyclohexene, geraniol (Table 1, entries 1 and 19), and linalool showed a low yield of 30% (Table 1, entry 7), while 2-cyclohexen-1-one, benzaldehyde and styrene (table 1, entries 3, 5 and 9) were hydrogenated with yields over 90% and electrochemical efficiencies* of 69, 35 and 48% respectively. Acetophenone (Table 1, entry 7) was not reactive under these conditions. The conjugated olefins, 1,3-cyclohexadiene and trans-trans-2,4-hexadien-1-ol (Table 1, entries 11 and 13), were hydrogenated in good yields but with different reactivity and selectivity. The former was more reactive giving cyclohexane (88%) as the major product, while the latter was less reactive, giving a mixture of dihydrogenated products (mixture of isomers, 52%) and tetrahydrogenated product (1-hexanol, 31%). The electrochemical efficiency was also different with values of 94% and 42% respectively. We can observe here a different behaviour for the hydrogenation of a conjugated (1,3-cyclohexadiene) and non-conjugated system (cyclohexene). This may occur due to the differences in adsorption strength of substrates on the catalyst surface. Citral, with a conjugated double bond, showed low reactivity (Table 1, entry 15).



*:  Electrochemical efficiency = the relation between hydrogen electrochemical generation (Q = n (mol) x e^- x 96487 (C.mol^-1)) versus hydrogenated substrate yield. Electrical charge passed is  Q = I (A) x t (s) .

[Vitus_Verdegast]

Let's discuss the possibility of using this extremely simple setup for attempting to reduce nitroalkenes, oximes, imines, thus applying it to phenethylamine chemistry.

First, is there anyone with access to these electrochemical journals in the references section? I have access to the other journals, it would be very nice if someone could post the former here in this thread.

It is very strange that they work here in an undivided cell, no? I would like to know about the exact reason why no anodic oxidation occurs here. If we can collect all those references, I'm sure we'll get more clues on how it could be applied for our favorite purposes.

Consider again the easiness and cheapness of this setup, and the scalability..