Journal of Scientific & Industri al Research Vol. 58, May 1999, pp327-331

Electroorganic Synthesis: An Edge Over Conventional Chemical Methods

. Krishna Nand Singh' and Ajay Kumar Shukla

Department of Applied Chemistry, Institute of Technology, Banaras Hindu University, Varanasi 221005, India

Electroorganic approach has currently fascinated academicians and industrial researchers because. of its high potential prospects for industrial ventures. The present . review d~scnbes the salIent features of thIS Important methodology with emphasis on the advantages over conventIOnal chemIcal methods.

Introduction

Electroorganic chemistry is a multidisciplinary science overlapping the vast field of organic chemistry, biochemistry, physical chemistry, chemical engineering, and material science. As a branch of organic chemistry, its principal aim ' is to enhance synthetic possibilities for academic and industrial ventures. In the present context, organic electrochemistry, apart from the synthesis of organic compounds, has relevance to the nature of electron transfer processes, the generation and study of transient intermediates, elucidation and confirmation of structures, initiation of polymerization, investigation into the nature and synthesis of catalysts, studies related to biological redox systems, degradation of toxic wastes, etc. Many electroanalytical methods (e.g. polarography, cyclic voltammetry, derivative linear sweep voltammetry, and voltammetry at ring disc electrodes) can be used to elucidate the mechanism of the electrochemical reaction and to select the reaction parameters on rational grounds. Many workers have earlier described l -4 these aspects of organic electrochemistry.

Advantages of Electrochemistry

In recent years, the number of investigations within organic electrochemistry has been increasing. There are various reasons for this expansion. The electrochemistry affords a facile and precise way to generate highly energetic intermediates via control of the electrode potential. It is important to note here that one electron exchanged over a potential difference of 1 V amounts to injecting 1 e V in a molecule, i.e., approximately 23 kcallmole. Owing to the possibility of performing electrochemistry over potential difference of several volts, it is easily perceptible that the method may involve

* AuthorJor correspondence

energies comparable to those of most chemical bonds and largely beyond most activation energies. Thus, highly energetic intermediates may be generated under mild and precisely controlled conditions. Another useful aspect of electrochemical generation of intermediates . is related to the current flow through the cell. Indeed the current is a direct measure of production rate and therefore an important parameter in product selectivity. The cost of electricity is foreseen to increase less rapidly than that of traditional chemicals. Also the use of breeder reactors for the industrial generation of electricity is foreseeable in the not-too-distant future, with that the application of electroorganic synthesis would expand significantly. For an organic chemist the study of electrochemistry also gives the "spin-off" benefit that many of the electro analytical results (e.g. redox potentials and electron transfer mechanisms) may be useful for investigation of ordinary chemical reactions. Chief advantage of electrochemical reactions compared to chemical one is the effective contribution to pollution contro1.5-7 The electron is a non-polluting reagent and electrochemical reactions are easy to control automatically. Because of the direct electron transfer, it avoids the problem of separation and waste treatment of the often poisonous end products of the chemical oxidants or reductants. For example, in the preparation of methoxyacetic acid by oxidation of methoxiethanol at the nickel hydroxide electrode, the undesirable side product can be converted into the desirable side product. The economy of the process is thereby cost-effective, and the problem of waste separation and treatment is reduced. This is also displayed in the manufacture of chloroacetic acid by chlorination of acetic acid, there the waste product dichloroacetic acid, formed by the overchlorination, is cathodically converted to chloroacetic acid.

328 J SCIIND RES VOL 58 MAY 1999

Types of Electrochemical Reactions

Electroorganic reactions may be differentiated in numerous ways. However, a general classification is given below :

(a) Transfonnation of functional groups, (b) Coupling, (c)Cleavage,(d)Substitution, (e) Elimination,(f) Addition.

(a) Transformation of Functional Groups

The ftmctional group transfonnations utilizing electrochemical mode include, NH2 ~ N02, N02 ~

NHOH, N02 ~ NH2, S ~ S02, S ~ SO, CH20H ~

CHO, CH20H -) COOH, CHO ~ CH20E, CHO ~

COOH, CH2NH2 ~ CN, etc. In the reduction of nitro

functions, the advantage is the potential selective conversion to different products . Nitroalkanes are reduced in both hydrous and anhydrous media , glVll1g hydroxylamines and amines. Electroreduction of nitrobenzene occurs at. copper and nickel electrodes in neutral and basic hydrous methanolic solutions. Complete reduction of nitrobenzene to aniline requires six electrons8

. (Eq. 1).

PhN02 + 4c- + 3f·r---... ~ PhNHOH + OH -

PhNIIOH + k + 2W -- PhNH2 + Hj)

... ( I)

(b) Coupling

Two identical or dissimilar molecular species are joined to yield a dimer . For example, the electrohydrodimerization . of dimethyl maleate to butane tetracarboxylic acid has been eventually developed by Monsanto to a pilot-plant scale9 (Eq. 2).

o H C':-OCH3

2 )( -2 .25 Vvs SCE, It catOOde,2c~ H eocH3

6

.. . (2)

(c) Cleavage

Electrochemical reactions resulting in cleavage of bonds are synthetically useful in many · respects, e.g., deprotection of functional groups and generation of reactive intennediates. The electrochemical approach may be applied for the cleavage of bond, i.e., carbon-carbon bond, carbon-oxygen bond, carbon-sulphur bond, carbon­phosphorous bond, carbon-halogen bond, oxygen-oxygen bond, sulphur-sulphur bonds, oxygen-nitrogen bond, sulphur-nitrogen bond and sulphur-halogen bond. As an examp le, electrolytic cleavage of a carbon-carbon bond occurs in the course of both cathodic and anodic reactions. Benzoin in sulphuric acid-acetone containing some water is cleaved mainly to benzaldehyde (35 percent) and benzoic acid (45 percent) with small amount of benzil (5 percent) and tar (10 percent) IO (Eq. 3) .

Cifl.sCH (011) COCifls -2e-, ·1-I \ CiflsCHO + rCifls CO]

~H:P C611sC li0 + C()I lsCOOH + CiflsCOCOC(i! ls

(d) Su bstitution

... (3)

Anodic substitution reactions are .common in organic e lectrochemistry. The overall picture may be represented as follows (Eq. 4).

Dr Krishna Nand Singh, a recipient oj A/CTE career Award (1995) , is working as Lecturer in Applied Chemist/y, Banaras Hindu University since July 1993. Born on June 12, 1962, Dr Singh obtained his Master's degree in Organic ChemistfY Jrom Banaras Hindi University (BHU) in 1985 and qualified the CSJR-JRF (NET) exam in 1985. He was awarded doctoral degree by the BHU in /991 based on his research in Electroorganic Synthesis. Dr Singh has successJully completed some research projects (CSlR and AlCTE) and has several publications in reputed journals. His current research interests include Electroorganic Synthesis and Superoxide Research.

Ajay Kumar Shukla was born on October 05, 1973 in Varanasi, UP. He received his BSc and MSc (Chemistry) from Banaras Hindu University, Varanasi, in 1994 and 1996, respectively. He is now a Junior research Fellow (NET) oj CSIR, perceiving ·doctoral studies under the guidance oj Dr K N Singh on Electrochemical Synthesis in Department oj Applied Chemistry, Institute oJTechnology, Banaras Hindu University, Varanasi. .

SINGH & SHUKLA: ELECTROORGANIC SYNTHESIS 329

R-E +Nu-_R-Nu +~ + 2e-

Where Nu- = H20, ROH, CH3CN, OH~Rcf.RCOcf.CN~ SCN~Nd1 N~ OCN~ CsHsN, ~ etc.

... (4)

As an example, tetrahydrofuran has successfully been converted to 2-hydroxy- tetrahydrofuran by anodic oxidation in 1M-H2S04 at Pt electrode II (Eq. 5).

o + H:P-0 . + 2W + 2e-o 0 OH

.. . (5)

(e) Elimination o _

Reductive elimination of X and Y as X and Y , from

RRICX - CYR2R3 or RRI X CCH2CYR2R3, where X and Yare reasonably good leaving groups, may lead to the formation of alkenes or cyclopropanes. For example, the elimination of the hydroxyl and phenylthio groups from ~­hydroxy sulphides is a convenient route to olefins 12 (Eq. 6).

.. . (6)

(f) Addition

Anodic addition affords products, some of which are of industrial interest, such as propylene oxide, diols, and 1 ,4-dimethoxyfuran. 13,14 To improve the selectivity, mediators li ke TI3+, Pd2+, Ce4+ or Ru3+ are sometimes used. 15 The hydrogenation of C=C bond can be achieved cathodically. Anodic additions at the double bonds may be represented as follows (Eq. 7).

... (7)

To exemplify it, the anodic oxidation of olefins on platinum in the presence of fluoride ion leads to the addition which is otherwise inaccessible (Eq. 8).

.. . (8)

Electrochemical vs Chemical Reactions

Electrochemistry has been recognized as an esteemed method in organic synthesis often when other synthetic methods fail. Thus, electroorganic approach is often chosen as a last possibility which leads to success but also to disappointment due to high expectancy. There are features of electrochemistry that are unique to this field and have no counterpart in conventional chemistry. They result from the combination of electron-transfer with a chemical conversion. Thus, reactive intermediates can be generated which otherwise are not or, are in a more limited extent accessible such as radical ions. The following examples may be cited to make it more explicit.

Trichlorobromoalkane can be cathodically convel'ted in the presence of aldehydes to a dichloromethyl anion at O°C (route a) and be trapped to form a dichlorotetrahydrofuran,16 but for the metalloorganic route (route b) reaction temperature of -110°C is necessary (Eq. 9).

CI ",, ' He CI

CI-C-CH~HBrR -0- CH~-CH~HBrR ~ Clb CI o ll~C b R' 0 R

... (9)

The electrogenerated radicals can also be trapped by olefins to yield additive dimers and additive monomers. The product distribution can be controlled by the current density and olefin concentration (Eq. 10, paths a and b) and by the electrode potential or the substituent of the olefin (Eq 10, path c).

Ea R-tl-t-t-R I I Y I

,Y Y, b Y !{ + c=c --R-t-C • R-C-C-R /, II I I

Y C R-C- C-Nu

-eO, Nu- I I

... (10)

Chemically, these types of adducts are not or, are less easily accessible, and the products are those of a radical chain addition 17. 18 (Eq. II) .

Y Y \ / I R-X + C=C _ R-C-C-X

/" I I

...(11)

For the nucleophilic substitution of aromatic C-H bonds, electrochemically this can be done in one-step,19 whereas chemically multistep processes are necessary to

330 J SCllND RES VOL 58 MAY 1999

achieve the same goal. The anodic cyanation20 of 1-methylpyrrol yielding the 2-carbonitrile analog is carried out in MeOHlNaCN in a divided cell with a platinum anode set at + I.OY vs SCE (Eq. 12).

IOI-2e-, + cN~ 01 N NC N Me J\1e

(64%)

... (12)

The cleavage of 1,2 diols can be readily achieved at the nickel hydroxide electrode21 (Eq. 13); chemically the more expensive Pb(OAc)4 or 104 - must be used.

... (13)

The conversion of heteroatoms to different oxidation states, e.g., H2N -+ N02, S -+ SO or S02' and 0 2N -+

NHOH or NH2 ' is common in eiectroorganic approach. In

the reduction of nitro functions, the especial advantage is the potential selective conversion to cel1ain oxidation state . By controlled potential electrolysis, protecting groups can be selectively removed . This makes electrochemical deprotection a valuable supplement to chemical deprotection . For example, the 1,3-dithion derivative can be oxidized anodically at a platinum electrode in acetonitrile, effecting the deprotection 22

(Eq. 14).

... (14)

Electroorganic Chemistry of Industrial Interest

It was not until 1964 that a truly large J scale electroorganic manufacturing facility was put on stream. Electroorganic processes that have been investigated on a pilot-plant or a semicommercial scale cover a wide range of reaction types. The cathodic processes considered for commercialization include the reduction of phthalic acids to the corresponding dihydrophthalic acids23 , benzene to cyclohexadiene,24-26 naphthalene to dihydronaphthalene27

and the reduction coupling of acetone to yield 2, 3

dimethyl-2-butanedioI28 . Anodic processes examined at least through the pilot-plant stage include the oxidation of propylene to propylene oxide,29,3o the Kolbe synthesis of dimethyl sebacate from monomethyladipate,31-34 and oxidation of 1,4 butynediol to acetylene dicarboxylic acid . Electrochemical fluorination of organics has also been practised35 and the two electrofluorination products of industri al importance are perfluoroctanoic acid and perfluorooctane sulphonic acid .

In India over 40 electrochemical processes have been developed by the Central Electrochemical Research Institute (CECRI), Karaikudi and many of these have been commercialized on a relatively small-scale . Some of the industrial products include, p-amino benzoic acid, p­am inophenol, p-nitrobenzoic acid , succinic acid, benzyl alcohol , benzaldehyde, salicylaldehyde, benzidine and saccharin .

Conclusion

Therefore , elecrroorganic synthesis is a branch which needs to be encouraged for commercialization. No doubt some of the problems in commercialisation are challenging and need to be tackled with care .

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