Electrochemical Reactions and Mechanisms in Organic Chemistry 2000 - Grimshaw

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Transcript of Electrochemical Reactions and Mechanisms in Organic Chemistry 2000 - Grimshaw

Electrochemical Reactions and Mechanisms in Organic ChemistryElsevier, 2000 Author : James Grimshaw ISBN: 978-0-444-72007-8 Preface, Pages vii-viii Chapter 1 - Electrochemical Oxidation and Reduction of Organic Compounds, Pages 1-26 Chapter 2 - Oxidation of Alkanes, Haloalkanes and Alkenes, Pages 27-53 Chapter 3 - Reduction of Alkenes and Conjugated Alkenes, Pages 54-88 Chapter 4 - Reductive Bond Cleavage Processes-I, Pages 89-157 Chapter 5 - Reductive Bond Cleavage Processes-II, Pages 158-186 Chapter 6 - Oxidation of Aromatic Rings, Pages 187-238 Chapter 7 - Reduction of Aromatic Rings, Pages 239-260 Chapter 8 - Oxidation of Alcohols, Amines and Amides, Pages 261-299 Chapter 9 - Oxidation of Ketones, Aldehydes, and Carboxylic Acids, Pages 300-329 Chapter 10 - Reduction of Carbonyi Compounds, Carboxylic Acids and Their Derivatives, Pages 330-370 Chapter 11 - Reduction of Nitro, Nitroso, Azo and Azoxy Groups, Pages 371-396 Index, Pages 397-401

by kmno4

PREFACE This book is concerned with reactions carried out at an elects'ode on a preparative scale. The impact of organic electrochemistry on synthetic organic chemistry has a long history beginning with the Kolbe reaction, which is still in the repertoire in first year teaching. In the early 1900's electrochemical methods for the oxidative or reductive transformation of functional groups were actively pursued.. They offer the advantage of having no spent oxidant or reductant for disposal. However electrochernicat processes fell out of favour in the face of conventional chemical reactions because the outcome from electrochemistry was often far from predictable. Now that tlre mechanisms of these processes are generally well understood, many of the former pitfalls can be avoided. Electrochemical processes use the electron as a reagent and so avoid a chemical oxidant or reductant, "It~e environmental impact of electrochemistry needs to be assessed by looking at the global cell reaction. In the electrochemical cell, every oxidation step at the anode nmst be accompanied by a reduction at the cathode. During an oxidation, whatever is evolved at the cathode is m effect a spent reagent. The cathode reaction can be controlled to give a desirable product, even hydrogen for use as a fuek During a reduction process this spent reagent is produced at the anode. It can be oxygen, which is venmd to the atmosphere. Control of the reaction at the counter electrode gives to electrochemical processes the advantage of being non-polluting, relative to corresponding steps using a chemical reagent~ The discovery of the Baizer hydrodimerization process for preparation of adiponitrile from acrylonitrile led to a resurgence of interest in organic electrochemistry. This process synthesises adiponitrfle at the cathode mid the spent reagent is oxygen evolved at the anode. Its mmrense technical success prompted extensive investigations into reaction mechanisms in o~ganic electrochemistry with a view to improving the old fimctio~mI group interchange reactions. At the same time new reactions of potential use in organic synthesis have been discovered. In parallel with these investigations, significant improvements have been made m the design of electrochemical cells both for laboratory and for industrial scale use Electrons are transferred at an electrode singly, not in pairs, The primary reacfive species to be generated is either a delocalised radical-ion or a radical formed by cleavage of a

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0,7 0.6

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0,5 0

I

~

J 4

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I

! 8

2

6pH

Figure 3,1, Changes with ptf o f the lmltZwave potential (E~.) for the polarographic waves of N-ethylmaleinimide in alcoholic aqueous buffers. Wave I is due to a two-electron N'e,c~s, waves II and Ill are each due to one-electror~ processes. Data from ref. [67].

form the cc,c~-linkage, but the structure of the product is not well established. Also, reduction of the cinnamamide 15 in tetrahydrofuran with lithium ions present leads to a mixture of the meso- and ()-isomers of 3,4-diphenyladipic acid diarnide [56]. Electrochemical reduction of methyl cinnamate in methanol affords a mixture of the saturated ester and the meso- and ()-hydrodimers [68]~ Loss of the initially f o x e d radical-anion follows a first order rate law [69]. The rate-determining step becomes protonation of the radical-anion to give an allyl radical 16, which is reduced only at a more negative potential. Dimerization of the allyl radical leads to the hydrodinaers. The tautomeric alkyl radical 17 is more easily reduced than the starting ester and is converted to the saturated ester. The tautomerism 16 to 17 becomes very fast in acid solution so that methyl dihydrocirmamate becomes the major product from reduction of methyl cinnarnate in an aqueous buffer of pH 4 [70]. Hydrodimerization of dimethyl rnaleatc in methanol, at a pH where the rate of enol-keto tautomerization is slowed down, has been developed to a pilot scale process [71 ]. 'lqae ester 18 derived from camphoric aahydride is reduced in acid solution to give the dihydrocompound [72]. Reduction of the sterically hindered

68

I~EDUCTION OF ALKENES AND CONJUGATED ALKENES

norbornenedicarboxylate I9 gives only the dihydro derivative in both acid and alkaline solution [73].

Ph--~CH--C--OMe-~ "- + MeOH It O

..... ~

Ph--CH-CH=C~OMe ~"H 1:6 -~

+ MeO-

16

.........

ItI ;H

Ph--(~H-CHz--C~OMe II O 17

Ph~CHz--CH2--C~OMe 11 Oo II Ph--CH-CH~--C~OMe Ph~CH-CHe~C~OMe I| O

OH I Ph--CH-CH-'-C~OMe

I

Ph~CH-CH=C~OMe

[~O""I"~C..~C(CO~Et~ }18

Pb cathode , EtOH, H2SO 4

r~CQ2H L..~C~CH(CO2H)z

Electrochemical reduction of conjugated alkenoic acids in alkaline solution generally leads to the dihydrocompound and negligible amounts of the hydrodimer are fore-Led. Examples include the conversion of maleic or fumaric acid to succmic

Hg cathode pill -9

t9acid [74] and the conversion of cinnamic acid to dihydrocinnamic acid at a mercury cathode [75], The dihydrocompound is also usually obtained by reduction in acid solution, for example with maleic or fumaric acids [76]. The advantage of this electrochemical method lies in its selectivity in the presence of non-conjugated alkene bonds, for example in the reduction of 20 where one atkene bond is not reduced [77], The reaction will tolerate a halogen substituent in 2-chlorocinnamic acid [78] and the cyano substituent in 2-cyanocinnamic acid [79]. No stereoselec-

Activated A lkenes

69

PhCH:CH-C~CO2HCHPh20II

Pb cathode....

PhC~CH-CH~CO2H;

EtOH, HzSO4

CHzPh

I

tivity is shown in the reduction of cyclohexen-l,2-dicarboxylic acid to cyclohexane-l,2-dicarboxylic acid [80]. Tile few examples of hydrodimerization of alkenoic acids include reduction of sorbic acid in dioxan, sulphuric acid at a mercury cathode [81 ] and of cinnamic acid under similar conditions [68].T A B L E 3.5

Hydrodimers from the con~olled potential reduction of activated aLkenes in aqueous dimethylformamide containing tetraethylammonium toluene4sulphonate Ref. [82]. Substrate Hydrodimer Product Yield /% 87

PhCH=CHCN "CHa (CH3)2C=C..cN CH~=CHCH=CHCN PhC~CH=CHCN PhCOCH=CHCO2Et NCCH=CHCO2Et PhCH=CHCONMez

PhCH..C~t;~CN Dihydrocompound formed NC(CH=)~CN hydrogenation) (after N~..,C~H~COPh PhCOCH~CHCO2Et NCCHCH2CO2Et ",--,,)2 PhCH:z~NMe2

50 5577

84 82

a,fi-Unsaturated Aldehydes and Ketones

Only non-enolizable eneones give satisfactory yields of hydrodimer on reduction in aprotic solvents [83]. A suitable aqueous buffer is needed with enolizable eneones to control base catalysed side reactions of condensation and oligomerisation. The polarographic behaviour of eneones in buft~ers is illustrated using cyclo-

70

REDUCTION OF ALKENES AND CONJUGATED ALKENES

hexenone (Figure 3.2) [84, 85]. Mesityl oxide shows similar behaviour [86]. A one-electron wave is seen in acid solution and the half-wave potential varies with 1.72,08 15 t)

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1.0

1.3

/~ i

1.1 3

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12

0 L 3

6

9

pH

Figure 3,2. Variation of half-wavepotential (Ev,)and diffusion current constant (I) for the two polarographicwavesof cyclohexenonein aqueous buffers. Data fromref. [84].

pH, Early workers considered this behaviour due to equlibrium preprotonafon of the eneone followed by one-electron addition, However when electron and proton addition occur simultaneously, this is equivalent to the following two equlibria being established in the layer close to the electrode:C=C-C=O + e . ..... " C=G.C=O1 " " -

C=C-C=O-]'"

+ H+ ~

C.~C=C4DH~"

~ C=Co-C-OH

Protonation of the radical-anion occurs on oxygen to give an enol-radical. The latter species is a resonance hybrid. It takes pan in a fast irreversible radical-radical dimerization step and since the species has two potential radical sites, three structural isomers of the hydrodimer can be formed. The main product is formed from a transition state with minimum steric hin&ence between the radical centres. Around pH 6-8, two polarographic waves are seen and the sum of tile two wave heights corresponds to a one-electron process. The first wave is due to the two reactions above and decreases in height because protons are in low concentration and do not diffuse sufficiently fast to the electrode surface. The second wave is due to formation of the radical-anion followed by proton transfer from a general acid present as a component of the buffer. In alkaline solution, the concentration of acid component in the buffer decreases and th