Electrochemical allylation

Durandetti et al. have described iron-catalyzed electrochemical allylation of carbonyl compounds with allylic acetates (Equation (27)).333 In the case of aldehydes, slow addition of the corresponding aldehyde is required in order to avoid pinacol formation. With crotyl acetate (R3 = Me), the reaction proved to be highly regioselective, providing almost exclusively branched homoallylic alcohols 150. 

The mechanism of the Zn chloride-assisted, palladium-catalyzed reaction of allyl acetate (456) with carbonyl compounds (457) has been proposed [434]. The reaction involves electroreduction of a Pd(II) complex to a Pd(0) complex, oxidative addition of the allyl acetate to the Pd(0) complex, and Zn(II)/Pd(II) transmetallation leading to an allylzinc reagent, which would react with (457) to give homoallyl alcohols (458) and (459) (Scheme 157). Substituted -lactones are electrosynthesized by the Reformatsky reaction of ketones and ethyl a-bromobutyrate, using a sacrificial Zn anode in 35 92% yield [542]. The effect of cathode materials involving Zn, C, Pt, Ni, and so on, has been investigated for the electrochemical allylation of acetone [543]. 

Direct electrochemical allylation can be performed with some carbonyl compounds.Transition metal catalyzed couplings are however more efficient, notably in reactions involving ketones. 

Scheme 6. Reaction mechanism of the Pd-catalyzed electrochemical allylation of carbonyl compounds in the presence of zinc salts...

Table 13. Ni-catalyzed electrochemical allylation of carbonyl compounds...

Both the palladium- and the nickel-catalysis enables the use of allylic acetate as starting reagent. In the two approaches a zinc compound is evoked as key intermediate, though its formation has been demonstrated only indirectly. In the two methods allylic transposition is observed. The authors have then concluded that these electrochemical allylation reactions closely parallel the chemical allylation reactions involving allylzinc intermediates. 

The electrochemical allylation of carbonyl compounds by electroreductivc regeneration of a diallyltin reagent from allyl bromide and a Sn species leads to formation of homoallylic alcohols in yields of 70-90 % even in methanol or methanol/water (Table 7, No. 12) Bisaryl formation is possible also from aryl iodides or bromides in the presence of electro-generated Pd phosphane complexes (Table 7, No. 13) In the presence of styrenes, 1,3-butadienes, or phenyl acetylene the products of ArH addition are formed in this way (Table 7, No. 14) . The electroreductivc cleavage of allylic acetates is also possible by catalysis of an Pd°-complex (Table K No. 15)... 

Here again are fairly good prospects for the industrial use of this method in the area of the area of fine chemicals. For example, Torii et al. (Otsuka Patents 90 92>) have used electrochemical allyl halogenation for the synthesis of intermediates for P-lactam antibiotics. 

Symmetrical ketones are also prepared from organic halides and carbon dioxide catalyzed by 2,2 -bipyridine-nickel complexes42. Torii and coworkers found a synthetically useful electrochemical allylation, which involves the electroreductive generation of a dial-lyltin reagent in the presence of a catalytic amount of tin. A wide variety of aldehydes and ketones have been added in high yields under relatively mild conditions by this method (equation 19)43. The process was further improved by the addition of catalytic amounts of... 

Scheme 6.12 Electrochemical allylation between allylic halides and aldehydes.

Electrochemical Allylation of Carbonyl Compounds by Allylic Acetates... 

Scheme 15.20 Electrochemical allylation of cyclohexanone catalyzed by iron complexes.

Cydization of acetylenic ketones to allyl alcohots by one electfon reduction with Li/NHa, also electrochemically (Shorn) or by Smia (Molander)... 

From this value and known C—H bond dissociation energies, pK values can be calculated. Early application of these methods gave estimates of the p/Ts of toluene and propene of about 45 and 48, respectively. Methane was estimated to have a pAT in the range of 52-62. Electrochemical measurements in DMF have given the results shown in Table 7.3. These measurements put the pK of methane at about 48, with benzylic and allylic stabilization leading to values of 39 and 38 for toluene and propene, respectively. The electrochemical values overlap with the pATdmso scale for compounds such as diphenyl-methane and triphenylmethane. 

Allylation of aldehydes and ketones by reagents, electrochemically generated at the cadmium anode from allyl bromides, are also known30. 

Later, Torii et al. found that the tin-aluminum-mediated allylation can be carried out with the less expensive allyl chloride, instead of allyl bromide, when a mixture of alcohol-water-acetic acid was used as the solvent.77 When combined with stoichiometric amounts of aluminum powder, both stoichiometric and catalytic amounts of tin are effective. As reported by Wu et al., higher temperatures can be used instead of aluminum powder.78 Under such a reaction condition, allyl quinones were obtained from 1,4-quinones, followed by oxidation with ferric chloride. Allylation reactions in water/organic solvent mixtures were also carried out electrochemically, with the advantage that the allyltin reagent could be recycled.79... 

In the following, some selected classes of reactions and applications of Ni11 polyazamacrocyclic complexes are discussed. Other applications include, inter alia, the electrochemical reduction of allyl ethers1613 and the epoxidation of alkenes.1614... 

Tin compounds can be produced by the Nalco-type reactions39 as well as by Cd and Zn mediated methods42,43. The use of organotin compounds in electrodically induced transmetallations has also been described in a study in which Grignard-type allylation of carbonyl compounds has been carried out by electrochemically recycled allyltin reagents44. 

Schafer reported that the electrochemical oxidation of silyl enol ethers results in the homo-coupling products. 1,4-diketones (Scheme 25) [59], A mechanism involving the dimerization of initially formed cation radical species seems to be reasonable. Another possible mechanism involves the decomposition of the cation radical by Si-O bond cleavage to give the radical species which dimerizes to form the 1,4-diketone. In the case of the anodic oxidation of allylsilanes and benzylsilanes, the radical intermediate is immediately oxidized to give the cationic species, because oxidation potentials of allyl radicals and benzyl radicals are relatively low. But in the case of a-oxoalkyl radicals, the oxidation to the cationic species seems to be retarded. Presumably, the oxidation potential of such radicals becomes more positive because of the electron-withdrawing effect of the carbonyl group. Therefore, the dimerization seems to take place preferentially. 

Electroreduction of esters (R COOR ) is classified into three types of reaction. The first type of reaction is the reductive cleavage of a bond between oxygen and R. This type of reaction is similar to the cathodic reduction of alcohols (Sect. 7.2) and ethers (Sect. 7.3). That is, activated esters such as allylic and benzylic esters are electrochemically reduced to the corresponding hydrocarbons and acids (Schemes) [1,16,19-21]. 

The nickel-catalyzed electrochemical carboxylation of l-(perfluoroalkyl) aUcenes proceeds regioselectively to afford 4-fluoro-4-perfluoroalkyl-3-alkenoic acids via a Ni(II)-allyl complex and a final F elimination (Fig. 19) [107]. Electrochemical carboxylation of alkynes in the presence of... 

Allylic acetoxylation of cyclohexene (96) at 80 °C affords 3-acetoxycyclohexene (97) in 67% yield (Scheme 36). It was found that the catalytic double-bond isomerization of 3-phenylpropene proceeds by the action of an electrochemically generated 17-electron Co(II) species [132]. The cobalt(III)-mediated electrooxidative decomposition of chlorinated organics, that is, l,3-dichloro-2-propanol, 2-monochloro-propanol, and so on, has been performed... 

Pd-hydroquinone-mediated electrochemical 1,4-diacetoxylation of cyclohexa-1,3-diene (118), leading to 1,4-diacetoxycyclo-hex-2-ene (119), has been investigated (Scheme 46) [156]. Palladium-catalyzed indirect electrochemical monoacetoxylation of olefins has been attained in an MeCN/Ac0H-NaC104/Ac0Na/Pd(0Ac)2-Cu(OAc)2-(C) system. The acetoxylation of cyclohexene produces unsaturated esters with less current efficiency, giving a 1 1 mixture of allylic and vinylic products [118]. 

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