Electrogenerated bases

Other reactions of acidic sulphones with electrogenerated bases. . 1039... 

In most cases, the behaviour of sulphones at the cathodic interface (obviously rendered basic when insufficiently buffered due to the accumulation of electrogenerated bases) may be strongly modified by the presence of vicinal CH groups in the a position to the S02 group. These acidic groups may transfer protons and are often responsible for low yields of the cleavage processes or for undesirable isomerization reactions. 

Increased acidity of this hydrogen atom may lead to self-protonation reactions with electrogenerated bases (in aprotic solvents or in unbuffered solutions). 

It has been found50 that such a multielectron step does not exist with 58, which exhibits a classical two-electron scission. In general, allylic sulphones (59) without an unsaturated system in a suitable position are not reducible. Thus, they do not exhibit a cathodic step in protic solutions. However, in aprotic media the isomerization may be base catalyzed, since small amounts of electrogenerated bases from electroactive impurities, even at low concentration, may contribute to start the isomerization. Figure 10 shows the behaviour of t-butyl allylic sulphone which is readily transformed in the absence of proton donor. On the other hand, 60 is not isomerized but exhibits a specific step (Figure 10, curve a) at very negative potentials. 

This formation of an ethylenic sulphone at the cathodic interface when reducing / -acetoxysulphones leads to a peculiar feature in cyclic voltammetry. The activated olefin, itself a source of new electrogenerated bases, contributes to render54 the overall process autocatalyzed. 

There are many possibilities for acidic sulphones to react with electrogenerated bases, and give specific functionalization on the a carbon. The first significant example was reported by Baizer58 when reducing sulphone 70 in the presence of oxygen. 

A variety of important synthetic reactions can he promoted by electrogenerated bases (EGB). The Michael addition and alkylation of compounds with activated hydrogen atoms are two examples in which using EGB has been very successful. The electrochemical method is very successful for functionalization of levoglucosenone as shown in Scheme 4.28.198... 

Application of Electrogenerated Bases to Fluoro Organic Synthesis... 

The electrogenerated base from pyrrolidone also promotes the introduction of a gem-difluoromethylen unit to aromatic aldehydes to give difluoromethyl carbinols in high yields (Scheme 4.4) [33],... 

Quite recently, Troupel et al. have developed an effective synthesis of gem-difluoro-/ -oxonitriles using an electrogenerated base derived from phenyl bromide and a sacrificial magnesium anode together with a nickel cathode coated with a small deposit of cadmium as shown in Scheme 4.5 [34]. 

VI. RELATED PROCESSES A. Nitro Compounds as Electrogenerated Bases... 

Scheme 33 Base catalyzed condensation of aldehydes by electrogenerated bases (EGB s) R alkyl, yields 72-77%, very high current efficiency.

Cyclobutane (35) and its analogs were not described in literature so far. They can be regarded as an easy source both of a cyclobutyl radical [239] (by cleavage of C—S bond under electron transfer) and a dienophile [240]. Thus, (35) readily yields the corresponding cyclobutene (36) in the presence of electrogenerated bases (EGB)... 

Electrogenerated bases (see Chap. 14) can be prepared in situ and tailored for regioselective deprotonation. Regioselective alkylation of an unsymmetrical cyclic ketone was achieved by using an electrogenerated triphenylmethyl anion (Eq. 5, RX = CH3I,... 

The anion of methyl phenylacetate, formed by an electrogenerated base, was homocoupled with iodine or anodically mediated by iodide to afford dimethyl 2,3-diphenylsuccinate in high yield and high d, /-selectivity. This reaction probably does not involve free radicals but an iodination-nucleophilic substitution sequence [194,195]. 

Cathodic reduction of 1,3-diphenyl-propenone leads to l-hydroxy-2-benzoyl-3,4-diphenyl cyclopentanes with exclusive cis configuration of the two phenyl groups. With l-phenyl-l-pentene-3-one the cyclodimer 2-methyl-3,5-diphenyl-4-(l-propionyl)-cyclohexanone is formed with a 100% yield in an intramolecular Michael addition via an electrogenerated base. The substituents are all in the most stable equatorial position [277]. 

Trimethyl aconitate can be cyclodimer-ized in 75% yield and a high stereoselectivity to a pentamethyl l-(2-methoxy-2-oxoethyl)- ,2,3,4,5-cyclopentane pentacar-boxylate. Product formation is initiated by an electrogenerated base that induces a catalytic cycle of two successive Michael additions. The most stable out of 16 possible diastereomers is formed, which indicates that the tandem Michael addition is thermodynamically controlled [282]. 

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