Dienes anodic oxidation

The procedure described is essentially that of Belleau and Weinberg and represents the only known way of obtaining the title compound. One other quinone acetal, 1,4,9,12-t6traoxadispiro[4.2.4.2]tetradeea-6,13-diene, has been synthesized by a conventional method (reaction of 1,4-cyclohexanedione with ethylene glycol followed by bromination and dehydrobromination ) as well as by an electrochemical method (anodic oxidation of 2,2-(l,4-phenylenedioxy)diethanol ). Quinone acetals have been used as intermediates in the synthesis of 4,4-dimethoxy-2,5-cyclohexadienone,. syw-bishomoquinone, - and compounds related to natural products. ... 

The anodic oxidation of conjugated dienes is much more easily achieved than the oxidation of monoolefins since the conjugation of the --electron system lowers the oxidation potentials of the dienes. Several peak potentials for dienes are summarized in Table l1. 

The typical pattern of anodic oxidation of conjugated dienes is oxidative 1,2- or 1,4-addition of nucleophiles, though the selectivity usually depends on the structure of the diene and the reaction conditions (equation 1). 

When conjugated dienes are anodically oxidized with a graphite anode in MeCN in the presence of NaCKAt and AW -dimethylurea, a variety of 2-imidazolidinones are formed though the yields are not always high as exemplified in equation 47. 

The anodic oxidation of 1,2-dienes in methanol takes place stepwise at each double bond yielding a tetramethoxylated compound as one of the products (equation if. This result is reasonable since a 1,2-diene is not a conjugated diene. 

Compared with conjugated dienes, the electrochemistry of nonconjugated dienes is classified into two types, A and B11,12. In type A, the double bond of the diene behaves essentially the same as the double bond of a monoolefin in the anodic oxidation. A typical... 

Despite the fact that the electrochemical oxidation of most of the nonconjugated dienes generally does not give products which result from interaction of the double bonds with one another, the anodic oxidation l-acetoxy-l,6-heptadienes gives intramolecularly cyclized products, that is, the cyclohexenyl ketones (equation 15)13. The cyclization takes place through the electrophilic attack of the cation generated from enol ester moiety to the double bond. 

Compared with the anodic oxidation of a 1,3-diene, the cathodic reduction of a 1,3-diene may be less interesting since the resulting simple transformation to monoolefin and alkane is more conveniently achieved by a chemical method than by the electrochemical method. So far, only few reactions which are synthetically interesting have been studied15. The typical pattern of the reaction is the formation of an anion radical from 1,3-diene followed by its reaction with two molecules of electrophile as exemplified by the formation of the dicarboxylic acid from butadiene (equation 22)16. 

It is well known that the anodic oxidation of 1,3-dienes in nucelophilic solvents such as methanol and acetic acid gives mainly 1,4-addition products together with a small amount of 1,2-addition products [31]. If the 1,3-dienes substituted... 

The diastereoselectivity of the anodic oxidation of 1-silyl-1,3-dienes is interesting [32]. The anodic oxidation of 5-phenyl-1-trimethylsilyl-1,3-hexa-diene in methanol gives a mixture of two diastereomers in 4 1 ratio. On the basis... 

Electron-rich olefins with substituents Y = phenyl, vinyl, amino, or alkoxy can be coupled by anodic oxidation to tail-tail dimers being either deprotonated to dienes and/or substituted a to Y, depending on Y and the reaction conditions (Eq. 6). Alkyl substituted arenes can be dehydrodimer-ized to diphenyls or diphenylmethanes depending on the kind of substitution (Eq. 7). 

Dienophilic cyclic a-carbonylazo compounds (triazoline diones, indazolone, phthalazine dione) prepared in a flow cell, by anodic oxidation of the corresponding hydrazino compounds in acidic methanol or acetonitrile, react with dienes (Scheme 50) [72, 73]. 

Oxidation of conjugated dienes in CH3CN-NaC104 in the presence of 1,3-dimethylurea gives a mixture of the possible 4,5-disubstituted 1,3-di-methylimidazolidin-2-ones in about 40% yield.103 Anodic oxidation of 2,4-hexadiene, 1,3-butadiene, and 1,3-cyclohexadiene in CH3CN-H20-NaC104 yields diols, 2-oxazolines, and 3-pyrrolines.104 The product distribution is influenced by the supporting electrolyte. 

Anodic oxidation of Mn(OAc)2 (catalytic amounts) in the presence of nonacti-vated alkenes and ethyl acetoacetate provides a route to dihydrofurans (cf, 6, 356). This electrooxidation of Mn(OAc)2 has been extended to coupling of activated methylene compounds with alkenes and dienes. 

When using a carbon electrode, the anodic oxidation of conjugated dienes (7) such as isoprene, pipe-rylene, cyclopentadiene and 1,3-cyclohexadiene in methanol or acetic acid mainly gives oxidative 1,4-addition products (8 equation 13). For example, 1,3-cyclohexadiene gives l,4-dimethoxycyclohex-2-ene (9) in 47% yield (equation 14). 1,3-Cyclooctadiene, in a similar experiment, yields a considerable amount of the allylically substituted product. 

In route A, one electron is removed fiom cme double bond to generate a cation radical, and subsequent transannular reaction of the cation radical with the other double bond forms a new carbon-carbon tend. On the other hand, in route B, allylic substitution or oxidative addition at one double bond takes place without intramolecular interaction between the double bonds. As exemplified by the anodic oxidation of 4-vinylcyclohexene (11) in methanol (equation 16), such dienes as 4-vinylcyclohexene, limonene and 1,5-cyclooctadiene yield only products via route B. 

A more efficient and economic procedure for the preparation of / -acetamido nitro compounds from alkenes, conjugated dienes and silyl enol ethers is based on the electrochemical generation of nitronium tetrafluoroborate in acetonitrile solution by anodic oxidation of dinitrogen tetrox-ide, using a divided cell (Table 6, Method B)142-143. From 1,3-dienes, mixtures of regio- and diastereomers were obtained. 

The 1,2- and 1,4-addition reactions have been observed also for alkyl-substituted aromatics, but the yields are often low, in particular for benzene derivatives, owing to competing side-chain substitution reactions. Examples include the addition of MeOH to methyl-substituted benzenes [38-42], naphthalenes [43], and anthracenes [43,44]. In a similar fashion, the anodic oxidation of 1,3-dienes in MeOH [45,46] results in mixtures of 1,2- and 1,4-addition products accompanied by substitution products and methoxy-containing dimers and trimers [46]. Styrenes are oxidized in MeOH to the 1,2-addition product together with products formed by dimerization-addition [47,48]. Oxidation of allenes results in most cases in complicated product mixtures resulting from single and double addition reactions [49-52]. 

Anodic oxidation in DMF of the open-chain tetrapyrrole, 1, 8 -dimethyl-a,c-bila-diene, yielded porphyrin one methyl group is lost from a cyclic intermediate with four conjugated pyrrole subunits by attack of a nucleophile [32]. 

Formation of a cation radical of the diene may catalyze Diels-Alder reactions with certain dienophiles thus oxidation of substituted vinylindoles to the cation radical in the presence of a push-pull dienophile may form different heterocyclic compounds by the Diels-Alder reaction, for instance pyrido[l,2i7]indoles from vinylindoles and yS-enamino-esters [33]. Similarly, anodic oxidation in MeCN of certain oxazolidines in presence of vinyl ether leads to derivatives of oxazepines in a catalytic reaction in which the ring-opened radical cation of the oxazolidine adds to the vinyl ether and the radical cation of the resulting oxazepine oxidizes the oxazolidine [34]. 

The initial attack in the anodic oxidation of papaverine [75] probably involves a similar attack further oxidation and dimerization leads to the isolated product, 12,12 -bis-(2,3,9,10-tetramethoxyindolo[2,l-fl]isoquinolyl). An analogous reaction is the electrooxidation of a tetramethoxy-substituted 2-methyl-l-phenethyl-l,2,3,4-tetrahydroisoquinoline to a dibenzoquinolizinium derivative [76] and the oxidation of A,A -triphenyl-( -phenyle-nediamine to 9,10-diphenylphenazine [77]. Intramolecular Michel addition of nitrogen in a tetrahydroquinoline derivative to an o-quinone moity have resulted in the formation of a 5,6-dihydrodibenz[6,d]indolizine derivative [78]. A similar ring closure occurs during the oxidation of various catecholamines [79] and similar compounds [79] to indoles. Cyclic a-carbonylazo compounds, generated by anodic oxidation of the hydrazines, may be trapped by reaction with dienes to the expected heterocycles [80]. 

Allylic methoxylation is often the result of anodic oxidation of alkenes and dienes in methanolic media [36-38]. Cyclopropanes behave similarly [39]. The reaction is usually accompanied by products of addition and combination, and the products may all be accommodated in the mechanistic scheme given in Eq. (20). 

Acyclic olefins are stereoselectively and sometimes stereospecifically dimethoxylated [301,303]. Anodic oxidation of 1,5- and 1,6-dienes in methanol provides the corresponding 1,4-dimethoxylated six- and seven-membered rings, and the trans-isomers are predominantly formed in both cases, as in Eq. (45) [304]. 

Anodic oxidation of l,2-bis(trimethylsilyl)-l,2-diphenylethane in methanol afforded 1,2-dimethoxy-1,2-diphenylethane d, l/meso 10) [380]. y-Acyloxyvinylstannanes undergo copper mediated anodic homocoupling to afford 1,3-dienes without isomerization. Thus, optically active 1,3-dienes were prepared anodically from optically active alkenylstannanes [381]. 

The anodic oxidation of nonconjugated dienes has been classified into two categories ... 

Nitroacetamidation of conjugated dienes is efficient when a solution of nitronium tetrafluoroborate in acetonitrile obtained by anodic oxidation of dinitrogen tetroxide is used substantial amounts of both 1,2- and 1,4-adducts are usually obtained.34 Some conjugated nitroalkenes can be readily prepared by reaction of alkenes with sodium nitrite and iodine under mild conditions (Scheme 158).347... 

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