1.3- Cyclohexadiene anodic oxidation

When a palladium(II)-hydroquinone system is used as the mediator4 in the anodic oxidation of 1,3-cyclohexadiene in acetic acid, either trans- or cis- 1,4-diacetoxy-2-cyclohexene is formed with rather high selectivity, though the possible formation of 1,2-diacetoxylated compound is not discussed. 

Anodic oxidation of halogenated tyrosines was studied in connection with some sponge metabolites (cavemicolin model compounds). The methyl exter of 3,5-dibromotyrosine afforded four different products in a 41 10 26 23 ratio with 23% overall yield as a result of equilibration. (Scheme 44) [93JCS(P2)3117], A related compound was obtained as a mixture of stereoisomers 56 from a Diels-Alder reaction between N-acetyldehydroalanine methyl ester and l-methoxy-l,3-cyclohexadiene (87TL2371). 

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. 

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. 

The prototype cyclodimerization of 1,3-cyclohexadiene catalyzed by tris(4-bromo-phenyl)aminium ion (Ar3N ) proceeds smoothly in CH2CI2 at room temperature or below to a 70% yield of the Diels-Alder dimer [Eq. (51)] [116]. The direct anodic oxidation of 1,3-cyclohexadiene results in formation of the Diels-Alder product only in low yield, with the major product being a polymer linked through the 1,4-positions [119]. 

Oxidation of resorcinol dimethyl ether [35] in methanolic KOH produces among other products 5,5,6,6-tetramethoxy-1,3-cyclohexadiene (IV), and hexamethyl cis,cis-orthomuconate (V) (V) could be obtained from (IV) in 77% yield on anodic oxidation, conceivably according to ... 

Antkracyclinones. The use of latent quinone reagents such as 2-lithio-3,3,6,6-tetramethoxy-l,4-cyclohexadiene has been extended to a synthesis of an anthra-cyclinone. The starting material 1 was converted into the quinone bisketal 2 by anodic oxidation. The corresponding lithio compound was then condensed with dimethyl 3-methoxyphthalate (3). The reaction fortunately was stereoselective and resulted in 4 in satisfactory yield. The conversion of 4 to the anthracyclinone 5 was conducted in three steps without isolation of intermediates reductive hydrolysis to the hydroquinone, saponification, and finally cyclization with hydrogen fluoride. The overall yield of 5 from 3-bromo-2,5-dimethoxybenzalde-hyde, the precursor of 1, was 8%. ... 

Some typical results are shown in Table 2. The table shows that oxidation of conjugated dienes such as isoprene, piperylene (1,3-pentadiene), cyclopentadiene and 1,3-cyclohexadiene with a carbon anode in methanol or in acetic acid containing tetraethylammonium p-toluenesulfonate (EtjNOTs) as the supporting electrolyte yields mainly 1,4-addition products2. 1,3-Cyclooctadiene yields a considerable amount of the allylically substituted product. 

The products of electrochemical oxidation of conjugated dienes are considerably affected by the reaction conditions such as the material of the electrode, the supporting electrolyte and the solvent. The oxidation of butadiene with a graphite or carbon-cloth anode in 0.5 M methanolic solution of NaClCU mainly yields dimerized products along with small amounts of monomeric and trimeric compounds (equation 5)1. The use of platinum or glassy carbon mainly gives monomeric products. Other dienes such as isoprene, 1,3-cyclohexadiene, 2,4-hexadiene, 1,3-pentadiene and 2,3-dimethyl-l,3-butadiene yield complex mixtures of isomers of monomeric, dimeric and trimeric compounds, in which the dimeric products are the main products. 

If production of an oxidizing hole in the da orbital is the important factor in the photochemical reaction, then electrochemical veneration of such a hole should produce a highly reactive intermediate mat would mimic the initial step in the 3(da po) photoreaction. Several of the binuclear complexes undergo reversible one-electron oxidations in noncoordinating solvents (22-24). The complex Rh2(TMB)42+ possesses a quasireversible one-electron oxidation at 0.74 V (Electrochemical measurements for [Rh2(TMB)4](PF6)2 CH2CI2/TBAPF6 (0.1 M), glassy carbon electrode, 25°C, SSCE reference electrode). Electrochemical oxidation of Rh2(TMB)42+ in the presence of 1,4-cyclohexadiene exhibits an enhanced anodic current with loss of the cathodic wave, behavior indicative of an electrocatalytic process (25). Bulk electrolysis of Rh2(TMB)42+ in an excess of 1,4-cyclohexadiene results in the formation of benzene and two protons (Equation 4). 

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