Kolbe reductive coupling

The macrocyclic framework was constructed beginning with 4, available from (-i-)-citronellol, and 5 obtained from (-)-)-malic acid. Kolbe reductive coupling and reacylation then gave the coupled ester 6 in 42% yield. Reduction of this material to the triol followed by protection of the 1,2 diol as the acetonide and Collins oxidation gave the desired aldehyde 7. 

Carbon-Carbon Bond Formation - A number of anodic and cathodic coupling reactions are known. The Kolbe oxidation of carboxylate salts and pinacol formation from ketone reduction are familiar examples. Somewhat less well known is the reductive coupling of activated alkenes. 

Platinum—The high standard reduction potential for platinum makes it an ideal anode material (although Pt corrosion does occur during some oxidations reactions, such as the Kolbe oxidative coupling reaction [29, 58, 59]). For anode potentials greater than about 0.50 V (on the hydrogen scale), an oxide film covers the Pt surface, so the electrode material is often platinum oxide. Large anodes are often titanium coated with platinum in order to reduce costs. 

The new 23-hydroxy-epimers of cholesterol have been prepared by boro-hydride reduction of the 23-ketone, and Grignard reactions on the cyanohydrin of pregnenolone acetate have been used to prepare both epimers of 20a,22-dihydroxycholesterol. Using optically pure half-esters of methyl succinic acid in Kolbe electrolytic coupling reactions with various bile acids the corresponding 25-d- and 25-L-cholestanoic acids have been prepared. ... 

Diverse products may be obtained from the reaction of perfluoroiodoalkanes with olefins of the type Rp-CH CHa in the presence of copper (see p. 63). With copper powder in DMF, reductive coupling and dehydrofluorination may result in the formation of dienes of the type RpCF CH CH CFRp. >2 Kolbe electrolysis of 2/T-hexailuoroisobutyric acid at a platinum electrode yields the hexane (CFs)2CH-CH(CF3)2 (67%), together with (CFs)2CH2 and CFg-CHiCFg, and this hexane, when treated with potassium hydroxide in di-n-butyl ether, yields perfluoro-... 

The 3.8-nonadienoate 91, obtained by dimerization-carbonylation, has been converted into several natural products. The synthesis of brevicomin is described in Chapter 3, Section 2.3. Another royal jelly acid [2-decenedioic acid (149)] was prepared by cobalt carbonyl-catalyzed carbonylation of the terminal double bond, followed by isomerization of the double bond to the conjugated position to afford 149[122], Hexadecane-2,15-dione (150) can be prepared by Pd-catalyzed oxidation of the terminal double bond, hydrogenation of the internal double bond, and coupling by Kolbe electrolysis. Aldol condensation mediated by an organoaluminum reagent gave the unsaturated cyclic ketone 151 in 65% yield. Finally, the reduction of 151 afforded muscone (152)[123]. n-Octanol is produced commercially as described beforc[32]. 

The photo-Kolbe reaction is the decarboxylation of carboxylic acids at tow voltage under irradiation at semiconductor anodes (TiO ), that are partially doped with metals, e.g. platinum [343, 344]. On semiconductor powders the dominant product is a hydrocarbon by substitution of the carboxylate group for hydrogen (Eq. 41), whereas on an n-TiOj single crystal in the oxidation of acetic acid the formation of ethane besides methane could be observed [345, 346]. Dependent on the kind of semiconductor, the adsorbed metal, and the pH of the solution the extent of alkyl coupling versus reduction to the hydrocarbon can be controlled to some extent [346]. The intermediacy of alkyl radicals has been demonstrated by ESR-spectroscopy [347], that of the alkyl anion by deuterium incorporation [344]. With vicinal diacids the mono- or bisdecarboxylation can be controlled by the light flux [348]. Adipic acid yielded butane [349] with levulinic acid the products of decarboxylation, methyl ethyl-... 

Electrolytic generation of perfluoroalkyl radicals can also lead to coupling (Kolbe Reaction), and when carried out in the presence of an addend, like an olefin, can lead to coupling, reduction and disproportionation-type products of the adduct radicals, as well as occasionally-decent yields of simple adducts [183-188]. 

Orientation effects by the electrode surface have been invoked in a large number of other cases (for reviews of the stereochemistry of electrode processes, see Eberson and Homer, 1973 Fry, 1972a), for example in halide reduction (for a review, see Casanova and Eberson, 1973), formation of dimers in the Kolbe reaction (Hawkes et al., 1973), reduction of systems with double and triple bonds (Horner and Roder, 1969), and anodic coupling of 1-methylcorypalline... 

The anodic oxidation of RCOO produced by deprotonation of RCOOH occurs at about 2 V (vs. SCE), known as the Kolbe reaction, producing the radical coupling products of the alkyl groups of RCOO, R-R and CO2 [350]. Since the one-electron reduction potential of the singlet excited state AcrH+ (2.3 V vs. SCE) [351] is positive enough to oxidize RCOO, photoinduced electron transfer from RCOOH (R = H, Me, Et, /-Pr, /-Bu, C11H23, C15H31) to AcrH+ occurs in the... 

The methoxycarbonylmethyl radical I is obtained by Kolbe electrolysis from methyl-malonate [Eq. (4, path a)], and in homogeneous solution by reductive fragmentation of the hydroperoxide of dimethyl acetone dicarboxylate [Eq. (4, path b)]. Radical I forms with styrene the adduct II, which reacts by disproportionation to III and IV, by coupling with I to V and by dimerization to VI. The yields and product ratios for the electrolysis and the reductive fragmentation correspond surprisingly well, and no extensive polymerization was found for the homogeneous reaction. 

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