Dissolving metal, reductive coupling

Dissolving metal reduction of camphor produces a mixture of bor-neol, isobomeol, and pinacol coupling products (Scheme 39). The ratios of the stereoisomers are affected profoundly by whether the starting ketone is enantiomerically pure or racemic, implying the chirality recognition at the stage of ketyl radical (65). 

The electrochemical properties of many functional groups have been described in reviews by Steckhan, Degner (industrial uses of electrochemistry), Kariv-Miller,543 and Feoktistov. The synthetic applications of anodic electrochemistry has also been reviewed. There are interesting differences between dissolving metal reductions (secs. 4.9.B-G) and electrochemical reactions. Cyclohexanone, for example, can be reduced to cyclohexanol (sec. 4.9.B) or converted to the 1,2-diol (556) via pinacol coupling by controlling the reduction potential, the nature of the electrode and the reaction medium. 46 Presumably, the more concentrated conditions favor formation of cyclohexanol via reduction of the carbanion. More dilute solutions appear to favor the radical with reductive dimerization to 556. More important to this process, however, is the difference in reduction potential (-2.95 vs. -2.700 V) and the transfer of two Faradays per mole in the former reaction and four Faradays per mole in the latter. 

There are many synthetic applications for sulfide carbanions. Sulfide 304 required the addition of DABCO to facilitate deprotonation by butyllithium, but the resulting anion coupled readily with allylic chloride 305 under these conditions to give 306. The sulfide moiety was removed by a dissolving metal reduction (for example, see secs. 4.9.D, 4.9.F) to give dendrolasin, 307.The ability to remove the sulfur after activating... 

A eutectic mixture of sodium and potassium dissolves in THF to form an intense blue solution when 18-crown-6 is used as a complexing agent, enabling dissolving metal reductions to be conveniently performed in this solvent. Acetylenes are rapidly reduced to olefins in good yields at 0 "C but -stereoselec-tivity E Z = 3 1) is lower than with the classical procedures. Acetylenes are reduced to Z-olefins in almost quantitative yield by a zinc-copper couple in refluxing methanol. ... 

Since reductions by metal atoms often occur as one-electron processes, intermediate radicals are frequently involved in dissolving-metal reductions. When the reaction conditions are adjusted so that coupling of these intermediates competes favorably with other processes, the formation of carbon-carbon bonds will occur. A useful example which has been known for a long time is the reductive coupling of acetone to 2,3-dimethylbutane-2,3-diol (pinacol). Considerable effort has been... 

Rate-determining step, hydroformylation, 163 Reactivity, enantiomers, 286 Recognition, enantiomers, 278 Reduction and oxidation, 5 Reductive coupling, dissolving metal, 288 Reductive elimination, 5, 111 Resolution. See Kinetic resolution Rhenium-carbene complexes, 288 Rhodium-catalyzed hydrogenation, 17, 352 amino acid synthesis, 18, 352 BINAP, 20... 

Termination steps in SrnI reactions compete with the propagation steps and, although these processes have aroused considerable mechanistic and theoretical speculation (see Chapter 9 in ref. 18), their effects, with several important exceptions, are not significant. For example the self-coupling of aryl radicals (equation 11) does not appear to occur under the conditions used for the SrnI reaction. One potentially disruptive termination step is reduction of the intermediate aryl radical (equation 6). The source of the reducing electron can be a dissolving metal (or solvated electron), one of the radical anion intermediates in the reaction (ArNu- or ArX- ), an electrode, or the nucleophile itself. These termination... 

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