Cathodic coupling

Three modes for the formation of cathodic mixed coupling products can be visualized  

Addition of cathodically generated radicals to double bonds (Eq. (209))  

Here the cathodically generated hydroxymethyl radical adds to the double bond to form a new radical, which is subsequently reduced and protonated to give product. 

Addition of cathodically generated radical anions to activated double bonds  

This route is essentially the radical anion mechanism (Eq. (206)) with donor and acceptor being dissimilar. 

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TABLE 7. Cathodic coupling of 1,3-dienes with esters... 

Scheme 10 Intermolecular cathodic coupling of a,/S-unsaturated esters R subst. phenyl, yields 50-60%.

Scheme 11 Intramolecular cathodic coupling of a,/3-unsaturated esters yields 40-100%.

Scheme 22 Cathodic coupling of amides Rh alkyl, R methyl, yields 63-70%.

Since the electroreduction of ketones shown in Scheme 29 has been well established [1-3, 12, 62-65], one more recent interest in the electroreduction of carbonyl compounds is focused on the stereo-selective reduction of ketones. For example, the diastereo-selective cathodic coupling of aromatic ketones has been reported. In the presence of a chiral-supporting electrolyte, a low degree of enantioselectivity has been found [66] (Scheme 30). 

Scheme 30 Diastereoselective cathodic coupling of aromatic ketones in the presence of chiral supporting electrolyte, yield 91%, 25% ee.

Scheme 35 Intramolecular cathodic coupling of vinylsilanes with ketones yields 55%.

Cathodic coupling of ketones with an ethoxydimethylsilyl-substituted olefin affords oxasilacyclopentanes with excellent yield. This coupling is synthetically valuable as the coupling product is a... 

Scheme 39 Diastereoselective cathodic coupling of ketones with allylic alcohols yields 60-90%.

The cathodic coupling of 4H-pyran 4-thiones (20) in nonaqueous media was achieved in good or even high yields [185, 186] to afford tt-donors like bipyranylidenes. 

To allow anion radicals generated from structure (20) to dimerize more rapidly, this reaction was carried out in the presence of an electrophile. For example, in the presence of EtBr as electrophile, the cathodic coupling of structure (20) (X = O, = r2 = Ph, r3 = R = H) may be depicted as follows (Scheme 39). 

The cathodic coupling of prenyl chloride with diethyl fumarate and methyl crotonate proceeds with different regios-electivities as shown in Fig. 17 [94]. Also in this case, the change in the regioselectivity appears to be due to the reduction of the diethyl fumarate to a radical anion that undergoes a nucleophilic substitution at the prenyl chloride, and in the other case there is a 2e reduction of the prenyl... 

Vitamin Bi2-catalyzed intramolecular cathodic coupling leads to a regioselective 1,4-addition with formation of a spirocom-pound (Eq. 2) [95]. This chain reaction is initiated by the reduction of Co(III) to a Co(I) species, which reacts in an oxidative addition with the alkyl bromide. The resulting alkyl-Co(III)-Br species is then reduced to an alkyl anion that undergoes a Michael addition and yields Co(I) for the next cycle. 

Imines can be stereoselectively hy-drodimerized to 1,2-diamines with rac/ meso rahos of 0.9 to 1.1 similar to the cathodic coupling of carbonyl compounds to pinacols in an acidic medium [304]. With 4,6-dimethylpyrimid-2-one only the meso-diamine was obtained [305]. Electroreduction of diimines prepared from... 

Scheme 70 Cathodic coupling of 3-phenylpropylbromide with acrylonitrile.

Scheme 117 Catalytic cycle for cathodic coupling of aryl halides.

Scheme132 Cathodic coupling of cyclohexenyl bromide with chromium(lll) as mediator.

Scheme 133 Cathodic coupling of benzophenone (pinacolization) with chromium (I II) as mediator.

Scheme 134 Cathodic coupling of arylketones with chromium(IM) as mediator.

Cathodic coupling of ketones to tiimethylsily substituted allyl alcohols such as... 

The cathodic cyclocoupling reaction (equation 38) with a diene under similar conditions was also observed for 1-vinylcyclohexene (67a) and 1-vinylcycloheptene (67b), giving 68 as the final product with satisfactory yields. Thus, similar intermediate metallocycles can be expected for this reaction. On the other hand, for the cathodic coupling of styrenes the formation of a Mg compound was not proved, although a magnesium electrode was necessary in order to obtain a 2-phenylcyclopropanol-type product. 

Many other parameters tend to influence the corrosion of metals immersed in sea water. When two metals of different potentials are galvanically coupled, the acceleration of the attack on the less noble metal of the two is observed frequently. A small area of an anodic metal coupled to a large area of a second metal that is cathodic can be particularly dangerous. A useful guide to help predict unfavorable combinations is the galvanic series of metals in sea water (0). The reverse situation—namely, a small cathode coupled to an anode that is large in area—often proves satisfactory in service. 

Cathodic coupling of ketones with 64 takes place regioselectively at the /3-position to the silyl substituent to give 65 in good yields (equation 44). A radical anionic intermediate derived from the carbonyl compound is proposed107. 

Cathodic coupling proceeds via radicals or radical anions, which are reductively generated from suitable substrates, in general electrophiles, e.g., carbonyl compounds and activated olefins. These intermediates either dimerize (Eq. (182)) or add to activated double bonds to yield 1,4-radical anions, which are subsequently reduced to hydrodimers (Eq. (183) ). 

Cathodic coupling of a, jS unsaturated carbonyl compounds yields three types of dimers head-to-head (112, Eq. (189) ), tail-to-tail (113) and head-to-tail hydrodimers (114). 

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