Nitriles electroreduction

Nickel-bpy and nickel-pyridine catalytic systems have been applied to numerous electroreductive reactions,202 such as synthesis of ketones by heterocoupling of acyl and benzyl halides,210,213 addition of aryl bromides to activated alkenes,212,214 synthesis of conjugated dienes, unsaturated esters, ketones, and nitriles by homo- and cross-coupling involving alkenyl halides,215 reductive polymerization of aromatic and heteroaromatic dibromides,216-221 or cleavage of the C-0 bond in allyl ethers.222... 

The same electrochemical process was also used for the coupling between aldehydes or ketones and activated alkyl halides such as a-chloroesters, -nitriles, and -ketones as well as aya-dichloroesters.334 Electroanalytical studies have shown initial electroreduction of Fe(n) to Fe(i) and subsequent formation of an iron organometallic intermediate (e.g., a 7t-allyliron complex in Equation (27)) before reaction with the corresponding carbonyl compounds.335... 

Adducts (10) and (11) were each converted to the unsaturated nitrile (12) that is shown in Scheme 4. Electroreductive cyclization of (12) presented a major challenge given the highly hindered nature of the fully substituted yS-carbon of the a, yS-unsaturated subunit. Many organic transformations fail to occur when presented with a similarly crowded environment. Thus, it is significant that the controlled potential electroreductive cyclization reaction of (12) afforded a 90% isolated yield of the requisite [3.2.1] adduct (13). Steric factors clearly did not hinder this reaction, a result that is presumed to be generalizable to other electroreductive cyclizations. Cycloadduct (13) was... 

The electroreductive ring-opening of a,-epoxy esters (475) and nitriles (478) proceeds stepwise as shown in Schemes (164 and 165). In both cases, the intermediates (476) and (479) can be isolated when the mediator-assisted reductions are carried out at room temperature. The whole reduction can be completed at 50 °C. For example, the electrolysis of a phenylglycidic ester (475) (r1 = Ph, r2 = H) in an MeOH-NaCl04-(Pt) system in the presence of PhSeSePh at room temperature gives... 

As indicated by the conversion of 70 to 71, the electroreductive cyclization reaction provides as excellent method for the assembly of the bicyclo[3.2.1]oc-tane ring system. Several additional examples are portrayed in the following equations. In general, the use of an unsaturated nitrile rather than the corresponding ester is preferred, as this precludes lactone formation, and therefore... 

The massive zinc (rod or plate) reacts spontaneously with activated bromides provided the preliminary electroreduction of a catalytic amount of zinc salt (ZnBr2 or ZnCl2) occurs. Reactions are carried out in nitrile solvents (CH3CN, PhCN,. ..) or their mixture with dichloromethane. An undivided cell fitted with a zinc anode and an indifferent cathode (gold, nickel, carbon, zinc,. ..) is used. As observed with benzylic bromides, the activation leads to an organozinc compound able to react with either the nitrile solvent or an electrophile reagent. The process is depicted in equation 12. 

The most systematically investigated acyl anion equivalents have been the IMS ethers of aromatic and heteroaromatic aldehyde cyanohydrins, TBDMS-protected cyanohydrins, - benzoyl-protected cyanohydrins, alkoxycaibonyl-protected cyanohydrins, THP-protected cyanohydrins, ethoxyethyl-protect cyanohydrins, a-(dialkylamino)nitriles, cyanophosphates, diethyl l-(trimethylsiloxy)-phenyimethyl phosphonate and dithioacetals. Deprotonation di these masked acyl anions under the action of strong basie, usually LDA, followed by treatment with a wide varies of electrophiles is of great synthetic value. If the electrophUe is another aldehyde, a-hydroxy ketones or benzoins are formed. More recently, the acyl caibanion equivalents formed by electroreduction of oxazolium salts were found to be useful for the formation of ketones, aldehydes or a-hydroxy ketones (Scheme 4). a-Methoxyvinyl-lithium also can act as an acyl anion equivalent and can be used for the formation of a-hydroxy ketones, a-diketones, ketones, y-diketones and silyl ketones. - - ... 

It has also been shown that Ni complexes can successfully carry out intramolecular or intermolecular electroreductive addition of alkyl radicals to activated olefins. Thus bicyelic ketones 78 and 80 are conveniently prepared in good yields from 2-bromoalkyl-2-cyclohexenone 77 and 3-bromoalkyl-2-cyclohexenone 79, respectively [25]. Alkyl bromides also react with a,) -unsaturated esters and nitriles to give 1,4-addition products in moderate yields [26]. 

The electrochemistry of the triple bond TABLE 11. Some electroreductive decyanations of nitriles ... 

Shono and coworkers achieved electroreductive intra- and intermolecular couplings of ketones and nitriles in 2-propanol solutions containing Et4NTos using Sn cathodes at controlled potential (-2.8 V SCE). Intramolecular coupling of cyclic y- and (5-cyano ketones, besides good to excellent yields, proved to be cis stereoselective when a-hydroxy ketones with bicyclo[3.3.0] or [4.3.0] skeletons were formed. When the reactions were carried out at 65 °C instead of 25 °C, dehydration of hydroxy ketones occurred and the corresponding a,jS-unsaturated ketones were obtained. The presence of alkyl or 2-ethoxycarbonyl substituents did not hinder the cyclization. In Table 13 some representative examples are shown. 

Attempted intermolecular coupling of ketones and nitriles under conditions similar to those used for intramolecular coupling led to mixtures of two types of ketone-nitrile coupling products and alcohols resulting from ketone electroreduction. Product selectivity could be changed altering nitrile/solvent (2-propanol or ethanol) composition. Some results for cyclohexanone/acetonitrile reductions are shown in Scheme 27. 

Electroreduction of a,j5-unsaturated nitriles ra .y-PhCH=CHCN, PhCMe=CHCN and CH2=CHCN was investigated in acetonitrile solutions at a Hg cathode, to establish the most favourable experimental conditions for cyanomethylation to occur at the expense... 

Other reactions studied include reduction of the triple bond of a-acetylenic esters and nitriles by tributyltin hydride in methanol electroreductive cyclization of acetylenic halides at a mercury cathode the trimerization and tetramerization of cyclo-octyne in the presence of various transition metals the kinetics of bromination of alka-l,3-diynes, of permanganate oxidation of acetylenedicarboxylic acid, and of iodination of propiolic acid the participation of the triple bond in reactions of various acetylenes of the general formula (225) and the trimerization of but-2-yne with tolyl-palladium chloride to give a [Pg.48]

Solid zinc activation has been used with success for the formation of stable organozinc compounds from functionalized benzylic bromides [8] and has been applied to the condensation of activated halides with nitriles [9] (Blaise reaction) or carbonyl compounds (10] (Reformatsky reaction). In this case, solid zinc is activated by catalytic generated zinc formed by electroreduction of anhydrous ZnBr2 solution in acetonitrile as solvent. Results and practical approaches have been reported [llj. 

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