Aldehydes electrochemical coupling

Durandetti, M., Nedelec, J.-Y., Perichon, J. An electrochemical coupling of organic halide with aldehydes, catalytic in chromium and nickel salts. The Nozaki-Hiyama-Kishi reaction. Org. Lett. 2001, 3, 2073-2076. 

Cheng has published a convenient and synthetically useful alternative method to the NHK reaction for the arylation of aromatic aldehydes in a mild and selective way with nickel(ll) bromide/zinc/dppe mediated protocol for the synthesis of diaryl carbinols. Durandetti reported an electrochemical coupling of aryl halides with aldehydes for the synthesis of diaryl carbinols which was catalytic in chromium and nickel salts. Comins utilized the NHK reaction to prepare 5-(l-hydroxyalkyl)-2,3-dihydro-4-pyridones, which were then explored in reductive, oxidative and substitutive reactions. " The first asymmetric catalytic synthesis of 5y -alk-l-ene-3,4-diols was developed the regio-, diastereo- and enantioselective addition of 3-chloropropenyl pivaloate to aldehydes was made possible by exploiting Salen r(II) species in a catalytic version of the NHK reaction. ... 

In the future, further studies should be addressed to improve the chemose-lectivity and diastereoselectivity of the reductive coupling process, especially searching for novel reagents and milder experimental conditions. As a matter of fact, a few novel reductive couphng procedures which showed improved efficiency and/or stereoselectivity have not been further apphed to optically active imines. For example, a new electrochemical procedure which makes use of the spatially addressable electrolysis platform with a stainless steel cathode and a sacrificial aluminum anode has been developed for imines derived from aromatic aldehydes, and the use of the N-benzhydryl substituent allowed 1,2-diamines to be obtained with good yields and dl-to-meso ratios... 

Head-to-head coupling ought to be more important in a, 0-unsaturated aldehydes, in which steric hindrance about the carbonyl carbon (head) is less than in the corresponding ketones. Indeed, jS, 0-dimethylacrolein (142) affords 143 (a dhmeso mixture), 144, and 145 in 24,67, and 9% yield, respectively, upon electrochemical reduction at pH 5.0 139). Tail-to-tail coupling does not occur ... 

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... 

Co(III)] complexes. For example, the coupling of 3-halocholestanes (333) and Michael acceptors affords epimeric mixtures of the 3-homologated steroids (334). The electrochemical nucleophilic acylation of a, 3-unsaturated aldehydes, a,3-unsaturated ketones and a,(3-unsaturated nitriles with acyl anhydrides affords adducts (335) in moderate yields.226a-b Similarly, the reduction of N-methyloxazolinium salts (336) affords die A, O-acetal intermediates (337) which are readily hydrolyzed (Scheme 102).226c... 

Although molybdenum and tungsten enzymes carry the name of a single substrate, they are often not as selective as this nomenclature suggests. Many of the enzymes process more than one substrate, both in vivo and in vitro. Several enzymes can function as both oxidases and reductases, for example, xanthine oxidases not only oxidize purines but can deoxygenate amine N-oxides [82]. There are also sets of enzymes that catalyze the same reaction but in opposite directions. These enzymes include aldehyde and formate oxidases/carboxylic acid reductase [31,75] and nitrate reductase/nitrite oxidase [83-87]. These complementary enzymes have considerable sequence homology, and the direction of the preferred catalytic reaction depends on the electrochemical reduction potentials of the redox partners that have evolved to couple the reactions to cellular redox systems and metabolic requirements. 

Electrochemical preparation of a functionalized benzylic zinc reagent and its coupling with a functionalized aromatic aldehyde preparation of 2-(4-cyanophenyl)-1-(4-methoxyphenyl)ethanol9... 

Organic halogen compounds have been used as synthetic precursors in electrochemical synthesis. They can be converted into aldehydes, ketones, alcohols and may be coupled with C—C bond formation in each case. Since this area is the subject of another chapter in the present volume (by Casanova and Prakash Reddy) the coverage here entails a brief description of synthetically useful coupling reactions that have been reported recently. 

Reduction of an aryl halide at a cadmium-modified nickel cathode in DMF containing TBABF4 leads to a formylation reaction between aryl carbanions and the solvent [186]. Two papers [187,188] have appeared in which reduction of aryl halides gives an aryl carbanion, which, by acting as a base to deprotonate a suitable nitrile, can cause coupling of the nitrile with esters, aldehydes, and ketones. Electrochemical trimethylsilylation of aryl halides can be effected at a stainless-steel or carbon-cloth cathode in THF-HMPA containing TEABF4 and trimethylchlorosilane [189]. 

Electrochemistry can be coupled with other physical methods such as fluorescence spectroscopy. An XO-based electrochemiluminescent biosensor for hypoxanthine has been reported. The enzyme was immobilized in a carbon paste electrode with bovine serum albumin cross-linked with glutar-aldehyde. The working principle of the biosensor is illustrated in Scheme 5.6. As already shown (eqn (5.3a)), H2O2 is produced by the catalytic reaction between hypoxanthine and XO immobilized on the electrode surface. In an alkaline or neutral solution, luminol is electrochemically oxidized to a compound that reacts spontaneously with H2O2 to generate chemiluminescent luminol and the ensuing luminescence was used to quantify the amount of hypoxanthine present. 

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