Halides, aryl, arylation electrochemical

Cross coupling between an aryl halide and an activated alkyl halide, catalysed by the nickel system, is achieved by controlling the rate of addition of the alkyl halide to the reaction mixture. When the aryl halide is present in excess, it reacts preferentially with the Ni(o) intermediate whereas the Ni(l) intermediate reacts more rapidly with an activated alkyl halide. Thus continuous slow addition of the alkyl halide to the electrochemical cell already charged with the aryl halide ensures that the alkyl-aryl coupled compound becomes the major product. Activated alkyl halides include benzyl chloride, a-chloroketones, a-chloroesters and amides, a-chloro-nitriles and vinyl chlorides [202, 203, 204], Asymmetric induction during the coupling step occurs with over 90 % distereomeric excess from reactions with amides such as 62, derived from enantiomerically pure (-)-ephedrine, even when 62 is a mixture of diastereoisomcrs prepared from a racemic a-chloroacid. Metiha-nolysis of the amide product affords the chiral ester 63 and chiral ephedrine is recoverable [205]. 

When the nickel complexes [NiX2(PPh3)2] (X = Cl, Br) are reduced electrochemically in the presence of C02 and aryl halides, aryl carboxylate anions are formed.585 The reduction leads to nickel(O) complexes which then undergo oxidative addition of the aryl halide followed by insertion of C02. Further reduction then gives the product and regenerates nickel(O) (Scheme 55). 

Vinyl and aryl halides have been electrochemically coupled using nickel complex catalysts and these methods have been reviewed previously825. 

Electrochemical oxidation of 4-aryl-substituted thiane in aqueous organic solvents containing various halide salts as electrolytes gave selectively the trans-sulfoxide (lOe). Under acidic conditions a preferential formation of the cis-sulfoxide was attained328. The stereoselective potential of this method for the oxidation of cyclic sulfides139,329 is apparent (equation 123). 

In certain cases, Michael reactions can take place under acidic conditions. Michael-type addition of radicals to conjugated carbonyl compounds is also known.Radical addition can be catalyzed by Yb(OTf)3, but radicals add under standard conditions as well, even intramolecularly. Electrochemical-initiated Michael additions are known, and aryl halides add in the presence of NiBr2. Michael reactions are sometimes applied to substrates of the type C=C—Z, where the co-products are conjugated systems of the type C=C—Indeed, because of the greater susceptibility of triple bonds to nucleophilic attack, it is even possible for nonactivated alkynes (e.g., acetylene), to be substrates in this... 

The ratio ARH/ARj (monoalkylation/dialkylation) should depend principally on the electrophilic capability of RX. Thus it has been shown that in the case of t-butyl halides (due to the chemical and electrochemical stability of t-butyl free radical) the yield of mono alkylation is often good. Naturally, aryl sulphones may also be employed in the role of RX-type compounds. Indeed, the t-butylation of pyrene can be performed when reduced cathodically in the presence of CgHjSOjBu-t. Other alkylation reactions are also possible with sulphones possessing an ArS02 moiety bound to a tertiary carbon. In contrast, coupling reactions via redox catalysis do not occur in a good yield with primary and secondary sulphones. This is probably due to the disappearance of the mediator anion radical due to proton transfer from the acidic sulphone. 

Although more hydrolytically sensitive than the phosphine boranes, diorganochlorophosphines can be more accessible than diorganophosphines and are not pyrophoric. Thus, the reaction of a chlorophosphine with an aryl halide or aryl triflate in the presence of zinc as a reducing agent and (DPPE)NiCl2 as catalyst provides a convenient procedure for P—C coupling (Equation (49)).150 A related nickel-catalyzed process driven by electrochemical reduction has also been reported 151... 

The reduction of organic halides is of practical importance for the treatment of effluents containing toxic organic halides and also for valuable synthetic applications. Direct electroreduction of alkyl and aryl halides is a kinetically slow process that requires high overpotentials. Their electrochemical activation is best achieved by use of electrochemically generated low-valent transition metal catalysts. Electrocatalytic coupling reactions of organic halides were reviewed in 1997.202... 

Fig. 5 Electrochemical reduction of aryl halides showing the variation of the apparent transfer coefficient with the scan rate. iodobenzene, O bromobenzene, V 1-iodonaphthalene, O 4-methyliodobenzene, at 298 K, iodobenzene at 329 K.

The synthesis of metalloporphyrins which contain a metal-carbon a-bond can be accomplished by a number of different methods(l,2). One common synthetic method involves reaction of a Grignardreagent or alkyl(aryl) lithium with (P)MX or (PMX)2 where P is the dianion of a porphyrin macrocycle and X is a halide or pseudohalide. Another common synthetic technique involves reaction of a chemically or electrochemically generated low valent metalloporphyrin with an alkyl or aryl halide. This latter technique is similar to methods described in this paper for electrosynthesis of cobalt and rhodium a-bonded complexes. However, the prevailing mechanisms and the chemical reactions... 

Cobalt Porphyrins. The primary synthetic method for generating cobalt porphyrins with a metal carbon a-bond is to react a chemically or electrochemically generated cobalt(I) anion, [(P)Co] , with an alkyl or an aryl halide(19-26). [(P)Co] is stable and... 

As revealed in the following three sections, work dealing with the electrochemical oxidation of halogenated organic compounds is centered largely on the behavior of alkyl, alicyclic, and aryl halides. A review by Becker [4] provides an especially good perspective of developments in these areas. 

Recently, chloro-, bromo-, and iodoben-zenes have been subjected to electroreduction using Ni(0) complex mediators to yield biphenyl. NiCl2L2 and NiBr2L2 [L= P(Ph)3, (Ph)2PCH2CH2P(Ph)2] have been used as catalysts [259-265]. Pro-tic media such as alcohols, that is, methanol, ethanol or alcohol-water mixtures are found to be suitable solvents for achieving the electrosynthesis of biaryls from aryl halides according to a procedure that involves a catalytic process by nickel-2,2 -bipyridine complexes [266]. Electrochemical cross-coupling between... 

P, y-Unsaturated esters (184) have been synthesized by a one-step electrochemical procedure from a-chloroesters (183) and aryl or vinyl halides (Scheme 73b) [294, 295]. This novel electroreductive cross-coupling method is based on the use of a Ni(II)(bpy) catalyst and a sacrificial aluminum anode in a one-compartment cell (Scheme 73). The whole cathodic process progresses at —1.2 V (SCE) (Scheme 73c),... 

In the case of stepwise electron-transfer bond-breaking processes, the kinetics of the electron transfer can be analysed according to the Marcus-Hush theory of outer sphere electron transfer. This is a first reason why we will start by recalling the bases and main outcomes of this theory. It will also serve as a starting point for attempting to analyse inner sphere processes. Alkyl and aryl halides will serve as the main experimental examples because they are common reactants in substitution reactions and because, at the same time, a large body of rate data, both electrochemical and chemical, are available. A few additional experimental examples will also be discussed. 

Fig. 9 Electrochemical reductive cleavage of aryl halides in a poor H-atom donor solvent. Cyclic voltammetry as a function of the scan rate, v. E, Electrode potential i, current. Reduction (cathodic) currents are represented as being upwards.

The simplest way of generating and observing aryl halide anion radicals is to use an electrochemical technique such as cyclic voltammetry. With conventional microelectrodes (diameter in the millimetre range), the anion radical can be observed by means of its reoxidation wave down to lifetimes of 10" s. Under these conditions, it is possible to convert, upon raising the scan rate, the irreversible wave observed at low scan rates into a one-electron chemically reversible wave as shown schematically in Fig. 9. Although this does not provide any structural information about RX , besides the standard potential at which it is formed, it does constitute an unambiguous proof of its existence. Under these conditions, the standard potential of the RX/RX " couple as well as the kinetics of the decay of RX-" can be derived from the electrochemical data. Peak potential shifts (Fig. 9) can also be used... 

The reduction of aryl-substituted vinyl halides by electrochemically generated aromatic anion radicals has also been investigated in DMF (Gatti et al., 1987). Counter-diffusion behaviour at low driving forces (pp. 34, 35) does not appear as clearly as in the case of aryl halides (Fig. 11). However, analysis of the log k vs E° plot according to a quadratic activation-driving force relationship gave standard potential and intrinsic barrier values that... 

Alkoxide or aryloxide anions are also reputed to be inactive in Sr I reactions. There is, however, one example of such a reaction at an sp carbon the nitro-derivative of 4-nitrocumyl reacts with phenoxide and 1-methyl-2-naphthoxide ions yielding the corresponding ethers (Kornblum et al., 1967). A similar reaction has been reported for halobenzenes in t-butyl alcohol upon stimulation by sodium amalgam (Rajan and Sridaran, 1977). This reaction could not, however, be reproduced (Rossi and Pierini, 1980) and other attempts to make phenoxide ions react at sp carbons have been equally unsuccessful (Ciminale et al, 1978 Rossi and Bunnett, 1973 Semmelhack and Bargar, 1980). It has been found, more recently, that phenoxide ions react with a series of aryl halides under electrochemical induction, but that the coupling occurs at the p- or o-phenolic carbon rather than at the phenolic oxygen (Alam et al, 1988 Amatore et al, 1988). This is... 

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