Sacrificial anodes Reformatsky reactions

The use of Fe, indium, and zinc as sacrificial anodes in the electrochemically assisted Reformatsky reaction of ethyl 2-bromoalkanoates with succinic anhydride is found to be effective, giving l-ethyl-3-oxohexanedioates in 49 80%... 

The mechanism of the Zn chloride-assisted, palladium-catalyzed reaction of allyl acetate (456) with carbonyl compounds (457) has been proposed [434]. The reaction involves electroreduction of a Pd(II) complex to a Pd(0) complex, oxidative addition of the allyl acetate to the Pd(0) complex, and Zn(II)/Pd(II) transmetallation leading to an allylzinc reagent, which would react with (457) to give homoallyl alcohols (458) and (459) (Scheme 157). Substituted -lactones are electrosynthesized by the Reformatsky reaction of ketones and ethyl a-bromobutyrate, using a sacrificial Zn anode in 35 92% yield [542]. The effect of cathode materials involving Zn, C, Pt, Ni, and so on, has been investigated for the electrochemical allylation of acetone [543]. 

The Reformatsky reaction can also be performed electrochemically either directly or using a mediator. Ni-catalysis has proven to be an efficient way to prepare j3-hydroxy ester or nitrile from the corresponding a-chlorocompounds (Table 14) [94]. Here again the first step is the oxidative addition of the cathodically generated Ni°bpy to the halocompound. The nature of the sacrificial anode also plays a crucial role in this reaction, though the formation of an organozinc intermediate has not been fully demonstrated. 

Reduction of a range of allyl and benzyl chlorides at a stainless steel cathode in dimethylformamide in the presence of carbonyl compounds and using a sacrificial anode of aluminium or zinc, leads to a Reformatsky-type reaction in 40-80 % yields. Allyl halide give products by reaction at both the a- and y-positions. Tetra-chioromethane and bromotrifluoromethane take part in similar reactions provided a... 

Electrochemical Reformatsky reaction promoted by sacrificial anodes 803... 

Finally, the use of ultrasound to entrain Reformatsky reactions deserves mentioning.37 Since a conventional laboratory cleaning bath is usually sufficiently effective, this easily adaptable method should find wider use in particular for large scale preparations. The same may hold true for the use of sacrificial anodes and/or for Reformatsky-type reactions triggered by electrochemical means.38... 

The electrosynthesis of /Miydroxy esters (220), 2,3-epoxy esters (221) and /Miydroxy nitriles (222) was achieved under nickel-catalyzed conditions, obviating Reformatsky reaction 70(equation 112-114). The reaction proceeds in excellent yield when a sacrificial zinc rod is used as the anode. A mechanism has been proposed which involves reduction of a Ni(II) complex to a Ni(0) complex, oxidative addition of the a-chloroester to the Ni(0) complex, and a zinc (II)/Ni(II) exchange, leading to an organozinc reagent, in analogy to the Reformatsky reactions. 

Application of zinc as a sacrificial anode often overcomes the difficulties with activation of zinc in the Reformatsky reaction and the problems with uncontrolled exothermic reactions. When zinc or indium is employed for the reaction, the amount of electricity used is less than the theoretical, and this suggests that the anode acts as activated metal and reacts with the bromo compound [175]. 

In an electrochemical version of the modified Reformatsky reaction a sacrificial indium anode is used. Using this method, fully substituted /3-lactones are obtained directly from the proper substrates. 

A one-pot preparation of 4-iodomethyloxetan-2-one is reported (93SL899). Electrochemically generated organometallic compounds react with ketones in a Reformatsky reaction to give esters or lactones (93AG1218) and the formation of P-lactones is favoured when a sacrificial indium anode is used (94JOC3161). 

Furthermore, 2,2-difluoro-3-hydroxyesters are readily obtained from ClCFjCOOMe and carbonyl compounds by electrolysis in a one-compartment cell using a sacrificial zinc anode and a nickel complex as catalyst. The catalytic cycle for this reaction is shown in Scheme 9 with nickel zinc exchange being a key step. In this process, the CHjCyDMF solvent (9 1) system leads to suppression of undesired Claisen condensation and an increase in the yield of 2,2-difluoro-3-hydroxyester formation. It is notable that high yields are obtained even with ketones and enolizable aldehydes, which are not good participants in the Reformatsky reaction alternative for producing these substances. 

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