Electro-organic reactions

Electroorganic reactions. Part 58. Revisiting the cleavage of oxalate ester radical-anions

“…This is unusual but benzylic radicals are known to survive further reduction where the first electron transfer to the substrate is at a less negative potential than the reduction potential of the benzylic radical formed by subsequent reaction 13,14 . This situation could apply here because the reduction potentials of the oxalate esters [E 0 (1)] are relatively low (Table 2) and could plausibly be less negative that those required for benzylic radical reduction [E 0 (2)]. We propose that the reactions are completed by hydrogen abstraction from the solvent (DMF).…”

“…2,3,4 This is difficult to achieve by direct reduction of alcohols because very negative potentials are required 5 and in any event the hydroxyl anion is a poor leaving group. Derivatives such as tosylates 6 , methanesulfonates 7 , diethylphosphates 8 , and acetates 9 are more easily cleaved by cathodic reduction but only at very negative potentials.…”

“…Conversion of hydroxyl into oxalate esters (R 1 OOC.COOR 2 ) was found greatly to reduce the reduction potential and for benzylic or allylic alcohols and vicinal diols cathodic cleavage, in aprotic solvent, of the R-O function took place. 2,3,4 In these cases electron transfer is to the central dicarbonyl function so the oxalate group acts as both electrophore and leaving group. In practice competing hydrolysis by adventitious water in the basic conditions of electrolysis precluded preparative-scale electrolysis of preformed esters.…”

“…However, in situ formation by transesterification during co-electrolysis of benzylic or allylic alcohols and diethyl oxalate led to † For Part 57 see reference 1 efficient 1F cleavage of hydroxyl in the added alcohol. 3,4 Coulometry, qualitative cyclic voltammetry and product studies were best explained 2 in terms of the initially formed radicalanions cleaved to radical (R ). In this context we report here on the direct observation by cyclic voltammetry of several relevant radical-anions and the measurement of their rates of cleavage and of substituent effects on the same.…”

Electrosynthetic C−O Bond Activation in Alcohols and Alcohol Derivatives

“…Circumvention of this side‐reaction was achieved by addition of an excess of diethyl oxalate (Figure 17, top right), analogous to the abovementioned transesterification protocol for deoxygenation of primary alcohols [109] . Under such conditions, a range of benzylic alcohols could successfully engage in the reductive transesterification–deoxygenation sequence [116, 117] . Furthermore, the benzylic alcohol substrates could be prepared by electroreduction of related carbonyl compounds in a one‐pot fashion [117] .…”