**3. Reductive Transformations**

Reductive organocatalytic electrosynthesis dates back to the late 1960s, when Grimshaw and co-workers described the use of alkaloids in catalytic amounts for the reduction of 4-methylcoumarins to optically active 3,4-dihydro-4-methylcoumarins in a divided cell at constant potential [79,80]. The transformation resulted in only modest chiral induction and moderate yields, the latter due to competing dimerization of the intermediate coumarin radical. Nevertheless, the organocatalytic strategy inspired the work of other groups working in this field [81–83] and chiral amines have been used as additives in catalytic amounts for asymmetric reduction of various ketones, carboxylic acids and oximes as well as for reductive dehalogenation [84–97]. Electrocatalytic reductions have also been carried out using metal catalysts, including a RhIII polypyridyl complex for the hydrogenation of acetophenone with modest chiral induction [98]. Other early electroreductive metal-catalyzed enantioselective processes include cobalt catalysis in the form of vitamin B12 [99–101], as well as nickel catalysis [102] for dehalogenations, isomerizations, reductive cleavages, etc.

## *3.1. Carboxylation Using CO2*

The electroreductive use of carbon dioxide (CO2) as a C1 synthon is a strategy that receives continued interest for direct reduction to fuels and bulk chemicals, as well as for incorporation in more complex organic molecules [103–107]. In the context of asymmetric electrosynthesis, both organocatalytic and metal-catalyzed examples can be found. In similarity with the early asymmetric electroreductive protocols, alkaloids have found their use in asymmetric electroreductive carboxylation of ketones with carbon dioxide. Lu and co-workers utilized catalytic amounts of cinchonidine and cinchonine to form (*R*)-or (*S*)-atrolactic acid with *ee*'s up to 30%, respectively, using a stainless steel cathode and Mg sacrificial anode in an undivided cell and constant current conditions [108]. Furthermore, cinchonine was used in catalytic amounts for the electrocarboxylation of 4-methylpropiophenone to form the (*S*)-configured product (up to 33% *ee*), using a similar experimental setup [109]. A few years later, the same group reported moderate yields and *ee*'s (up to 49% *ee*) in the electrocarboxylation of 2-acetonaphthone under constant current conditions, using 2.5 mol% of cinchonidine with a sacrificial Mg anode and stainless steel cathode in an undivided cell under atmospheric CO2 pressure at 0 ◦C [110]. In successive work, the group utilized a metal-catalyzed strategy for the electrocarboxylation of 1-phenylethyl chloride, employing a CoII-(R,R)(salen) complex (15 mol%) [111]. Optically active 2-phenylpropionic acid could be obtained in 37% yield and 83% *ee* under potentiostatic conditions in an undivided cell with glassy carbon cathode and Mg anode at 50 ◦C (Scheme 13), while no reaction was observed in absence of a Co source. With the support of CV analysis, the transformation was proposed to proceed via one-electron transfer to give an anionic [CoI-(R,R)(salen)] complex, which could react with the substrate to form a CoIII organocobalt intermediate. One-electron reduction of this species to a CoII anionic complex followed by direct nucleophilic attack to CO2 ensures the asymmetric induction. However, if homolytic cleavage of the Co-R bond occurs, the generated [PhCH(CH)3] radical can prompt a competing background reaction (upon reduction to the corresponding anion and attack to CO2) to give the racemic acid. Detection of styrene by GC-MS and the observation of dimer as major side-product supported the proposed formation of radical intermediates.

**Scheme 13.** Cobalt-catalyzed reductive carboxylation of benzylic chlorides.

In 2018, Mei and co-workers developed a Pd-catalyzed electrocarboxylation of allyl esters, providing α-aryl carboxylic acids in high yields and regioselectivity [112]. In the report, the enantioselective variant of the method was developed using Pd(OAc)2 (7.5 mol%) in combination with a chiral bidentate triarylphosphine ligand (8 mol%) under constant current in an undivided cell with a Pt cathode and a Mg sacrificial anode, under 1 atmosphere of CO2. Moderate yield (66%) and enantioselectivity (67% *ee*) could be obtained (Scheme 14). No reaction was observed when the current was replaced with common chemical reductants as Mn<sup>0</sup> or Zn0, whereas the reaction proceeded even in the absence of the Pd catalyst, albeit with low conversion. The use of EtOH as an additive was crucial to gran<sup>t</sup> higher yields and regioselectivities, although further insight about its role was not disclosed. Based on CV measurements, the authors proposed that initial reduction of PdII to Pd<sup>0</sup> followed by oxidative addition of the allyl acetate generates a cationic π-allylpalladiumII complex. This species equilibrates to the favored terminal η1-allylpalladiumII species, which is reduced at the cathode. The resulting nucleophilic Pd<sup>0</sup> complex reacts with CO2 to form a magnesium carboxylate salt with Mg<sup>2</sup>+ from the anode, while Pd<sup>0</sup> re-enters the catalytic cycle. Direct attack of the η1-allylpalladiumII species to CO2 was ruled out since the transformation did not occur in absence of electric current.

**Scheme 14.** Cathodically driven Pd-catalyzed carboxylation of cinnamyl acetate.
