Electrochemical Dehydration Reaction

Abstract

Electrochemical dehydration reaction is a fascinating and underexplored field of research, which has started to attract significant attention in recent years. Dehydration reactions are characterized by the formal removal of water in the course of the transformation, and they are among the most fundamental types of reactions found throughout chemistry. Examples are esterification reactions, amidation reactions, and the synthesis of carbon-heteroatom multiple bonds. In general, dehydration reactions are not considered to be redox reactions, because no oxidation states change in the substrate from which water is eliminated or in the dehydration reagent that is utilized. At first glance, there does not seem to be a link between dehydration reactions and redox chemistry. In recent years, however, it has been demonstrated that dehydration reactions can be carried out by electrolysis. Given the enormous importance of dehydration reactions from academic to technical scale, electrochemical dehydration reactions offer a more sustainable approach to such transformations. In this review, the recent progress is surveyed and the opportunities of this new and evolving field are highlighted. Electrochemical dehydration reactions are an interesting new discipline in the emerging domain of electroorganic chemistry, which is currently experiencing a remarkable renaissance to establish itself as a 21st-century technique.

Keywords: carboxylic acids; dehydrative reactions; electrolysis; sulfonic acids; sustainable chemistry.

Scheme 1

Some examples of electrochemical dehydration reactions.

Scheme 2

Comparison of Kolbe electrolysis (left) with electrochemical dehydration of carboxylic acids to their carboxylic anhydrides (right).^{[ 19} , ^{28 ]}

Scheme 3

Examples from the electrochemical dehydration of carboxylic acids to their anhydrides. RT = room temperature.^{[ 19 ]}

Scheme 4

Examples from the electrochemical dehydration of dicarboxylic acids to their cyclic anhydrides.^{[ 28 ]} BDD = boron‐doped diamond. DMAP = 4‐(dimethylamino)pyridine.^{[ 59 ]}

Scheme 5

Proposed mechanism for the electrochemical dehydration of carboxylic acids to their anhydrides.^{[ 28 ]} By ¹⁸O isotope labeling of the carboxylic acid starting material, ¹⁸O‐labeled sulfate was detected in the reaction solution after electrolysis.

Scheme 6

Examples from the electrochemical esterification of carboxylic acids with alcohols to esters.^{[ 31} , ^{32 ]} PMB = p‐methoxy benzene. The scale‐up was carried out via electrolysis under constant cell voltage.

Scheme 7

Proposed mechanism of the phenothiazine‐mediated electrochemical catalytic esterification of carboxylic acids with alcohols to esters.^{[ 31} , ³² , ^{34 ]} PMB = p‐methoxy benzene.

Scheme 8

Examples from the electrochemical dehydration of carboxylic acids to carboxylic anhydrides for the synthesis of amides, esters, and thioesters.^{[ 34 ]}

Scheme 9

Examples from the electrochemical dehydration of aldoximes to nitriles.^{[ 38 ]}

Scheme 10

Proposed mechanism for the mediated electrochemical dehydration of aldoximes to nitriles via nitrile oxide intermediates.^{[ 38 ]}

Scheme 11

Examples from the halogen‐free electrochemical dehydration of aldoximes to nitriles. C_Gr = graphite. MTES = methyl triethylammonium methylsulfate.^{[ 43} , ^{44 ]}

Scheme 12

Examples from the electrochemical Beckmann rearrangement. HFIP = 1,1,1,3,3,3‐hexafluoroisopropanol, DCE = dichloroethane.^{[ 46 ]}

Scheme 13

Proposed reaction mechanism of the electrochemical Beckmann rearrangement.^{[ 46 ]}

Scheme 14

Examples from the synthesis of N‐sulfonyl amidines by electrochemical dehydrative coupling of primary sulfonamides with derivatives of DMF. C_Gr = graphite.^{[ 47 ]}

Scheme 15

The proposed reaction mechanism of the electrochemical dehydrative coupling of primary sulfonamides with derivatives of DMF to N‐sulfonyl amidines.^{[ 47 ]}

Scheme 16

Examples from the electrochemical dehydration of sulfonic acids to their sulfonic anhydrides and their subsequent reactions with alcohols or amines to sulfonates or sulfonamides, respectively.^{[ 49 ]}

Scheme 17

Proposed mechanism of the electrochemical dehydration of sulfonic acids to their anhydrides.^{[ 49 ]}

Scheme 18

Electrochemical synthesis of cyclic sulfites from diols and SO₂. DBU = 1,8‐diazabicyclo[5.4.0]undec‐7‐ene.^{[ 50 ]}

Scheme 19

Proposed reaction mechanism of the electrochemical synthesis of cyclic sulfites from diols and SO₂.^{[ 50 ]} DBU = 1,8‐diazabicyclo[5.4.0]undec‐7‐ene.

Scheme 20

Mechanistic classification of electrochemical dehydration reactions.