Dehydrogenative Esterification and Dehydrative Etherification by Coupling of Primary Alcohols Based on Catalytic Function Switching of an Iridium Complex

Abstract

Esters and ethers are widely used in modern organic synthesis. In recent years, the synthesis of esters by the dehydrogenative coupling of primary alcohols has been reported to be an environmentally benign process. In this situations, if a single catalyst can be used for the synthesis of either esters or ethers by making slight alterations to the reaction conditions, the value of the synthetic methodology would increase. In this study, we successfully developed a new catalytic function switching system: not only dehydrogenative esterification but also dehydrative etherification under environmentally friendly conditions were accomplished by the employment of a single iridium catalyst based on catalytic function switching. Using benzyl alcohol as a starting material, the esterification product, benzyl benzoate, and the etherification product, dibenzyl ether, were obtained in 92 % and 89 % yields, respectively, by employing same iridium catalyst precursor bearing a dihydroxybipyridine ligand, under optimized conditions.

Dehydrative Coupling of Alcohols by Iridium(III) Complexes with N-Heterocyclic-Pyridine Chelating Ligands Decorated with Naphthalene-Diimide

“…The 1 H NMR spectrum of 5A is consistent with the asymmetric substitution of the NDI moiety, as evidenced by the appearance of an AB system centered at 8.77 ppm assigned to two pairs of unequivalent protons of the naphthalene core of the NDI unit. As for complexes 2A and 2B, in 5A, the ligand is coordinated to the Ir(III) center via a (C NHC ,C pyr )-chelating form, as evidenced by the two doublets observed at 8.25 and 6.95 ppm assigned to the protons of the pyridine ring, and to the resonances at 164.6 and 163.8 ppm that appear on the 13 C NMR spectrum, assigned to the Ir−C carbene 13 and Ir−C (pyridine) carbons, respectively.…”

“…17 Albrecht and co-workers observed the homocoupling etherification of benzyl alcohol using a Ir(III)Cp* complex with a chelating pyridine-NHC ligand, but the reaction was not selective (benzaldehyde was also produced), and the reaction conditions used were harsher than those used by us (150 °C, catalyst loading 2 mol %). 10d In a more recently published work, Fujita and co-workers described the homocoupling of a series of primary alcohols at a lower temperature (120 °C) using a series of Cp*Ir(III) complexes, but the reaction required a hydrogen atmosphere and a catalyst loading of 1 mol %, 13 and the product yields were slightly below 90%. As for the activity of [IrCp*Cl 2 ] 2 , in our previous studies, we showed that it was able to produce the cross-coupling etherification of benzyl alcohol and n-butanol under the exact same reaction depicted in Table 1, affording 90% of the cross-coupling product, although a 2 mol % of catalyst loading (based on the amount of metal) was required.…”

“…The direct coupling of alcohols to ethers is an atom-economical and environmental benign process, as it produces water as the only byproduct, thus avoiding the generation of copious amounts of salt byproducts resulting from the use of inorganic bases and organic halides and alkoxides, which are generated by the currently used industrial scale methods . Several authors, ,,, including us, have used Ir­(III) complexes as catalysts for the etherification by cross-coupling of primary alcohols. Two different redox neutral reaction mechanisms have been proposed for the Ir­(III) etherification of alcohols.…”