Insights into the Oxidative Dehydrogenation of Amines with Nanoparticulate Iridium Oxide

Abstract

The aerobic oxidation of amines offers a promising route towards many versatile chemical compounds. Within this contribution, we extend our previous investigations of iridium oxide-catalyzed alcohol oxidation to amine substrates. In addition to demonstrating the versatility of this catalyst, particular attention is focused on the mechanisms of the reaction. Herein, we demonstrate that although amines are oxidized slower than the corresponding alcohols, the catalyst has a preference for amine substrates, and oxidizes various amines at turnover frequencies greater than other systems found in the open literature. Furthermore, the competition between double amine dehydrogenation, to yield the corresponding nitrile, and amine-imine coupling, to yield the corresponding coupled imine, has been found to arise from a competitive reaction pathway, and stems from an effect of substrate-to-metal ratio. Finally, the mechanism responsible for the formation of N-benzylidene-1-phenylmethanamine was examined, and attributed to the coupling of free benzyl amine substrate and benzaldehyde, formed in situ through hydrolysis of the primary reaction product, benzyl imine.

Integrated Experimental and Theoretical Study of Shape-Controlled Catalytic Oxidative Coupling of Aromatic Amines over CuO Nanostructures

“…This result can be rationalized by considering the coupling reaction between an in-situ-generated imine intermediate and benzylamine, which is favored in the presence of additional amine substrates over a second dehydrogenation process leading to the formation of a benzonitrile derivative. The trend observed for our reactions is consistent with the activity trend experimentally observed by Hermans et al 51 To assess whether our CuO nanocatalyst is indeed heterogeneous in nature, we carried out hot filtration tests by performing oxidative coupling of benzylamine under optimized reaction conditions for 10 or 14 h using the Cu-1 nanocatalyst (Figure 5B). In one test, after 10 h of the catalytic reaction, the catalyst was separated by centrifugation, and the reaction was continued; however, conversion did not go beyond 25%.…”

tert-Butyl hydroperoxide (TBHP)-mediated oxidative self-coupling of amines to imines over a α-MnO2 catalyst

“…21 Hermans and co-workers did not detect it in GC-FID/MS and thought that it was instable in reaction or analysis. 25 Jones and co-workers recently proposed the same intermediate but still did not have experimental proof. 11 At first we wondered whether benzaldimine could survive the high reaction and temperature analyses or the sampling method.…”

“…1 Among the existing methods for imine syntheses, the oxidative self-coupling of amines to imines has attracted more attention. Relatively high yields have been achieved using homogeneous catalysts, such as Cu/TEMPO, 2 Cu/ABNO, 3 and TBHPQ, 4 and heterogeneous catalysts, such as precious metal catalysts (Pt, 5 Ru, 6 Pd, 7 Au, 8 Ir 9 ), vanadium oxides, 10 Cu-Ce-O, 11 Fe/MOF, 12 GO, 13 Cu, 14 and photocatalysts. 15 However, all of the above methodologies either require complicated catalyst preparation and reaction setup, involve cumbersome work-up or show limited compatibility for the inactivated amines, such as the normal aliphatic amine and benzyl amines substituted by electron-withdrawing groups.…”

Mechanistic insights into the oxidative dehydrogenation of amines to nitriles in continuous flow

“…5,13,18 We combine the results from the Hammett relationship study in continuous flow with (to the best of our knowledge) the first in situ investigations of the oxidative dehydrogenation of amines over heterogeneous catalysts. 5,13,18 We combine the results from the Hammett relationship study in continuous flow with (to the best of our knowledge) the first in situ investigations of the oxidative dehydrogenation of amines over heterogeneous catalysts.…”

“…Previous investigations of the oxidative dehydrogenation of amines using heterogeneous catalysts have been limited to applications of catalysts to various reactants and kinetic studies of batch reactions. 5,13,18 We combine the results from the Hammett relationship study in continuous flow with (to the best of our knowledge) the first in situ investigations of the oxidative dehydrogenation of amines over heterogeneous catalysts. We use results from Hammett investigations and in situ infrared (IR) spectroscopy to suggest an updated catalytic mechanism for the oxidative dehydrogenation of amines over RuO 2 /Al 2 O 3 catalysts.…”