Dyes, Radicals and Nitrous Oxide Adducts Derived from N-Heterocyclic Carbenes

N-Heterocyclic carbenes are able to form covalent adducts with nitrous oxide (N2O) under ambient conditions. This thesis summarizes the studies of these adducts and their reactivity. A ditopic carbanionic N-heterocyclic carbene was found to react with nitrous oxide, resulting in a stable covalent adduct with two N2O groups attached to the heterocycle. Mesoionic N-heterocyclic carbenes derived from C2-arylated 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene, as well as a triazolylidene, also react with N2O. Crystallographic analyses of all complexes reveal bent N2O groups, which can adopt a cis or trans configuration. Resulting complexes of imidazole- or triazole-based mesoionic carbenes and nitrous oxide can be converted into azo dyes by reaction with arenes in the presence of AlCl3 or HCl. The azo coupling can be achieved with electron-rich aromatic compounds such as mesitylene, trimethoxybenzene, azulene, or dibenzo-18-crown-6. The latter coupling reaction allows for an easy preparation of a colorimetric sensor for potassium or sodium ions. The putative intermediate of the reaction with acid, a rare diazohydroxide, has been isolated and structurally characterized. Using imidazolium compounds with two N2O groups, it is possible to prepare amine-substituted azo dyes, or dyes with two azoarene groups attached to the heterocycle. The synthesis and the characterization of a new class of neutral aminyl radicals is also reported. Monoradicals were obtained by the reduction of azoimidazolium dyes with potassium. Structural, spectroscopic and computational data suggest that the spin density is centered on one of the nitrogen atoms of the former azo group. The reduction of a dimeric dye with an octamethylbiphenylene bridge between the azo groups resulted in the formation a biradical with largely independent unpaired electrons. Both, the monoradicals and the biradical were found to display high stability in solution as well as in the solid state.

Neutral Aminyl Radicals Derived from Azoimidazolium Dyes

José Luis Bila, Léonard Yannick Maurice Eymann, Philip Pattison, Henrik Moodysson Rønnow, Rosario Scopelliti, Kay Severin, Andrzej Sienkiewicz, Euro Solari, Alexander Tskhovrebov, Laurent Alexis Clément Vannay, Matthew Wodrich, Ivica Zivkovic

The synthesis and characterization of a new class of neutral aminyl radicals is reported. Monoradicals were obtained by reduction of azoimidazolium dyes with potassium. Structural, spectroscopic, and computational data suggest that the spin density is centered on one of the nitrogen atoms of the former azo group. The reduction of a dimeric dye with an octamethylbiphenylene bridge between the azo groups resulted in the formation of a biradical with largely independent unpaired electrons. Both the monoradicals and the biradical were found to display high stability in solution as well as in the solid state.

Amer Chemical Soc2016

Synthetic chemistry with nitrous oxide and triazenes

Iris Roswitha Landman

Nitrous oxide (N2O) has gained much interest because of its physiological effects ("laughing gas") and its negative environmental impact ("greenhouse gas", "ozone-depleting substance"): It has a lifetime of more than 100 years in the atmosphere. Its persistent character shows how challenging it is to convert N2O into more valuable products. From a synthetic point of view, N2O has potential as diazo (N2) transfer reagent, but applications in synthetic organic chemistry are scarce. The main challenges are to capture N2O, avoid the loss of N2, and to overcome poor-selectivity and -yields. Herein, we describe two examples of the incorporation of two nitrogen atoms into the final product. Firstly, we use N2O as diazo transfer reagent in the preparation of triazolopyridines, which are valuable starting materials for heterocyclic compounds. The reactions can be performed under mild conditions and give synthetically interesting triazoles in moderate to good yields. Secondly, general methods to N1 acylated triazenes were missing previously. We developed routes to 1-acyl triazenes via acid-catalyzed hydration, or by gold- or iodine-catalyzed oxidation of 1-alkynyl triazenes. The latter compounds are prepared from N2O. The properties of the N1 acylated triazenes are distinct compared to other triazenes, displaying different physical and chemical properties. Under strong acidic conditions, 1-acyl triazenes act as acylation reagents. Finally, despite that triazene chemistry is known for more than 160 years, the preferred coordination site of acids still remained a matter of debate (N1, N2, or N3). In search of experimental evidence, we have analyzed triflic acid, B(C6F5)3, and PdCl2 adducts of triazenes by IR, NMR spectroscopy and single crystal X-ray crystallography. For all cases, we found the acid coordinating to the N1 atom of the triazene. This preference should be taken into account for future acid- and transition metal-catalyzed reactions with triazenes. Altogether, this thesis contributes to triazene chemistry and synthetic chemistry with N2O. In this way, we provide further evidence that N2O can be used as a diazo donor to get to synthetically interesting N-containing products.

EPFL2022

Synthetic chemistry with nitrous oxide and triazenes

Iris Roswitha Landman

EPFL2022