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Synthesis of Indenyl Triazenes by Rhodium-Catalyzed Annulation Reactions

Rosario Scopelliti, Kay Severin, Farzaneh Fadaei Tirani, Carl Thomas Bormann, Florian Gérald Abela

The synthesis of vinyl triazenes by metal-catalyzed processes is attractive, but only few examples have been described so far. Here, we describe Rh-catalyzed [3+2] annulation reactions of arylboronic acid derivatives and alkynyl triazenes. These reactions provide access to functionalized indenyl triazenes. The regioselectivity of the annulations is mostly high, but strongly dependent on the substituents on the alkyne. The triazene group in the products can be cleaved under acidic condition. In contrast to what has been reported for other vinyl triazenes, rearrangements are observed instead of direct substitution reactions.

WILEY-V C H VERLAG GMBH2020

Divergent Synthesis of Densely Substituted Arenes and Pyridines via Cyclotrimerization Reactions of Alkynyl Triazenes

Kay Severin, Nicolai Cramer, Carl Thomas Bormann, Florian Gérald Abela, Jin Fay Tan, Frederick Mark Chadwick

Densely substituted fused aromatic triazenes can be prepared by [2 + 2 + 2] cyclotrimerization reactions of 1-alkynyl triazenes. The CpRu-catalyzed cyclization proceeds well with both simple alkynyl triazenes and tethered 1-diynyl triazenes. Attractively, the methodology can be extended to pyridine synthesis by replacing an alkyne with a nitrile. The reaction is regioselective and yields the sterically more hindered product. The triazene group precisely installed on the synthesized aryl and pyridyl ring is a highly versatile moiety, which is effortlessly converted into the most important and frequently used functional aryl substituents, including fluorides. It is also suited for intramolecular transformations to afford a variety of valuable heterocycles. The coordination chemistry of alkynyl triazenes and CpRuCl was studied and led to the structural characterization of a CpRuCl(η2-alkyne) complex, a CpRuCl(η4-cyclobutadiene) complex, and an unusual dinuclear Ru complex with a bridging tetramethylfulvene ligand. Complexes of this type are potentially involved in catalyst deactivation pathways.

2019

Investigations of the Chemistry of 1-Alkynyltriazenes

Florian Gérald Abela

The chemistry of aromatic triazenes is well known, and has been explored for more than a century. These compounds have emerged as very useful intermediates, because of their unique reactivity. This functionality can notably be used to access many other functional groups, and it is present in certain bioactive compounds. Recently, our laboratory has reported a novel synthesis of triazenes using nitrous oxide, which allows the synthesis of 1-alkynyltriazenes. These compounds are poorly known, as their synthesis is not trivial by other routes. This thesis aims to explore the reactivity of these compounds. We have shown that the chemical reactivity of 1-alkynyltriazenes parallels what has been observed for ynamides. The similarity in reactivity of these two classes of compounds is demonstrated by addition reactions with acids and by cycloaddition reactions with ketenes and tetracyanoethene. The presence of the reactive triazene groups in the products allowed for subsequent transformations. We also studied the reactivity of these substrates in Ru-catalyzed [2+2] cycloadditions. The triazenes serve as unique vinyl cation surrogates. Under acidic conditions, the triazene functionality can be replaced by a variety of groups including halides, alkoxides, sulfoxides, amides, arenes and heteroarenes, providing efficient access to a pool of chiral and achiral polycyclic compounds The formation of very reactive cations using cyclic alkenenyl triazenes is described in a short study. It seems that the strained vinyl cation can be formed, but that its high reactivity prevents it to react selectively with weak nucelophiles. In the final part of this work, first results of a rhodium-catalyzed reaction with bifunctional boronic acids are presented. We show that 1-alkynyltriazenes react in the presence of rhodium to form indene derivatives. The unique reactivity of the triazenes lead to the formation of unexpected compounds. Overall, our results suggest that 1-alkynyltriazenes will become valuable reagents in synthetic organic chemistry.

EPFL2018