Copper-catalysed perarylation of cyclopentadiene: synthesis of hexaarylcyclopentadienes

While hexaphenylsilacyclopentadiene (hexaphenylsilole) is viewed as an archetypal Aggregation-Induced Emission (AIE) luminogen, its isostructural hydrocarbon surrogate hexaphenylcyclopentadiene has strikingly never been investigated in this context, most probably due to a lack of synthetic availability. Herein, we report a straightforward synthesis of hexaphenylcyclopentadiene, via the direct perarylation of cyclopentadiene upon copper(i) catalysis under microwave activation, with the formation of six new C-C bonds in a single synthetic operation. Using zirconocene dichloride as a convenient source of cyclopentadiene and a variety of aryl iodides as coupling partners, this copper-catalysed cross-coupling reaction gave rise to a series of unprecedented hexaarylcyclopentadienes. The latter are direct precursors of extended π-conjugated polycyclic compounds, and their cyclodehydrogenation under Scholl reaction conditions yielded helicenic 17,17-diarylcyclopenta[l,l']diphenanthrenes. These structurally complex polyannelated fluorene derivatives can now be prepared in only two synthetic steps from cyclopentadiene.

Copper(i)–catalyzed intramolecular asymmetric C-arylation of acyclic β-ester amides: enantioselective formation of chiral oxindoles

A highly efficient intramolecular asymmetric C-arylation of acyclic β-ester amides is demonstrated.

Bis-Phenoxo-CuII2 Complexes: Formal Aromatic Hydroxylation via Aryl-CuIII Intermediate Species

Ullmann-type copper-mediated arylC-O bond formation has attracted the attention of the catalysis and organometallic communities, although the mechanism of these copper-catalyzed coupling reactions remains a subject of debate. We have designed well-defined triazamacrocyclic-based aryl-CuIII complexes as an ideal platform to study the C-heteroatom reductive elimination step with all kinds of nucleophiles, and in this work we focus our efforts on the straightforward synthesis of phenols by using H2O as nucleophile. Seven well-defined aryl-CuIII complexes featuring different ring size and different electronic properties have been reacted with water in basic conditions to produce final bis-phenoxo-CuII2 complexes, all of which are characterized by XRD. Mechanistic investigations indicate that the reaction takes place by an initial deprotonation of the NH group coordinated to CuIII center, subsequent reductive elimination with H2O as nucleophile to form phenoxo products, and finally air oxidation of the CuI produced to form the final bis-phenoxo-CuII2 complexes, whose enhanced stability acts as a thermodynamic sink and pushes the reaction forward. Furthermore, the corresponding triazamacrocyclic-CuI complexes react with O2 to undergo 1e− oxidation to CuII and subsequent C-H activation to form aryl-CuIII species, which follow the same fate towards bis-phenoxo-CuII2 complexes. This work further highlights the ability of the triazamacrocyclic-CuIII platform to undergo aryl-OH formation by reductive elimination with basic water, and also shows the facile formation of rare bis-phenoxo-CuII2 complexes.

Characterization and Reactivity Studies of a Terminal Copper-Nitrene Species

High-valent terminal copper-nitrene species have been postulated as key intermediates in copper-catalyzed aziridination and amination reactions. The high reactivity of these intermediates has prevented their characterization for decades, thereby making the mechanisms ambiguous. Very recently, the Lewis acid adduct of a copper-nitrene intermediate was trapped at -90 °C and shown to be active in various oxidation reactions. Herein, we describe for the first time the synthesis and spectroscopic characterization of a terminal copper(II)-nitrene radical species that is stable at room temperature in the absence of any Lewis acid. The azide derivative of a triazamacrocyclic ligand that had previously been utilized in the stabilization of aryl-Cu^III intermediates was employed as an ancillary ligand in the study. The terminal copper(II)-nitrene radical species is able to transfer a nitrene moiety to phosphines and abstract a hydrogen atom from weak C-H bonds, leading to the formation of oxidized products in modest yields.