Copper(III) Metallacyclopentadienes via Zirconocene Transfer and Reductive Elimination to an Isolable Phenanthrocyclobutadiene

Abstract

Despite the widespread use of copper catalysis for the formation of C–C bonds, debate about the mechanism persists. Reductive elimination from Cu­(III) is often invoked as a key step, yet examples of its direct observation from isolable complexes remain limited to only a few examples. Here, we demonstrate that incorporation of bulky mesityl (Mes) groups into the α-positions of a phenanthrene-appended zirconacyclopentadiene, Cp2Zr­(2,5-Mes2-phenanthro­[9,10]­C4), enables efficient oxidative transmetalation to the corresponding, formal Cu­(III) metallacyclopentadiene dimer. The dimer was quantitatively converted to a structurally analogous anionic monomer [ n Bu4N]­{Cl2Cu­(2,5-Mes2-phenanthro­[9,10]­C4)} upon treatment with [ n Bu4N]­[Cl]. Both metallacycles undergo quantitative reductive elimination upon heating to generate phenanthrocyclobutadiene and a Cu­(I) species. Due to the steric protection provided by the mesityl groups, this cyclobutadiene was isolated and thoroughly characterized to reveal antiaromaticity comparable to that of free cyclobutadiene, which imbues it with a small highest occupied molecular orbital–lowest unoccupied molecular orbital energy gap of 1.85 eV and accessible reduced and oxidized electronic states.

Copper–Carbon Homolysis Competes with Reductive Elimination in Well-Defined Copper(III) Complexes

“…5 Additionally, Tilley recently published a study on Cu III metallacyclopentadiene dimer complexes, which can undergo reductive elimination to generate phenanthrocyclobutadiene in quantitative yield. 6 More recently, Sanford has reported the characterization of Cu III −aryl intermediates in aminoquinoline-directed C−H functionalization. 7 Despite significant progress in understanding the reductive elimination step in Cu catalysis, the involvement of Cu III species in Cu catalysis remains speculative.…”

“…Our group and the Shen group have successfully synthesized alkyl– and aryl–Cu III –CF 3 complexes capable of participating in C–CF 3 bond-forming reactions . Additionally, Tilley recently published a study on Cu III metalla­cyclo­penta­diene dimer complexes, which can undergo reductive elimination to generate phenanthro­cyclo­buta­diene in quantitative yield . More recently, Sanford has reported the characterization of Cu III –aryl intermediates in amino­quinoline-directed C–H functionalization …”

Formal [2+1] Cycloadditions of a Metallacyclopentadiene Unit with Alkynes: Constructing Tetracyclic Conjugated Frameworks with Bridgehead Metals

“…As shown in Figure 2, a broad scope of alkynes with either electron-withdrawing or electron-donating substituents in different positions of benzene rings perform successfully, resulting in complexes 3-26 in moderate yields. Common substituents, such as methyl (3)(4)(5), methoxyl (6)(7)(8), halogen (9)(10)(11)(12)(13)(14)(15), tert-butyl (16), formyl (17), acetyl group (18), carboxyl (19), methoxycarbonyl (20), trifluoromethyl (21), and pinacolborato (22), hydroxyl (23), alkynyl (24-25), naphthalene ( 26) are all tolerant. Apart from substituted phenyl groups, 3-ethynylthiophene and alkyl-substituted alkynes also afford the desired products 27-29.…”

“…Metallacycles in which metal fragments formally replace at least one carbon unit in the frameworks of carbocyclic compounds have gained broad interests and play a substantial role in organic, organometallic, and coordination chemistry. [1][2] Metallacyclopentadiene complexes constitute an important class of five-membered metallacycles [3][4] and represent one of the key intermediates in the transition-metal-catalyzed cyclotrimerization [5][6] and polymerization of alkynes. [7] Thus, research on stoichiometric reactions of metallacyclopentadienes with alkynes would not only enrich the reaction modes of metallacycles but also promote the mechanism study of cyclotrimerization and polymerization.…”

High-Valent Ni and Cu Complexes of a Tetraanionic Bis(amidateanilido) Ligand

“…Mononuclear Ni III compounds are relatively rare, generally supported by anionic, – amidate, , and organometallic ligation . Mononuclear Cu III compounds are comparatively more common, with the high-valent center coordinated by anionic β-diketiminate, , thiolates, – amidates, – and organometallics. – Phenoxide coordination is particularly interesting: salen, or bis­(iminophenoxide), and its related bis­(amidatephenoxide) ligands can support either M II -phenoxyl radical or M III -phenoxide forms depending on the substitution around the aromatic phenoxide moiety, illustrating its possible redox noninnocent nature. – In contrast, a recently reported bis­(iminoanilido) ligand, in which the phenoxide donor of the salen ligand is substituted with its isoelectronic anilido counterpart, is capable of supporting a Cu III center, while also demonstrating redox noninnocent behavior at more positive oxidation potentials. – The Ni-analogue of the same ligand has also been reported, existing as a ligand-oxidized Ni II -anilinyl species . While Cu-bis­(iminophenoxide) and Cu-bis­(iminoanilido) are neutral owing to the dianionic nature of their respective ligands, Cu-bis­(amidatephenoxide) is dianionic as its ligand is tetraanionic upon complete deprotonation.…”