Ligand-Induced Regioselectivity in Metal-Catalyzed Aryne Reactions Using Borylaryl Triflates as Aryne Precursors

Ring Expansion via Palladium-Catalyzed Aryne Insertion into C-C Bonds of Benzocyclobutenones

Insertion of arynes into cyclic C-C bonds provides a straightforward approach for ring expansion with an ortho-phenylene moiety; however, the current scope of substrates has been restricted to enolizable ketones. Here we report the first example of aryne-insertion into non-enolizable ketones through Pd-catalyzed C-C bond activation of benzocyclobutenones. 2-Haloaryl boronates were employed as an unconventional aryne precursor. The reaction shows a wide substrate scope and high functional group tolerance. The mechanistic studies reveal an interesting dual role of the Pd catalyst for both aryne generation and C-C bond activation.

Copper Catalysis with Arynes: Unlocking Site-Selective Arylation of Pyrazoles

Arynes are among the most reactive species in organic chemistry-six-membered rings so strained that their energy rivals that of a hand grenade. 1 Since their discovery in 1902, chemists have used arynes to achieve innovative transformations and access diverse natural products, however, their application for catalytic cross-coupling remains unrealized. 2 A major challenge in late-stage functionalization is the selective N -arylation of unsymmetric pyrazoles to create a core found in blockbuster medicines worth over nineteen billion dollars annually. 3 Traditional cross-coupling methods usually favor one type of regioisomer and thus, limit late-stage access to alternatives that could speed up drug discovery. 4,5 Here, we show that copper catalysis harnesses arynes to achieve switchable arylation of pyrazoles. By tuning metallotautomers via ligand choice, we direct N -arylation to either nitrogen site in a pyrazole, unlocking site-selective control. 6,7 Mechanistic studies reveal how steric and electronic forces guide regioselectivity and turn an unpredictable process into a precise synthetic tool.

Zirconium-mediated carbon-fluorine bond functionalisation through cyclohexyne "umpolung"

Polarity reversal, or "umpolung", is a widely acknowledged strategy to allow organic functional groups amenable to react in alternative ways to the usual preference set by their electronic features. In this article, we demonstrate that cyclohexyne umpolung, realized through complexation to zirconocene, makes the small strained cycloalkyne amenable to C-F bond functionalisation. Such strong bond activation chemistry is unprecedented in "free" aryne and strained alkyne chemistry. Our study also reveals that the reactivity of the Zr-cyclohexyne complex is highly sensitive to the degree of fluorination of the heteroarene. In addition, parasitic reactions of the ancillary ligand PMe3 were observed when pentafluoropyridine was the substrate.

Investigations via Kinetics and Multivariate Linear Regression Models of the Mechanism and Origins of Regioselectivity in a Palladium-Catalyzed Aryne Annulation

The synthetic potential of unsymmetrically substituted arynes is not yet fully realized due to regioselectivity issues. Although many models exist to predict the regioselectivity of arynes, these models do not hold for metal-mediated reactions. Previously, we reported a way to induce regioselectivity in a metal-catalyzed aryne annulation reaction by using bulky monodentate phosphine ligands. Reported herein is a mechanistic investigation into the operative catalytic cycle within this transformation. Additionally, the molecular parameters responsible for regioselectivity have been examined via LFERs and multivariate linear regression. This model shows the interdependence on both the steric and electronic properties of the aryne and the size of the phosphine ligand for regioselectivity.

Nickel binding enables isolation and reactivity of previously inaccessible 7-aza-2,3-indolynes

N-Heteroaromatics are key elements of pharmaceuticals, agrochemicals, and materials. N-Heteroarynes provide a scaffold to build these essential molecules but are underused because five-membered N-heteroarynes have been largely inaccessible on account of the strain of a triple bond in that small of a ring. On the basis of principles of metal-ligand interactions that are foundational to organometallic chemistry, in this work we report the stabilization of five-membered N-heteroarynes in the nickel coordination sphere. A series of 1,2-bis(dicyclohexylphosphino)ethane nickel 7-azaindol-2,3-yne complexes were synthesized and characterized crystallographically and spectroscopically. Ambiphilic reactivity of the nickel 7-azaindol-2,3-yne complexes was observed with multiple nucleophilic, electrophilic, and enophilic coupling partners.

Insights into the Regioselectivity of Metal-Catalyzed Aryne Reactions

The synthetic potential of unsymmetrically substituted aryne intermediates is significantly hindered by regioselectivity issues. Current methods for inducing regioselectivity all rely on substrate control and are focused on non-metallated arynes. Before our initial disclosure, there was no systematic study regarding the regioselectivity of metal-catalyzed aryne reactions. By exploiting ligand control, we have induced regioselectivity in a palladium-catalyzed aryne annulation to form phenanthridinones (up to 9:91 r.r.). Through this study we have investigated: ligand effects, influence of steric perturbation, and the impact of the aryne precursor.

Ligand-Controlled Regioinduction in a PHOX-Ni Aryne Complex

Phosphinooxazoline (PHOX) ligands have been used to control the regio- and enantioselectivity in a wide variety of metal-catalyzed reactions. Despite their widespread use, PHOX ligands have never been studied in metal-aryne complexes. Herein we report the first example of a PHOX-Ni aryne complex. As demonstrated in other systems, the differentiated P versus N donors and different steric environments of the unsymmetric ligand are able to induce regiocontrol. A 81:19 mixture of o-methoxy substituted aryne complexes is observed. Single-crystal X-ray crystallographic analysis, UV/vis spectroscopy, and cyclic voltammetry are used to gain further insight into the molecular and electronic structure of these complexes. Lastly, a methylation/deuteration sequence shows retention of the PHOX ligand-induced regiocontrol in the difunctionalized products and that the regiospecificity of these difunctionalizations is due to the trans influence of the P donor.