Modular bismacycles for the selective C–H arylation of phenols and naphthols

Given the important role played by 2-hydroxybiaryls in organic, medicinal and materials chemistry, concise methods for the synthesis of this common motif are extremely valuable. In seeking to extend the synthetic chemists’ lexicon in this regard, we have developed an expedient and general strategy for the ortho-arylation of phenols and naphthols using readily-available boronic acids. Our methodology relies on in situ generation of a uniquely reactive Bi(V) arylating agent from a bench stable Bi(III) precursor via telescoped B-to-Bi transmetallation and oxidation. By exploiting reactivity 2 that is orthogonal to conventional metal-catalyzed manifolds, diverse aryl and heteroaryl partners can be rapidly coupled to phenols and naphthols under mild conditions. Following arylation, highyielding recovery of the Bi(III) precursor allows for its efficient re-use in subsequent reactions. Mechanistic interrogation of each key step of the methodology informs its practical application and provides fundamental insight into the under-exploited reactivity of organobismuth compounds

Organobismuth Redox Manifolds: Versatile Tools for Synthesis

Bismuth’s ability to manoeuvre between oxidation states supports several distinct reaction manifolds. Recent advances in the design, synthesis and application of organobismuth reagents and catalysts illustrate the potential of these redox manifolds as complementary tools to conventional transition metal-based synthesis strategies

Bismuth(V)-Mediated C–H Arylation of Phenols and Naphthols

We recently reported a general and practical strategy for the Bi(V)-mediated C–H arylation of phenols and naphthols. Our telescoped protocol proceeds via transmetallation from readily available arylboronic acids to a stable Bi(III) precursor, oxidation to a reactive Bi(V) intermediate, and subsequent ortho-selective phenol arylation. The process exhibits broad scope with respect to both components and tolerates functionality that is incompatible with conventional cross-coupling methods. Preliminary investigations provide insight into the mechanism of each key reaction step

Development and Scale-Up of a New Sulfone-Based Bismacycle as a Universal Precursor for Bi(V)-Mediated Electrophilic Arylation

The scope and practical utility of bismuth(V)-mediated electrophilic arylation have been greatly improved by the recent development of user-friendly protocols based on modular bismacycle reagents. Here, we report the scalable synthesis of a new bench-stable bismacycle bromide and demonstrate that it can be used as a “universal precursor” in electrophilic arylation. Relative to established syntheses of related bismacycles, the new protocol benefits from improved step- and vessel-economy, reduced production time, and the complete elimination of cryogenic temperatures and undesirable solvents (Et2O and CH2Cl2). The synthesis is complemented by a robust, chromatography-free purification procedure that was developed by using design of experiments. We show that this process is highly reproducible at the 100 mmol scale, with two independent experiments giving 61 and 62% yields of isolated material. We anticipate that this efficient method for the synthesis of a new bismacycle precursor will expedite both (a) wider uptake of existing bismuth-mediated arylation methods by the synthetic community and (b) ongoing efforts to develop new bismuth-mediated transformations

Expanding Ligand Space: Preparation, Characterization, and Synthetic Applications of Air-Stable, Odorless Di-tert-alkylphosphine Surrogates

The di-tert-alkylphosphino motif is common to many best-in-class ligands for late-transition-metal catalysis. However, the structural diversity of these privileged substructures is currently limited by the need to manipulate highly toxic, highly reactive reagents and intermediates in their synthesis. In response to this longstanding challenge, we report an umpolung strategy for the synthesis of structurally diverse di-tert-alkylphosphine building blocks via SN1 alkylation of in situ generated PH3 gas. We show that the products—which are isolated as air-stable, odorless phosphonium salts—can be used directly in the preparation of key synthetic intermediates and ligand classes. The di-tert-alkylphosphino building blocks that are accessible using our methodology therefore enable facile expansion of extant ligand classes by modification of a previously invariant vector; we demonstrate that these modifications affect the steric and electronic properties of the ligands and can be used to tune their performance in catalysis

Modular Synthesis of α,α-Diaryl α-Amino Esters via Bi(V)-Mediated Arylation/SN2-Displacement of Kukhtin–Ramirez Intermediates

We report a concise and modular approach to α,α-diaryl α-amino esters from readily available α-keto esters. This mild, one-pot protocol proceeds via ketone umpolung, with in situ formation of a Kukhtin–Ramirez intermediate preceding sequential electrophilic arylation by Bi(V) and SN2 displacement by an amine. The methodology is compatible with a wide range of anilines and primary amines - including derivatives of drugs and proteinogenic amino acids - Bi(V) arylating agents, and α-keto ester substrates

Au-catalyzed biaryl coupling to generate 5- to 9-membered rings: turnover-limiting reductive elimination versus π-complexation

The intramolecular gold–catalyzed arylation of arenes by aryltrimethylsilanes has been investigated from both a mechanistic and preparative aspect. The reaction generates five to nine membered rings, and of the 44 examples studied, ten include a heteroatom (N, O). The tethering of the arene to the arylsilane not only provides a tool to probe the impact of the conforma-tional flexibility of Ar–Au–Ar intermediates, via systematic modulation of the length of aryl-aryl linkage, but also the ability to arylate neutral and electron-poor arenes - substrates that do not react at all in the intermolecular process. Rendering the arylation intramolecular also results in phenomenologically simpler reaction kinetics, and overall these features have facili-tated a detailed study of linear free energy relationships, kinetic isotope effects, and the first quantitative experimental data on the effects of aryl electron-demand and conformational freedom on the rate of reductive elimination from diaryl gold(III) species. The turnover-limiting step for the formation of a series of fluorene derivatives is sensitive to the electronics of the arene and changes from reductive elimination to π-complexation for arenes bearing strongly electron-withdrawing substitu-ents (σ >0.43). Reductive elimination is accelerated by electron-donating substituents (□ = -2.0) on one or both rings, with the individual σ-values being additive in nature. Longer and more flexible tethers between the two aryl rings results in faster reductive elimination from Ar-Au(X)-Ar, and to the π-complexation of the arene by Ar-AuX2 becoming the turnover-limiting step

Synthesis of Highly Functionalized Bismacycles via Post-Transmetallation Modification of Arylboronic Acids

Bismacycles featuring a sulfone-bridged scaffold have recently been developed as versatile and convenient electrophilic arylating agents. Here, we report that the exocyclic aryl group, which is ultimately transferred to a nucleophilic coupling partner, can be functionalized through cross-coupling, heteroatom substitutions, oxidations and reductions, and protecting group manipulations. This “postsynthetic modification” approach provides concise and divergent access to complex aryl bismacycles. The utility of the functionalized bismacycles in electrophilic arylation of C–H and O–H bonds is demonstrated

Synthesis of Highly Functionalized Bismacycles via Post-Transmetallation Modification of Aryboronic Acids

Bismacycles featuring a sulfone-bridged scaffold have recently been developed as versatile and convenient electrophilic arylating agents. Here, we report that the exocyclic aryl group, which is ultimately transferred to a nucleophilic coupling partner, can be functionalized through cross-coupling, heteroatom substitutions, oxidations and reductions, and protecting group manipulations. This “postsynthetic modification” approach provides concise and divergent access to complex aryl bismacycles. The utility of the functionalized bismacycles in electrophilic arylation of C−H and O−H bonds is demonstrated

Coaxial Dielectric Spectroscopy as an In-Line Process Analytical Technique for Reaction Monitoring

The suitability of broadband dielectric spectroscopy (DS) as a tool for in-line (in situ) reaction monitoring is demonstrated. Using the esterification of 4-nitrophenol as a test-case, we show that multivariate analysis of time-resolved DS data-collected across a wide frequency range with a coaxial dip-probe-allows reaction progress to be measured with both high precision and high accuracy. In addition to the workflows for data collection and analysis, we also establish a convenient method for rapidly assessing the applicability of DS to previously untested reactions or processes. We envisage that, given its orthogonality to other spectroscopic methods, its low cost, and its ease of implementation, DS will be a valuable addition to the process chemist's analytical toolbox