Tuning the Electronic Properties of Azophosphines as Ligands and Their Application in Base-Free Transfer Hydrogenation Catalysis.

The design and tuning of new ligands is crucial for unlocking new reactivity at transition metal centers. Azophosphines have recently emerged as a new class of 1,3-P,N ligands in ruthenium piano-stool complexes. This work shows that the azophosphine synthesis can tolerate -aryl substituents with strongly electron-donating and electron-withdrawing -R groups and that the nature of this R group can affect the spectroscopic and structural properties of the azophosphines, as measured by NMR spectroscopy, UV-vis spectroscopy, single-crystal X-ray diffraction, and DFT studies.

Regiodivergent Gold-Catalyzed Rearrangement-Addition Reactions of Sulfenylated Propargylic Carboxylates with Indoles.

Sulfenylated propargylic carboxylates were introduced to investigate the influence of sulfur substitution in gold-catalyzed alkyne activation pathways. Regiodivergent gold-catalyzed rearrangement and indole capture reactions proceed under mild conditions to give functionalized indole products bearing sulfenylated ()-enol carboxylate motifs. Pathways involving both 1,2- and 1,3-carboxylate migrations are achieved selectively, with indole being added in a 1,4 relationship to the sulfenyl group in each case.

Regiodivergent Synthesis of 4- and 5-Sulfenyl Oxazoles from Alkynyl Thioethers.

The regiodivergent synthesis of 4- and 5-sulfenyl oxazoles from 1,4,2-dioxazoles and alkynyl thioethers has been achieved. Gold-catalysed conditions are used to favour the formation of 5-sulfenyl oxazoles via β-selective attack of the nitrenoid relative to the sulfenyl group. In contrast, 4-sulfenyl oxazoles are formed by α-selective reaction under Brønsted acid conditions from the same substrates.

A laterally-fused N-heterocyclic carbene framework from polysubstituted aminoimidazo[5,1-]oxazol-6-ium salts.

A polysubstituted 3-aminoimidazo[5,1-]oxazol-6-ium framework has been accessed from a new nitrenoid reagent by a two-step ynamide annulation and imidazolium ring-formation sequence. Metalation with Au(I), Cu(I) and Ir(I) at the C2 position provides an L-shaped NHC ligand scaffold that has been validated in gold-catalysed alkyne hydration and arylative cyclisation reactions.

Modular synthesis of α-arylated carboxylic acids, esters and amides photocatalyzed triple C-F bond cleavage of methyltrifluorides.

α-Arylated carboxylic acids, esters and amides are widespread motifs in bioactive molecules and important building blocks in chemical synthesis. Thus, straightforward and rapid access to such structures is highly desirable. Here we report an organophotocatalytic multicomponent synthesis of α-arylated carboxylic acids, esters and amides from exhaustive defluorination of α-trifluoromethyl alkenes in the presence of alkyltrifluoroborates, water and nitrogen/oxygen nucleophiles.

Gold-Catalyzed Annulations with Nucleophilic Nitrenoids Enabled by Heteroatom-Substituted Alkynes.

The combination of a nucleophilic nitrene equivalent, a triple bond and a π-acid catalyst has underpinned numerous efficient transformations for the preparation of azacycles. This personal account details our efforts in developing an annulation strategy. Adding a nucleophilic nitrenoid to an activated alkyne can generate carbenoid character that is then quenched by a cyclisation onto the nitrenoid substituent.

A Radical-Initiated Fragmentary Rearrangement Cascade of Ene-Ynamides to [1,2]-Annulated Indoles via Site-Selective Cyclization.

Straightforward access to [1,2]-annulated indoles, key substructures in natural products, is highly desirable yet challenging. Herein, a radical triggered fragmentary cyclization cascade reaction of ene-ynamides is presented, providing a rapid access into [1,2]-annulated indoles by an intermolecular radical addition, intramolecular cyclization, desulfonylative aryl migration, and site-selective C()-N cyclization sequence. DFT calculations support oxidation of N-centered radical species to cations prior to the C-N bond formation, followed by an unusual aza-Nazarov cyclization.

Gold(I)-Catalyzed Synthesis of 3-Sulfenyl Pyrroles and Indoles by a Regioselective Annulation of Alkynyl Thioethers.

The combination of nucleophilic nitrenoids and π-acid catalysis has emerged as a powerful tool in heterocycle synthesis. Accessing more varied heterocycle-substitution patterns by maintaining the same reaction pathways across different alkynes remains a challenge. Here we show that Au(I) catalysis of isoxazole-based nitrenoids with alkynyl thioethers provides controlled access to (3 + 2) annulation by a regioselective addition β to the sulfenyl group.

Gold-Catalyzed Intermolecular Alkyne Oxyarylation for C3 Functionalization of Benzothiophenes.

C3-selective C-C bond formation on benzothiophenes is challenging, and few direct functionalization methods are available. A gold-catalyzed reaction of alkynes with benzothiophene -oxides provides regioselective entry into C3-alkylated benzothiophenes with the C7-alkylated isomer as the minor product. This oxyarylation reaction works with alkyl and aryl alkynes and substituted and unsubstituted benzothiophenes.

A Gold Carbene Manifold to Prepare Fused γ-Lactams by Oxidative Cyclisation of Ynamides.

Gold-catalysed oxidative cyclisation reactions of ynamides offer great promise in γ-lactam synthesis but are limited by preferential over-oxidation to form α-keto imides. Evaluating the factors that might limit N-cyclisation pathways led to effective gold-catalysed conditions that allow access to different fused γ-lactams on changing the ynamide N-substituent and accommodate previously incompatible substitution patterns. New and efficient methods for the synthesis of functionalised 3-aryl indoles and cyclohepta[c]pyrrol-1-one derivatives are presented.

Alkynyl Thioethers in Gold-Catalyzed Annulations To Form Oxazoles.

Non-oxidative, regioselective, and convergent access to densely functionalized oxazoles is realized in a functional-group tolerant manner using alkynyl thioethers. Sulfur-terminated alkynes provide access to reactivity previously requiring strong donor-substituted alkynes such as ynamides. Sulfur does not act in an analogous donor fashion in this gold-catalyzed reaction, thus leading to complementary regioselective outcomes and addressing the limitations of using ynamides.

Alkynyl sulfoxides as α-sulfinyl carbene equivalents: gold-catalysed oxidative cyclopropanation.

Alkynyl sulfoxides are shown to act as α-sulfinyl metallocarbene synthons under oxidative gold catalysis, enabling reactions that are not available from diazo-precursors. This strategy is exemplified in the synthesis of fused α-sulfinyl cyclopropanes.

Divergent C-H Insertion-Cyclization Cascades of N-Allyl Ynamides.

Gold carbene reactivity patterns were accessed by ynamide insertion into a C(sp(3) )H bond. A substantial increase in molecular complexity occurred through the cascade polycyclization of N-allyl ynamides to form fused nitrogen-heterocycle scaffolds. Exquisite selectivity was observed despite several competing pathways in an efficient gold-catalyzed synthesis of densely functionalized C(sp(3) )-rich polycycles and a copper-catalyzed synthesis of fused pyridine derivatives.

Regioselective functionalisation of dibenzothiophenes through gold-catalysed intermolecular alkyne oxyarylation.

A protocol has been developed for direct Csp(3)-Csp(2) bond formation at the 4- and 6-positions of dibenzothiophenes using a gold(I) catalyst with terminal alkynes and dibenzothiophene-S-oxides. The sulfoxide acts as a traceless directing group to avoid the need to prefunctionalise at carbon. The iterative use of this protocol is possible and has been employed in the preparation of novel macrocyclic structures.

A direct route into fused imidazo-diazines and imidazo-pyridines using nucleophilic nitrenoids in a gold-catalyzed formal [3 + 2]-dipolar cycloaddition.

Pyridinium N-(heteroaryl)aminides can be employed as robust and practical synthetic equivalents of nucleophilic 1,3-N,N-dipoles in a formal cycloaddition onto electron-rich alkynes under gold catalysis. Convergent and regioselective access to five types of imidazo-fused heteroaromatics is provided from the appropriate aminide. The efficient transformation accommodates significant structural variation around the aminide, ynamide, or indolyl-alkyne reactants and tolerates sensitive functional groups.

1,2-N-migration in a gold-catalysed synthesis of functionalised indenes by the 1,1-carboalkoxylation of ynamides.

Unique α-hemiaminal ether gold carbene intermediates were accessed by a gold-catalysed 1,1-carboalkoxylation strategy and evolved through a highly selective 1,2-N-migration. This skeletal rearrangement gave functionalised indenes, and isotopic labelling confirmed the rare C-N bond cleavage of the ynamide moiety. The effect of substituents on the migration has been explored, and a model is proposed to rationalise the observed selectivity.

A gold-catalysed fully intermolecular oxidation and sulfur-ylide formation sequence on ynamides.

An efficient C-O, C-S and C-C bond-forming sequence leads to functionalised compounds bearing sulfur-substituted quaternary carbons. Ynamides are employed as diazo-equivalents to access the [2,3]-sigmatropic rearrangements of allyl sulfonium ylides by a three-component chemoselective oxidation and intermolecular ylide formation.

Highly regioselective synthesis of 2,4,5-(hetero)aryl substituted oxazoles by intermolecular [3+2]-cycloaddition of unsymmetrical internal alkynes.

A robust N-nucleophilic 1,3-N,O-dipole equivalent reacts with unsymmetrical internal alkynes under gold catalysis. Conjugation from a remote nitrogen lone pair enables and controls this convergent and highly regioselective process.

Isotopic labelling studies for a gold-catalysed skeletal rearrangement of alkynyl aziridines.

Isotopic labelling studies were performed to probe a proposed 1,2-aryl shift in the gold-catalysed cycloisomerisation of alkynyl aziridines into 2,4-disubstituted pyrroles. Two isotopomers of the expected skeletal rearrangement product were identified using (13)C-labelling and led to a revised mechanism featuring two distinct skeletal rearrangements. The mechanistic proposal has been rationalised against the reaction of a range of (13)C- and deuterium-labelled substrates.

Site-specific introduction of gold-carbenoids by intermolecular oxidation of ynamides or ynol ethers.

Ynamides and ynol ethers undergo intermolecular gold-catalysed reaction with a nucleophilic oxidant to access metal-carbenoid reactivity patterns. A site-specific oxidation/1,2-insertion cascade is used for a general access to functionalised α,β-unsaturated carboxylic acid derivatives and vinylogous carbimates.

Gold-catalysed room-temperature cycloisomerisation of alkynes and unactivated enolisable ketones.

The cycloisomerisation of simple keto-alkynes proceeds at room temperature under the mild conditions of gold catalysis. Bicyclic fused and spiro compounds can be obtained by overall 5-exo and 6-exo carbon-carbon bond-forming cyclisations.