Xinjun Luan

RuII-Catalyzed Vinylative Dearomatization of Naphthols via a C(sp2)–H Bond Activation Approach

Intermolecular annulation reactions of 1-aryl-2-naphthols with internal alkynes proceed efficiently in the presence of a Ru catalyst and a Cu oxidant to generate spirocyclic compounds by sequential cleavage of the C(sp(2))-H bond, migratory insertion of the alkyne, and dearomatization of the naphthyl ring. Various spirocyclic molecules bearing an all-carbon quaternary stereocenter could be obtained by this novel method with good yields and excellent regioselectivity, and the current process tolerates a variety of synthetically important functional groups.

A chiral bis-sulfoxide ligand in late-transition metal catalysis; rhodium-catalyzed asymmetric addition of arylboronic acids to electron-deficient olefins.

A bis-sulfoxide with a binaphthyl backbone is introduced as a readily available, chiral ligand entity in late-transition metal catalysis. Ligand p-tol-BINASO [where p-tol-BINASO is 1,1‘-binaphthalene-2,2‘-diyl-bis-(p-tolylsulfoxide)] is obtained in pure form in one single synthetic step from relatively cheap, commercially available starting materials. Precatalyst [{(P,R,R)-p-tol-BINASO}RhCl]2 was synthesized in high yield and structurally characterized by X-ray diffraction, and structural data were compared to the free ligand. The precatalyst shows both excellent reactivity and selectivity in the asymmetric 1,4-addition of arylboronic acids to cyclic α,β-unsaturated ketones and esters.

Contemporaneous Dual Catalysis by Coupling Highly Transient Nucleophilic and Electrophilic Intermediates Generated in Situ

We report herein a new process, which we call contemporaneous dual catalysis, that selectively couples two highly reactive catalytic intermediates while overcoming competing trapping by stoichiometric reactive species also present in the reaction. The reaction proceeds via the convergence of a vanadium-catalyzed propargylic rearrangement and a palladium-catalyzed allylic alkylation. It generates a variety of α-allylated α,β-unsaturated ketones, which are not readily accessible by other means. Notably, this dual catalysis is achieved using low catalyst loadings (1.0 mol % [Pd], 1.5 mol % [V]) and gives good to excellent yields (up to 98%) of the desired products.

Identification and Characterization of a New Family of Catalytically Highly Active Imidazolin-2-ylidenes

A new class of easily accessible and stable imidazolin-2-ylidenes has been synthesized where the side chains are comprised of substituted naphthyl units. Introduction of the naphthyl groups generates C 2 -symmetric ( rac) and C s- symmetric ( meso) atropisomers, and interconversion between the isomers is studied in detail both experimentally and computationally. Complete characterization of the carbenes includes rare examples of crystallographically characterized saturated NHC structures. Steric properties of the ligands and an investigation of their stability are also presented. In catalysis, the new ligands show versatility comparable to the most widely used NHCs IMes/SIMes or IPr/SIPr. Excellent catalytic results are obtained when either the NHC salts (ring-opening alkylation of epoxides), NHC-modified palladium compounds (C-C and C-N cross-couplings), or NHC-ruthenium complexes (ring-closing metathesis, RCM) are employed. In several cases, this new ligand family provides catalytic systems of higher reactivity than that observed with previously reported NHC compounds.

Matching the Chirality of Monodentate N-heterocyclic Carbene Ligands: A Case Study on Well-Defined Palladium Complexes for the Asymmetric α-Arylation of Amides

N-Heterocyclic carbene ligands derived from C(2)-symmetric diamines with naphthyl side chains are introduced as chiral monodentate ligands, and their palladium complexes (NHC)Pd(cin)Cl are prepared. These compounds exist as a mixture of diastereomers, and the palladium complexes can be successfully separated and their absolute stereochemistry assigned. When used in the asymmetric intramolecular alpha-arylation of amides, oxindoles with quaternary carbon centers can be obtained in high yield and selectivity when correctly matching the chirality of the NHC complexes.

Highly Chemo- and Enantioselective Synthesis of 3-Allyl-3-aryl Oxindoles via the Direct Palladium-Catalyzed α-Arylation of Amides

A new NHC x Pd-catalyzed asymmetric alpha-arylation of amides is reported that gives direct access to synthetically valuable, allylated oxindoles with quaternary carbon centers. The reaction is made possible by the introduction of a new chiral NHC ligand. The palladium complexes derived therefrom combine excellent reactivity with high chemo- and enantioselectivity for the title transformation.

Contemporaneous Dual Catalysis: Chemoselective Cross-Coupling of Catalytic Vanadium–Allenoate and π-Allylpalladium Intermediates

This paper presents a detailed investigation of a dual catalytic system that combines a vanadium-catalyzed Meyer-Schuster rearrangement and a palladium-catalyzed allylic alkylation. The implementation of this novel reaction relies on matching the formation rates of vanadium-allenoate and π-allylpalladium intermediates with their bimolecular coupling rate in order to minimize the undesired protonation or O-alkylation of the catalytically generated intermediates. Chemoselectivity in this dual catalytic process was successfully achieved by adjusting ligand structure and catalyst loading ratios of the vanadium and palladium catalysts. A great range of coupling partners for both the propargyl alcohol and allyl carbonate components are readily accommodated in this new transformation, which in turn provides a novel avenue to a variety of α-allylated α,β-unsaturated ketones, esters, and amides in moderate to excellent isolated yields.

Impact of NHC ligand conformation and solvent concentration on the ruthenium-catalyzed ring-closing metathesis reaction

Two saturated N-heterocyclic carbene ligands with substituted naphthyl side chains were used for the preparation of Blechert-type ruthenium metathesis precatalysts. The resulting conformers of the complexes were separated and unambiguously assigned by X-ray diffraction studies. All new complexes were compared in terms of activity to the original, SIMes-derived Blechert catalyst and were shown to be superior. A study on the impact of solvent concentration in RCM reactions using the most active of these new catalysts ultimately led to the ring closing of a variety of substrates at very low catalyst loadings.

Ru(II)-catalyzed oxidative spiroannulation of 2-arylphenols with alkynes via a C-H activation/dearomatization strategy.

An intermolecular spiroannulation reaction of appropriately substituted 2-arylphenols with internal alkynes has been developed by using a Ru(II) catalyst and an oxidant. This transformation was realized by a phenol-directed C-H activation, migratory insertion of the alkyne, and subsequent dearomatization of the phenolic ring, providing a broad range of highly functionalized spirocyclic compounds in moderate yields with high regioselectivity.

Palladium(II)-Catalyzed Oxidative Dearomatization of Free Naphthols with Two Alkyne Units

Readily available 2-naphthols undergo [2 + 2 + 1] spiroannulation reactions with alkynes in the presence of a Pd(II) catalyst and an oxidant. This process relies on C-H functionalization and naphthyl ring dearomatization at the 1-position of 2-naphthols to provide a variety of spirocyclic compounds. Using alkyl-aryl alkynes as the coupling partners led to regioisomeric mixtures in favor of the head-to-tail isomer bearing a quaternary carbon stereocenter.