Direct Transformation of Arenols Based on C—O Activation

Nickel-Catalyzed Chemoselective Carbomagnesiation for Atroposelective Ring-Opening Difunctionalization

There is a pressing need for methods that can connect enantioenriched organic compounds with readily accessible building blocks via asymmetric functionalization of unreactive chemical bonds in organic synthesis and medicinal chemistry. Herein, the asymmetric chemoselective cleavage of two unactivated C(Ar)-O bonds in the same molecule is disclosed for the first time through an unusual nickel-catalyzed carbomagnesiation. This reaction facilitates the evolution of a novel atroposelective ring-opening difunctionalization. Utilizing readily available dibenzo bicyclic substrates, diverse valuable axially chiral biaryls are furnished with high efficiencies. Synthetic elaborations showcase the application potential of this method. The features of this method include good atom-economy, multiple roles of the nucleophile, and a simple catalytic system that enables the precise magnesiation of an α-C(Ar)-O bond and arylation of a β-C(Ar)-O bond.

Enantioselective nickel-catalyzed Mizoroki-Heck cyclizations of amide electrophiles

Amide cross-couplings that rely on C-N bond activation by transition metal catalysts have emerged as valuable synthetic tools. Despite numerous discoveries in this field, no catalytic asymmetric variants have been disclosed to date. Herein, we demonstrate the first such transformation, which is the Mizoroki-Heck cyclization of amide substrates using asymmetric nickel catalysis. This proof-of-concept study provides an entryway to complex enantioenriched polycyclic scaffolds and advances the field of amide C-N bond activation chemistry.

Direct Stannylation and Silylation of Arylmethanols by Palladium Catalysis

A direct transformation of non-preactivated benzyl alcohols to benzyl stannanes and benzyl silanes was realized through Pd-catalyzed C(sp³)-O activation process. By using versatile tin and silicon sources, these reactions exhibit a broad substrate scope and a high efficiency under mild conditions, affording functionalized benzyl and allylic stannanes and benzylsilanes with high yields. The successful implementation of gram-scale stannylation/silylation as well as the one-pot Stille coupling reaction demonstrates the potential application of this method in organic synthesis. Both experimental and theoretical investigations reveal the mechanistic details of this reaction.

Catalytic Amination of Phenols with Amines

Given the wide prevalence and ready availability of both phenols and amines, aniline synthesis through direct coupling between these starting materials would be extremely attractive. Herein, we describe a rhodium-catalyzed amination of phenols, which provides concise access to diverse anilines, with water as the sole byproduct. The arenophilic rhodium catalyst facilitates the inherently difficult keto-enol tautomerization of phenols by means of π-coordination, allowing for the subsequent dehydrative condensation with amines. We demonstrate the generality of this redox-neutral catalysis by carrying out reactions of a large array of phenols with various electronic properties and a wide variety of primary and secondary amines. Several examples of late-stage functionalization of structurally complex bioactive molecules, including pharmaceuticals, further illustrate the potential broad utility of the method.

Nickel-Catalyzed Enantioselective Arylative Activation of Aromatic C-O Bond

The pioneering nickel-catalyzed cross-coupling of C-O electrophiles was unlocked by Wenkert in the 1970s; however, the transition-metal-catalyzed asymmetric activation of aromatic C-O bonds has never been reported. Herein the first enantioselective activation of an aromatic C-O bond is demonstrated via the catalytic arylative ring-opening cross-coupling of diarylfurans. This transformation is facilitated via nickel catalysis in the presence of chiral N-heterocyclic carbene ligands, and chiral 2-aryl-2'-hydroxy-1,1'-binaphthyl (ArOBIN) skeletons are delivered axially in high yields with high ee. Moreover, this versatile skeleton can be transformed into various synthetic useful intermediates, chiral catalysts, and ligands by using the CH- and OH-based modifiable sites. This chemistry features mild conditions and good atom economy.

Selective deoxygenative alkylation of alcohols via photocatalytic domino radical fragmentations

The delivery of alkyl radicals through photocatalytic deoxygenation of primary alcohols under mild conditions is a so far unmet challenge. In this report, we present a one-pot strategy for deoxygenative Giese reaction of alcohols with electron-deficient alkenes, by using xanthate salts as alcohol-activating groups for radical generation under visible-light photoredox conditions in the presence of triphenylphosphine. The convenient generation of xanthate salts and high reactivity of sequential C–S/C–O bond homolytic cleavage enable efficient deoxygenation of primary, secondary and tertiary alcohols with diverse functionality and structure to generate the corresponding alkyl radicals, including methyl radical. Moreover, chemoselective radical monodeoxygenation of diols is achieved via selective formation of xanthate salts.

The generation of alkyl radicals through deoxygenation of abundant alcohols via photoredox catalysis is of interest. In this study, the authors report a one-pot strategy for visible-light-promoted photoredox coupling of alcohols with electron-deficient alkenes, assisted by carbon disulfide and triphenylphosphine.

Mechanism and Selectivity Control in Ni- and Pd-Catalyzed Cross-Couplings Involving Carbon-Oxygen Bond Activation

Transition-metal-catalyzed C-O bond activation provides a useful strategy for utilizing alcohol- and phenol-derived electrophiles in cross-coupling reactions, which has become a research field of active and growing interest in organic chemistry. The synergy between computation and experiment elucidated the mechanistic model and controlling factors of selectivities in these transformations, leading to advances in innovative C-O bond activation and functionalization methods.Toward the rational design of C-O bond activation, our collaborations with the Jarvo group bridged the mechanistic models of C(sp²)-O and C(sp³)-O bond activations. We found that the nickel catalyst cleaves the benzylic and allylic C(sp³)-O bonds via two general mechanisms: the stereoinvertive S_N2 back-side attack model and the stereoretentive chelation-assisted model. These two models control the stereochemistry in a wide array of stereospecific Ni-catalyzed cross-coupling reactions with benzylic or allylic alcohol derivatives. Because of the catalyst distortion, the ligands can differentiate the competing stereospecific C(sp³)-O bond activations. The PCy₃ ligand interacts with nickel mainly through σ-donation, and the Ni(PCy₃) catalyst can undergo facile bending of the substrate-nickel-ligand angle, which favors the stereoretentive benzylic C-O bond activation. The N-heterocyclic carbene SIMes ligand has additional d(metal)-p(ligand) back-donation with nickel, which leads to an extra energy penalty for the same angle bending. This results in the preference of stereoinvertive benzylic C-O bond activation under Ni/SIMes catalysis. In addition to ligand control, a Lewis acid can increase the selectivity for stereoinvertive C(sp³)-O activation by stabilizing the S_N2 back-side attack transition state. The oxygen leaving group complexes with the MgI₂ Lewis acid in the stereoinvertive activation, leading to the exclusive stereoinvertive Kumada coupling of benzylic ethers. We also identified that the competing C(sp³)-O bond activation models have noticeable differences in charge separation. This leads to the solvent polarity control of the stereospecificity in C(sp³)-O activations. Low-polarity solvents favor the neutral stereoretentive C-O bond activation, while high-polarity solvents favor the zwitterionic stereoinvertive cleavage.In sharp contrast to the nickel catalysts, the C(sp²)-O bond activation under palladium catalysis mainly proceeds via the classic three-membered ring oxidative addition mechanism instead of the chelation-assisted mechanism. This is due to the lower oxophilicity of palladium, which disfavors the oxygen coordination in the chelation-assisted-type activation. The three-membered ring activation model selectively cleaves the weak C-O bond, resulting in the exclusive chemoselectivity of acyl C-O bond activation in Pd-catalyzed cross-coupling reactions with aryl carboxylic acid derivatives. This explains the overall acylation in the Pd-catalyzed Suzuki-Miyaura coupling with aryl esters. In collaboration with the Szostak group, we revealed that the three-membered ring model applies in the Pd-catalyzed C-O bond activation of carboxylic acid anhydride, which stimulated the development of a series of Pd-catalyzed decarbonylative functionalizations of aryl carboxylic acids.

Carbon dioxide-promoted palladium-catalyzed dehydration of primary allylic alcohols: access to substituted 1,3-dienes

A carbon dioxide promoted dehydration reaction of primary allylic alcohols gives synthetically important substituted 1,3-dienes in good yields under milder conditions compared with the reaction using a heterogeneous catalyst or carbon monoxide.

Nickel-catalyzed Suzuki–Miyaura cross-coupling of C–F bonds

The efficient Suzuki–Miyaura cross-coupling of ortho-fluoro aromatic amides with aryl boronates is described. The reaction proceeds effectively, even at 60 °C. The reaction exhibits a good tolerance for functional groups and a broad scope for aromatic amides.

Nickel-catalyzed C-O/N-H, C-S/N-H, and C-CN/N-H annulation of aromatic amides with alkynes: C-O, C-S, and C-CN activation

The Ni-catalyzed reaction of ortho-phenoxy-substituted aromatic amides with alkynes in the presence of LiO^tBu as a base results in C–O/N–H annulation with the formation of 1(2H)-isoquinolinones. The use of a base is essential for the reaction to proceed. The reaction proceeds, even in the absence of a ligand, and under mild reaction conditions (40 °C). An electron-donating group on the aromatic ring facilitates the reaction. The reaction was also applicable to carbamate (C–O bond activation), methylthio (C–S bond activation), and cyano (C–CN bond activation) groups as leaving groups.

The Ni-catalyzed reaction of ortho-phenoxy-substituted aromatic amides with alkynes in the presence of LiO^tBu as a base results in C–O/N–H annulation with the formation of 1(2H)-isoquinolinones.