Ni-Catalyzed C(sp3)-O Arylation of α-Hydroxy Esters

An SN1-Approach to Cross-Coupling: Deoxygenative Arylation Facilitated by the β-Silicon Effect

We report a dual metal-catalyzed method for the cross-coupling of unprotected alcohols by exploiting the β-Si effect. This deoxygenative Suzuki-Miyaura reaction tolerates a range of primary, secondary, and tertiary alcohol substrates along with diverse functional groups and heterocycles. Mechanistic experiments including KIE, VTNA, and Eyring analyses suggest the existence of a carbocation intermediate on the reaction pathway, consistent with a rare SN1 pathway for the activation of an electrophile in cross-coupling reactions. A novel bis-imidazolium N-heterocyclic carbene (NHC) ligand was found to be optimal for reactivity, and nickel(0)-, nickel(I)- and nickel(II)- complexes of this ligand were isolated and characterized. In contrast to more well-established shorter chain ligands, these long-chain NHCs are found to have characteristically large bite angles, which may be critical for enabling the deoxygenative arylation of aliphatic alcohols.

A Dual Ni/Photoredox Cross-Coupling Approach toward Mandelic Acids

Mandelic acid derivatives represent a valuable class of compounds due to their wide use in synthetic organic chemistry and the pharmaceutical sector. Herein, we report a novel reductive Ni/photoredox cross-coupling of readily accessible, bench stable N-alkoxyphthalimides and aryl halides to prepare unprotected mandelic acid ester derivatives. Mechanistic experiments suggest that this cross-coupling likely proceeds via a pathway that is distinct from previous reports using similar redox-active alkoxy radical precursors.

Photocatalytic O- to S-Rearrangement of Tertiary Cyclopropanols

Despite the importance of heteroatom-substituted cyclopropane derivatives in drug design and organic synthesis, cyclopropanethiols remain critically underexplored. Inspired by the wide use of the Newman-Kwart rearrangement to access valuable thiophenols from phenol feedstocks, we report the development of a photocatalytic approach for efficient ambient temperature aliphatic O- to S-rearrangement on tertiary cyclopropanol derivatives. After demonstrating that a range of cyclopropanethiols-that are difficult to access by other methods-can be obtained with this strategy, we show that these rearranged products can be easily hydrolyzed and further derivatized. We conclude this study with mechanistic findings that enabled an initial extension of this approach toward other classes of aliphatic alcohols.

Redox-Active Thiocarbonyl Auxiliaries in Ni-Catalyzed Cross-Couplings of Aliphatic Alcohols

ConspectusThe Barton-McCombie deoxygenation reaction first established the use of O-alkyl thiocarbonyl derivatives as powerful redox-active agents for C(sp3)-O reduction. In recent years, first-row transition metals capable of engaging with alkyl radical intermediates generated from O-alkyl thiocarbonyl derivatives using alternative stoichiometric radical precursors have been developed. Given the ability of select Ni catalysts to both participate in single-electron oxidative addition pathways and intercept alkyl radical intermediates, our group has investigated the use of O-alkyl thiocarbonyl derivatives as electrophiles in novel cross-coupling reactions. After describing related work in this area, this Account will first summarize our entry point into this field. Here, we used the cyclopropane ring as a reporter of leaving group reactivity to aid in the design and optimization of a novel redox-active O-thiocarbamate leaving group for C(sp3)-O arylation. Motivation for this pursuit was driven by the propensity of the cyclopropane ring to undergo ring opening under polar (2e) oxidative addition pathways or to be maintained under single-electron (1e) conditions. Using these guiding principles, we developed a method for the deoxygenative arylation of cyclopropanol derivatives using a Ni catalyst without the need for a stoichiometric external reductant or photocatalyst. We next summarize our evaluation of an alternative redox-active O-thiocarbonyl imidazole auxiliary in a related deoxygenative cross-coupling. This work demonstrated an extension of our initial approach to the deoxygenative arylation of primary and secondary aliphatic alcohol derivatives. A brief mechanistic investigation revealed that this reaction likely proceeds via a distinct mechanism involving direct homolytic C(sp3)-O bond cleavage. We conclude this Account with a summary of work aimed toward a unique approach for thiocarboxylic acid derivative synthesis. This project was inspired by the efficiency of thionoester generation under most of the reaction conditions evaluated in our prior investigations. Using alcohol, amine, or thiol starting materials, which were activated with convenient thiocarbonyl sources in a single step, we optimized for a Ni-catalyzed cross-coupling capable of providing access to a range of thionoester, thioamide, or dithioester products. In summary, our work has revealed the potential of redox-active thiocarbonyl auxiliaries in Ni-catalyzed cross-couplings with C(sp3)-O electrophiles. We anticipate that the continued investigation of aliphatic thiocarbonyl derivatives as radical precursors with alternative single-electron inputs will be an area of continued growth in the years to come.

Palladium-catalyzed coupling of amides and cyclopropanols for the synthesis of γ-diketones

A palladium catalytic method has been developed for the coupling of amides and cyclopropanols to γ-diketones, through simultaneous C-N and C-C activation. Heteroatom ligand exchange and heteroatom-to-carbon ligation mode switching enable the achievement of molecular cross-coupling in an amide N-atom structural context-dependent manner, avoiding any stoichiometric organometallic reagent or base.

Dehydroxylative Alkylation of α-Hydroxy Carboxylic Acid Derivatives via a Spin-Center Shift

A strategically distinct dehydroxylative alkylation reaction of α-hydroxy carboxylic acid derivatives with alkenes is developed. The reaction starts with the attack of a 4-dimethylaminopyridine (DMAP)-boryl radical to the carbonyl oxygen atom, followed by a spin-center shift (SCS) to trigger the C-O bond scission. The resulting α-carbonyl radicals couple with a wide range of alkenes to furnish various alkylated products. This strategy allows for the efficient conversion of a wide array of α-hydroxy amides and esters derived from several biomass molecules and natural products to value-added compounds. Experimental and computational studies verified the reaction mechanism.

Cucurbit[6]uril supported β-Ni(OH)2 nanoparticles as a heterogeneous catalyst for the synthesis of quinazolines via acceptorless dehydrogenative coupling of alcohols with nitriles

Synthesis of a series of quinazolines using β-Ni(OH)2-CB[6] as a heterogeneous nanocatalyst.