Direct β-Alkenylation of Ketones via Pd-Catalyzed Redox Cascade

Pd(II)-catalyzed cyclization of 2-methyl aromatic ketones with maleimides through weak chelation assisted dual C-H activation

A palladium-catalyzed dual C-H functionalization of substituted aromatic ketones and ester with maleimides leading to tricyclic heterocyclic molecules with good to excellent yield is reported. In this protocol, weak chelation of the carbonyl groups has been successfully utilized for the selective activation of the ortho-methyl C(sp3)-H bond instead of the ortho-C(sp2)-H bond in the presence of an external bidentate ligand Ac-Ile-OH. The reaction proceeds through two-fold C-H activation to generate a five-membered cyclic ring. The first C-H activation takes place selectively at the benzylic position followed by a second C-H bond activation at the meta position. The protocol demonstrates compatibility among diverse substituted aromatic ketones and ester as well as various substituted maleimides. Further derivatization of the tricyclic ketone to an alcohol exhibits the synthetic applicability of the protocol. Also, a plausible reaction mechanism has been proposed.

Palladium-Catalyzed β-C(sp3)-H Arylation of Silyl Prop-1-en-1-ol Ethers with Aryl Halides: Entry to α,β-Unsaturated Ketones

A palladium(0)-catalyzed β-C(sp3)-H arylation of silyl prop-1-en-1-ol ethers with aryl halides for the synthesis of α,β-unsaturated ketones is presented. In contrast to the reported β-C(sp3)-H arylation of ketones, the chemoselectivity of this current method relies on the Pd(0) catalytic systems and reaction temperatures: While using the Pd(dba)2/DavePhos/KF system at 80 °C resulted in β-C(sp3)-H monoarylation to produce β-monoarylated α,β-unsaturated ketones, harnessing the Pd(OAc)2/t-Bu XPhos/K2HPO4 system at 110 °C induced β-C(sp3)-H diarylation to afford β,β-diarylated α,β-unsaturated ketones. The method provides a versatile route that uses readily available ketone-derivatized α-nonsubstituted silyl prop-1-en-1-ol ethers as the alkene sources and is characterized by a good functional group compatibility, a broad substrate scope, and an excellent selectivity.

Ru(OAc)3-Catalyzed Regioselective Difunctionalization of Alkynes: Access to (E)-2-Bromo-1-alkenyl Sulfonates

A new approach is proposed for the divergent and regioselective synthesis of (E)-2-bromo-1-phenylvinyl trifluoromethanesulfonates through alkyne difunctionalization by employing a compatible system of abundantly available alkynes, N-bromosuccinimide (NBS), and trimethylsilyl trifluoromethanesulfonate (TMSOTf) catalyzed by ruthenium(III) acetate [Ru(OAc)3]. It is a novel method for the preparation of vinyl triflate and it offers a fundamental basis for the development of advanced functional compounds, including drugs and organic functional materials. Unlike previously reported methods, the proposed protocol can tolerate a broad range of functional groups. Alkynes derived from bioactive molecules, such as l(-)-borneol, demonstrate the potential value of this new reaction in organic synthesis.

Site-Selective Ligand Bridging among Multiple Internal Coordination Sites of a Metallomacrocycle and Its Conformational Regulation

Metallomacrocycles with internal coordination sites have a high potential to precisely control the positions of the guest ligands and the overall shape of the assemblies by utilizing the directionality and reversibility of the coordination bonds. However, when such coordinative hosts possess multiple coordination sites, it was difficult to control to which coordination sites the incoming guest ligands bind, because such systems often result in a random, uncontrolled mixture. The metallomacrocycle that we now report, a hexanuclear palladium complex of hexapap possessing six internal coordination sites, can take two different conformations depending on the guests. One is an Alternate conformation, in which six coordination sites of pap alternatively point to Up-Down-Up-Down-Up-Down. The other is a Twisted conformation, in which the coordination sites direct Up-Middle-Down-Up-Middle-Down. Interestingly, linear ditopic α,ω-diamines are captured in three distinct cross-linking modes, and regulations between the two macrocyclic conformations have been realized by the lengths of the diamines. Furthermore, the heteroleptic site-selective bridging of two kinds of diamines has been achieved. It has been demonstrated that a slight difference in the diamine lengths leads to a significant change in the structure and selection of the produced host-guest macrocyclic complexes.

Direct synthesis of alkylated 4-hydroxycoumarin derivatives via a cascade Cu-catalyzed dehydrogenation/conjugate addition sequence

An efficient approach for the direct synthesis of alkylated 4-hydroxycoumarin derivatives via a Cu-catalyzed cascade dehydrogenation/conjugate addition sequence starting from simple saturated ketones and 4-hydroxycoumarins has been developed. This protocol features excellent functional-group tolerance, easy scale-up, and a broad substrate scope including bioactive molecules. More importantly, a series of marketed drugs, such as warfarin, acenocoumarol, coumachlor, and coumafuryl, can be obtained by this method.

Iridium-Catalyzed Intramolecular β-C-H Alkenylation of Ketones with Alkynes via a Hydride-Transfer Approach

Direct functionalization of carbonyl β C-H bonds without using directing groups has not been a trivial task, and it is even more challenging to realize the corresponding atom-economical transformations with common alkenes or alkynes as the coupling partner. Here, we describe the development of an iridium-catalyzed intramolecular direct β-alkenylation of ketones with regular alkynes. The reaction is redox neutral, avoids strong acids or bases, and tolerates various functional groups. The combined experimental and computational mechanistic studies reveal a hydride-transfer pathway, involving ketone α,β-desaturation, iridium-hydride-mediated alkyne insertion, conjugate addition, and α-protonation.

Photoredox/nickel dual catalyzed stereospecific synthesis of distal alkenyl ketones

The selective C-C bond deconstruction/refunctionalization via a photoredox/nickel dual-catalyzed hydroalkylation of alkynes is developed under mild reaction conditions. In this protocol, a broad range of alkyl- and aryl-alkynes could react smoothly with cycloalkanols, affording the corresponding distal and site-specific vinyl-substituted ketones with high yields and excellent regioselectivities. Moreover, DFT calculations verified that the electron-rich behavior of aromatics and weak Brønsted bases have a common effect on the photocatalytic oxidant ring-opening of cyclobutanols.

Cu-catalyzed coupling of indanone oxime acetates with thiols to 2,3-difunctionalized indenones

A Cu-catalyzed coupling reaction of indanone oxime acetates with thiols has been developed for the synthesis of 2,3-functionalized 1-indenones. This protocol has several features including easy mild reaction conditions, stabilized enamine products, good tolerance of functional groups, and no external oxidants. This reaction enables direct derivatization on the indanone ring to provide valuable functionalized indenones at room temperature.

Ru-Catalyzed (E)-Specific ortho-C-H Alkenylation of Arenecarboxylic Acids by Coupling with Alkenyl Bromides

In the presence of [p-cymene)RuCl₂]₂, (E)-configured alkenyl bromides couple with aromatic carboxylates to form ortho-vinylbenzoic acids. This C-H vinylation proceeds in high yields without any activating phosphine ligands and has an excellent functional group tolerance. Starting from commonly available (E/Z )-mixtures of alkenyl bromides, (E)-configured vinyl arenes or dienes are formed exclusively. Mechanistic studies show that this selectivity is achieved because the (E)-configured alkenyl bromides undergo a smooth coupling, whereas the (Z)-isomers are rapidly eliminated with the formation of alkynes.

Dehydrogenative Pd and Ni Catalysis for Total Synthesis

The development of novel synthetic methods remains a cornerstone in simplifying complex molecule synthesis. Progress in the field of transition metal catalysis has enabled new mechanistic strategies to achieve difficult chemical transformations, increased the value of abundant chemical building blocks, and pushed the boundaries of creative and strategic route design to improve step economy in multistep synthesis. Methodologies to introduce an olefin into saturated molecules continue to be essential transformations because of the plethora of reactions available for alkene functionalization. Of particular importance are dehydrogenation reactions adjacent to electron-withdrawing groups such as carbonyls, which advantageously provide activated olefins that can be regioselectively manipulated. Palladium catalysis occupies a central role in the most widely adopted carbonyl dehydrogenation reactions, but limits to the scope of these protocols persist.In this Account, we describe our group's contributions to the area of transition-metal-catalyzed dehydrogenation using palladium catalysis and more sustainable and economical nickel catalysis. These metals are used in conjunction with allyl and aryl halides or pseudohalides that serve as oxidants to access a unique mechanistic approach for one-step α,β-dehydrogenation of various electron-withdrawing groups, including ketones, esters, nitriles, amides, carboxylic acids, and electron-deficient heteroarenes. The pivotal reaction parameters that can be modified to influence reaction efficiency are highlighted, including base and oxidant structure as well as ligand and salt additive effects. This discussion is expected to serve as a guide for troubleshooting challenging dehydrogenation reactions and provide insight for future reaction development in this area.In addition to enabling dehydrogenation reactions, our group's allyl-Pd and -Ni chemistry can be used for C-C and C-X bond-forming reactions, providing novel disconnections with practical applications for expediting multistep synthesis. These transformations include a telescoped process for ketone α,β-vicinal difunctionalization; an oxidative enone β-functionalization, including β-stannylation, β-silylation, and β-alkylation; and an oxidative cycloalkenylation between unstabilized ketone enolates and unactivated alkenes. These bond-forming methodologies broaden the range of transformations accessible from abundant ketone, enone, and alkene moieties. Both the dehydrogenation and C-C and C-X bond-forming methodologies have been implemented in our group's total synthesis campaigns to provide step-efficient synthetic routes toward diverse natural products.Through the lens of multistep synthesis, the utility and robustness of our dehydrogenation and dehydrogenative functionalization methodologies can be better appreciated, and we hope that this Account will inspire practitioners to apply our methodologies to their own synthetic challenges.

Intramolecular β-Alkenylation of Cyclohexanones via Pd-Catalyzed Desaturation-Mediated C(sp3)-H/Alkyne Coupling

Site-selective C-C bond formation through the direct coupling of C(sp³)-H bonds with unsaturated hydrocarbons represents an atom-economical and redox-neutral way to functionalize chemically inert positions, such as those β to a carbonyl group. While most existing β-functionalization methods utilize a directing group (DG) strategy, here we report a Pd-catalyzed intramolecular β-alkenylation of ketones using alkynes as the coupling partner without the aid of DGs. Mediated by a ketone desaturation process, the reaction is redox-neutral and avoids using strong acids or bases. The resulting cis-5,6-fused bicycles can be diversely derivatized with excellent selectivity. Mechanistic studies imply an unusual "hydride-transfer" chain-like pathway, which involves the cyclometalation of an enyne intermediate and protonation of the resulting Pd enolate followed by an intermolecular hydride transfer through the desaturation of another substrate.

Palladium-Catalyzed β-Arylation of Cyclic α,β-Unsaturated O-Methyl Oximes with Aryl Iodides

We report a Pd-catalyzed β-arylation of cyclic α,β-unsaturated O-methyl oximes with aryl iodides. This reaction shows complete regioselectivity and excellent functional group tolerance. β-Arylation of 2-cyclohexen-1-one O-methyl oxime (existing as 2 : 1 E/Z mixture) with certain aryl iodides such as 4-iodoanisole affords only β-arylated (E)-O-methyl oximes.