Pd/Xiang-Phos-catalyzed enantioselective intermolecular carboheterofunctionalization of norbornene and norbornadiene

Sadphos as Adaptive Ligands in Asymmetric Palladium Catalysis

ConspectusPalladium catalysis, as one of the most important strategies in asymmetric synthesis, has continuously attracted the attention of organic chemists. With the development of chiral ligands, increasingly challenging reactions and substantial progress in asymmetric catalysis are being realized.Since 2014, we have focused on exploiting a series of sulfinamide phosphine ligands called "Sadphos," including Ming-Phos, Xu-Phos, Xiao-Phos, Xiang-Phos, TY-Phos, PC-Phos, GF-Phos, and WJ-Phos. These ligands can be easily prepared in two to four steps using commercial materials. These new types of ligands have shown remarkable performance in transition-metal-catalyzed reactions, especially in Pd-catalyzed transformations. X-ray diffraction analysis, mechanistic studies, and density functional theory calculations have revealed that Sadphos ligands can coordinate with the Pd0 and PdII species in the Pd0/P, Pd0/P,S, or PdII/P,O modes.This Account summarizes our recent efforts toward palladium-catalyzed enantioselective reactions using Sadphos ligands. These ligands were found to be privileged and very crucial to promote the reactions by increasing the reactivity and enantioselectivity. Ming-Phos is an effective ligand in Pd-catalyzed asymmetric coupling and intramolecular Heck reactions, providing highly enantioselective trisubstituted allenes, axially chiral anilides, gem-diarylmethine silanes, and disubstituted dihydroisoquinolinones. Incorporation of an electron-rich cyclohexyl group in the phosphine moiety afforded Xu-Phos, which showed a unique effect in a series of asymmetric transformations, including reductive Heck, dearomative Mizoroki-Heck, tandem Heck/Suzuki coupling, carboiodination, carboamination, and cross-coupling reactions. Using a similar strategy, our group synthesized more electron-rich TY-Phos and Xiang-Phos ligands bearing t-butyl and 1-adamantyl group at P atoms, respectively. Regarding stereoelectronic features, these two characteristic ligands were the best choice to satisfy the requirements of the palladium-catalyzed fluoroarylation of gem-difluoroalkenes, intermolecular α-arylation of aldehydes, carboetherification of alkenyl oximes, and carboheterofunctionalization of 2,3-dihydrofurans. Compared with the aforementioned Sadphos ligands, the attractive features of Xiao-Phos, including high nucleophilicity originating from the CH2PPh2 group and the ortho-substituent effect at the side of the aryl ring, are presumably responsible for its efficiency. The Pd/Xiao-Phos catalyst system shows good performance in a series of cross-coupling reactions of secondary phosphine oxides, affording P-stereogenic products bearing multiple types of molecular skeletons. The modification of the basic Sadphos backbone by introducing a xanthene skeleton motivated us to design and synthesize monophosphines, named PC-Phos and GF-Phos. PC-Phos is effective in various reactions, including arylation of sulfenate anions, denitrogenative cyclization of benzotriazoles, and dearomatization of indoles. The practicability of GF-Phos was validated in the Pd-catalyzed asymmetric three-component coupling of N-tosylhydrazones, aryl halides, and terminal alkynes, as well as in the cross-coupling of N-tosylhydrazones and vinyl iodides with pendent amines. In addition, ferrocene-derived WJ-Phos was employed in the palladium-catalyzed Suzuki-Miyaura cross-coupling reaction, affording axially chiral biaryl monophosphine oxides in excellent enantiomeric excesses.

Palladium/TY-Phos-Catalyzed Asymmetric Heck/Tsuji-Trost Reaction of o-Bromophenols with 1,3-Dienes

2,3-Dihydrobenzofurans are crucial building blocks in the synthesis of natural products and pharmaceutical molecules. However, their asymmetric synthesis has been a long-standing formidable challenge so far. In this work, we developed a highly enantioselective Pd/TY-Phos-catalyzed Heck/Tsuji-Trost reaction of o-bromophenols with various 1,3-dienes, allowing expedient access to chiral substituted 2,3-dihydrobenzofurans. This reaction features excellent regio- and enantiocontrol, high functional group tolerance, and easy scalability. More importantly, the demonstration of this method as a highly valuable tool for the construction of optically pure natural products (R)-tremetone and fomannoxin is highlighted.

Manipulations of phenylnorbornyl palladium species for multicomponent construction of a bridged polycyclic privileged scaffold

Hexahydromethanocarbazole is a privileged scaffold in the discovery of new drugs and photoactive organic materials due to its good balance between structural complexity and minimized entropy penalty upon receptor binding. To address the difficulty of synthesizing this highly desirable bridged polycyclic scaffold, we designed a convenient multicomponent reaction cascade as intercepted Heck addition/C-H activation/C-palladacycle formation/electrophilic attack of ANP/N-palladacycle formation/Buchwald amination. A distinguishing feature of this sophisticated strategy is the successive generation of two key phenylnorbornyl palladium species to control the reaction flow towards desired products. DFT calculations further reveal the crucial roles of Cs₂CO₃ and 5,6-diester substitutions on the norbornene reactant in preventing multiple side-reactions. This innovative method exhibits a broad scope with good yields, and therefore will enable the construction of natural-product-like compound libraries based on hexahydromethanocarbazole.

Synthesis of W-Phos Ligand and Its Application in the Copper-Catalyzed Enantioselective Addition of Linear Grignard Reagents to Ketones

The asymmetric catalytic addition of linear Grignard reagents to ketones has been a long-standing challenge in organic synthesis. Herein, a novel family of PNP ligands (W-Phos) was designed and applied in copper-catalyzed asymmetric addition of linear Grignard reagents to aryl alkyl ketones, allowing facile access to versatile chiral tertiary alcohols in good to high yields with excellent enantioselectivities (up to 94 % yield, 96 % ee). The process can also be used to synthesize chiral allylic tertiary alcohols from more challenging α,β-unsaturated ketones. Notably, the potential utility of this method is demonstrated in the gram-scale synthesis and modification of various densely functionalized medicinally relevant molecules.

Synthesis of dihydropyrazoles enabled by Pd-catalyzed carboamination of alkenyl hydrazones with alkenyl and aryl halides

The synthesis of dihydropyrazoles enabled by Pd-catalyzed carboamination of alkenyl hydrazones with alkenyl or aryl halides is described. This method provides a practical and efficient route towards the synthesis of functionalized dihydropyrazoles.

Palladium-Catalyzed Synthesis of Tricyclic Indoles via a N-S Bond Cleavage Strategy

In palladium/norbornene (Pd/NBE) chemistry, the "ortho effect" has been proven to be a key factor in the process of β-carbon elimination to extrude NBE. Herein, we found that the o-iodoaniline protected by a p-methoxybenzenesulfonyl group can recover the "ortho effect" and then undergo N-S bond cleavage with vinyl palladium, thus achieving a highly selective C-N coupling reaction in the Catallani-Lautens reaction system. On the basis of this discovery, a one-step synthesis of highly functionalized tricyclic indole derivatives was realized.

Palladium-Catalyzed Asymmetric Tandem Denitrogenative Heck/Tsuji-Trost of Benzotriazoles with 1,3-Dienes

The asymmetric denitrogenative cycloaddition has emerged as a powerful tool to build chiral aza-heterocyles. However, only one example of asymmetric denitrogenative cycloaddition of benzotriazole with unsaturated hydrocarbons has been explored so far, because the ring-opening of benzotriazole to generate α-imino metal carbenoid species is a thermodynamically unfavorable process. We herein report an efficient asymmetric denitrogenative cycloaddition of benzotriazoles with cyclic and acyclic 1,3-dienes enabled by Pd and chiral sulfonamide phosphine ligand. A variety of substituted hexahydrocarbazoles and indolines were delivered in good yields with high ee values. Interestingly, a pair of enantiomers could be obtained with the use of Xu1 and PC2 with the same absolute configuration. The synthetic utilities of the optically active hexahydrocarbazoles were also showcased.

Recent advances in palladium-catalyzed (hetero)annulation of C[double bond, length as m-dash]C bonds with ambiphilic organo(pseudo)halides

Palladium has proven to be effective in catalyzing the (hetero)annulation of C[double bond, length as m-dash]C bonds with ambiphilic organo(pseudo)halides. Through the employment of appropriate ambiphilic coupling partners, efficient annulation of a variety of allenes, 1,3-dienes, strained alkenes, styrenes, and other C[double bond, length as m-dash]C bond variants can be achieved to provide direct access to numerous useful hetero- and carbocyclic scaffolds. In this Feature Article, we summarize palladium-catalyzed (hetero)annulation methods reported since 2005 (spanning just over 15 years) and discuss outstanding challenges in this area of study.

Metal-Free C-C/C-N/C-C Bond Formation Cascade for the Synthesis of (Trifluoromethyl)sulfonylated Cyclopenta[b]indolines

A bis(triflyl)ethylation [triflyl = (trifluoromethyl)sulfonyl] inserted into a sequential cyclization cascade resulted in the direct formation of gem-bis(triflyl)ated cyclopenta[b]indolines from anilide-derived allenols and alkenols. This catalyst- and irradiation-free sequence facilitated the efficient preparation of functionalized tricyclic indoline cores bearing two contiguous stereocenters. The formed cyclopenta[b]indolines can be easily transformed into a wide variety of triflylated indolines, including the tetracycle ring system found in polyveoline.

Anti-selective [3+2] (Hetero)annulation of non-conjugated alkenes via directed nucleopalladation

2,3-Dihydrobenzofurans and indolines are common substructures in medicines and natural products. Herein, we describe a method that enables direct access to these core structures from non-conjugated alkenyl amides and ortho-iodoanilines/phenols. Under palladium(II) catalysis this [3 + 2] heteroannulation proceeds in an anti-selective fashion and tolerates a wide variety of functional groups. N-Acetyl, -tosyl, and -alkyl substituted ortho-iodoanilines, as well as free -NH2 variants, are all effective. Preliminary results with carbon-based coupling partners also demonstrate the viability of forming indane core structures using this approach. Experimental and computational studies on reactions with phenols support a mechanism involving turnover-limiting, endergonic directed oxypalladation, followed by intramolecular oxidative addition and reductive elimination.