Asymmetric Synthesis of γ-Lactams Containing α,β-Contiguous Stereocenters via Pd(II)-Catalyzed Cascade Methylene C(sp3)–H Alkenylation/Aza-Wacker Cyclization

Pd(II)-Catalyzed Desymmetrizing gem-Dimethyl C(sp3)-H Alkenylation/Aza-Wacker Cyclization Directed by PIP Auxiliary

Desymmetrization of gem-dimethyl groups has been developed as an efficient pathway to achieve asymmetric C(sp3)-H functionalization. Herein, we described a Pd(II)-catalyzed desymmetrizing gem-dimethyl C(sp3)-H alkenylation/aza-Wacker cyclization directed by a bidentate 2-pyridinylisopropyl auxiliary. Chiral α-methyl γ-lactams were obtained in good yields (up to 82%) and high enantioselectivities (up to 91.5% ee).

Recent advances in Pd-catalyzed asymmetric cyclization reactions

Over the past few decades, there have been major developments in transition metal-catalyzed asymmetric cyclization reactions, enabling the convenient access to a wide spectrum of structurally diverse chiral carbo- and hetero-cycles, common skeletons found in fine chemicals, natural products, pharmaceuticals, agrochemicals, and materials. In particular, a plethora of enantioselective cyclization reactions have been promoted by chiral palladium catalysts owing to their outstanding features. This review aims to collect the latest advancements in enantioselective palladium-catalyzed cyclization reactions over the past eleven years, and it is organized into thirteen sections depending on the different types of transformations involved.

Asymmetric Construction of Multifunctional γ-Lactams from 1,3-Dienes and α-Ketoamides via Pd(0)-π-Lewis Base Catalysis

A straightforward approach for the asymmetric synthesis of multifunctionalized γ-lactams, including those bearing two tetrasubstituted stereogenic centers, has been developed through a palladium-catalyzed vinylogous addition/allylic amination process between 1,3-dienes and α-ketoamides. This protocol features advantages of ready substrate availability, broad applicability, high efficiency, and excellent stereoselectivity, making it an attractive complementary tool to the previous strategies.

Palladium-catalyzed intramolecular aza-Wacker-type cyclization of vinyl cyclopropanecarboxamides to access conformationally restricted aza[3.1.0]bicycles

A palladium(ii)-catalyzed intramolecular oxidative aza-Wacker-type reaction of vinyl cyclopropanecarboxamides to access a series of conformationally restricted highly substituted aza[3.1.0]bicycles is reported. The transformation proceeded through a typical aza-Wacker reaction mechanism to forge a new C-N bond with oxygen as the terminal oxidant. The desired fused heterocycles were obtained in moderate yields. The process is tolerant of a range of functional aryl groups under mild conditions.

Copper-Catalyzed Diastereo- and Enantioselective Decarboxylative [3 + 2] Cyclization of Alkyne-Substituted Cyclic Carbamates with Azlactones: Access to γ-Butyrolactams Bearing Two Vicinal Tetrasubstituted Carbon Stereocenters

A copper-catalyzed diastereo- and enantioselective decarboxylative [3 + 2] cyclization reaction of alkyne-substituted cyclic carbamates with azlactones has been established. A range of optically pure γ-butyrolactams bearing two vicinal tetrasubstituted carbon stereocenters were obtained in high yields with good to excellent stereoselectivities (up to 99% yield, 99:1 dr, and 99% ee). This is the first example of asymmetric synthesis γ-butyrolactams containing sterically congested vicinal tetrasubstituted stereocenters via a decarboxylative cyclization pathway.

Diastereoselective palladium-catalyzed functionalization of prochiral C(sp3)-H bonds of aliphatic and alicyclic compounds

We highlight the reported developments of the palladium-catalyzed C-H activation and functionalization of the inactive/unreactive prochiral C(sp³)-H bonds of aliphatic and alicyclic compounds. There exist numerous classical methods for generating contiguous stereogenic centers in a compound with a high degree of stereocontrol. Along similar lines, the Pd(II)-catalyzed, directing group-aided functionalization of inactive prochiral/diastereotopic C(sp³)-H bonds have been exploited to accomplish the stereoselective construction of stereo-arrays in organic compounds. We present a concise discussion on how specific strategies consisting of Pd(II)-catalyzed, directing group-aided C(sp³)-H functionalization have been utilized to generate two or more stereogenic centers in aliphatic and alicyclic compounds.

Diastereoselective Addition of Prochiral Nucleophilic Alkenes to α-Chiral N-Sulfonyl Imines

The Lewis-acid-promoted addition of prochiral E- and Z-allyl nucleophiles to chiral α-alkoxy N-tosyl imines is described. Alkene geometry is selectively transferred to the newly formed carbon-carbon bond, resulting in stereochemical control of C1, C2, and C3 of the resulting 2-alkoxy-3-N-tosyl-4-alkyl-5-hexene products. A computational analysis to elucidate the high selectivity is also presented. This methodology was employed in the synthesis of two naturally occurring isomers of clausenamide.

Coordination-assisted, transition-metal-catalyzed enantioselective desymmetric C–H functionalization

Recent advances in transition-metal-catalyzed enantioselective desymmetric C–H functionalization are summarized.

Transition-Metal-Catalyzed Annulations Involving the Activation of C(sp3 )-H Bonds

The selective functionalization of C(sp³)−H bonds using transition‐metal catalysis is among the more attractive transformations of modern synthetic chemistry. In addition to its inherent atom economy, such reactions open unconventional retrosynthetic pathways that can streamline synthetic processes. However, the activation of intrinsically inert C(sp³)−H bonds, and the selection among very similar C−H bonds, represent highly challenging goals. In recent years there has been notable progress tackling these issues, especially with regard to the development of intermolecular reactions entailing the formation of C−C and C−heteroatom bonds. Conversely, the assembly of cyclic products from simple acyclic precursors using metal‐catalyzed C(sp³)−H bond activations has been less explored. Only recently has the number of reports on such annulations started to grow. Herein we give an overview of some of the more relevant advances in this exciting topic.

Radical Cyclization of 1,n-Enynes and 1,n-Dienes for the Synthesis of 2-Pyrrolidone

2-Pyrrolidones have aroused enormous interest as a useful structural moiety in drug discovery; however, not only does their syntheses suffer from low selectivity and yield, but also it requires high catalyst loadings. The radical cyclization of 1,n-enynes and 1,n-dienes has demonstrated to be an attractive method for the synthesis of 2-pyrrolidones due to its mild reaction conditions, fewer steps, higher atom economy, excellent functional group compatibility, and high regioselectivity. Furthermore, radical receptors with unsaturated bonds (i. e. 1,n-enynes and 1,n-dienes) play a crucial role in realizing radical cyclization because of the ability to selectively introduce one or more radical sources. In this review, we discuss representative examples of methods involving the radical cyclization of 1,n-enynes and 1,n-dienes published in the last five years and discuss each prominent reaction design and mechanism, providing favorable tools for the synthesis of valuable 2-pyrrolidone for a variety of applications.

2-(Pyridin-2-yl)isopropyl (PIP) Amine: An Enabling Directing Group for Divergent and Asymmetric Functionalization of Unactivated Methylene C(sp3)-H Bonds

Directing group (DG) assistance provides a good solution to the problems of reactivity and selectivity, two of the fundamental challenges in C(sp³)-H activation. However, the activation of unbiased methylene C(sp³)-H bonds remains challenging due to the high heterolytic bond dissociation energy and substantial steric hindrance. Two main strategies have been developed thus far, that is, use of a strongly coordinating bidentate DG pioneered by Daugulis and use of a weakly coordinating monodentate DG accelerated by pyridine-type ligands, as disclosed by Yu. The seminal work by Daugulis sparked significant interest in the application of the monoanionic bidentate auxiliary in aliphatic C-H activation reactions. Our research has focused on enabling the divergent functionalization and enantiotopic differentiation of unactivated methylene C-H bonds. Inspired by the structure of bidentate 8-aminoquinoline and the accelerating effect of the gem-dimethyl moiety in cyclometalations, we developed a strongly coordinating bidentate 2-(pyridine-yl)isopropyl (PIP) amine DG consisting of a pyridyl group, a gem-dimethyl moiety, and an amino group, which enabled the divergent functionalization of unactivated β-methylene C(sp³)-H bonds to forge C-O, C-N, C-C, and C-F bonds with palladium catalysts. The exclusive β-selectivity was ascribed to the preferential formation of kinetically favored [5,5]-bicyclic palladacycle intermediates. DFT calculations revealed that the well-designed gem-dimethyl group was responsible for the lowered energy and compressed bite angle of the key transition state related to C-H cleavage.More recently, the combination of PIP amine with axially chiral ligands was found to promote asymmetric functionalization of unbiased methylene C(sp³)-H bonds, a challenging research topic in the area of C-H activation that remains to be addressed. Two different types of axially chiral ligands, namely, non-C₂-symmetric chiral phosphoric acids (CPAs) and 3,3'-disubstituted BINOLs, have been developed. The former enabled Pd(II)-catalyzed inter- and intramolecular arylation of unbiased methylene C(sp³)-H bonds with high enantioselectivity, whereas the latter promoted a series of asymmetric functionalization reactions, such as alkynylation, arylation, alkenylation/aza-Wacker cyclization, and intramolecular amidation. The unexpectedly high stereocontrol compared with other bidentate DGs might be attributable to steric communication between the ligand and gem-dimethyl moiety of PIP amine. Thus far, the combination of PIP amine DG with 3,3'-disubstituted BINOL ligands is arguably the most general strategy for asymmetric functionalization of unbiased methylene C(sp³)-H bonds. Finally, the ease of installation and removal of PIP under mild conditions and synthetic applications are described.

Pd(II)-Catalyzed enantioselective arylation of unbiased methylene C(sp3)-H bonds enabled by a 3,3'-F2-BINOL ligand

Palladium-catalyzed asymmetric functionalization of unbiased methylene C(sp3)-H bonds is a long-standing challenge. Here, we report a Pd(ii)-catalyzed highly enantioselective arylation of unbiased methylene C(sp3)-H bonds enabled by a strongly coordinating bidentate 2-pyridinylisopropyl (PIP) directing group and an easily accessible 3,3'-F2-BINOL chiral ligand. The use of aryl iodides with the combination of 3,3'-F2-BINOL was beneficial for high enantiocontrol. A range of aliphatic amides and aryl iodides were tolerated, providing the desired arylated products in high enantioselectivities (up to 96% ee). The PIP directing group could be removed under mild conditions without erosion of enantiopurity.