Selective Cross-Dehydrogenative C(sp3)-H Arylation with Arenes

Pd/Chiral Phosphoric Acid-Enabled Asymmetric Intramolecular Double C-H Activation Reaction for the Synthesis of P-Stereogenic Benzophosphole Oxides

A Pd-catalyzed intramolecular asymmetric double C-H activation reaction was reported, and a BINOL skeleton-based chiral phosphoric acid (CPA) was identified as a suitable ligand for this transformation. A broad range of P-stereogenic benzophosphole oxides were synthesized in moderate to excellent yield with high levels of enantioselectivity (≤96% ee).

Pd(II)-Catalyzed Asymmetric [2+2] Annulation for the Construction of Chiral Benzocyclobutenes

Asymmetric de novo synthesis of benzocyclobutenes (BCBs) via catalytic intermolecular reaction is highly desired for efficient access to this important class of compounds, yet such a strategy remains unmet challenge. Here, we report a Pd/Pyrox-catalyzed asymmetric [2+2] annulation between arylboronic acids and functionalized alkenes, providing an unprecedented efficient protocol to access various enantio-enriched BCBs in a modular and versatile manner under mild conditions. A broad substrate scope with excellent enantioselectivity has been achieved under the current protocol. The isolation and characterization of the key chiral palladacycle intermediate, together with DFT calculations, provides strong evidence for the catalytic pathway including an enantiodetermining arylpalladation step.

Palladium-Catalyzed [3 + 2] Annulation of Aromatic Amides with Maleimides through Dual C-H Activation

A palladium-catalyzed [3 + 2] annulation of substituted aromatic amides with maleimides providing tricyclic heterocyclic molecules in good to moderate yields through weak carbonyl chelation is reported. The reaction proceeds via a dual C-H bond activation where the first C-H activation takes place selectively at the benzylic position followed by a second C-H bond activation at the meta position to afford a five-membered cyclic ring. An external ligand Ac-Gly-OH has been used to succeed in this protocol. A plausible reaction mechanism has been proposed for the [3 + 2] annulation reaction.

Rh(III)-catalyzed [4 + 1] cyclization of aryl substituted pyrazoles with cyclopropanols via C-H activation

A rhodium-catalyzed formal [4 + 1]-cyclization reaction of aryl substituted pyrazoles with cyclopropanols via C-H bond activation/cyclization processes to selectively construct a series of carbonyl functionalized pyrazolo[5,1-a]isoindoles is described. The reaction features good functional group compatibility and a broad substrate scope with respect to both cyclization components with up to 84% yields. Mechanistic studies indicated that the C-H cleavage might be the rate-determining step in this transformation.

Palladium-catalyzed cross-coupling of unreactive C(sp3)-H bonds with azole C(sp2)-H bonds by using bromide as a traceless directing group

A palladium-catalyzed intermolecular cross-coupling of unreactive C(sp³)-H bonds and azole C(sp²)-H bonds with bromide as a traceless directing group is described. The judicious selection of the bulky and electron-rich phosphine ligand is the key for the success of this cascade process. The protocol features a broad substrate scope, excellent regioselectivity, and good functional group tolerance.

Access to β-Alkylated γ-Functionalized Ketones via Conjugate Additions to Arylideneisoxazol-5-ones and Mo(CO)6-Mediated Reductive Cascade Reactions

1,4-Conjugate addition of ((chloromethyl)sulfonyl)benzenes to arylideneisoxazol-5-ones, followed by one-pot, N-selective trapping in the presence of electrophiles, was investigated. This strategy led to the synthesis of new, stable N-protected isoxazol-5-ones in good yields and high diastereolectivity. The study of the reactivity of obtained products in the presence of the Mo(CO)₆/H₂O system allowed the development of a cascade reaction leading to novel methyl ketones in high yields and unchanged dr bearing an uncommon chloromethinearylsulfonyl end group.

Transition-Metal-Catalyzed, Coordination-Assisted Functionalization of Nonactivated C(sp3)-H Bonds

Transition-metal-catalyzed, coordination-assisted C(sp³)-H functionalization has revolutionized synthetic planning over the past few decades as the use of these directing groups has allowed for increased access to many strategic positions in organic molecules. Nonetheless, several challenges remain preeminent, such as the requirement for high temperatures, the difficulty in removing or converting directing groups, and, although many metals provide some reactivity, the difficulty in employing metals outside of palladium. This review aims to give a comprehensive overview of coordination-assisted, transition-metal-catalyzed, direct functionalization of nonactivated C(sp³)-H bonds by covering the literature since 2004 in order to demonstrate the current state-of-the-art methods as well as the current limitations. For clarity, this review has been divided into nine sections by the transition metal catalyst with subdivisions by the type of bond formation. Synthetic applications and reaction mechanism are discussed where appropriate.

Recent progress in the oxidative coupling of unactivated Csp3-H bonds with other C-H bonds

The oxidative coupling between two carbon-hydrogen (C-H) bonds offers the most straightforward pathway to construct C-C bonds from hydrocarbons without pre-functionalization, exhibiting high step- and atom-economy. This article features the recent advances in the oxidative coupling of unactivated Csp³-H with different hybrid C-H bonds, including Csp-H, Csp²-H and Csp³-H bonds. The substrate scope, proposed mechanism and application of these reactions are summarized, intending to provide insights toward developing novel and efficient protocols for Csp³-C bond construction.

Transition metal catalyzed C-H bond activation by exo-metallacycle intermediates

exo-Metallacycles have become the key reaction intermediates in activating various remote C(sp²)-H and C(sp³)-H bonds in the past decade and aided in achieving unusual site-selectivity. Various novel exo-chelating auxiliaries have assisted metals to reach desired remote C-H bonds of different alcohol and amine-derived substrates. As a result, a wide range of organic transformations of C-H bonds like halogenation, acetoxylation, amidation, sulfonylation, olefination, acylation, arylation, etc. were accessible using the exo-metallacycle strategy. In this review, we have summarized the developments in C-H bond activation via four-, five-, six-, seven- and eight-membered exo-metallacycles and the key reaction intermediates, including the mechanistic aspects, are discussed concisely.

Palladium-Catalyzed Intramolecular Cross-Coupling of Unactivated C(sp3)-H and C(sp2)-H Bonds

Direct C-H/C-H coupling represents an appealing method for the construction of C-C bonds, and the cross-coupling of unactivated C(sp³)-H and C(sp²)-H bonds is challenging and remains to be investigated. We have developed the Pd-catalyzed intramolecular coupling of inert C(sp³)-H and C(sp²)-H bonds. The reaction proceeded by o-methyl oxime-directed aryl C(sp²)-H activation and subsequent alkyl C(sp³)-H cleavage, generating C(sp²),C(sp³)-palladacycles as the key intermediates. Dihydrobenzofurans and indanes were formed as the final products.

Pd-catalysed β-selective C(sp3)-H arylation of simple amides

An efficient Pd-catalysed β-C(sp³)-H arylation of diverse native amides with aryl iodides was developed. This protocol overcomes the necessity of the Thorpe-Ingold effect and features broad substrate scope and good functional group tolerance. The potential application of this protocol is collectively demonstrated by gram-scale synthesis and the synthesis of several bioactive molecules.

Intramolecular Oxidative Coupling between Unactivated Aliphatic C-H and Aryl C-H Bonds

Direct oxidative coupling of different inert C-H bonds is the most straightforward and environmentally benign method to construct C-C bonds. In this paper, we developed a Pd-catalyzed intramolecular oxidative coupling between unactivated aliphatic and aryl C-H bonds. This chemistry showed great potential to build up fused cyclic scaffolds from linear substrates through oxidative couplings. Privileged chromane and tetralin scaffolds were constructed from readily available linear starting materials in the absence of any organohalides and organometallic partners.

Rapid Construction of Tetralin, Chromane, and Indane Motifs via Cyclative C-H/C-H Coupling: Four-Step Total Synthesis of (±)-Russujaponol F

The development of practical C-H/C-H coupling reactions remains a challenging yet appealing synthetic venture because it circumvents the need to prefunctionalize both coupling partners for the generation of C-C bonds. Herein we report a cyclative C(sp³)-H/C(sp²)-H coupling reaction of free aliphatic acids enabled by a cyclopentane-based mono-N-protected β-amino acid ligand. This reaction uses inexpensive sodium percarbonate (Na₂CO₃·1.5H₂O₂) as the sole oxidant and generates water as the only byproduct. A range of biologically important scaffolds, including tetralins, chromanes, and indanes, can be easily prepared by this protocol. Finally, the synthetic application of this methodology is demonstrated by the concise total synthesis of (±)-russujaponol F in a four-step sequence starting from readily available phenylacetic acid and pivalic acid through sequential functionalizations of four C-H bonds.

C(sp3)-H Monoarylation of Methanol Enabled by a Bidentate Auxiliary

With the assistance of a practical directing group (COAQ), the first catalytic protocol for the palladium-catalyzed C(sp³)-H monoarylation of methanol has been developed, offering an invaluable synthesis means to establish extensive derivatives of crucial arylmethanol functional fragments. Furthermore, the gram-scale reaction, broad substrate scope, excellent functional group compatibility, and even the practical synthesis of medicines further demonstrate the usefulness of this strategy.

Copper-catalyzed oxidative benzylic C(sp3)–H amination: direct synthesis of benzylic carbamates

A Cu(i)–diimine ligand combined with a N–F source allows the C–H abstraction and incorporation of a carbamate functional group in the hydrocarbons at the benzylic position.