Diastereoselective Pd(II)-Catalyzed sp3 C–H Arylation Followed by Ring Opening of Cyclopropanecarboxamides: Construction of anti β-Acyloxy Carboxamide Derivatives

Construction of Biaryl Sulfonamides via Pd(II)-Catalyzed Cross-Coupling of C(sp2)-H Bonds with Iodobenzenesulfonamides

This study describes the utility of Pd(II)-catalyzed C-H arylation of benzamides for constructing biaryl sulfonamides. Sulfonamides are known for their promising applications in pharmaceuticals and agrochemicals. A literature review revealed that biaryl sulfonamides were generally constructed via the traditional cross-coupling reactions. We report a progressive method for obtaining biaryl sulfonamides via the Pd(II)-catalyzed bidentate directing group (8-aminoquinoline or picolinamide)-assisted cross-coupling of sp2 C-H bonds of aromatic carboxamides with iodobenzenesulfonamides. After the C-H arylation reactions, we attempted the removal of the 8-aminoquinoline from the synthesized biaryl scaffolds possessing the carboxamide and sulfonamide moieties using triflic acid. In some cases, we observed the occurrence of decarboxylation and Friedel-Crafts acylation, affording interesting aromatic scaffolds possessing the sulfonamide moiety. The current work contributes toward developing alternative ways for assembling various biaryl sulfonamides.

Pd(II)-catalyzed coupling of C-H bonds of carboxamides with iodoazobenzenes toward modified azobenzenes

In this paper, we report a synthetic protocol for the construction of biaryl motif-based or π-extended azobenzene and alkylated azobenzene derivatives via the Pd(II)-catalyzed bidentate directing group (DG)-aided C-H activation and functionalization strategy. In the past, the synthesis of biaryl motif-based azobenzenes was accomplished through the traditional cross-coupling reaction involving organometallic reagents and aryl halides or equivalent coupling partners. We have shown the direct coupling of C-H bonds of aromatic/aliphatic carboxamides (possessing a DG) with iodoazobenzenes as the coupling partners through the Pd(II)-catalyzed bidentate DG-aided, site-selective C-H functionalization method. Azobenzene-containing compounds are a versatile class of photo-responsive molecules that have found applications across branches of chemical, biological and materials sciences and are prevalent in medicinally relevant molecules. Accordingly, the synthesis of new and functionalized azobenzene-based scaffolds has been an attractive topic of research. Although the classical methods are efficient, they need pre-functionalized starting materials. This protocol involving the Pd(II)-catalyzed, directing group-aided site-selective C-H arylation of aromatic and aliphatic carboxamides using iodoazobenzene as the coupling partner affording azobenzene-based carboxamides is an additional route and also a contribution towards enriching the library of modified azobenzenes. We have also shown the photoswitching properties of representative compounds synthesized via the Pd(II)-catalyzed directing group-aided site-selective C-H functionalization method.

Pd(II)-catalyzed selective β-C-H functionalization of azobenzene carboxamides

We report a Pd(II)-catalyzed bidentate directing group 8-aminoquinoline-aided, site-selective β-C-H functionalization protocol for assembling modified azobenzene carboxamides. Considering the importance of azobenzenes in chemical sciences, this paper reports a new route for enriching the library of modified azobenzene motifs.

Construction of carbazole-based unnatural amino acid scaffolds via Pd(II)-catalyzed C(sp3)-H functionalization

We report the synthesis of carbazole-based unnatural α-amino acid and non-α-amino acid derivatives via a Pd(II)-catalyzed bidentate directing group 8-aminoquinoline-aided β-C(sp³)-H activation/functionalization method. Various N-phthaloyl, DL-, L- and D-carboxamides derived from their corresponding α-amino acids, non-α-amino acids and aliphatic carboxamides were subjected to the β-C(sp³)-H functionalization with 3-iodocarbazoles in the presence of a Pd(II) catalyst to afford the corresponding carbazole moiety installed unnatural amino acid derivatives and aliphatic carboxamides. Carbazole motif-containing racemic (DL) and enantiopure (L and D) amino acid derivatives including phenylalanine, norvaline, leucine, norleucine and 2-aminooctanoic acid with anti-stereochemistry and various non-α-amino acid derivatives including GABA have been synthesized. Removal of the 8-aminoquinoline directing group, deprotection of the phthalimide moiety and the preparation of carbazole amino acid derivatives containing free amino- and carboxylate groups are shown. The carbazole motif is prevalent in alkaloids and biologically active molecules and functional materials. Thus, this work on the synthesis of carbazole-based unnatural amino acid derivatives would enrich the libraries of unnatural amino acid derivatives and carbazoles.

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.

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.

Harnessing the Efficacy of 2-Pyridone Ligands for Pd-Catalyzed (β/γ)-C(sp3)-H Activations

Mechanisms of palladium-aminooxyacetic acid and 2-pyridone-enabled cooperative catalysis for the β- and γ-C(sp³)-H functionalizations of ketones are investigated with density functional theory. 2-Pyridone-assisted dissociation of the trimeric palladium acetate [Pd₃(OAc)₆] is found to be crucial for these catalytic pathways. The evolution of the [6,6]-membered palladacycles (Int-4) are elucidated and are active complexes in Pd(II/IV) catalytic cycles. Nevertheless, 2-pyridone acts as an external ligand, which accelerates β-C(sp³)-H activation. Computational investigations suggest that the C(sp³)-H bond activation is the rate-limiting step for both the catalytic processes. To overcome the kinetic inertness, an unsubstituted aminooxyacetic acid auxiliary is used for the β-C(sp³)-H activation pathway to favor the formation of the [5,6]-membered palladacycle intermediate, Int-IV. Among the several modeled ligands, 3-nitro-5-((trifluoromethyl)sulfonyl)pyridine-2(1H)-one (L8) is found to be highly valuable for both the (β/γ)-C(sp³)-H functionalization catalytic cycles. A favorable free energy pathway of late-stage functionalization of (R)-muscone paves the path to design other bioactive molecules.

Bidentate Directing Groups: An Efficient Tool in C-H Bond Functionalization Chemistry for the Expedient Construction of C-C Bonds

During the past decades, synthetic organic chemistry discovered that directing group assisted C-H activation is a key tool for the expedient and siteselective construction of C-C bonds. Among the various directing group strategies, bidentate directing groups are now recognized as one of the most efficient devices for the selective functionalization of certain positions due to fact that its metal center permits fine, tunable, and reversible coordination. The family of bidentate directing groups permit various types of assistance to be achieved, such as N,N-dentate, N,O-dentate, and N,S-dentate auxiliaries, which are categorized based on the coordination site. In this review, we broadly discuss various C-H bond functionalization reactions for the formation of C-C bonds with the aid of bidentate directing groups.

Mono-selective β-C–H arylation of N-methylated amino acids and peptides promoted by the 2-(methylthio)aniline directing group

2-(Methylthio)aniline (MTA) directed C(sp³)–H functionalisations are efficient and straightforward protocols for the selective β-modification of N-methylated amino acids.

A comprehensive overview of directing groups applied in metal-catalysed C-H functionalisation chemistry

The present review is devoted to summarizing the recent advances (2015-2017) in the field of metal-catalysed group-directed C-H functionalisation. In order to clearly showcase the molecular diversity that can now be accessed by means of directed C-H functionalisation, the whole is organized following the directing groups installed on a substrate. Its aim is to be a comprehensive reference work, where a specific directing group can be easily found, together with the transformations which have been carried out with it. Hence, the primary format of this review is schemes accompanied with a concise explanatory text, in which the directing groups are ordered in sections according to their chemical structure. The schemes feature typical substrates used, the products obtained as well as the required reaction conditions. Importantly, each example is commented on with respect to the most important positive features and drawbacks, on aspects such as selectivity, substrate scope, reaction conditions, directing group removal, and greenness. The targeted readership are both experts in the field of C-H functionalisation chemistry (to provide a comprehensive overview of the progress made in the last years) and, even more so, all organic chemists who want to introduce the C-H functionalisation way of thinking for a design of straightforward, efficient and step-economic synthetic routes towards molecules of interest to them. Accordingly, this review should be of particular interest also for scientists from industrial R&D sector. Hence, the overall goal of this review is to promote the application of C-H functionalisation reactions outside the research groups dedicated to method development and establishing it as a valuable reaction archetype in contemporary R&D, comparable to the role cross-coupling reactions play to date.

Combining transition metals and transient directing groups for C–H functionalizations

In the domain of synthetic chemistry, C–H bond activation has always remained in the spotlight for researchers over the last few decades. Although different strategies have been employed to chemically trigger unactivated C–H bonds, transition metal catalyzed directing group (DG) aided C–H bond activation is the most explored pathway of all because of its ability to perform diverse site selective functional metamorphosis. Despite its popularity, tedious synthetic methodology requiring additional steps for the installation and removal of DGs from the target substrate diminishes its efficacy. However, replacement of directing groups by transient directing groups (tDGs) reduces the hurdle to a greater extent without compromising the product yield and selectivity. In this report we have depicted the intense journey of transient directing groups with three (Rh, Ru, and Pd) prevalent second row transition metals.

This review is about the momentous evolution of transient directing group assisted C–H functionalizations promoted by three major second row transition metals.

Pd-Catalyzed C(sp2)–H aminocarbonylation using the Langlois reagent as a carbonyl source

An efficient Pd-catalyzed C(sp²)–H aminocarbonylation of aryl carboxamides assisted by an N,S-bidentate directing group was developed.

Diastereoselective Synthesis of Spirocyclopropanes under Mild Conditions via Formal [2 + 1] Cycloadditions Using 2,3-Dioxo-4-benzylidene-pyrrolidines

A highly diastereoselective cyclopropanation of cyclic enones with sulfur ylides was developed under catalyst-free conditions, producing multifunctional spirocyclopropanes in generally excellent yields (up to 99% yield and >99:1 d.r.). The asymmetric version of this method was realized by using an easily available chiral sulfur ylide, affording products with moderate to good stereoselectivity.