Palladium-Catalyzed Ortho-Alkoxylation of N-Benzoyl α-Amino Acid Derivatives at Room Temperature

Alcohols as Substrates in Transition-Metal-Catalyzed Arylation, Alkylation, and Related Reactions

Alcohols are abundant and attractive feedstock molecules for organic synthesis. Many methods for their functionalization require them to first be converted into a more activated derivative, while recent years have seen a vast increase in the number of complexity-building transformations that directly harness unprotected alcohols. This Review discusses how transition metal catalysis can be used toward this goal. These transformations are broadly classified into three categories. Deoxygenative functionalizations, representing derivatization of the C-O bond, enable the alcohol to act as a leaving group toward the formation of new C-C bonds. Etherifications, characterized by derivatization of the O-H bond, represent classical reactivity that has been modernized to include mild reaction conditions, diverse reaction partners, and high selectivities. Lastly, chain functionalization reactions are described, wherein the alcohol group acts as a mediator in formal C-H functionalization reactions of the alkyl backbone. Each of these three classes of transformation will be discussed in context of intermolecular arylation, alkylation, and related reactions, illustrating how catalysis can enable alcohols to be directly harnessed for organic synthesis.

Benzoyl Valine Quasiracemates: Pairing CF3 Quasienantiomers with H to t-Butyl

Understanding the interplay of structural features responsible for molecular assembly is essential for molecular crystal engineering. When assembling molecules with encoded motifs, first choice supramolecular strategies almost always include robust directional nonbonded contacts. Quasiracemic materials, considered near racemates since cocrystallization occurs with chemically unique components, lack a molecular framework or functional group restrictions, highlighting the importance of molecular shape to molecular assembly. Recently, our group reported quasiracemates derived from benzoyl leucine/phenylalanine derivatives with two points of chemical difference. In this study, we modified the chemical framework with valine and increased the scope of the work by imposing a larger variance in the side chain substituents. Pairing a CF3 component with quasienantiomers that differ iteratively from hydrogen to t-butyl offers an important view into the supramolecular landscape of these materials. Single-crystal X-ray crystallography and lattice energy assessments, coupled with conformational and crystal structure similarity searches, show an elevated degree of isomorphism for many of the targeted 17 racemates and quasiracemates. These benzoyl amino acid molecular architectures create extended hydrogen-bond patterns in the crystal that provide enhanced opportunities to study the shape space and molecular recognition profiles for a diverse family of quasienantiomeric components.

Palladium-catalyzed native α-amino acid derivative-directed arylation/oxidation of benzylic C-H bonds: synthesis of 5-aryl-1,4-benzodiazepin-2-ones

A Pd-catalyzed, native α-amino acid derivative-directed benzylic C-H bond arylation/oxidation with aryl iodides was developed. The natural amino acid auxiliary could serve as a desired building block for formation of 5-aryl-1,4-benzodiazepin-2-ones after removal of the trifluoroacetyl protecting group. The bifunctional reaction probably proceeded through a sequential benzylic arylation/oxidation process.

Synthesis of Indole-Fused Six-, Seven-, or Eight-Membered N,O-Heterocycles via Rhodium-Catalyzed NH-Indole-Directed C-H Acetoxylation/Hydrolysis/Annulation

We report herein the facile synthesis of indole-fused six-, seven-, or eight-membered N,O-heterocycles through rhodium-catalyzed C-H acetoxylation/hydrolysis/annulation. The notable features of this method include C-H acetoxylation using NH-indole as the intrinsic directing group, high functional group compatibility, and construction of indole-fused medium-sized rings.

Pd(II)-Catalyzed Chemoselective Acetoxylation of C(sp2)-H and C(sp3)-H Bonds in Tertiary Amides

Palladium-catalyzed chemoselective C(sp²)-H and C(sp³)-H acetoxylation of synthetically useful tertiary amides is reported under relatively mild reaction conditions. This protocol proceeds through the assistance of a weakly coordinated directing group (C═O) and requires low catalyst (1.0 mol %) loading. Diverse functionalities, such as C(sp²)-Cl, C(sp³)-Cl, -CF₃, -COOEt, and -NO₂ groups, including morpholinyl, piperazinyl, and pyrrolidinyl heterocycles, are compatible under the reaction conditions. Further functionalization of this protocol is demonstrated by hydrolysis to alcohols, alcohol-acids, as well as reduction to tertiary amines. A preliminary kinetic isotope effect study supported the rate-limiting C-H bond activation process.

The role of hypervalent iodine(iii) reagents in promoting alkoxylation of unactivated C(sp3)-H bonds catalyzed by palladium(ii) complexes

Although Pd(OAc)2-catalysed alkoxylation of the C(sp3)-H bonds mediated by hypervalent iodine(iii) reagents (ArIX2) has been developed by several prominent researchers, there is no clear mechanism yet for such crucial transformations. In this study, we shed light on this important issue with the aid of the density functional theory (DFT) calculations for alkoxylation of butyramide derivatives. We found that the previously proposed mechanism in the literature is not consistent with the experimental observations and thus cannot be operating. The calculations allowed us to discover an unprecedented mechanism composed of four main steps as follows: (i) activation of the C(sp3)-H bond, (ii) oxidative addition, (iii) reductive elimination and (iv) regeneration of the active catalyst. After completion of step (i) via the CMD mechanism, the oxidative addition commences with an X ligand transfer from the iodine(iii) reagent (ArIX2) to Pd(ii) to form a square pyramidal complex in which an iodonium occupies the apical position. Interestingly, a simple isomerization of the resultant five-coordinate complex triggers the Pd(ii) oxidation. Accordingly, the movement of the ligand trans to the Pd-C(sp3) bond to the apical position promotes the electron transfer from Pd(ii) to iodine(iii), resulting in the reduction of iodine(iii) concomitant with the ejection of the second X ligand as a free anion. The ensuing Pd(iv) complex then undergoes the C-O reductive elimination by nucleophilic attack of the solvent (alcohol) on the sp3 carbon via an outer-sphere SN2 mechanism assisted by the X- anion. Noteworthy, starting from the five coordinate complex, the oxidative addition and reductive elimination processes occur with a very low activation barrier (ΔG ‡ 0-6 kcal mol-1). The strong coordination of the alkoxylated product to the Pd(ii) centre causes the regeneration of the active catalyst, i.e. step (iv), to be considerably endergonic, leading to subsequent catalytic cycles to proceed with a much higher activation barrier than the first cycle. We also found that although, in most cases, the alkoxylation reactions proceed via a Pd(ii)-Pd(iv)-Pd(ii) catalytic cycle, the other alternative in which the oxidation state of the Pd(ii) centre remains unchanged during the catalysis could be operative, depending on the nature of the organic substrate.

Advances in Transition-Metal-Catalyzed C–H Bond Oxygenation of Amides

C–O bond formation represents a fundamental chemical transformation in organic synthesis to develop valuably oxygenated (hetero)arenes. Particularly, the direct and regioselective C–H bond oxygenation of privileged amides, using a transition metal catalyst and a mild oxygenating source, is a step-economy and attractive approach. During the last decade, considerable progress has been realized in the direct C–H oxygenation of primary, secondary, and tertiary amides. This Short Review compiles the advances in transition-metal-catalyzed oxygenation of C(sp2)–H and C(sp3)–H bonds on various amides with diverse oxygenation sources. The review is categorized into two different major sections: (i) C(sp2)–H oxygenation and (ii) C(sp3)–H oxygenation. Each section is discussed based on the directing group (monodentate and bidentate) attached to the amide derivatives.

1 Introduction

2 C(sp2)–H Oxygenation

2.1 Monodentate Directed

2.2 Bidentate Directed

3 C(sp3)–H Oxygenation

3.1 Monodentate Directed

3.2 Bidentate Directed

4 Conclusion and Outlook

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.

Silver-Mediated Methoxylation of Aryl C(sp2)–H Bonds Directing by DMEDA

The first example of silver-mediated phosphine-promoted methoxylation of aryl C(sp2)–H bonds with the commercially available reagent for the preparation of alkyl aryl ethers has been developed. This protocol is characterized by mild reaction conditions, broad substrate scope, and high regioselectivity.

Copper Salts/TBAB-Catalyzed Chemo- and Regioselective β-C(sp3)-H Acyloxylation of Aliphatic Amides

An efficient Cu(II)-catalyzed and tetrabutylammonium bromide (TBAB)-promoted strategy for highly regioselective and chemoselective C(sp³)-H acyloxylation of aliphatic amides is described. Acyloxylation occurs selectively at the β position with a broad substrate scope of carboxylic acids and aliphatic amides and good functional group compatibility. Notably, the competing reaction of intramolecular dehydrogenative amidation and intermolecular acyloxylation could be efficiently controlled by the amount of copper salt and the addition of TBAB. The intramolecular dehydrogenative amidation product was obtained in high yield when the amount of copper salts was increased. However, when TBAB was used as an additive, a preference for acyloxylation over intramolecular amidation was observed and the acyloxylated products were obtained in good yield. Preliminary studies were carried out to gain insights into the mechanism.

Ni(ii)-catalyzed mono-selective ortho-arylation of unactivated aryl C–H bonds utilizing amino acids as a directing group

The nickel(ii)-catalyzed ortho-arylation of unactivated C–H bonds utilizing amino acids as directing groups with aryl iodides or bromides as coupling electrophiles is described.

Cobalt-catalyzed peri-selective alkoxylation of 1-naphthylamine derivatives

A cobalt-catalyzed C(sp²)-H alkoxylation of 1-naphthylamine derivatives has been disclosed, which represents an efficient approach to synthesize aryl ethers with broad functional group tolerance. It is noteworthy that secondary alcohols, such as hexafluoroisopropanol, isopropanol, isobutanol, and isopentanol, were well tolerated under the current catalytic system. Moreover, a series of biologically relevant fluorine-aryl ethers were easily obtained under mild reaction conditions after the removal of the directing group.

Pd-catalyzed intramolecular C(sp2)–H amination of phenylalanine moieties in dipeptides: synthesis of indoline-2-carboxylate-containing dipeptides

Indoline-2-carboxylate-containing dipeptides were synthesized from dipeptides via Pd-catalyzed intramolecular C–H amination.

Copper-Catalyzed Direct Acyloxylation of C(sp2)-H Bonds in Aromatic Amides

Copper-catalyzed ortho-acyloxylation of the sp² C-H bond of aryl amides with carboxylic acids is reported. Benzoic acids, cinnamic acids, and aliphatic acids can be involved, and the desired products were obtained in moderate to good yields. This procedure is compatible with a wide range of functional groups and heteroarenes without the use of any ligands or additives. This method provides an operationally simple approach for the synthesis of benzoate and cinnamate.