Activation of Aryl Carboxylic Acids by Diboron Reagents towards Nickel-Catalyzed Direct Decarbonylative Borylation

Site-selective decarbonylative [4 + 2] annulation of carboxylic acids with terminal alkynes by C-C/C-H activation strategy and cluster catalysis

Cycloaddition and annulation strategies are among the most powerful methods for creating molecular complexity in organic molecules. In this manuscript, we report a highly site-selective palladium-catalyzed decarbonylative [4 + 2] cyclization of carboxylic acids with terminal alkynes by a sequential C-C/C-H bond activation. Most notably, this method represents the first use of carboxylic acids as the ubiquitous and underdeveloped synthons for intramolecular cycloadditions by decarbonylative C-C bond cleavage. The method provides a solution to the long-standing challenge of the regioselective synthesis of substituted naphthalenes by cycloaddition. Mechanistic studies show that this reaction occurs through a sequential process involving the formation of key palladacycle by a sequential C-C/C-H bond activation and highly regioselective alkyne insertion enabled by cluster catalysis. Wide substrate scope for both carboxylic acids and terminal alkynes is demonstrated with high functional group tolerance. Moreover, this reaction is scalable and applicable to the synthesis of functionalized molecules featuring bioactive fragments. This reaction advances the toolbox of redox-neutral carboxylic acid interconversion to cycloaddition processes. We anticipate that this approach will find broad application in organic synthesis, drug discovery and functionalized material research fields.

Pd-Catalyzed Decarbonylative sp2 C-H Arylation: Construction of Five- and Six-Membered (Hetero)Cyclic Compounds

The cyclic compounds have wide applications in the design and synthesis of drugs and materials; thus, their efficient construction attracts much attention from the synthetic community. In this letter, we report an efficient method for preparing cyclic compounds starting from the readily available carboxylic acids. This reaction takes place through intramolecular decarbonylative sp2 C-H arylation, enabling efficient synthesis of a wide range of five- and six-membered cyclic compounds. Both carbo- and heterocycles can be produced under the reaction conditions. Moreover, this reaction features a wide substrate scope with high functional group tolerance. The scale-up experiments also show its practicality in organic synthesis. Those experimental results indicate that this reaction would find wide applications in the synthetic community.

Efficient Synthesis and Structural Analysis of Chiral 4,4'-Biazulene

4,4'-Biazulene is a potentially attractive key component of an axially chiral biaryl compound, however, its structure and properties have not been clarified owing to the lack of its efficient synthesis. We report a breakthrough in the reliable synthesis of 4,4'-biazulene, which is achieved by the access to azulen-4-ylboronic acid pinacol ester and 4-iodoazulene as novel key synthetic intermediates for the Suzuki-Miyaura cross-coupling reaction. The X-ray crystallographic analysis of 4,4'-biazulene confirmed its axial chirality. The enantiomers of 4,4'-biazulene were successfully resolved by HPLC on the chiral stationary phase column. The kinetic experiments and DFT calculations indicate that the racemization energy barrier of 4,4'-biazulene is comparable to that of 1,1'-binaphthyl.

Ligand Relay for Nickel-Catalyzed Decarbonylative Alkylation of Aroyl Chlorides

Transition metal-catalyzed direct decarboxylative transformations of aromatic carboxylic acids usually require high temperatures, which limit the substrate's scope, especially for late-stage applications. The development of the selective decarbonylative of carboxylic acid derivatives, especially the most fundamental aroyl chlorides, with stable and cheap electrophiles under mild conditions is highly desirable and meaningful, but remains challenging. Herein, a strategy of nickel-catalyzed decarbonylative alkylation of aroyl chlorides via phosphine/nitrogen ligand relay is reported. The simple phosphine ligand is found essential for the decarbonylation step, while the nitrogen ligand promotes the cross-electrophile coupling. Such a ligand relay system can effectively and orderly carry out the catalytic process at room temperature, utilizing easily available aroyl chlorides as an aryl electrophile for reductive alkylation. This discovery provides a new strategy for direct decarbonylative coupling, features operationally simple, mild conditions, and excellent functional group tolerance. The mild approach is applied to the late-stage methylation of various pharmaceuticals. Extensive experiments are carried out to provide insights into the reaction pathway and support the ligand relay process.

NHC-Catalyzed Reaction of Carboxylic Acids Using Allene Ketones as Substrates and Activating Reagents

We present a new reaction between carboxylic acids and allene ketones mediated by N-heterocyclic carbene (NHC) catalysts, which exhibit, in principle, nearly perfect atom economy. In this new approach, allene ketones act as both an activating reagent and a reactant. All atoms in the substrates end up in the product without the need for coupling reagents. The present study aims to encourage further explorations of NHC catalytic reactions with alternative activation strategies and better atom economy.

Recent Expedition in Pd- and Rh-Catalyzed C(Ar) -B Bond Formations and Their Applications in Modern Organic Syntheses

Transition metal-catalyzed borylation has emerged as a powerful and versatile strategy for synthesizing organoboron compounds. These compounds have found widespread applications in various aspects, including organic synthesis, materials science, and medicinal chemistry. This review provides a concise summary of the recent advances in palladium- and rhodium-catalyzed borylation from 2013 to 2023. The review covers the representative examples of catalysts, substrates scope and reaction conditions, with particular emphasis on the development of catalyst systems, such as phosphine ligands, NHC-carbene, and more. The diverse array of borylative products obtained for further applications in Suzuki-Miyaura coupling, and other transformations, are also discussed. Future directions in this rapidly evolving field, with the goal of designing more efficient, selective borylation methodologies are highlighted, too.

Nickel Catalyzed Carbonylation/Carboxylation Sequence via Double CO2 Incorporation

A carbonylation-carboxylation synthetic sequence, via double CO2 fixation, is described. The productive merger of a Ni-catalyzed cross-electrophile coupling manifold, with the use of AlCl3, triggered a cascade reaction with the formation of three consecutive C-C bonds in a single operation. This strategy traces an unprecedented synthetic route to ketones under Lewis acid assisted carbon dioxide valorization. Computational insights revealed a unique double function of AlCl3, and labeling (13CO2) experiments validate the genuine incorporation of CO2 in both functional groups.

Palladium-Catalyzed Decarbonylative Annulation of 2-Arylbenzoic Acids with Internal Alkynes toward Phenanthrenes

A palladium-catalyzed decarbonylative annulation of 2-arylbenzoic acids with internal alkynes via C(sp2)-H activation has been developed. A series of phenanthrenes were produced in moderate to good yield with good functional group tolerance. The mechanism study indicated that the C(sp2)-H activation should be the rate-determining step during the reaction.

Formation of C(sp2)-C(sp3) Bonds Instead of Amide C-N Bonds from Carboxylic Acid and Amine Substrate Pools by Decarbonylative Cross-Electrophile Coupling

Carbon-heteroatom bonds, most often amide and ester bonds, are the standard method to link together two complex fragments because carboxylic acids, amines, and alcohols are ubiquitous and the reactions are reliable. However, C-N and C-O linkages are often a metabolic liability because they are prone to hydrolysis. While C(sp2)-C(sp3) linkages are preferable in many cases, methods to make them require different starting materials or are less functional-group-compatible. We show here a new, decarbonylative reaction that forms C(sp2)-C(sp3) bonds from the reaction of activated carboxylic acids (via 2-pyridyl esters) with activated alkyl groups derived from amines (via N-alkyl pyridinium salts) and alcohols (via alkyl halides). Key to this process is a remarkably fast, reversible oxidative addition/decarbonylation sequence enabled by pyridone and bipyridine ligands that, under reaction conditions that purge CO(g), lead to a selective reaction. The conditions are mild enough to allow coupling of more complex fragments, such as those used in drug development, and this is demonstrated in the coupling of a typical Proteolysis Targeting Chimera (PROTAC) anchor with common linkers via C-C linkages.

A one-pot protocol for iron-catalyzed decarbonylative borylation of aryl and alkyl carboxylic acids

We present a protocol for the eco-friendly synthesis of aryl and alkyl boronic esters from aryl and alkyl carboxylic acids. We describe steps for aryl and alkyl carboxylates preparation. We further detail procedures for the synthesis of borylated products using aryl and alkyl carboxylates through iron-catalyzed decarbonylation at 100°C-130°C, followed by purification of the crude products by flash column chromatography. For complete details on the use and execution of this protocol, please refer to Wen et al. (2022).¹.

Photocatalytic direct borylation of carboxylic acids

The preparation of high value-added boronic acids from cheap and plentiful carboxylic acids is desirable. To date, the decarboxylative borylation of carboxylic acids is generally realized through the extra step synthesized redox-active ester intermediate or in situ generated carboxylic acid covalent derivatives above 150 °C reaction temperature. Here, we report a direct decarboxylative borylation method of carboxylic acids enabled by visible-light catalysis and that does not require any extra stoichiometric additives or synthesis steps. This operationally simple process produces CO₂ and proceeds under mild reaction conditions, in terms of high step economy and good functional group compatibility. A guanidine-based biomimetic active decarboxylative mechanism is proposed and rationalized by mechanistic studies. The methodology reported herein should see broad application extending beyond borylation.

Catalysis with Diboron(4)/Pyridine: Application to the Broad-Scope [3 + 2] Cycloaddition of Cyclopropanes and Alkenes

In contrast to the extensive but non-recyclable use of tetraalkoxydiboron(4) compounds as stoichiometric reagents in diverse reactions, this article reports an atom-economical reaction using a commercial diboron(4) as the catalyst. The key to success was designing a catalytic cycle for radical [3 + 2] cycloaddition involving a pyridine cocatalyst to generate from the diboron(4) catalyst and reversibly mediate the transfer of boronyl radicals. In comparison with known [3 + 2] cycloaddition with transition metal-based catalysts, the current reaction features not only metal-free conditions, inexpensive and stable catalysts, and simple operation but also remarkably broadened substrate scope. In particular, previously unusable cyclopropyl ketones without an activating group and/or alkenes with 1,2-disubstitution and 1,1,2-trisubstitution patterns were successfully used for the first time. Consequently, challenging cyclopentane compounds with various levels of substitution (65 examples, 57 new products, up to six substituents at all five ring atoms) were readily prepared in generally high to excellent yield and diastereoselectivity. The reaction was also successfully applied in concise formal synthesis of an anti-obesity drug and building natural product-like complex bridged or spirocyclic compounds. Mechanistic experiments and computational investigation support the proposed radical relay catalysis featuring a pyridine-assisted boronyl radical catalyst. Overall, this work demonstrates the first approach to use tetraalkoxydiboron(4) compounds as catalysts and may lead to the development of new, green, and efficient transition metal-like boron-catalyzed organic reactions.

Decarboxylative Borylation and Cross-Coupling of (Hetero)aryl Acids Enabled by Copper Charge Transfer Catalysis

We report a copper-catalyzed strategy for arylboronic ester synthesis that exploits photoinduced ligand-to-metal charge transfer (LMCT) to convert (hetero)aryl acids into aryl radicals amenable to ambient-temperature borylation. This near-UV process occurs under mild conditions, requires no prefunctionalization of the native acid, and operates broadly across diverse aryl, heteroaryl, and pharmaceutical substrates. We also report a one-pot procedure for decarboxylative cross-coupling that merges catalytic LMCT borylation and palladium-catalyzed Suzuki-Miyaura arylation, vinylation, or alkylation with organobromides to access a range of value-added products. The utility of these protocols is highlighted through the development of a heteroselective double-decarboxylative C(sp²)-C(sp²) coupling sequence, pairing copper-catalyzed LMCT borylation and halogenation processes of two distinct acids (including pharmaceutical substrates) with subsequent Suzuki-Miyaura cross-coupling.