Redox‐Neutral Decarbonylative Cross‐Couplings Coming of Age

Abstract

Carboxylica cids represent quintessential substrates for organic synthesis. [1] Diverse electrophilic and nucleophilic reactions of carboxylic acids form the foundation of daily research by practicing organic chemists [2] and feature as ap illar of all undergraduate organic chemistry textbooks. [3] The steadily increasing demandf or efficient and sustainable carbon-carbon bondforming protocols has given rise to the exponential growth of transition-metal-catalyzed cross-coupling reactions, as firmly manifestedb yt he 2010 NobelP rize in Chemistry. [4] Following the inspiring breakthrough reported by Gooßen and co-workers in 2006, [5] traditionally,t he cross-couplingo fc arboxylic acids is addressed by decarboxylation. [6] Typically,t his involves generation of arenecarboxylate speciesa nd extrusion of carbon dioxide, which converts carboxylic acids into aryl nucleophiles. [7] This approachh as provene xtremely valuable, as it allows carboxylic acids to serve as ubiquitous, orthogonal, and readily accessible cross-coupling partners in paradigms under decarboxylative regimen. However,d espite the profound impact on chemical synthesis, decarboxylative cross-couplings have been ac hallenge, owing to high energy requirements for the decarboxylation step, [5] which often requires stoichiometric metal additives, substrates that favor decarboxylation, and typically high temperatures. [6] Recently,s ignificant progressh as been made in the developmento fc ross-coupling reactions of carboxylic acids via redox-neutral decarbonylative pathways ( Figure 1).In this approach, the carboxylic acid is first converted into an activated,e asily accessible acyl carboxylic acid derivative, followed by metal insertion, transmetallation/decarbonylation, and reductivee limination, thus mimicking the classical M 0/II (M = metal) cross-coupling mechanism under redox-neutral conditions ( Figure 2). Altogether,this provides ap owerful alternative approach to the decarboxylative cross-couplingo fc arboxylic acids [6] as wella st he traditional cross-coupling of aryl halides. [4] Of course,d ecarbonylative cross-couplingsa re not new. [4] One of the first efficient examples of ad ecarbonylative crosscoupling was reported by de Vries and co-workersi n1 998 and involved the use of symmetrical anhydrides in aP dCl 2 /LiClcocatalyzed decarbonylative Heck cross-coupling. [8,9] Subsequently,t his approachw as refinedb yG ooßen and co-workers who utilized aromatice sters and vinylice sters thus minimizing generation of toxic halide waste. [10] The idea of forming aryl electrophiles from carboxylic acids by decarbonylation laid dormant until 2012w hen Itami and co-workersr eported that relatively unactivated phenolic esters can be induced to undergo selectived ecarbonylation/CÀCc ross-coupling with acidic heterocycles utilizingN i(cod) 2 (cod = 1,5-cycooctadiene) and dcype [1,2-bis(dicyclohexylphosphino)ethane] as ak ey bidentate ligand. [11] In 2015, our group introduced decarbonylative cross-coupling of amides by selectiveN ÀCb ond activati...

Highly-chemoselective step-down reduction of carboxylic acids to aromatic hydrocarbons via palladium catalysis

“…(2) Furthermore, the broad scope of the reaction and the orthogonal mechanism for removing the carboxylic acid group expand the utility of carboxylic acids as directing groups in organic synthesis. (3) Perhaps most importantly, the transformation encompasses a general manifold for decarbonylative redox-neutral cross-coupling of ubiquitous carboxylic acids via a unified mechanism that provides a range of new compelling methods for manipulation of this privileged functional group 25…”

Nickel-Catalyzed Decarbonylative Stannylation of Acyl Fluorides under Ligand-Free Conditions

“…Conventionally, transition metal-catalyzed transformations of acyl fluorides with organometallic reagents (Zn, Si, and B) have focused on the synthesis of biaryl ketones in a carbonyl-group retentive manner [4,5,6]. In a sharp contrast, recently, decarbonylative transformations of carboxylic acid derivatives [7,8], especially acyl fluorides have been studied intensively [9]. Sakai and Ogiwara have disclosed that the auxiliary ligand of the palladium catalyst can control the reaction type of reduction of acyl fluorides [10].…”

Synthesis of Biaryls via Decarbonylative Palladium-Catalyzed Suzuki-Miyaura Cross-Coupling of Carboxylic Acids

“…The latter example is particularly noteworthy as it highlights compatibility of the present process with highly activated phenolic esters, which can be reacted under forcing Ni catalysis (Muto et al., 2015). This unique selectivity is predicated on selective activation of carboxylic acid derivatives enabled by transition metal catalysis (resonance energy, PhC(O)–Opiv = 5.1 kcal/mol versus PhC(O)–OPh, 9.3 kcal/mol, barrier to rotation) (Zhao and Szostak, 2019).

Figure 3Scope of the Decarbonylative Suzuki-Miyaura Cross-Coupling of Carboxylic Acids: Carboxylic Acid Scope(A) Scope of carboxylic acids.(B) Late-stage functionalization.(C) Sequential cross-coupling.Conditions: Carboxylic acid (1.0 equiv.

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“…This report implements a decarbonylative version of Suzuki-Miyaura cross-coupling with loss of carbon monoxide that enables to directly engage carboxylic acids in a redox-neutral pathway to generate biaryls with high selectivity using a well-defined Pd(0)/(II) catalytic cycle (–CO, Figure 1C) (Zhao and Szostak, 2019). As (1) significantly more carboxylic acids than aryl halides are commercially available and (2) carboxylic acids form an intrinsic part of advanced bioactive products and functional materials, undoubtedly the direct Suzuki-Miyaura cross-coupling of carboxylic acids as electrophilic components represents a modular approach to precisely construct biaryl building blocks.…”