Natural Product Synthesis via Palladium-Catalyzed Carbonylation

A divergent intermediate strategy yields biologically diverse pseudo-natural products

The efficient exploration of biologically relevant chemical space is essential for the discovery of bioactive compounds. A molecular design principle that possesses both biological relevance and structural diversity may more efficiently lead to compound collections that are enriched in diverse bioactivities. Here the diverse pseudo-natural product (PNP) strategy, which combines the biological relevance of the PNP concept with synthetic diversification strategies from diversity-oriented synthesis, is reported. A diverse PNP collection was synthesized from a common divergent intermediate through developed indole dearomatization methodologies to afford three-dimensional molecular frameworks that could be further diversified via intramolecular coupling and/or carbon monoxide insertion. In total, 154 PNPs were synthesized representing eight different classes. Cheminformatic analyses showed that the PNPs are structurally diverse between classes. Biological investigations revealed the extent of diverse bioactivity enrichment of the collection in which four inhibitors of Hedgehog signalling, DNA synthesis, de novo pyrimidine biosynthesis and tubulin polymerization were identified from four different PNP classes.

Pd-catalysed synthesis of oxomalonamides through adjacent triple carbonylation of tertiary amines

The monocarbonylation and dicarbonylation reactions are well-established reactions in carbonylation chemistry. This work reports unusual oxidative adjacent triple carbonylation of the tertiary amine towards oxomalonamide synthesis using a Pd catalyst. The protocol involves the use of inert tertiary amines as an active reservoir of secondary amines through sp3 C-N bond activation using O2 as an ideal oxidant.

Competing Mechanisms in Palladium-Catalyzed Alkoxycarbonylation of Styrene

Palladium-catalyzed carbonylation is a versatile method for the synthesis of various aldehydes, esters, lactones, or lactams. Alkoxycarbonylation of alkenes with carbon monoxide and alcohol produces either saturated or unsaturated esters as a result of two distinct catalytic cycles. The existing literature presents an inconsistent account of the procedures favoring oxidative carbonylation products. In this study, we have monitored the intermediates featured in both catalytic cycles of the methoxycarbonylation of styrene PhCH=CH2 as a model substrate, including all short-lived intermediates, using mass spectrometry. Comparing the reaction kinetics of the intermediates in both cycles in the same reaction mixture shows that the reaction proceeding via alkoxy intermediate [PdII]-OR, which gives rise to the unsaturated product PhCH=CHCO2Me, is faster. However, with an advancing reaction time, the gradually changing reaction conditions begin to favor the catalytic cycle dominated by palladium hydride [PdII]-H and alkyl intermediates, affording the saturated products PhCH2CH2CO2Me and PhCH(CO2Me)CH3 preferentially. The role of the oxidant proved to be crucial: using p-benzoquinone results in a gradual decrease of the pH during the reaction, swaying the system from oxidative conditions toward the palladium hydride cycle. By contrast, copper(II) acetate as an oxidant guards the pH within the 5-7 range and facilitates the formation of the alkoxy palladium complex [PdII]-OR, which favors the oxidative reaction producing PhCH=CHCO2Me with high selectivity. Hence, it is the oxidant, rather than the catalyst, that controls the reaction outcome by a mechanistic switch. Unraveling these principles broadens the scope for developing alkoxycarbonylation reactions and their application in organic synthesis.

Palladium-Catalyzed Carbonylative Difunctionalization of Unactivated Alkenes Initiated by Unstabilized Enolates

This report describes the first example of palladium-catalyzed carbonylative difunctionalization of unactivated alkenes initiated by enolate nucleophiles. The approach involves initiation by an unstabilized enolate nucleophile under an atmospheric pressure of CO and termination with a carbon electrophile. This process is compatible with a diverse range of electrophiles, including aryl, heteroaryl, and vinyl iodides to yield synthetically useful 1,5-diketone products, which were demonstrated to be precursors for multi-substituted pyridines. A PdI -dimer complex with two bridging CO units was observed although its role in catalysis is not yet understood.

Palladium-Catalyzed Carbonylations: Application in Complex Natural Product Total Synthesis and Recent Developments

Carbon monoxide is a cheap and abundant C1 building block that can be readily incorporated into organic molecules to rapidly build structural complexity. In this Perspective, we outline several recent (since 2015) examples of palladium-catalyzed carbonylations in streamlining complex natural product total synthesis and highlight the strategic importance of these carbonylation reactions in the corresponding synthesis. The selected examples include spinosyn A, callyspongiolide, perseanol, schizozygane alkaloids, cephanolides, and bisdehydroneostemoninine and related stemona alkaloids. We also provide our perspective about the recent advancements and future developments of palladium-catalyzed carbonylations.

Pd-Catalyzed Asymmetric 5-exo-trig Cyclization/Cyclopropanation/Carbonylation of 1,6-Enynes for the Construction of Chiral 3-Azabicyclo[3.1.0]hexanes

We herein disclose a mild and efficient access to chiral 3-azabicyclo[3.1.0]hexanes via a Pd-catalyzed asymmetric 5-exo-trig cyclization/cyclopropanation/carbonylation of 1,6-enynes. Various nucleophiles, such as alcohols, phenols, amines and water, are well compatible with the reaction system. This reaction forms three C-C bonds, two rings, two adjacent quaternary carbon stereocenters as well as one C-O/C-N bond with excellent regio- and enantioselectivities. The products could be further functionalized to generate a library of 3-azabicyclo[3.1.0]hexane frameworks.

Synthesis of Bis(3-indolyl)methanes Mediated by Potassium tert-Butoxide

The indole moiety is an important N-heterocycle found in natural products, and a key structural component of many value-added chemicals including pharmaceuticals. In particular, bis(3-indolyl)methanes (BIMs) are an important subgroup of indoles, composed of two indole units. Herein, we report the development of a simple method to access BIMs derivatives in yields of up to 77 % by exploiting a tBuOK-mediated coupling reaction of indoles and benzyl alcohols.

Chemistry of azo-imine based palladium complexes: a brief review

This paper provides a general overview of palladium complexes incorporating numerous multidentate azo-imine ligands, their structural diversity, synthetic protocol, interesting properties such as redox properties, biological activity and their further application on C – N coupling reactions, C – C cross-coupling reactions (e.g., Suzuki and Heck) and catalytic activity on different organic transformations. A detailed literature survey revealed that no brief review has yet been published on the azo-azomethine-based palladium complex. Therefore, the present review incorporates the research undertaken on the synthetic strategies and coordinating behaviour of various azo-imine ligands with palladium metal centre and a few important properties of metal complexes.

Macrolactonization of methyl 15-hydroxypentadecanoate to cyclopentadecanolide using KF-La/γ-Al2O3 catalyst

It has been a challenge to synthesize macrolide musk in excellent yields with high purity. KF-La/γ-Al₂O₃ catalyst was prepared from a highly basic mesoporous framework using a mild method. The prepared KF-La/γ-Al₂O₃ catalyst was employed for the synthesis of cyclopentadecanolide from methyl 15-hydroxypentadecanoate. The morphology and structure of prepared catalysts were characterized using XRD, TG-DTG, SEM, EDX, TEM, BET and CO₂-TPD. The results revealed that the K₃AlF₆ and LaOF are produced on the surface of KF-La/γ-Al₂O₃, and LaO can promote the dispersion of KF on the surface of Al₂O₃. Catalysts pore size main distribution ranges between 10 and 30 nm, the maximum CO₂ desorption temperature is 715°C when the La loading is 25%. Because F^- ion has a higher electronegativity than O^2- ion, the KF-promoted metal oxide (Al₂O₃ or/and La₂O₃) contained more strong basic sites, compared with that of the corresponding metal oxide. The yield of cyclopentadecanolide obtained at 0.5 g KF-25La/γ-Al₂O₃ catalyst and a reaction temperature of 190°C for 7 h were 58.50%, and the content after reactive distillation is 98.8%. The KF-La/γ-Al₂O₃ catalyst has a larger pore size and basic strength, which is more conducive to the macrolactonization of long-chain hydroxy ester.

Visible-light-induced defluorinative carbonylative coupling of alkyl iodides with α-trifluoromethyl substituted styrenes

A visible-light-mediated defluorinative carbonylative cross-coupling of alkyl iodides with α-trifluoromethyl styrenes has been developed. The reaction occurs at room temperature under blue light irradiation, and various gem-difluoroalkenes were obtained in moderate to good yields. Synthetic transformations of the obtained product were performed as well.

Palladium-Catalyzed Reductive Aminocarbonylation of o-Iodophenol-Derived Allyl Ethers with o-Nitrobenzaldehydes to 3-Alkenylquinolin-2(1H)-ones

An attractive palladium-catalyzed reductive aminocarbonylation reaction of allylic ethers has been explored for the synthesis of 3-alkenylquinolin-2(1H)-one derivatives. With Mo(CO)₆ as both CO surrogate and reductant, a variety of 3-alkenylquinolin-2(1H)-ones were obtained in good to excellent yields from o-iodophenol-derived allyl ethers with o-nitrobenzaldehydes as the nitrogen sources. This reaction proceeds through a cascade pathway and does not rely on high-pressure CO gas as needed in former allylic carbonylation reactions. This strategy provides a new pathway for the construction of 3-alkenylquinolin-2(1H)-ones.

Synthesis of unprotected glyco-alkynones via molybdenum-catalyzed carbonylative Sonogashira cross-coupling reaction

Herein we report a novel Mo-catalyzed carbonylative Sonogashira cross-coupling between 2-iodoglycals and terminal alkynes. The reaction displays major improvements compared to a related Pd-catalyzed procedure previously published by our group, such as utilizing unprotected sugar derivatives as starting materials and tolerance to substrates bearing chelating groups. In this work we also demonstrate the utility of the glyco-alkynone products as platform for further functionalization by synthesizing glyco-flavones via Au-catalyzed 6-endo-dig cyclization.

Palladium-catalyzed enantioselective carbonylation reactions

Carbonylation, one of the most powerful approaches to the preparation of carbonylated compounds, has received significant attention from researchers active in various fields. Indeed, impressive progress has been made on this subject over the past few decades. Among the various types of carbonylation reactions, asymmetric carbonylation is a straightforward methodology for constructing chiral compounds. Although rhodium-catalyzed enantioselective hydroformylations have been discussed in several elegant reviews, a general review on palladium-catalyzed asymmetric carbonylations is still missing. In this review, we summarize and discuss recent achievements in palladium-catalyzed asymmetric carbonylation reactions. Notably, this review’s contents are categorized by reaction type.

Nickel-Catalyzed Hydroarylation Reaction: A Useful Tool in Organic Synthesis

This article describes the recent advances in the field of nickel-catalyzed hydroarylation reaction of alkenes, alkynes, and arenes. All reactions proceeded either through internal hydride transfer or in presence of...

Palladium-catalyzed desulfonylative aminocarbonylation of benzylsulfonyl chlorides with o-aminobenzaldehydes/o-aminoacetophenones for the synthesis of quinoin-2(1H)-ones

A straightforward and efficient synthesis of quinoin-2(1H)-ones has been explored via a palladium-catalyzed desulfonylative aminocarbonylation of benzylsulfonyl chlorides with o-aminobenzaldehydes/o-aminoacetophenones.

Palladium-catalyzed tandem Heck/carbonylation/aminocarbonylation en route to chiral heterocyclic α-ketoamides

An unprecedented tandem carbonylation/aminocarbonylation triggered by palladium-catalyzed enantioselective Heck-type exocyclopalladation delivering chiral heterocyclic α-ketoamides has been developed.

Palladium-Catalyzed Reductive Desulfonative Aminocarbonylation of Benzylsulfonyl Chlorides with Nitroarenes to Phenylacetamides

Herein, we demonstrate a new procedure for the direct using benzylsulfonyl chlorides as versatile C(sp3) electrophiles in a palladium-catalyzed reductive desulfonative aminocarbonylation reaction. Using nitroarenes as readily accessible and stable...

Diastereoselective Alkene Hydroesterification Enabling the Synthesis of Chiral Fused Bicyclic Lactones

Palladium-catalysed diastereoselective hydroesterification of alkenes assisted by the coordinative hydroxyl group in the substrate afforded a variety of chiral γ-butyrolactones bearing two stereocenters. Employing the carbonylation-lactonization products as the key intermediates, the route from the alkenes with single chiral center to chiral THF-fused bicyclic γ-lactones containing three stereocenters was developed.

Palladium-Catalyzed Enantioselective Intramolecular Heck Carbonylation Reactions: Asymmetric Synthesis of 2-Oxindole Ynones and Carboxylic Acids

Herein, we report a Pd-catalyzed enantioselective domino Heck carbonylation reaction of o-iodoacrylanilides with terminal alkynes and water as the nucleophiles, affording a diversity of β-carbonylated 2-oxindole derivatives bearing a 3,3-disubstituted all-carbon quaternary stereocenter, in high yields (55-99 %) with good to excellent enantioselectivities (up to 99 % ee). The synthetic utilities of the protocol were demonstrated in the gram-scale synthesis of 2-oxindole-derived ynone 3 ea and carboxylic acid 4 a, as well as the facile synthesis of chiral 2-oxindoles with a pyrazole or isoxazole moiety.

Palladium-Catalyzed Reductive Aminocarbonylation of Benzylammonium Triflates with o-Nitrobenzaldehydes for the Synthesis of 3-Arylquinolin-2(1H)-ones

A palladium-catalyzed straightforward procedure for the synthesis of 3-arylquinolin-2(1H)-ones has been developed. The synthesis proceeds through a palladium-catalyzed reductive aminocarbonylation reaction of benzylic ammonium triflates with o-nitrobenzaldehydes, and a wide range of 3-arylquinolin-2(1H)-ones was obtained in moderate to good yields with very good functional group compatibility.

Supported Palladium Catalyzed Carbonylative Coupling Reactions using Carbon Monoxide as C1 Source

The carbonylative reactions of aryl halides, boronic acids, amines, activated alkene and alkynes under CO and supported palladium catalyzed conditions are very popular reactions for the synthesis of bioactive molecules, pharmaceuticals, polymers, peptides, intermediates and fine chemicals synthesis. Due to cost effectiveness and easy handling of recyclable supported palladium catalyst, it became more popular among researchers either working in academic institute or industry. In recent years, irrespective of poisoning effect of CO with palladium as major limitation, several advancements have been done through surface selection, designing and condition improvement to achieve high yield in the area of carbonylative coupling reactions. We hope this review will be helpful as a ready reference of last 20 years in the field of CO insertion reactions using diverse range of supported palladium catalysts under carbon monoxide or its sources as C1 source.

Palladium-catalyzed benzylic C(sp3)-H carbonylative arylation of azaarylmethyl amines with aryl bromides

A highly selective palladium-catalyzed carbonylative arylation of weakly acidic benzylic C(sp3)-H bonds of azaarylmethylamines with aryl bromides under 1 atm of CO gas has been achieved. This work represents the first examples of use of such weakly acidic pronucleophiles in this class of transformations. In the presence of a NIXANTPHOS-based palladium catalyst, this one-pot cascade process allows a range of azaarylmethylamines containing pyridyl, quinolinyl and pyrimidyl moieties and acyclic and cyclic amines to undergo efficient reactions with aryl bromides and CO to provide α-amino aryl-azaarylmethyl ketones in moderate to high yields with a broad substrate scope and good tolerance of functional groups. This reaction proceeds via in situ reversible deprotonation of the benzylic C-H bonds to give the active carbanions, thereby avoiding prefunctionalized organometallic reagents and generation of additional waste. Importantly, the operational simplicity, scalability and diversity of the products highlight the potential applicability of this protocol.

Nickel-Catalyzed One-Pot Carbonylative Synthesis of 2-Mono- and 2,3-Disubstituted Thiochromenones from 2-Bromobenzenesulfonyl Chlorides and Alkynes

A nickel-catalyzed one-pot carbonylation reaction of 2-bromobenzenesulfonyl chlorides with alkynes for the synthesis of thiochromenones has been established. Both terminal and internal alkynes were suitable substrates in this carbonylative transformation, and a broad range of 2-mono- and 2,3-disubstituted thiochromenone products were obtained in moderate to good yields with quite high functional group compatibility. Notably, this procedure presents the first example of nickel-catalyzed carbonylative synthesis of thiochromenones with 2-bromobenzenesulfonyl chlorides as a promising sulfur precursor.

Hypothesis-Driven Palladium-Catalyzed Transformations for the Construction of New Molecular Architectures

The development of new synthetic protocols to access diverse molecular scaffolds from readily available starting compounds is of significance in both academia and industry. Towards this, the catalysis by transition metals has been employed as a powerful tool to access molecules with broad structural and functional diversity. An overview of the recent literature manifested the tremendous potential of transition metal-catalyzed processes in advancing organic synthesis in a new direction. This account compiles new conceptual advancements in the palladium-catalyzed Alder-ene type cycloisomerization reactions, C-H functionalizations, and one-pot multicatalytic processes, which have become essential tools to access new classes of molecules.

A novel construction of acetamides from rhodium-catalyzed aminocarbonylation of DMC with nitro compounds

Dimethyl carbonate (DMC), an environment-friendly compound prepared from CO2, shows diverse reactivities. In this communication, an efficient procedure using DMC as both a C1 building block and solvent in the aminocarbonylation reaction with nitro compounds has been developed. W(CO)6 acts both a CO source and a reductant here.

Palladium-catalysed carboformylation of alkynes using acid chlorides as a dual carbon monoxide and carbon source

Hydroformylation, a reaction that installs both a C-H bond and an aldehyde group across an unsaturated substrate, is one of the most important catalytic reactions in both industry and academia. Given the synthetic importance of creating new C-C bonds, the development of carboformylation reactions, wherein a new C-C bond is formed instead of a C-H bond, would bear enormous synthetic potential to rapidly increase molecular complexity in the synthesis of valuable aldehydes. However, the demanding complexity inherent in a four-component reaction, utilizing an exogenous CO source, has made the development of a direct carboformylation reaction a formidable challenge. Here, we describe a palladium-catalysed strategy that uses readily available aroyl chlorides as a carbon electrophile and CO source, in tandem with a sterically congested hydrosilane, to perform a stereoselective carboformylation of alkynes. An extension of this protocol to four chemodivergent carbonylations further highlights the creative opportunity offered by this strategy in carbonylation chemistry.

Palladium-catalyzed carbonylative cyclization of benzyl chlorides with anthranils for the synthesis of 3-arylquinolin-2(1H)-ones

An efficient carbonylative procedure for the synthesis of 3-arylquinoin-2(1H)-ones has been established. Through a palladium-catalyzed aminocarbonylation of benzyl chlorides with anthranils, a variety of 3-arylquinoin-2(1H)-one products were obtained in moderate to excellent yields with good functional group tolerance.

Palladium catalyzed synthesis of indolizines via the carbonylative coupling of bromopyridines, imines and alkynes

We report herein the development of a palladium-catalyzed, multicomponent synthesis of indolizines. The reaction proceeds via the carbonylative formation of a high energy, mesoionic pyridine-based 1,3-dipole, which can undergo spontaneous cycloaddition with alkynes. Overall, this provides a route to prepare indolizines in a modular fashion from combinations of commercially available or easily generated reagents: 2-bromopyridines, imines and alkynes.

Access to 2-pyridinylamide and imidazopyridine from 2-fluoropyridine and amidine hydrochloride

Under catalyst-free conditions, an efficient method to synthesize 2-pyridinylamides has been developed, and the protocol uses inexpensive and readily available 2-fluoropyridine and amidine derivatives as the starting materials. Simultaneously, the copper-catalysed approach to imidazopyridine derivatives has been established with high chemoselectivity and regiospecificity. The results suggest that the nitrogen-heterocycles containing iodide substituents can also be compatible for the reaction via the cascade Ullmann-type coupling, and the nucleophilic substitution reaction provides the target products in a one-pot manner.

Development of Facile and Simple Processes for the Heterogeneous Pd-Catalyzed Ligand-Free Continuous-Flow Suzuki–Miyaura Coupling

The Suzuki–Miyaura coupling reaction is one of the most widely utilized C–C bond forming methods to create (hetero)biaryl scaffolds. The continuous-flow reaction using heterogeneous catalyst-packed cartridges is a practical and efficient synthetic method to replace batch-type reactions. A continuous-flow ligand-free Suzuki–Miyaura coupling reaction of (hetero)aryl iodides, bromides, and chlorides with (hetero)aryl boronic acids was developed using cartridges packed with spherical resin (tertiary amine-based chelate resin: WA30)-supported palladium catalysts (7% Pd/WA30). The void space in the cartridge caused by the spherical catalyst structures enables the smooth flow of a homogeneously dissolved reaction solution that consists of a mixture of organic and aqueous solvents and is delivered by the use of a single syringe pump. Clogging or serious backpressure was not observed.

Pd-catalyzed amidation of 1,3-diketones with CO and azides via a nitrene intermediate

An efficient Pd-catalyzed amidation of 1,3-diketones has been developed using carbon monoxide and organic azides. This reaction provides a step-economic approach to produce β-ketoamides from readily available compounds under mild ligand-, oxidant-, and base-free conditions. The mechanistic studies showed that the reaction occurred through an in situ generated isocyanate intermediate.

Rhodium-Catalyzed Carbonylative Synthesis of Aryl Salicylates from Unactivated Phenols

A rhodium-catalyzed carbonylative transformation of unactivated phenols to aryl salicylates is described. This protocol is characterized by utilizing 1,3-rhodium migration as the key step to provide direct access to synthesize o-hydroxyaryl esters. Various desired aryl o-hydroxybenzoates were produced in moderate to excellent yields with bis(dicyclohexylphosphino)ethane (DCPE) as the ligand. Interestingly, diphenyl carbonate was formed as the main product when 1,3-bis(diphenylphosphino)propane (DPPP) was used as the ligand. A plausible reaction mechanism is proposed.

Rapid Organocatalytic Formation of Carbon Monoxide: Application towards Carbonylative Cross Couplings

Herein, the first organocatalytic method for the transformation of non‐derivatized formic acid into carbon monoxide (CO) is introduced. Formylpyrrolidine (FPyr) and trichlorotriazine (TCT), which is a cost‐efficient commodity chemical, enable this decarbonylation. Utilization of dimethylformamide (DMF) as solvent and catalyst even allows for a rapid CO generation at room temperature. Application towards four different carbonylative cross coupling protocols demonstrates the high synthetic utility and versatility of the new approach. Remarkably, this also comprehends a carbonylative Sonogashira reaction at room temperature employing intrinsically difficult electron‐deficient aryl iodides. Commercial ¹³C‐enriched formic acid facilitates the production of radiolabeled compounds as exemplified by the pharmaceutical Moclobemide. Finally, comparative experiments verified that the present method is highly superior to other protocols for the activation of carboxylic acids.