Origin of Enantio- and Chemoselectivity in the Synthesis of Spirocycles via Palladium/Xu-Phos-Catalyzed Cascade Heck/Remote C(sp2)-H Alkylation: A Computational Mechanistic Study

Density functional theory (DFT) calculations were performed to study the mechanism and factors affecting the enantio-, regio-, and chemoselectivities in the palladium/Xu-Phos-catalyzed cascade Heck/remote C(sp2)-H alkylation reaction. The active catalyst is found to be able to sustain coordination with P and S atoms and can adapt its coordination mode to accommodate the significant steric hindrance between the ligand and substrate, unlike previous findings that showed coordination with P and O atoms. The reaction is established to occur in sequence through the oxidative addition of the aryl iodide to Pd(0), intramolecular alkene insertion, C(sp2)-H bond activation, and C(sp2)-C(sp3) bond reductive elimination. The C(sp2)-C(sp3) bond reductive elimination is identified as the rate-determining step, and the intramolecular alkene insertion as the enantioselectivity-determining step. The high enantioselectivity originates from the stronger electronic interaction between the catalyst and substrate; the exclusive 5-exo-regioselectivity is due to the stronger nucleophilicity of the terminal alkene carbon atom, and the chemoselectivity of C-H activation over carboiodination is driven by thermodynamics.

Synthesis of spirooxindoles via formal acetylene insertion into a common palladacycle intermediate

A palladium-catalyzed spirocyclization reaction is reported, which is proposed to arise via insertion of an oxabicycle into a palladacycle, formed from carbocyclization and a C-H functionalization sequence. Mechanistic studies suggest the insertion is diastereoselective and a post-catalytic retro-Diels-Alder step furnishes an alkene, wherein the oxibicycle has served as an acetylene surrogate. Aryl iodides and carbamoyl chlorides were compatible as starting materials under the same reaction conditions, enabling the convergent and complementary synthesis of spirooxindoles, as well as other azacycles. These spirooxindoles allowed further transformations that were previously unaccessible.

Intermolecular Difunctionalization of C,C-Palladacycles Obtained by Pd(0)-Catalyzed C-H Activation

C,C-Palladacycles are an important class of organometallic compounds in which palladium is σ-bonded to two carbon atoms. They have three notable features that make them attractive in organic synthesis and organometallic chemistry: (1) C,C-Palladacycles are reactive intermediates that can be accessed via Pd(0)-catalyzed C-H activation of organic halides. Compared to Pd(II)-catalyzed heteroatom-directed C-H activation, C-H activation catalyzed by Pd(0) has some distinct advantages. In this type of catalytic reaction, the halo groups of readily available organic halides act as traceless directing groups. Furthermore, this strategy avoids the use of stoichiometric external oxidants. (2) C,C-Palladacycles have differentiated reactivities from common open-chain Pd(II) species. In particular, C,C-palladacycles have high reactivity toward electrophiles including alkyl halides. This unique reactivity can be utilized to develop novel reactions. (3) C,C-Palladacycles have two C-Pd bonds, providing a unique platform for developing novel reactions.Although a number of reactions of C,C-palladacycles had been developed prior to our work, the scope was largely limited to intramolecular cyclization reactions. Although Catellani reactions are intermolecular reactions of C,C-palladacycles, only one of the C-Pd bonds is functionalized. Our laboratory has sought to develop intermolecular difunctionalization reactions of C,C-palladacycles that exploit their unique reactivity and open new possibilities in organic synthesis. Aiming to develop synthetically useful reactions, we primarily focus on ring-forming reactions. In this Account, we summarize our laboratory's efforts to exploit intermolecular difunctionalization reactions of C,C-palladacycles that are obtained through Pd(0)-catalyzed C-H activation. We have developed a wide array of new reactions that represent facile and efficient methods for the synthesis of cyclic organic compounds, including functional materials and drug molecules. A range of C,C-palladacycles have been studied, including C(aryl),C(aryl)-palladacycles from 2-halobiaryls, C(aryl),C(alkyl)-palladacycles from ortho-iodo-tert-butylbenzenes or ortho-iodoanisole derivatives, and those obtained by cascade reactions. C,C-Palladacycles have been found to react with a variety of oxidants to furnish Pd(IV) intermediates, such as alkyl halides, aryl halides, diazo compounds, and N,N-di-tert-butyldiaziridinone, ultimately affording various cyclic structures, including 5-10-membered rings, carbo- and azacycles, spirocycles, and fused rings. Furthermore, novel reactivity of C,C-palladacycles has been discovered. For example, we found that C,C-palladacycles have unusually high reactivity toward disilanes, which can be leveraged to disilylate a variety of C,C-palladacycles with very high efficiency. These results should provide inspiration to develop other C-Si bond-forming reactions in the future. We hope that this Account will stimulate further research into the rich chemistry of C,C-palladacycles, in particular reactions that find practical applications in the synthesis of bioactive and functional molecules and those that advance the state of the art in C-H functionalization.

Pd(II)-Catalyzed Synthesis of Polycyclic Heteroarenes via an Aminopalladation/C-H Activation/Dealkylation/Decarboxylative Cyclization Cascade

Until now, cascade reactions involving five-membered C,C-palladacycles rely heavily on Pd(0)-catalyzed C-H functionalization of aryl halides initiated by carbopalladation. Herein, we report a novel Pd(II)-catalyzed cascade decarboxylative cyclization of o-alkynylanilines initiated by aminopalladation. In this protocol, o-alkynylanilines undergo sequential anti aminopalladation, C-H activation, and dealkylation to form C,C-palladacycles, which are then trapped by o-bromobenzoic acids or 8-bromo-1-naphthoic acid to produce diverse polycyclic heteroarenes, such as dibenzo[a,c]carbazoles and multiple arene-fused cyclohepta[1,2-b]indoles.

Palladium-Catalyzed Sequential Heck Reactions of Olefin-Tethered Aryl Iodides with Alkenes

An efficient approach to functionalized (E)-3-cinnamyl-3-methyl-2,3-dihydrobenzofurans and (E)-(3-methyl-2,3-dihydrobenzofuran-3-yl)but-2-enones has been developed through a Pd-catalyzed one-pot cascade process involving two sequential Heck reactions, that is, an intramolecular Heck reaction of olefin-tethered aryl iodides and an intermolecular Heck reaction with substituted styrenes and α,β-unsaturated ketones. As a result, a series of desired products were obtained in moderate to good yields and with exclusive E-form selectivities.

Research advances in palladium-catalysed intermolecular C-H annulation of aryl halides with various aromatic ring precursors

Transition-metal-catalysed C-H functionalization has emerged as a powerful approach for the transformation of organic molecules due to its high atom and step economy. Palladium-catalysed intermolecular C-H annulation of aryl halides, especially those involving annulation of a five-membered C,C-palladacycle with coupling reagents, have attracted considerable attention in the past decades. This review summarizes the progress on palladium-catalysed intermolecular C-H annulation of aryl halides with various aromatic ring precursors. Mechanistically, five-membered C,C-palladacycles as intermediates are involved in the majority of reactions.

Pd-Catalyzed C-H Functionalization of Indole-Containing Alkene-Tethered Aryl Halides with Alkynes To Construct Indole Alkaloid Scaffolds

A convenient and straightforward approach for the construction of indole alkaloid scaffolds from indole-containing alkene-tethered aryl halides and alkynes through a sequential C-H activation, five-membered palladacycle formation, and alkyne insertion process has been described. The approach provides a series of indole alkaloid compounds in moderate to excellent yields with good functional tolerance.

Accessing Unsymmetrically Linked Heterocycles through Stereoselective Palladium-Catalyzed Domino Cyclization

A palladium-catalyzed strategy is presented to synthesize unsymmetrically linked heterocycles within stereoselective tetrasubstituted olefins. This reaction is proposed to occur via a vinyl-Pd^II intermediate capable of initiating the cyclization of various alkyne-tethered nucleophiles. Products are formed in up to 96 % yield and excellent stereoselectivities are obtained using low catalyst loadings. This transformation was scalable up to 1 mmol and mechanistic studies suggest a syn-carbopalladation of the carbamoyl chloride followed by Pd^II -catalyzed cyclization of alkyne-tethered nucleophiles.

Tandem 1,6-addition/cyclopropanation/rearrangement reaction of vinylogous para-quinone methides with 3-chlorooxindoles: construction of vicinal quaternary carbon centers

A novel tandem 1,6-addition/cyclopropanation/rearrangement reaction of vinylogous para-quinone methides with 3-chlorooxindoles has been developed, providing dispirooxindole–cyclopentane–cyclohexadienones with vicinal quaternary carbon centers.

Regioselective Synthesis of Spirocyclic Pyrrolines via a Palladium-Catalyzed Narasaka-Heck/C-H Activation/[4+2] Annulation Cascade Reaction

A novel palladium-catalyzed spirocyclization through sequential Narasaka-Heck, C-H activation and [4+2] annulation has been developed. In this reaction, cheap and readily available 2-chlorobenzoic acid or ethyl phenylpropiolate was employed as...

Decarboxylative cyclization of o-chlorobenzoic acids with C,C-palladacycles formed by an aminopalladation/dealkylation strategy to access dibenzo[a,c]carbazoles

A novel decarboxylative cyclization of o-chlorobenzoic acids with C,C-palladacycles formed by an aminopalladation/dealkylation strategy for the assembly of dibenzo[a,c]carbazoles has been reported.

A Modular Approach for the Palladium-Catalyzed Synthesis of Bis-heterocyclic Spirocycles

A simple and modular approach toward bis-heterocyclic spirocycles using palladium catalysis is reported. The enclosed methodology leverages a Mizoroki-Heck-type reaction to generate a neopentylpalladium species. This species can undergo intramolecular C-H activation on a wide array of (hetero)aryl C-H bonds, generating a variety of [4.4] and [4.5] bis-heterocyclic spirocycles in up to 95% yield. A diverse range of bis-heterocyclic spirocycles were possible, with 24 examples and 18 different combinations of heterocycles were synthesized. Biologically relevant aza-heterocycles such as purine, pyrazole, (benz)imidazole, (aza)indole, and pyridine were readily incorporated into the spirocyclic core. The reaction was readily scalable to 1 mmol using a lower catalyst loading and number of base equivalents, and the product was purified without the use flash column chromatography.

Synthesis of Spiroindenyl-2-Oxindoles through Palladium-Catalyzed Spirocyclization of 2-Bromoarylamides and Vinyl Bromides

An expeditious approach to the construction of spiroindenyl-2-oxindoles was developed via a palladium-catalyzed spirocyclization reaction of 2-bromoarylamides with vinyl bromides. The reaction formed spiropalladacycles as the intermediates via carbopalladation and the C–H functionalization of 2-bromoarylamides. The spiropalladacycles reacted with vinyl bromides to form spiroindenyl-2-oxindoles. A Heck process rather than vinylic C–H functionalization was involved in the reaction.

Palladium-catalyzed cascade carbonylative annulation between alkene-tethered aryl iodides and carbon monoxide

Rapid construction of molecules bearing all-substituted quaternary stereocenters represents a highly significant but challenging task in organic synthesis. Herein, we report a novel palladium-catalyzed cascade between alkene-tethered aryl iodides and carbon monoxide, which has resulted in a practical and powerful method for the synthesis of complex polycyclic molecules containing aryl-substituted quaternary stereocenters. Mechanistic studies suggested that the reaction proceeded via a Heck-type carbonylative cyclization, followed by a ketene-involved Friedel-Crafts acylation.

Experimental and Computational Studies of Palladium-Catalyzed Spirocyclization via a Narasaka-Heck/C(sp3 or sp2)-H Activation Cascade Reaction

The first synthesis of highly strained spirocyclobutane-pyrrolines via a palladium-catalyzed tandem Narasaka-Heck/C(sp³ or sp²)-H activation reaction is reported here. The key step in this transformation is the activation of a δ-C-H bond via an in situ generated σ-alkyl-Pd(II) species to form a five-membered spiro-palladacycle intermediate. The concerted metalation-deprotonation (CMD) process, rate-determining step, and energy barrier of the entire reaction were explored by density functional theory (DFT) calculations. Moreover, a series of control experiments was conducted to probe the rate-determining step and reversibility of the C(sp³)-H activation step.

Palladium-Catalyzed Domino Heck/Sulfination: Synthesis of Sulfonylated Hetero- and Carbocyclic Scaffolds Using DABCO-Bis(sulfur dioxide)

The synthesis of a broad variety of hetero- and carbocyclic scaffolds via a Pd-catalyzed domino Heck/SO₂ insertion reaction is reported. This reaction utilizes DABSO, a safe and easy-to-handle alternative to SO₂ gas. The reaction proceeds through a sulfinate intermediate, which can act as a lynchpin for the in situ generation of sulfones, sulfonamides, and sulfonyl fluorides. Good yields and scalability are demonstrated.

Visible-Light-Mediated Additive-Free Decarboxylative Ketonization Reaction of Acrylic Acids: An Access to α-Thiocyanate Ketones

Visible-light-mediated additive-free decarboxylative functionalization of acrylic acids has been developed. The reaction uses inexpensive organic dye 9,10-dicyanoanthracene as a photocatalyst and uses the ubiquitous dioxygen as both an oxygen source and an oxidant. Through this mild and environmentally friendly method, a series of important α-thiocyanate ketones can be generated from easily available acrylic acids and ammonium thiocyanate. In addition, the facile transformation of product α-thiocyanate ketones makes this method have great potential for application in organic and pharmaceutical chemistry.

Transition metal-catalyzed synthesis of spirooxindoles

Spirooxindole is a principal bioactive agent and is observed in several natural products including alkaloids. They are broadly studied in the pharmaceutical field and have a significant role in the evolution of drugs such as anti-viral, anti-cancer, anti-microbial etc. In organic chemistry, an indispensable role is presented by transition metal catalysts. An effective synthetic perspective to spirooxindoles is the use of transition metals as the catalyst. This review discusses the synthesis of spirooxindoles catalyzed by transition metals and covers literature up to 2020.

Palladium-catalyzed domino spirocyclization of [60]fullerene: synthesis of diverse [60]fullerene-fused spiro[4,5]/[5,5] derivatives

Herein a new, general and practical method for the spirocyclization of [60]fullerene through a palladium-catalyzed domino Heck/C-H activation reaction is presented. A wide range of novel [60]fullerene-fused spirocyclic derivatives can be easily and flexibly synthesized with a broad substrate scope and excellent functional-group tolerance. A plausible mechanism involving an alkyl Pd(ii) species as a key intermediate has been proposed.

A palladium-catalyzed Heck/[4 + 1] decarboxylative cyclization cascade to access diverse heteropolycycles by using α-bromoacrylic acids as C1 insertion units

A novel palladium-catalyzed Heck/[4 + 1] decarboxylative cyclization cascade of alkene-tethered aryl iodides by employing α-bromoacrylic acids as C1 insertion units is reported.

Stereoselective synthesis and applications of spirocyclic oxindoles

This review summaries recent synthetic developments towards spirocyclic oxindoles and applications as valuable medicinal and synthetic targets.

Copper-catalyzed regioselective [3+2] annulation of malonate-tethered acyl oximes with isatins

A copper-catalyzed regioselective [3+2] annulation of malonate-tethered acyl oximes with isatins was developed, affording valuable 2,3-dihydrooxazole-spirooxindoles in moderate to good yields with excellent diastereoselectivity.

The role of allyl ammonium salts in palladium-catalyzed cascade reactions towards the synthesis of spiro-fused heterocycles

There is a continuous need for designing new and improved synthetic methods aiming at minimizing reaction steps while increasing molecular complexity. In this respect, catalytic, one-pot cascade methodologies constitute an ideal tool for the construction of complex molecules with high chemo-, regio-, and stereoselectivity. Herein, we describe two general and efficient cascade procedures for the synthesis of spiro-fused heterocylces. This transformation combines selective nucleophilic substitution (S_N2′), palladium-catalyzed Heck and C–H activation reactions in a cascade manner. The use of allylic ammonium salts and specific Pd catalysts are key to the success of the transformations. The synthetic utility of these methodologies is showcased by the preparation of 48 spiro-fused dihydrobenzofuranes and indolines including a variety of fluorinated derivatives.

Synthetic methods aiming at minimizing reaction steps while increasing molecular complexity are highly sought after by organic chemists. Here, the authors report two cascade procedures combining nucleophilic substitution, palladium-catalyzed Heck and C–H activation reactions for the synthesis of spiro-fused heterocycles.

Pd-Catalyzed Domino Reactions Involving Alkenes To Access Substituted Indole Derivatives

Palladium-catalyzed domino reactions are advanced tools in achieving various nitrogen-containing heterocycles in an efficient and economical manner due to the reduced number of steps in the process. This review highlights recent advances in domino processes aimed at the synthesis of indole derivatives and polycyclic systems containing the indole nucleus in intra/intra- or intra/intermolecular reactions. In particular, we consider domino processes that involve a double bond in a step of the sequence, which allow the issue of regioselectivity in the cyclization to be faced and overcome. The different sections in this review focus on the synthesis of the indole nucleus and functionalization of the scaffold starting from different substrates that have been identified as activated starting materials, which involve a halogenated moiety or unactivated unsaturated systems. In the former case, the reaction is under Pd(0) catalysis, and in the second case a Pd(II) catalytic species is required and then an oxidant is necessary to reconvert the Pd(0) into the active Pd(II) species. On the other hand, the second method has the advantage that it uses easy available and inexpensive substrates.

1 Introduction

2 Indole Scaffold Synthesis

2.1 Activated Substrates

2.2 Unactivated Substrates

3 Functionalization of Indole Scaffold

3.1 Activated Substrates

3.2 Unactivated Substrates

4 Conclusions

Palladium-catalyzed domino Heck-disilylation and -borylation of alkene-tethered 2-(2-halophenyl)-1H-indoles: access to diverse disilylated and borylated indolo[2,1-a]isoquinolines

A chemoselective domino Heck-disilylation and -borylation reaction for generating various disilylated and borylated tetracyclic indolo[2,1-a]isoquinolines has been developed.

Palladium-catalyzed intermolecular C–H silylation initiated by aminopalladation

The intermolecular disilylation reaction of C,C-palladacycles obtained through aminopalladation has been developed.

Pd-Catalyzed one-pot synthesis of vinylsilanes via a three-component tandem reaction

A palladium-catalyzed three-component tandem reaction for stereoselective assembly of various tri- or tetrasubstituted vinylsilanes is established.