Enhanced Catalytic Performance of Indenediide Palladium Pincer Complexes for Cycloisomerization: Efficient Synthesis of Alkylidene Lactams

Boosting the Reactivity of Bis-Lactones to Enable Step-Growth Polymerization at Room Temperature

The development of new sustainable polymeric materials endowed with improved performances but minimal environmental impact is a major concern, with polyesters as primary targets. Lactones are key monomers thanks to ring-opening polymerization, but their use in step-growth polymerization has remained scarce and challenging. Herein, we report a powerful bis(γ-lactone) (γSL) that was efficiently prepared on a gram scale from malonic acid by Pd-catalyzed cycloisomerization. The γ-exomethylene moieties and the spiro structure greatly enhance its reactivity toward ring-opening and enable step-growth polymerization under mild conditions. Using diols, dithiols, or diamines as comonomers, a variety of regioregular (AB)n copolymers with diverse linkages and functional groups (from oxo-ester to β-thioether lactone and β-hydroxy-lactame) have been readily prepared. Reaction modeling and monitoring revealed the occurrence of an original trans-lactonization process following the first ring-opening of γSL. This peculiar reactivity opens the way to regioregular (ABAC)n terpolymers, as illustrated by the successive step-growth polymerization of γSL with a diol and a diamine.

Gold-Zinc Cooperative Catalysis for Seven-exo-dig Hydrocarboxylation of Internal Alkynes

Seven-exo-dig hydrocarboxylation of nonactivated internal alkynes with conformationally flexible linker chains was achieved with cooperative gold-zinc catalysts composed of an imidazo[1,5-a]pyridinylidene ligand including a bipyridine coordination site at the C5 position. A proximity effect of the gold and zinc sites was essential for their high catalytic activity, in which the internal alkyne activated by the cationic gold species was attacked by the carboxylic acid deprotonated by the basic zinc site. Using a gold(I)-complex with a bulky aromatic N-substituent, 2,6-dibenzhydryl-4-methylphenyl group, for the NHC ligand facilitated seven-membered ring formation while minimizing intermolecular hydrocarboxylation as an undesired side reaction. The reaction mechanism was investigated by quantum chemical calculations.

K2S2O8-Mediated Radical Cyclization of 1,6-Enyne for the Synthesis of Diiodonated γ-Lactams

A novel and convenient K₂S₂O₈-mediated diiodo cyclization of 1,6-enynes for the facile synthesis of functionalized γ-lactam derivatives has been developed. This reaction features mild and transition-metal-free conditions, which offer a green and efficient entry to synthetically important γ-lactam scaffolds. Mechanistic studies suggest that iodide radicals initiate the cascade cyclic transformation.

Endo-Selective Intramolecular Alkyne Hydroamidation Enabled by NiH Catalysis Incorporating Alkenylnickel Isomerization

Intramolecular alkyne hydroamidation represents a straightforward approach for the access to synthetically valuable cyclic enamides. Despite some advances made in this realm, the ability to attain a precise regiocontrol still remains challenging, especially for endo cyclization that leads to six-membered and larger azacyclic rings. Herein, we report a NiH-catalyzed intramolecular hydroamidation of alkynyl dioxazolones that allows for an excellent endo selectivity, thus affording a range of six- to eight-membered endocyclic enamides with a broad scope. Mechanistic investigations revealed that Ni(I) catalysis is operative in the current system, proceeding via regioselective syn-hydronickelation, alkenylnickel E/Z isomerization, and Ni-centered inner-sphere nitrenoid transfer. In particular, the key alkenylnickel isomerization step, which previously lacked mechanistic understandings, was found to take place through the η²-vinyl transition state. The synthetic value of this protocol was demonstrated by diastereoselective modifications of the obtained endocyclic enamides to highly functionalized δ-lactam scaffolds.

A Facile Oxidation of Tertiary Amines to Lactams by Using Sodium Chlorite: Process Improvement by Precise pH Adjustment with CO2

By using cheap and innocuous sodium chlorite, a series of tertiary amines have been oxidized to the corresponding lactams with good selectivity and high yield. In this method, neither transition-metal catalyst nor oxidant was used. In the oxidation step, the pH of the sodium chlorite was precisely adjusted to pH around 6 using CO2, such pH is a compromise between oxidative properties, chemical stability, and unwanted precipitation. In addition, buffer salts are not necessary, which allows this oxidation reaction to be performed under safe and environmentally benign conditions.

Platinum Assisted Tandem P-C Bond Cleavage and P-N Bond Formation in Amide Functionalized Bisphosphine o-Ph2PC6H4C(O)N(H)C6H4PPh2-o: Synthesis, Mechanistic, and Catalytic Studies

The reactions of amide functionalized bisphosphine o-Ph₂PC₆H₄C(O)N(H)C₆H₄PPh₂-o (1) with platinum salts are described. Treatment of 1 with [Pt(COD)Cl₂] yielded a chelate complex, [PtCl₂{o-Ph₂PC₆H₄C(O)N(H)C₆H₄PPh₂-o}κ²-P,P] (2), which on subsequent treatment with LiHMDS formed a novel 1,2-azaphospholene-phosphine complex [Pt(C₆H₅)Cl{o-C₆H₄{C(O)N(o-PPh₂(C₆H₄))P(Ph)}}κ²-P,P] (3) involving a tandem P-C bond cleavage and P-N bond formation. The same complex 3 on passing dry HCl gas afforded the dichloro complex [PtCl₂{o-C₆H₄{C(O)N(o-PPh₂(C₆H₄))P(Ph)}}κ²-P,P] (5). Complex 2 upon refluxing in toluene or treatment of 1 with [Pt(COD)Cl₂] in the presence of a base at room temperature resulted in the pincer complex [PtCl{o-Ph₂PC₆H₄C(O)N(C₆H₄PPh₂-o)}κ³-P,N,P] (4). Reaction of 1 with [Pt(COD)ClMe] at room temperature also afforded the pincer complex [PtMe{o-Ph₂PC₆H₄C(O)N(C₆H₄PPh₂-o)}κ³-P,N,P] (6). Mechanistic studies on 1,2-azaphospholene formation showed the reductive elimination of LiCl to form a phosphonium salt that readily adds one of the P-C bonds oxidatively to the in situ generated Pt⁰ species to form a chelate complex 3. The analogous palladium complex [PdCl₂{o-C₆H₄{C(O)N(o-PPh₂(C₆H₄))P(Ph)}}κ²-P,P] (7) showed excellent catalytic activity toward N-alkylation of amines with alcohols with a very low catalyst loading (0.05 mol %), and the methodology is very efficient toward the gram-scale synthesis of many N-alkylated amines.

Radical Cyclization of 1,n-Enynes and 1,n-Dienes for the Synthesis of 2-Pyrrolidone

2-Pyrrolidones have aroused enormous interest as a useful structural moiety in drug discovery; however, not only does their syntheses suffer from low selectivity and yield, but also it requires high catalyst loadings. The radical cyclization of 1,n-enynes and 1,n-dienes has demonstrated to be an attractive method for the synthesis of 2-pyrrolidones due to its mild reaction conditions, fewer steps, higher atom economy, excellent functional group compatibility, and high regioselectivity. Furthermore, radical receptors with unsaturated bonds (i. e. 1,n-enynes and 1,n-dienes) play a crucial role in realizing radical cyclization because of the ability to selectively introduce one or more radical sources. In this review, we discuss representative examples of methods involving the radical cyclization of 1,n-enynes and 1,n-dienes published in the last five years and discuss each prominent reaction design and mechanism, providing favorable tools for the synthesis of valuable 2-pyrrolidone for a variety of applications.

Pd-catalysed general access to 7-membered N/O-heterocyclic compounds as potential agents against inflammation

A Pd-catalysed regioselective synthesis of 4,5-disubstituted 7-membered N/O-heterocycles was achieved via the 7-endo-dig cyclization followed by C-C bond formation of 2-(1-alkynyl)phenylacetamide. The ligand/additive free cascade reaction proceeded in the presence of PdCl₂ in aqueous MeCN when the separate and individual use of methyl vinyl ketone and allyl bromide generally afforded an O- and N-heterocycle, respectively. The pharmacological assay was performed to identify the first example of a 1H-benzo[d]azepin-2(3H)-one based novel inhibitor of PDE4B.

Sequential Pd0 - and PdII -Catalyzed Cyclizations: Enantioselective Heck and Nucleopalladation Reactions

An enantioselective consecutive cyclization/coupling process, catalyzed by palladium is reported. Stereoinduction arises from an enantioselective carbopalladation, generating an intermediate which promotes a nucleopalladation step. The dual cyclization sequence was compatible with a variety of alkyne-tethered oxygen- and nitrogen-centered nucleophiles, and a variety of alkenyl-tethered aryl iodides, to forge numerous bisheterocycles in good yields and high regio- and enantioselectivities.

Novel approach to benzo-fused 1,2-azaphospholene involving a Pd(II)-assisted tandem P-C bond cleavage and P-N bond formation reaction

New bisphosphine o-Ph2PC6H4C(O)N(H)C6H4PPh2-o (1) (Bala-HariPhos) showed a unique reactivity towards Pd(ii) resulting in a 1,2-azaphospholene complex, involving a tandem P-C bond cleavage, P-N bond formation and cyclization process via the elimination of PhH. Mechanistic details were investigated using NMR spectroscopy, DFT calculations and kinetic data, and by SCXRD analysis. It involves the reductive elimination from a tautomerised complex to form a phosphonium salt followed by oxidative addition.

Metal-ligand-Lewis acid multi-cooperative catalysis: a step forward in the Conia-ene reaction

An original multi-cooperative catalytic approach was developed by combining metal–ligand cooperation and Lewis acid activation. The [(SCS)Pd]₂ complex featuring a non-innocent indenediide-based ligand was found to be a very efficient and versatile catalyst for the Conia-ene reaction, when associated with Mg(OTf)₂. The reaction operates at low catalytic loadings under mild conditions with HFIP as a co-solvent. It works with a variety of substrates, including those bearing internal alkynes. It displays complete 5-exo vs. 6-endo regio-selectivity. In addition, except for the highly congested ^tBu-substituent, the reaction occurs with high Z vs. E stereo-selectivity, making it synthetically useful and complementary to known catalysts.

An original multi-cooperative catalytic approach was developed by combining metal–ligand cooperation and Lewis acid activation.

Metal-Catalyzed Synthesis and Transformations of β-Haloenol Esters

In the last years there has been an increasing interest in the search for protocols to obtain β-haloenol esters in an efficient and selective manner as they are versatile building blocks in synthetic organic chemistry. In this article, metal-catalyzed transformations allowing the access to both acyclic and cyclic (i.e., haloenol lactones) β-haloenol esters are reviewed. Metal-catalyzed reactions in which these molecules participate as substrates are also discussed.

Organochalcogen ligands in catalysis of oxidation of alcohols and transfer hydrogenation

Organochalcogen compounds have been used as the building blocks for the development of a variety of catalysts that have been studied comprehensively during the last two decades for several chemical transformations. Transfer hydrogenation (reduction of carbonyl compounds to alcohols) and oxidation of alcohols (conversion of alcohols to their respective ketones and aldehydes) are also among such chemical transformations. Some compilations are available in the literature on the development of catalysts, based on organochalcogen ligands, and their applications in Heck reaction, Suzuki reaction, and other related aspects. Some review articles have also been published on different aspects of oxidation of alcohols and transfer hydrogenation. However, no such article is available in the literature on the syntheses and use of organochalcogen ligated catalysts for these two reactions. In this perspective, a survey of developments pertaining to the synthetic aspects of such organochalcogen (S/Se/Te) based catalysts for the two reactions has been made. In addition to covering the syntheses of chalcogen ligands, their metal complexes and nanoparticles (NPs), emphasis has also been placed on the efficient conversion of different substrates during catalytic reactions, diversity in catalytic potential and mechanistic aspects of catalysis. It also includes the analysis of comparison (in terms of efficiency) between this unique class of catalysts and efficient catalysts without a chalcogen donor. The future scope of this area has also been highlighted.

Palladium pincer complexes featuring an unsymmetrical SCN indene-based ligand with a hemilabile pyridine sidearm

A new unsymmetrical indene-based pro-ligand featuring thiophosphinoyle and methylpyridine sidearms 2 was prepared. Coordination and cyclometalation in the presence of [PdCl2(PhCN)2] and PS-DIEA afforded three well-defined 2-indenyl SCN pincer complexes 3a-c. The lability of the pyridine moiety has been evidenced upon treatment with triphenylphosphine and 2,6-dimethylphenylisocyanide. In addition, reversible C-Pd bond cleavage has been demonstrated under Brønsted acid/base conditions. The indenediide SCN pincer complex 4 was prepared by deprotonation of 3a in the presence of triphenylphosphine. Preliminary catalytic tests on the cycloisomerization of 4-pentynoic acid have underlined the impact of the pyridine sidearm on the catalytic activity.

Self-assembled M12L24 nanospheres as a reaction vessel to facilitate a dinuclear Cu(i) catalyzed cyclization reaction

The application of large M12L24 nanospheres allows the pre-concentration of catalysts to reach high local concentrations, facilitating reactions that proceed through dinuclear mechanisms. The mechanism of the copper(i)-catalyzed cyclization of 4-pentynoic acid has been elucidated by means of a detailed mechanistic study. The kinetics of the reaction show a higher order in copper, indicating the formation of a bis-Cu intermediate as the key rate determining step of the reaction. This intermediate was further identified during catalysis by CIS-HRMS analysis of the reaction mixture. Based on the mechanistic findings, an M12L24 nanosphere was applied that can bind up to 12 copper catalysts by hydrogen bonding. This pre-organization of copper catalysts in the nanosphere results in a high local concentration of copper leading to higher reaction rates and turnover numbers as the dinuclear pathway is favored.

A highly active diradical cobalt(iii) catalyst for the cycloisomerization of alkynoic acids

The first cobalt-catalysed cycloisomerisation of alkynoic acids is reported, thanks to the design of a well-defined diradical cobalt(iii) catalyst, in the absence of any additives. The high efficiency, regioselectivity and chemoselectivity are comparable to those of noble metal-based systems. The unique reactivity might be attributed to second coordination sphere effects.

Visible light-induced aerobic oxidative cross-coupling of glycine esters with α-angelicalactone: a facile pathway to γ-lactams

A rapid synthesis of substituted γ-lactams is achieved via metal-free photocatalyzed aerobic oxidative dehydrogenative formal [2 + 3] cyclization of glycine esters with α-angelicalactone.

A case study of proton shuttling in palladium catalysis

Thanks to mechanistic studies, the catalytic performance of SCS indenediide Pd pincer complexes has been spectacularly enhanced using catechol additives as proton shuttles.

The mechanism of alkynoic acid cycloisomerization with SCS indenediide Pd pincer complexes has been investigated experimentally and computationally. These studies confirmed the cooperation between the Pd center and the backbone of the pincer ligand, and revealed the involvement of a second molecule of substrate. It acts as a proton shuttle in the activation of the acid, it directs the nucleophilic attack of the carboxylic acid on the π-coordinated alkyne and it relays the protonolysis of the resulting vinyl Pd species. A variety of H-bond donors have been evaluated as external additives, and polyols featuring proximal hydroxyl groups, in particular catechol derivatives, led to significant catalytic enhancement. The impact of 4-nitrocatechol and 1,2,3-benzenetriol is particularly striking on challenging substrates such as internal 4- and 5-alkynoic acids. Endo/exo selectivities up to 7.3/1 and 60-fold increase in reactivity were achieved.

Amide and amine nucleophiles in polar radical crossover cycloadditions: synthesis of γ-lactams and pyrrolidines

In this work we present a direct catalytic synthesis of γ-lactams and pyrrolidines from alkenes and activated unsaturated amides or protected unsaturated amines, respectively. Using a mesityl acridinium single electron photooxidant and a thiophenol cocatalyst under irradiation, we are able to directly forge these important classes of heterocycles with complete regiocontrol.