A case study of proton shuttling in palladium catalysis

The Mechanism of Rh(I)-Catalyzed Coupling of Benzotriazoles and Allenes Revisited: Substrate Inhibition, Proton Shuttling, and the Role of Cationic vs Neutral Species

Direct coupling of benzotriazole to unsaturated substrates such as allenes represents an atom-efficient method for the construction of biologically and pharmaceutically interesting functional structures. In this work, the mechanism of the N2-selective Rh complex-catalyzed coupling of benzotriazoles to allenes was investigated in depth using a combination of experimental and theoretical techniques. Substrate coordination, inhibition, and catalyst deactivation was probed in reactions of the neutral and cationic catalyst precursors [Rh(μ-Cl)(DPEPhos)]2 and [Rh(DPEPhos)(MeOH)2]+ with benzotriazole and allene, giving coordination, or coupling of the substrates. Formation of a rhodacycle, formed by unprecedented 1,2-coupling of allenes, is responsible for catalyst deactivation. Experimental and computational data suggest that cationic species, formed either by abstraction of the chloride ligand or used directly, are relevant for catalysis. Isomerization of benzotriazole and cleavage of its N-H bond are suggested to occur by counteranion-assisted proton shuttling. This contrasts with a previously proposed scenario in which oxidative N-H addition at Rh is one of the key steps. Based on the mechanistic analysis, the catalytic coupling reaction could be optimized, leading to lower reaction temperature and shorter reaction times compared to the literature.

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.

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.

Pyridine Wingtip in [Pd(Py-tzNHC)2]2+ Complex Is a Proton Shuttle in the Catalytic Hydroamination of Alkynes

The cationic palladium(II) complex 1 of pyridyl-mesoionic carbene ligand catalyzes Markovnikov-selective intermolecular hydroamination between anilines and terminal alkynes into the corresponding imines. The reaction proceeds at room temperature, in the absence of additives, with exquisite selectivity and diverse functional group tolerance. The key intrinsic feature of the catalyst is the pyridine wingtip confined to the proximity of the alkynophilic metal active site, which mimics the function of enzyme-like architectures by assisting entropically favored proton transfers.

Bambusurils as a mechanistic tool for probing anion effects

Bambusuril macrocycles have high affinity towards anions (X⁻) such as PF₆⁻ and SbF₆⁻ or BF₄⁻ and ClO₄⁻. Therefore, addition of bambusurils to reaction mixtures containing these anions effectively removes the free anions from the reaction process. Hence, comparing reactions with and without addition of bambusurils can demonstrate whether the anions actively participate in the reaction mechanism or not. We show this approach for gold(i) mediated addition of methanol to an alkyne. The reaction mechanism can proceed via monoaurated intermediates (e.g., in catalysis with [(IPr)AuX]) or via diaurated intermediates (e.g., in catalysis with [(PPh₃)AuX]). We show that anions X⁻ slightly affect the reaction rates, however the effect stays almost the same even after their encapsulation in the cavity of bambusurils. We also demonstrate that X⁻ affects the overall reaction rate in the very same way as the reaction rate of the protodeauration step. All results are consistent with the indirect effect of X⁻ by the acidity of the conjugated acid HX on the rate-determining step. There is no evidence that a direct involvement of X⁻ would affect the reaction rate.

We present bambusurils, macrocycles which strongly bind anions of different sizes, as a mechanistic tool to probe counter ion effects in metal catalysis.

Lactone Synthesis by Enantioselective Orthogonal Tandem Catalysis

In this work, we report enantioselective orthogonal tandem catalysis for the one pot conversion of Meldrum's acid derivatives and alkynes into δ-lactones. This new transformation, which resembles a formal [4+2] cycloaddition with concomitant decarboxylation and loss of acetone, proceeds in high yields and excellent enantioselectivity (up to 99 % ee) over a broad substrate scope. The products are densely functionalized and ripe for further transformations, as demonstrated here by both ring-opening reactions and reduction to saturated lactones. It was discovered that a new and serendipitously formed Ag^I -Me-StackPhos complex efficiently catalyzes the highly selective 6-endo-dig cyclization, completely reversing the regiochemistry that has been previously reported in related systems. More generally, in this study we identify a pair of compatible catalysts for alkyne difunctionalization that operate concurrently, which enable the alkyne to act as both a nucleophile and an electrophile in sequential one-pot transformations.

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.

Protodepalladation as a Strategic Elementary Step in Catalysis

Protodepalladation is the redox-neutral conversion of a C-Pd(II) bond to a C-H bond promoted by a Brønsted acid. It can be viewed as the microscopic reserves of Pd(II)-mediated C-H cleavage. In the context of catalytic reaction development, protodepalladation offers a means of converting organopalladium(II) intermediates to organic products without a change in oxidation state at the metal center. Hence, when integrated into catalytic cycles, it can be a uniquely enabling elementary step. The goal of this Review is to provide an overview of protodepalladation, including exploration of different reactions types, discussion of literature examples, and analysis of mechanistic features. Our hope is that this review will stimulate other researchers in the field to pursue new applications of this underexploited step in catalysis.

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.

The role of proton shuttling mechanisms in solvent-free and catalyst-free acetalization reactions of imines

Catalyst-free and solvent-free reactions of the type NuH + E → Nu–EH are NuH-catalyzed processes in which Grotthuss-like proton shuttling pays a key role.

Enantioselective Nazarov cyclization of indole enones cooperatively catalyzed by Lewis acids and chiral Brønsted acids

Enantioselective control of the chirality of a tertiary α-carbon in the products of a Nazarov cyclization of enones is challenging because the reaction involves an enantioselective proton transfer process. We herein report the use of cooperative catalysis using Lewis acids and chiral Brønsted acids to control the stereochemistry of the tertiary α-carbon in the products of this reaction. Specifically, with ZnCl₂ and a chiral spiro phosphoric acid as catalysts, we realized the first enantioselective construction of cyclopenta[b]indoles with chiral tertiary α-carbons via Nazarov cyclization of indole enone substrates with only one coordinating site. Mechanistic studies revealed that the chiral spiro phosphoric acid acts as a multifunctional catalyst: it co-catalyzes the cyclization of the dienone and enantioselectively catalyzes a proton transfer reaction of the enol intermediate. This new strategy of enantioselective control by means of cooperative catalysis may show utility for other challenging asymmetric cyclization reactions.

A computational study on the mechanism of ynamide-mediated amide bond formation from carboxylic acids and amines

This paper reports a computational study elucidating the reaction mechanism for ynamide-mediated amide bond formation from carboxylic acids and amines. The mechanisms have been studied in detail for ynamide hydrocarboxylation and the subsequent aminolysis of the resulting adduct by an amine. Ynamide hydrocarboxylation is kinetically favorable and thermodynamically irreversible, resulting in the formation of a key low-lying intermediate CP1 featuring geminal vinylic acyloxy and sulfonamide groups. The aminolysis of CP1 by the amine is proposed to be catalyzed by the carboxylic acid itself that imparts favourable bifunctional effects. In the proposed key transition state TSaminolysis-acid-iso2, the amine undergoes direct nucleophilic substitution at the acyl of CP1 to replace the enolate group in a concerted way, which is promoted by secondary hydrogen bonding of carboxylic acid with both the amine and CP1. These secondary interactions are suggested to increase the nucleophilicity of the amine and to activate the C_acyl-O bond to be cleaved, thereby stabilizing the aminolysis transition state. The concerted aminolysis mechanism is competitive with the classic stepwise nucleophilic acyl substitution mechanism that features sequential amine addition to acyl/intramolecular proton transfer/C-O bond cleavage and a key tetrahedral intermediate. Based on the mechanistic model, the carboxylic acid substrate effect and studies of more acidic CF₃SO₃H as the catalyst are in good agreement with the experimental observations, lending further support for the mechanistic model. The bifunctional catalytic effect of the carboxylic acid substrate may widely play a role in related amide bond-forming reactions and peptide formation chemistry.

Light-Driven Enantioselective Organocatalytic β-Benzylation of Enals

A photochemical organocatalytic strategy for the direct enantioselective β-benzylation of α,β-unsaturated aldehydes is reported. The chemistry capitalizes upon the light-triggered enolization of 2-alkyl-benzophenones to afford hydroxy-o-quinodinomethanes. These fleeting intermediates are stereoselectively intercepted by chiral iminium ions, transiently formed upon condensation of a secondary amine catalyst with enals. Density functional theory (DFT) studies provided an explanation for why the reaction proceeds through an unconventional Michael-type addition manifold, instead of a classical cycloaddition mechanism and subsequent ring-opening.

Role of Lewis acid additives in a palladium catalyzed directed C-H functionalization reaction of benzohydroxamic acid to isoxazolone

Metallic salts as well as protic additives are widely employed in transition metal catalyzed C-H bond functionalization reactions to improve the efficiency of catalytic protocols. In one such example, ZnCl₂ and pivalic acid are used as additives in a palladium catalyzed synthesis of isoxazolone from a readily available benzohydroxamic acid under one pot conditions. In this article, we present some important mechanistic insights into the role of ZnCl₂ and pivalic acid, gained by using density functional theory (M06) computations. Two interesting modes of action of ZnCl₂ are identified in various catalytic steps involved in the formation of isoxazolone. The conventional Lewis acid coordination wherein zinc chloride (ZnCl₂·(DMA)) binds to the carbonyl group is found to be more favored in the C-H activation step. However, the participation of a hetero-bimetallic Pd-Zn species is preferred in reductive elimination leading to C_aryl-N bond formation. Pivalic acid helps in relay proton transfer in C-H bond activation through a cyclometallation deprotonation (CMD) process. The explicit inclusion of ZnCl₂ and solvent N,N-dimethyl acetamide (DMA) stabilizes the transition state and also helps reduce the activation barrier for the C-H bond activation step. The electronic communication between the two metal species is playing a crucial role in stabilizing the C_aryl-N bond formation transition state through a Pd-Zn hetero-bimetallic interaction.