The thiazolium catalyzed benzoin condensation with mild base does not involve a “dimer” intermediate

Free carbenes from complementarily paired alkynes

Carbenes (R1R2C:) like radicals, arynes and nitrenes constitute an important family of neutral, high-energy, reactive intermediates-fleeting chemical entities that undergo rapid reactions. An alkyne (R3C≡CR4) is a fundamental functional group that houses a high degree of potential energy; however, the substantial kinetic stability of alkynes renders them conveniently handleable as shelf-stable chemical commodities. The ability to generate metal-free carbenes directly from alkynes, fuelled by the high potential (that is, thermodynamic) energy of the latter, would constitute a considerable advance. We report here that this can be achieved simply by warming a mixture of a 2-alkynyl iminoheterocycle (a cyclic compound containing a nucleophilic nitrogen atom) with an electrophilic alkyne. We demonstrate considerable generality for the process: many shelf-stable alkyne electrophiles engage many classes of (2-alkynyl)heterocyclic nucleophiles to produce carbene intermediates that immediately undergo many types of transformations to provide facile and practical access to a diverse array of heterocyclic products. Key mechanistic aspects of the reactions are delineated.

On the Redox Properties of the Dimers of Thiazol-2-ylidenes That Are Relevant for Radical Catalysis

We report the isolation and study of dimers stemming from popular thiazol-2-ylidene organocatalysts. The model featuring 2,6-di(isopropyl)phenyl (Dipp) N-substituents was found to be a stronger reducing agent (E_ox = -0.8 V vs SCE) than bis(thiazol-2-ylidenes) previously studied in the literature. In addition, a remarkable potential gap between the first and second oxidation of the dimer also allows for the isolation of the corresponding air-persistent radical cation. The latter is an unexpected efficient promoter of the radical transformation of α-bromoamides into oxindoles.

Rate and equilibrium constants for the addition of triazolium salt derived N-heterocyclic carbenes to heteroaromatic aldehydes

Heteroaromatic aldehydes are often used preferentially or exclusively in a range of NHC-catalysed processes that proceed through the generation of a reactive diaminoenol or Breslow Intermediate (BI), with the reason for their unique reactivity currently underexplored. This manuscript reports measurement of rate and equilibrium constants for the reaction between N-aryl triazolium NHCs and heteroaromatic aldehydes, providing insight into the effect of the NHC and heteroaromatic aldehyde structure up to formation of the BI. Variation in NHC catalyst and heteroaromatic aldehyde structure markedly affect the observed kinetic parameters of adduct formation, decay to starting materials and onward reaction to BI. In particular, large effects are observed with both 3-halogen (Br, F) and 3-methyl substituted pyridine-2-carboxaldehyde derivatives which substantially favour formation of the tetrahedral intermediate relative to benzaldehyde derivatives. Key observations indicate that increased steric hindrance leads to a reduction in both k 2 and k -1 for large (2,6-disubstituted)-N-Ar groups within the triazolium scaffold, and sterically demanding aldehyde substituents in the 3-position, but not in the 6-position of the pyridine-2-carboxaldehyde derivatives. As part of this study, the isolation and characterisation of twenty tetrahedral adducts formed upon addition of N-aryl triazolium derived NHCs into heteroaromatic aldehydes are described. These adducts are key intermediates in NHC-catalysed umpolung addition of heteroaromatic aldehydes and are BI precursors.

Formation of Breslow Intermediates under Aprotic Conditions: A Computational Study

The mechanism of formation of the Breslow intermediate (BI) under aprotic conditions is investigated with density functional theory (DFT) calculations. The zwitterionic adduct (ZA) is formed by the first addition of an imidazolinylidene to benzaldehyde. The forward reaction is found to proceed through the second addition of the ZA to another benzaldehyde, and subsequent proton migration gives a hemiacetal. The bimolecular reaction enables the conversion of the ZA to a more reactive hemiacetal, which is further decomposed to the BI with the assistance of the ZA. During the ZA-assisted process, the hemiacetal and the BI act as hydrogen bond donors to stabilize the ZA. The hydrogen bond interactions between the ZA and the BI or hemiacetal are analyzed. The DFT computations demonstrate that along the proposed route, the proton migration leading to the hemiacetal intermediate is the rate-determining step (ΔG^⧧ = 21.2 kcal mol^-1). The bimolecular mechanism provides an alternative pathway to explain BI formation under aprotic conditions.

Kinetic and structure-activity studies of the triazolium ion-catalysed benzoin condensation

Steady-state kinetic and structure-activity studies of a series of six triazolium-ion pre-catalysts 2a-2f were investigated for the benzoin condensation. These data provide quantitative insight into the role of triazolium N-aryl substitution under synthetically relevant catalytic conditions in a polar solvent environment. Kinetic behaviour was significantly different to that previously reported for a related thiazolium-ion pre-catalyst 1, with the observed levelling of initial rate constants to νmax at high aldehyde concentrations for all triazolium catalysts. Values for νmax for 2a-2f increase with electron withdrawing N-aryl substituents, in agreement with reported optimal synthetic outcomes under catalytic conditions, and vary by 75-fold across the series. The levelling of rate constants supports a change in rate-limiting step and evidence supports the assignment of the Breslow-intermediate forming step to the plateau region. Correlation of νmax reaction data yielded a positive Hammett ρ-value (ρ = +1.66) supporting the build up of electron density adjacent to the triazolium N-Ar in the rate-limiting step favoured by electron withdrawing N-aryl substituents. At lower concentrations of aldehyde, both Breslow-intermediate and benzoin formation are partially rate-limiting.

N-Heterocyclic Carbene Organocatalysis: With or Without Carbenes?

In this work the mechanism of the aldehyde umpolung reactions, catalyzed by azolium cations in the presence of bases, was studied through computational methods. Next to the mechanism established by Breslow in the 1950s that takes effect through the formation of a free carbene, we have suggested that these processes can follow a concerted asynchronous path, in which the azolium cation directly reacts with the substrate, avoiding the formation of the carbene intermediate. We hereby show that substituting the azolium cation, and varying the base or the substrate do not affect the preference for the concerted reaction mechanism. The concerted path was found to exhibit low barriers also for the reactions of thiamine with model substrates, showing that this path might have biological relevance. The dominance of the concerted mechanism can be explained through the specific structure of the key transition state, avoiding the liberation of the highly reactive, and thus unstable carbene lone pair, whereas activating the substrate through hydrogen-bonding interactions. Polar and hydrogen-bonding solvents, as well as the presence of the counterions of the azolium salts facilitate the reaction through carbenes, bringing the barriers of the two reaction mechanisms closer, in many cases making the concerted path less favorable. Thus, our data show that by choosing the exact components in a reaction, the mechanism can be switched to occur with or without carbenes.

3-Benzylbenzothiazolylidene Carbene Catalyzed Isomerization of Dimethyl Maleate to Dimethyl Fumarate: Experimental and Theoretical Results

Background:

N-Heterocyclic Carbenes (NHCs) have emerged as ubiquitous species having applications in a broad range of fields, including organocatalysis and organometallic chemistry. Since Arduengo and co-workers first isolated a bottlable NHC, namely imidazol-2-ylidene derivative, these nucleophilic species have attained a prominent place in synthetic organic chemistry. The NHC-induced non-asymmetric catalysis has turned out to be a really fruitful area of research in recent years.

Methods and Results:

The quantitative aspects of the experimental and theoretical investigation of isomerization of dimethyl maleate to dimethyl fumarate catalyzed by an N-heterocyclic carbene (NHC), namely 3-benzylbenzothiazolylidene are being reported for the first time. Dimethyl maleate on treating with 3-benzylbenzothiazolylidene carbene (10 mol%), generated in situ from the reaction of 3- benzylbenzothiazolium bromide with triethylamine in diethyl ether at room temperature under nitrogen atmosphere isomerizes quantitatively to dimethyl fumarate. Theoretical investigation of a model reaction scheme at the wB97XD/6-31+G(d) level reveals that initial attack of the carbene, which is the ratedetermining step, is followed by rotation about the C-C bond in preference to a higher activation free energy path involving proton abstraction. The species so formed splits off the carbene to yield dimethyl fumarate. Eyring equation has been used to rationalize the effect of temperature on the isomerization rate.

Conclusions and Perspective:

3-Benzylbenzothiazolylidene carbene catalyzes the isomerization of dimethyl maleate to its trans-isomer. This carbene can be used in other catalytic reactions, such as acyloin condensation and Stetter reaction.

The Mechanism of N-Heterocyclic Carbene Organocatalysis through a Magnifying Glass

The term "N-Heterocyclic carbene organocatalysis" is often invoked in organic synthesis for reactions that are catalyzed by different azolium salts in the presence of bases. Although the mechanism of these reactions is considered today evident, a closer look into the details that have been collected throughout the last century reveals that there are many open questions and even contradictions in the field. Emerging new theoretical and experimental results offer solutions to these problems, because they show that through considering alternative reaction mechanisms a more consistent picture on the catalytic process can be obtained. These novel perspectives will be able to extend the scope of the reactions that we call today N-heterocyclic carbene organocatalysis.

New frontiers in the transition-metal-free synthesis of heterocycles from alkynoates: an overview and current status

This review highlights the successful utilization of transition-metal-free approaches for the modular assembly of various heterocycles from alkynoates.

Catalytic coupling of biomass-derived aldehydes into intermediates for biofuels and materials

Catalytic upgrading of biomass-based aldehydes into chain-extended intermediates for downstream applications in biofuels, fine chemicals, and renewable materials, is reviewed.

Evidence for the spontaneous formation of N-heterocyclic carbenes in imidazolium based ionic liquids

We present a study of the reactions of aldehydes in ionic liquids which gives evidence for the spontaneous formation of N-heterocyclic carbenes in ionic liquids based on 1,3-dialkyl substituted imidazolium cations from the lack of a deuterium isotope effect on the reaction of these ionic liquids with aldehydes.

Gas Phase Studies of N-Heterocyclic Carbene-Catalyzed Condensation Reactions

N-Heterocyclic carbenes (NHCs) catalyze Umpolung condensation reactions of carbonyl compounds, including the Stetter reaction. These types of reactions have not heretofore been examined in the gas phase. Herein, we explore the feasibility of examining these reactions in the absence of solvent. A charge-tagged thiazolylidene catalyst is used to track the reactions by mass spectrometry. We find that the first Umpolung step, the addition of the NHC catalyst to a carbonyl compound to form the "Breslow intermediate", does not readily proceed in the gas phase, contrary to the case in solution. The use of acylsilanes in place of the carbonyl compounds appears to solve this issue, presumably because of a favorable Brook rearrangement. The second addition reaction, with enones, does not occur under our gas phase conditions. These reactions do occur in solution; the differential reactivity between the condensed and gas phases is discussed, and calculations are used to aid in the interpretation of the results.

Rate and equilibrium constants for the addition of N-heterocyclic carbenes into benzaldehydes: a remarkable 2-substituent effect

Rate and equilibrium constants for the reaction between N-aryl triazolium N-heterocyclic carbene (NHC) precatalysts and substituted benzaldehyde derivatives to form 3-(hydroxybenzyl)azolium adducts under both catalytic and stoichiometric conditions have been measured. Kinetic analysis and reaction profile fitting of both the forward and reverse reactions, plus onwards reaction to the Breslow intermediate, demonstrate the remarkable effect of the benzaldehyde 2-substituent in these reactions and provide insight into the chemoselectivity of cross-benzoin reactions.

Rate and Equilibrium Constants for the Addition of N-Heterocyclic Carbenes into Benzaldehydes: A Remarkable 2-Substituent Effect

Rate and equilibrium constants for the reaction between N‐aryl triazolium N‐heterocyclic carbene (NHC) precatalysts and substituted benzaldehyde derivatives to form 3‐(hydroxybenzyl)azolium adducts under both catalytic and stoichiometric conditions have been measured. Kinetic analysis and reaction profile fitting of both the forward and reverse reactions, plus onwards reaction to the Breslow intermediate, demonstrate the remarkable effect of the benzaldehyde 2‐substituent in these reactions and provide insight into the chemoselectivity of cross‐benzoin reactions.

Zwitterionic carbene adducts and their carbene isomers

N-heterocyclic carbenes (NHC) and abnormal NHCs (aNHC) form the stable adducts,1and2, with X (CH₂, SiH₂, NH, PH, O, S), which are excellent nucleophiles at X.

Experimental mechanistic studies of the tail-to-tail dimerization of methyl methacrylate catalyzed by N-heterocyclic carbene

We and others have previously reported the intermolecular umpolung reactions of Michael acceptors catalyzed by an N-heterocyclic carbene (NHC). The representative tail-to-tail dimerization of methyl methacrylate (MMA) has now been intensively investigated, leading to the following conclusions: (1) The catalysis involves the deoxy-Breslow intermediate, which is quite stable and remains active after the catalysis. (2) Addition of the intermediate to MMA and the final catalyst elimination are the rate-limiting steps. Addition of the NHC to MMA and the proton transfers are relatively very rapid. (3) The two alkenyl protons of the first MMA undergo an intermolecular transfer to C3 and C5 of the dimer. (4) The initial proton transfer is intermolecular. (5) Compared with the benzoin condensation, noticeable differences in the kinetics, reversibility, and stability of the intermediates are observed.

N-heterocyclic carbene catalyzed synthesis of oxime esters

A triazolium salt derived N-heterocyclic carbene catalyzes the redox esterification reaction between α-β-unsaturated aldehydes and oximes. The resulting saturated oxime esters were obtained in very good yields for a broad range of aliphatic, aromatic and heteroaromatic substrates.

The effect of the N-mesityl group in NHC-catalyzed reactions

The majority of N-heterocyclic carbene catalyzed reactions of α-functionalized aldehydes, including annulations, oxidations, and redox reactions, occur more rapidly with N-mesityl substituted NHCs. In many cases, no reaction occurs with NHCs lacking ortho-substituted aromatics. By careful competition studies, catalyst analogue synthesis, mechanistic investigations, and consideration of the elementary steps in NHC-catalyzed reactions of enals, we have determined that the effect of the N-mesityl group is to render the initial addition of the NHC to the aldehyde irreversible, thereby accelerating the formation of the Breslow intermediate. These studies rationalize the experimentally observed catalyst preference for all classes of NHC-catalyzed reactions of aldehydes and provide a roadmap for catalyst selection and design.

Organocatalytic hydroacylation of unactivated alkenes

N-Heterocyclic carbenes interact with aldehydes to generate the Breslow intermediate, a rendering of the prototypical electrophile into a nucleophile (umpolung). Recent work has indicated that these intermediates may also add to simple, unpolarized alkenes. The use of a chiral precatalyst leads to the generation of the derived adducts with high yields and very high selectivities.

α,β-Unsaturated acyl azoliums from N-heterocyclic carbene catalyzed reactions: observation and mechanistic investigation

Catalytically generated acyl azoliums I and their α,β-unsaturated counterparts II are thought to be key reactive intermediates in a rapidly growing number of transformations promoted by N-heterocyclic carbene (NHC) catalysts.[1 ] Acyl azoliums are invoked in the postulated catalytic cycles of nearly all of the new NHC-catalyzed reactions of α-functionalized aldehydes reported since 2004, in which they are generally assumed to possess the reactivity of an activated carboxylic acid, that is, analogous to an activated ester.[2 ] In NHC-catalyzed processes, they are most often obtained through internal redox reactions of functionalized aldehydes but have also been prepared by oxidations of the Breslow intermediates[3 ] or additions to ketenes.[4 ] Acyl azoliums I are important intermediates in thiamine pyrophosphate (ThPP) dependent enzymatic reactions.[5 ] Townsend et al. have recently proposed that unsaturated acyl azolium III is the key intermediate in clavulanic acid biosynthesis;[6 ] despite careful efforts, III or its analogues II have never been characterized or independently synthesized. Here, we document the observation and characterization of α,β-unsaturated acyl azoliums 1 and 2 (Scheme 1 ) and demonstrate that their corresponding hemiacetals (1 ′ and 2 ”) are the kinetically important intermediates in both their acylation and annulation reactions.

Theoretical investigations on the mechanism of benzoin condensation catalyzed by pyrido[1,2-a]-2-ethyl[1,2,4]triazol-3-ylidene

A new annulated N-heterocyclic carbene (NHC), pyrido[1,2-a]-2-ethyl[1,2,4]triazol-3-ylidene, has been synthesized and its good catalytic activity for benzoin condensation has been experimentally determined by You and co-workers recently [ Ma , Y. J. , Wei , S. P. , Lan , J. B. , Wang , J. Z. , Xie , R. G. , and You , J. S. J. Org. Chem. 2008 , 73 , 8256 ]. In this work, the mechanism of the title reaction has been intensively studied computationally by employing the density functional theory (B3LYP) method in conjunction with 6-31+G(d) and 6-311+G(2d,p) basis sets. Our results indicate that path A (in which a sequence of intermolecular proton transfers between two carbene/benzaldehyde coupling intermediates affords enamine) and path B (in which a t-BuOH assisted hydrogen transfer generates enamine) proposed on the basis of the Breslow mechanism are competitive for their similar barriers. In path A, the first intermolecular proton transfer between two N-heterocyclic carbene/benzaldehyde coupled intermediates to form tertiary alcohol and enolate anion is theoretically the rate-determining step with corresponding barrier (30.93 kcal/mol), while the t-BuOH assisted hydrogen transfer generating Breslow enamine is the rate-determining step with corresponding barrier (28.84 kcal/mol) in path B. The coupling of carbene and benzaldehyde, and the coupling of enamine and another benzaldehyde to form a C-C bond are partially rate-determining for their relatively significant barriers (24.06 and 26.95 kcal/mol, respectively), being the same in both paths A and B. Our results are in nice agreement with the experimental result in a kinetic investigation of thiazolium ion-catalyzed benzoin condensation performed by White and Leeper in 2001.

Artificial enzymes with thiazolium and imidazolium coenzyme mimics

Hydrophobic thiazolium and imidazolium coenzyme mimics in the presence of modified-polyethylenimine enzyme mimics catalyze the benzoin condensation 2300-3300 times faster than the coenzyme mimics alone. Polycationic enzyme mimics provide not only a hydrophobic binding domain for coenzyme and substrate, but also electrostatic stabilization of anionic species that arise along the reaction pathway of the benzoin condensation.