New evidence supporting bis(thiazolin-2-ylidene)s as the actual catalytic species in the benzoin condensation

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.

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.

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.

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.

N-Heterocyclic Carbenes as Organocatalysts

Organocatalyzed reactions represent an attractive alternative to metal-catalyzed processes notably because of their lower cost and benign environmental impact in comparison to organometallic catalysis. In this context, N-heterocyclic carbenes (NHCs) have been studied for their ability to promote primarily the benzoin condensation. Lately, dramatic progress in understanding their intrinsic properties and in their synthesis have made them available to organic chemists. This has resulted in a tremendous increase of their scope and in a true explosion of the number of papers reporting NHC-catalyzed reactions. Here, we highlight the ever-increasing number of reactions that can be promoted by N-heterocyclic carbenes.

Nucleophilic carbenes in asymmetric organocatalysis

The coenzyme thiamine (vitamin B1), a natural thiazolium salt, is involved in many enzymatic catalyses. Since it has been proposed that the catalytically active species of these reactions is a nucleophilic carbene, many chemists have tried to perform enzyme mimetic asymmetric carbene catalysis. After a long and difficult search, stable carbenes are finally isolated, characterized, and in the chemist's hands. The experiments of decades have finally resulted in successful enantioselective benzoin condensations and enantioselective intramolecular Stetter reactions as important examples of carbene catalyzed asymmetric nucleophilic acylation processes.

Asymmetric Benzoin Condensation Catalyzed by Chiral Rotaxanes Tethering a Thiazolium Salt Moiety via the Cooperation of the Component: Can Rotaxane Be an Effective Reaction Field?

Although some reactions on rotaxanes have been reported, the characteristic features of the rotaxanes providing unique reaction fields have hardly been studied, especially as catalyst. In our continuous studies on interlocked molecules such as rotaxanes and catenanes, we have noticed the importance of such interlocked structures with high freedom in functionalized materials such as molecular catalyst. For catalytic asymmetric benzoin condensations, two optically active rotaxanes possessing thiazolium salt moieties were prepared using the binaphthyl group as the chiral auxiliary. The benzoin condensations of aromatic aldehydes catalyzed by the chiral rotaxanes as catalysts gave optically active benzoins with ca. 30% ee in moderate to high chemical yields depending upon the structure of rotaxane and the reaction conditions employed. From the results, two intrarotaxane chirality transfers are confirmed: (i) through-space chirality transfer from wheel to axle and (ii) through-bond chirality transfer controlled with an achiral wheel. Because these asymmetric reaction fields are specific to the rotaxane structure, the importance and possibility of the "rotaxane field" as a particular reaction field is demonstrated in this work.