Ab initio and density functional theory evidence on the rate-limiting step in the Morita-Baylis-Hillman reaction

Acyl Transfer-Driven Rauhut-Currier Dimerization of Morita-Baylis-Hillman Ketones

A serendipitous Rauhut-Currier dimerization of 1,1-disubstituted activated olefins derived from Morita-Baylis-Hillman adducts was observed in the presence of DABCO. The reaction is driven by the migration of an acyl group and produces multifunctionalized enol esters in yields greater than 90% in most cases, without necessitating column chromatographic purification. The acyl transfer is thought to proceed via a transition state typical of a Morita-Baylis-Hillman (MBH) reaction, supported by a brief mechanistic study involving computational calculations.

Morita-Baylis-Hillman Reaction: How Do Optimal Enzyme Active Sites Compare with Organocatalysts

Morita-Baylis-Hillman reaction attracts significant attention for the synthesis of highly functionalized compounds. It requires multiple catalytic elements for efficient catalysis, making it an appealing target for the design of a...

Molecular Insights on Solvent Effects in Organic Reactions as Obtained through Computational Chemistry Tools

Molecular understanding of the role of protic solvents in a gamut of organic transformations can be developed using density functional and ab initio computational studies focused on the reaction mechanism. Inclusion of explicit solvent molecules in the vital TSs has been proven to be valuable toward improving the energetic estimates of organocatalytic as well as transition-metal-catalyzed organic reactions. Herein, we provide an overview of the importance of an explicit-implicit solvation model using a number of interesting examples.

Towards a converged strategy for including microsolvation in reaction mechanism calculations

A major part of chemical conversions is carried out in the fluid phase, where an accurate modeling of the involved reactions requires to also take into account solvation effects. Implicit solvation models often cover these effects with sufficient accuracy but can fail drastically when specific solvent-solute interactions are important. In those cases, microsolvation, i.e., the explicit inclusion of one or more solvent molecules, is a commonly used strategy. Nevertheless, microsolvation also introduces new challenges-a consistent workflow as well as strategies how to systematically improve prediction performance are not evident. For the COSMO and COSMO-RS solvation models, this work proposes a simple protocol to decide if microsolvation is needed and how the corresponding molecular model has to look like. To demonstrate the improved accuracy of the approach, specific application examples are presented and discussed, i.e., the computation of aqueous pK_a values and a mechanistic study of the methanol mediated Morita-Baylis-Hillman reaction.

B(C6F5)3-catalyzed dehydrogenative cyclization of N-tosylhydrazones and anilines via a Lewis adduct: a combined experimental and computational investigation

Tris(pentafluorophenyl)borane-catalyzed dehydrogenative-cyclization of N-tosylhydrazones with aromatic amines has been disclosed. This metal-free catalytic protocol is compatible with a range of functional groups to provide both symmetrical and unsymmetrical 3,4,5-triaryl-1,2,4-triazoles. Mechanistic experiments and density functional theory (DFT) studies suggest an initial Lewis adduct formation of N-tosylhydrazone with B(C6F5)3 followed by sequential intermolecular amination of the borane adduct with aniline, intramolecular cyclization and frustrated Lewis pair (FLP)-catalyzed dehydrogenation for the generation of substituted 1,2,4-triazoles.

Morita-Baylis-Hillman reaction of a chiral aziridine aldehyde

The Morita-Baylis-Hillman reaction of (R)-1-((R)-1-phenylethyl)aziridine-2-carbaldehyde with alkyl acrylate was carried out under various conditions by changing solvents, bases, and alcohol additives. The reaction at room temperature under neat conditions (no solvent) with quinuclidine as an amine nucleophile, in the presence of benzyl alcohol as an additive, afforded a product, γ-(aziridin-2-yl)-β-hydroxy-α-methylene butanoate, in 97% yield with a diastereomeric ratio of anti and syn as 86 : 14.

Mechanism and reactivity in the Morita-Baylis-Hillman reaction: the challenge of accurate computations

A systematic density functional theory exploration of various reactive steps together with benchmark coupled cluster results are used to propose an accurate model of the mechanism of the Morita-Baylis-Hillman (MBH) reaction in organic chemistry. This reaction has attracted considerable interest from the synthetic and mechanistic points of view in recent years, with both computational and experimental mechanistic studies. It has recently (R. E. Plata and D. A. Singleton, J. Am. Chem. Soc., 2015, 137, 3811-3826) been correctly pointed out that previous computational studies failed to reproduce known mechanistic features of the reaction. The same study argued that computation is simply not able at the present time to provide accurate models for such reactions. This second claim is shown by our present work to overstate the problem: by using current 'state of the art' methodology, our results are fully consistent with observed behavior within the expected error bars of 1-5 kcal mol^-1, far smaller than the errors reported in Plata and Singleton's study. On the basis of exhaustive calculations reported here, we suggest that our proposed approaches for modeling electronic structure, solvation, and entropy should be able to provide accurate predictions for many more reactions. We also suggest that reactions like the MBH reaction, where solvation and entropy effects are particularly large, are among the hardest for computational mechanistic studies.

Isotope exchange reaction in tritium-contaminated vacuum pump oil: mechanism and HTO effect

HTO is not only a reactant, but also acts as the proton shuttle in the transition state.

A computational investigation of the solvent-dependent enantioselective intramolecular Morita–Baylis–Hillman reaction of enones

A DFT study of the enantioselective organocatalytic intramolecular Morita–Baylis–Hillman (IMBH) reaction of enones under the influence of 1,1,1,3,3,3-hexafluoroisopropanol is reported. This study establishes an excellent fit between the experiment and theory.

A mechanistic study on guanidine-catalyzed chemical fixation of CO2 with 2-aminobenzonitrile to quinazoline-2,4(1H,3H)-dione

Both basicity of TMG and acidity of the [TMGH]⁺ guanidinium are crucial for a reaction.

Morita-Baylis-Hillman Reaction of β,β-Disubstituted Enones: An Enantioselective Organocatalytic Approach for the Synthesis of Cyclopenta[b]annulated Arenes and Heteroarenes

The first enantioselective organocatalytic intramolecular Morita-Baylis-Hillman (MBH) reaction of sterically highly demanding β,β-disubstituted enones is presented. The MBH reaction of β,β-disubstituted-α,β-unsaturated electron-withdrawing systems was previously considered to be unfeasible. Towards this end, designer substrates, which under simple and practical reaction conditions generate a variety of cyclopenta[b]annulated arenes and heteroarenes in excellent enantiopurities and near-quantitative yields in remarkably short reaction times, are described. The reason for the unusually facile nature of this reaction is attributed to the synergy guided and entropically favored intramolecular reaction. Further, this strategy provides easy access to a substantial number of bioactive natural products and pharmaceutically significant compounds.

A case study of the mechanism of alcohol-mediated Morita Baylis-Hillman reactions. The importance of experimental observations

The mechanism of the Morita Baylis-Hillman reaction has been heavily studied in the literature, and a long series of computational studies have defined complete theoretical energy profiles in these reactions. We employ here a combination of mechanistic probes, including the observation of intermediates, the independent generation and partitioning of intermediates, thermodynamic and kinetic measurements on the main reaction and side reactions, isotopic incorporation from solvent, and kinetic isotope effects, to define the mechanism and an experimental mechanistic free-energy profile for a prototypical Morita Baylis-Hillman reaction in methanol. The results are then used to critically evaluate the ability of computations to predict the mechanism. The most notable prediction of the many computational studies, that of a proton-shuttle pathway, is refuted in favor of a simple but computationally intractable acid-base mechanism. Computational predictions vary vastly, and it is not clear that any significant accurate information that was not already apparent from experiment could have been garnered from computations. With care, entropy calculations are only a minor contributor to the larger computational error, while literature entropy-correction processes lead to absurd free-energy predictions. The computations aid in interpreting observations but fail utterly as a replacement for experiment.

Kinetic and mechanistic investigations of the Baylis–Hillman reaction in ionic liquids

We report here a quantitative study of the kinetics and mechanism of the Baylis–Hillman reaction in the presence of ionic liquids as solvent media.

Microsolvated transition state models for improved insight into chemical properties and reaction mechanisms

Over the years, several methods have been developed to effectively represent the chemical behavior of solutes in solvents. The environmental effects arising due to solvation can generally be achieved either through inclusion of discrete solvent molecules or by inscribing into a cavity in a homogeneous and continuum dielectric medium. In both these approaches of computational origin, the perturbations on the solute induced by the surrounding solvent are at the focus of the problem. While the rigor and method of inclusion of solvent effects vary, such solvation models have found widespread applications, as evident from modern chemical literature. A hybrid method, commonly referred to as cluster-continuum model (CCM), brings together the key advantages of discrete and continuum models. In this perspective, we intend to highlight the latent potential of CCM toward obtaining accurate estimates on a number of properties as well as reactions of contemporary significance. The objective has generally been achieved by choosing illustrative examples from the literature, besides expending efforts to bring out the complementary advantages of CCM as compared to continuum or discrete solvation models. The majority of examples emanate from the prevalent applications of CCM to organic reactions, although a handful of interesting organometallic reactions have also been discussed. In addition, increasingly accurate computations of properties like pK(a) and solvation of ions obtained using the CCM protocol are also presented.

A unified mechanistic view on the Morita-Baylis-Hillman reaction: computational and experimental investigations

The thermodynamic properties and reaction mechanism of the Morita-Baylis-Hillman (MBH) reaction have been investigated through experimental and computational techniques. The impossibility to accelerate this synthetically valuable transformation by increasing the reaction temperature has been rationalized by variable-temperature experiments and MP2 theoretical calculations of the reaction thermodynamics. An increase in temperature results in a switching of the equilibrium to the reactants occurring at even moderate temperature levels. The complex reaction mechanism for the MBH reaction has been investigated through an in-depth analysis of the suggested alternative pathways, using the M06-2X computational method. The results provided by this theoretical approach are in agreement with all the experimental/kinetic evidence such as reaction order, acceleration by protic species (methanol, phenol), and autocatalysis. In particular, the existing controversy about the character of the key proton transfer in the MBH reaction (Aggarwal versus McQuade pathways) has been resolved. Depending on the specific reaction conditions both suggested pathways are competing mechanisms, and depending on the amount of protic species and the reaction progress (early or late stage) either of the two mechanisms will be favored.

Highly chemoselective Baylis-Hillman and aldol reactions of 2H-thiopyran-4(3H)-one using tertiary amine catalysts in aqueous media

For the first time, the Baylis-Hillman (BH) reaction of 2H-thiopyran-4(3H)-one is investigated, and surprisingly, the reaction of 2H-thiopyran-4(3H)-one with aldehydes in the presence of different tertiary amines shows excellent chemo- and regioselectivity in water. At room temperature, DBU affords BH adducts, but with DABCO, aldol products were obtained. In the case of DABCO, Et(3)N, or DMAP, domino aldol-rearrangement reactions occurred at 45-50 °C.

Computational investigation on the mechanism and the stereoselectivity of Morita-Baylis-Hillman reaction and the effect of the bifunctional catalyst N-methylprolinol

The mechanism of the Morita-Baylis-Hillman (MBH) reaction between formaldehyde and methyl vinyl ketone (MVK) catalyzed by N-methylprolinol was investigated using density functional theory (DFT) method. The overall reaction includes two steps: C-C bond formation and hydrogen migration. In the presence of water, the hydrogen migration occurs via a six-membered ring transition state and the corresponding energy barrier decreases dramatically, and therefore the RDS is the C-C bond formation step. The calculations indicate that the C-C bond formation step controls the stereochemistry of the reaction. In this step, the hydrogen bonding induces the direction of the attack of enamine to aldehyde from the -OH group side of N-methylprolinol. The energy-favored transition states are mainly stabilized by hydrogen bonding, while the chirality of the products is affected by the hydrogen bonding and the steric hindrance. The calculations correctly reproduce the major product in (R)-configuration, which is consistent with the experimental observation.

The performance of computational techniques in locating the charge separated intermediates in organocatalytic transformations

The formation of zwitterionic adducts between neutral nucleophiles such as NMe(3) and PMe(3) with neutral electrophiles such as methyl vinyl ketone (MVK) has been studied with a wide variety of theoretical methods. It has been found that hybrid density functional methods such as B3LYP are not capable of describing these zwitterionic structures as minima on the potential energy surface. This is also true for combinations of MP2 theory with basis sets lacking diffuse basis functions. The mPW1K hybrid functional, in contrast, correctly describes zwitterionic adducts as true intermediates on the PES. On the basis of this insight, a new version of the G3 compound energy scheme has been developed for the accurate description of zwitterionic structures. It has also been verified that modifications of the B2-PLYP double-hybrid functional are equally capable of the proper description of zwitterionic adducts. The applicability of this latter class of methods to a larger dataset involving combinations of different nucleophiles and electrophiles has been documented.

Enantioselective, organocatalytic Morita-Baylis-Hillman and Aza-Morita-Baylis-Hillman reactions: stereochemical issues

Conscious of the importance that stereochemical issues may have on the design of efficient organocatalyts for both Morita-Baylis-Hillman and aza-Morita-Baylis-Hillman reaction we have analyzed them in this minireview. The so-called standard reactions involve "naked" enolates which therefore should lead to the syn adducts as the major products, irrespective of the E, Z stereochemistry of the enolate. Accordingly, provided the second step is rate determining step, the design of successful bifunctional or polyfunctional catalysts has to consider the geometrical requirements imposed by the transition structures of the second step of these reactions. On the other hand, MBH and aza-MBH reactions co-catalyzed by (S)-proline and a secondary or tertiary amine (co-catalyst) involve the aldol-type condensation of either a 3-amino-substituted enamine, dienamine, or both, depending on the cases. A Zimmerman-Traxler mechanism defines the stereochemical issues regarding these co-catalyzed condensations which parallel those of the well established (S)-proline catalyzed aldol-like reactions.

Mechanistic insights and the role of cocatalysts in Aza-Morita-Baylis-hillman and Morita-Baylis-Hillman reactions

The mechanism of the trimethylamine or trimethylphosphine catalyzed aza-Morita-Baylis-Hillman (MBH) reaction between acrolein and mesyl imine is investigated by using ab initio and density functional methods. All key transition states are located at the CBS-4M as well as at the mPW1K/6-31+G** levels of theories. To account for the experimentally known rate enhancements through the use of polar protic cocatalysts, transition state models with explicit cocatalysts are considered. Inclusion of polar protic cocatalysts is found to have a profound influence in decreasing the activation barriers associated with the key elementary steps. The protic cocatalysts such as water, methanol, and formic acid are identified as effective in promoting a relay proton transfer. Interestingly, the efficiency of the relay mechanism results in relatively better stabilization of the proton transfer transition state as compared to the addition of enolate to the electrophile (C-C bond formation). The cocatalyst bound models suggest that the proton transfer could become the rate-determining step in the aza-MBH reaction under polar protic conditions. A comparison of the aza-MBH reaction with the analogous MBH reaction is also attempted to bring out the subtle differences between these two reactions. Enhanced kinetic advantages arising from the nature of the activated electrophile are noticed for the aza-MBH reaction. The difference in the relative energies between the transition states for the proton transfer and the C-C bond formation steps with bound cocatalyst(s) is found to be more pronounced in the aza-MBH reaction. In general, the reported results underscore the importance of considering explicit solvents/cocatalysts in order to account for the likely role of the specific interactions between reactants and solvents/cocatalysts.