Organocatalytic enantioselective construction of Si-stereocenters: recent advances and perspectives

Silicon-stereogenic chiral organosilanes have found increasing applications in synthetic chemistry, medicinal chemistry, and materials science. In this context, various asymmetric catalytic methods have been established for the diverse synthesis of silicon-stereogenic silanes. In particular, asymmetric organocatalysis is emerging as an important and complementary synthetic tool for the enantioselective construction of silicon-stereocenters, along with the rapid development of chiral-metal catalyzed protocols. Its advent provides a powerful platform to achieve functionalized silicon-stereogenic organosilanes with structural diversity, and should lead to great development in chiral organosilicon chemistry. In this Tutorial Review, we highlight these latest achievements from two aspects: desymmetrizations of prochiral tetraorganosilanes and dynamic kinetic asymmetric transformations of racemic organosilanes by employing five organocatalytic activation modes. The advantages, limitations and synthetic value of each protocol, as well as the synthetic opportunities still open for further exploration, are also discussed.

Asymmetric Auto-Tandem Catalysis with Chiral Organosuperbases: Intramolecular Cyclization/Enantioselective Direct Mannich-Type Addition Sequence

An enantioselective addition reaction of alkynyl esters with imines was developed under asymmetric autotandem catalysis with a chiral Brønsted base. A chiral bis(guanidino)iminophosphorane organosuperbase, which has much higher basicity than that of conventional chiral organic bases, efficiently promoted both the intramolecular cyclization for the construction of a benzofuran ring or a benzothiophene ring and the sequential enantioselective direct Mannich-type reaction of diarylmethane derivatives, affording enantio-enriched diarylalkane derivatives that are otherwise difficult to access.

Organocatalysis as an enabling tool for enantioselective ring-opening reactions of cyclopropanes

The rich reactivity profile of cyclopropanes has been extensively explored to trigger new organic transformations that enable unusual disconnective approaches to synthesize molecular motifs that are not easily reached through conventional reactions. In particular, the chemistry of cyclopropanes has received special attention in the last decade, with multiple new approaches that capitalize on the use of organocatalysis for the activation of the cyclopropane scaffold. This situation has also opened the possibility of developing enantioselective variants of many reactions that until now were only carried out in an enantiospecific or diastereoselective manner. Our group has been particularly active in this field, focusing more specifically on the use of aminocatalysis and Brønsted acid catalysis as major organocatalytic activation manifolds to trigger new unprecedented transformations involving cyclopropanes that add to the current toolbox of general methodologies available to organic chemists for the enantioselective synthesis of chiral compounds.

Shaping Chirality via Stereoselective, Organocatalytic [4+2] Cycloadditions involving Heterocyclic ortho-Quinodimethanes

Polycyclic compounds bearing a complex heterocyclic core such as an aromatic heterocycle "fused" with one or more functionalized rings, are widespread leading molecules in the domain of synthetic organic chemistry and pharmaceuticals. Although many synthetic methodologies have been devised to access achiral, fused heteroaromatic scaffolds, or related chiral variants adorned with out-of-cycle stereogenic elements, equally efficient strategies to afford chiral heterocycles featuring in-cycle stereocenters, exist to a lesser extent and presently represent a growing field of investigation. The mild, organocatalytic generation of elusive ortho-quinodimethane intermediates (oQDMs), derived from suitable heteroaromatic carbonyl- or carbonyl-like pronucleophiles has recently proved successful in the synthesis of such peculiar chiral architectures via stereoselective [4+2] cycloadditions. This review provides an overview of the most important advances attained in this field over the last decade.

Construction of 1,3-Nonadjacent Stereogenic Centers Through Enantioselective Addition of α-Thioacetamides to α-Substituted Vinyl Sulfones Catalyzed by Chiral Strong Brønsted Base

An enantioselective addition reaction for the construction of 1,3-nonadjacent stereogenic centers is developed by means of a chiral strong Brønsted base catalyst. The chiral sodium ureate catalyst efficiently promoted the reaction of α-thioacetamides as less acidic pronucleophiles with vinyl sulfones having a variety of α-substituents including aryl, alkyl and halo groups, and α-phenylacrylates, accomplishing the construction of various 1,3-nonadjacent stereogenic centers in highly diastereo- and enantioselective manners. This is a rare example of the construction of 1,3-nonadjacent stereogenic centers with less acidic pronucleophiles. In addition, the application of Michael acceptors having various types of α-substituents in a single catalyst system is achieved for the first time, demonstrating the utility of the present catalyst system for the construction of 1,3-nonadjacent stereogenic centers.

Core Structure-Oriented Asymmetric α-Allenylic Alkylation of Amino Acid Esters Enabled by Chiral Aldehyde/Palladium Catalysis

Aiming at the reported chiral synthons leading to manzacidins A and D, here we report a highly efficient catalytic asymmetric α-allenylic alkylation reaction of NH2-unprotected amino acid esters that is promoted by combined chiral aldehyde/palladium catalysis. Fifty examples of unnatural α,α-disubstituted amino acid esters are reported with good-to-excellent yields and stereoselectivities. Based on this methodology, a key intermediate leading to manzacidin C and its other three stereoisomers is prepared accordingly.

Brønsted Base-Catalyzed Enantioselective α-Functionalization of Carbonyl Compounds Involving π-Extended Enolates

Chiral Brønsted base (BB) catalyzed asymmetric transformations constitute an important tool for synthesis. A meaningful fraction of these transformations proceeds through transiently generated enolate intermediates, which display quite versatile reactivity against a variety of electrophiles. Some years ago, our group became interested in developing BB-catalyzed asymmetric reactions of enolizable carbonyl substrates that involve π-extended enolates in which, besides control of reaction diastereo and enantioselectivity, the site-selectivity control is an additional issue in most cases. In the examples covered in this account the opportunities deployed, and the challenges posed, by these methods are illustrated, with a focus on the generation of quaternary carbon stereocenters. In the way, new bifunctional BB catalysts as well as achiral templates were developed that may find further applications.

Anion-Activated Bases and Nucleophiles Characterized by Photoelectron Spectroscopy

Negative ion photoelectron spectra at 20 K along with ab initio [CCSD(T)] and M06-2X density functional theory calculations are reported for a series of six basic and nucleophilic pyridine derivatives with an anionic substituent [i.e., 3- and 4-PyrBX3-, where X = F, 4-t-BuC6H4, 4-MeOC6H4, and 3,5-(MeO)2C6H3]. Vertical detachment energies (VDEs) of these charge-activated reagents span from 4.50-5.85 eV and are well reproduced by M06-2X/aug-cc-pVTZ and CCSD(T)/maug-cc-pVTZ computations. Surprisingly, the VDEs are found to correlate with the SN2 reactivity of the PPh4+ salts of the substituted pyridine anions with 1-iodooctane in dichloromethane. This provides an experimental measure of the nucleophilicity of these charge-activated anions, which represent a new class of chemical reagent.

Desymmetrization and Kinetic Resolution of Endoperoxides Using a Bifunctional Organocatalyst

Bifunctional thiourea/amine organocatalysts have been used for the desymmetrization of meso-endoperoxides using the Kornblum-DeLaMare reaction, giving 4-hydroxyketones in 78-98% yields with ≤98:2 enantioselectivity. The influence of the catalyst structure, solvent, and temperature was examined. The most promising catalyst was applied to the kinetic resolution of racemic endoperoxides to give enantioenriched materials (≤99:1 er).

Electrostatically Enhanced 3- and 4-Pyridyl Borate Salt Nucleophiles and Bases

A variety of electrostatically enhanced 3- and 4-pyridylborate salt catalysts are reported and show significant improvement over an activated noncharged neutral control compound. Their nucleophilicity in a stoichiometric SN2 reaction and catalytic performance in a urethane synthesis are evaluated along with three methods for rapidly evaluating the basicity of these species. That is, qualitative titrations in CH2Cl2 and CHCl3 were carried out, two separate solution-state IR studies in CCl4 and CDCl3 are reported, and the proton affinities of the anionic components of the salts were computed. Charge differences between the anion and its protonated zwitterionic conjugate acid are evaluated along with the highest occupied molecular orbitals of the anions in relationship to some of the surprising reactivity findings that were observed in the two kinetic studies.

Enantio- and Diastereoselective Mannich Reactions of ß-Dicarbonyls by Second Stage Diastereoconvergent Crystallization

The synthesis of chiral α-monosubstituted-ß-dicarbonyls is a challenging task in asymmetric catalysis due to the rapid, typically uncontrolled, product racemization or epimerization under most reaction conditions. For this reason, diastereoselective additions of unsubstituted ß-dicarbonyls to π-electrophiles are unusual. Herein, we disclose a simple catalytic crystallization-driven enantio- and diastereoselective Mannich reaction for the synthesis of stereodefined α-monosubstituted-ß-keto esters, dissymmetric ß-diesters, dissymmetric ß-diketones, and ß-keto amides that productively leverages product epimerization in solution. Mechanistic studies suggest a scenario where the initial enantioselective, diastereodivergent skeletal assembly is catalyzed by a chiral tertiary amine organocatalyst, which then facilitates second stage crystallization-induced diastereoconvergence to provide the challenging α-stereocenter in excellent stereoselectivity.

Bifunctional Iminophosphorane-Catalyzed Enantioselective Nitroalkane Addition to Unactivated α,β-Unsaturated Esters

Herein we describe the enantioselective intermolecular conjugate addition of nitroalkanes to unactivated α,β-unsaturated esters, catalyzed by a bifunctional iminophosphorane (BIMP) superbase. The transformation provides the most direct access to pharmaceutically relevant enantioenriched γ-nitroesters, utilizing feedstock chemicals, with unprecedented selectivity. The methodology exhibits a broad substrate scope, including β-(fluoro)alkyl, aryl and heteroaryl substituted electrophiles, and was successfully applied on a gram scale with reduced catalyst loading, and, additionally, catalyst recovery was carried out. The formal synthesis of a range of drug molecules, and an enantioselective synthesis of (S)-rolipram were achieved. Additionally, computational studies revealed key reaction intermediates and transition state structures, and provided rationale for high enantioselectivities, in good agreement with experimental results.

Asymmetric Organocatalysis in the Remote (3 + 2)-Cycloaddition to 4-(Alk-1-en-1-yl)-3-cyanocoumarins

The application of organocatalytic bifunctional activation in the remote (3 + 2)-cycloaddition between 4-(alk-1-en-1-yl)-3-cyanocoumarins and imines derived from salicylaldehyde is demonstrated. Products, bearing two biologically relevant units, have been obtained with good chemical and stereochemical efficiency. The stereochemical outcome of the process results from the application of a quinine-derived catalyst. Selected transformations of the cycloadducts leading to further chemical diversity have been demonstrated.

Catalytic Asymmetric α-Functionalization of α-Branched Aldehydes

Aldehydes constitute a main class of organic compounds widely applied in synthesis. As such, catalyst-controlled enantioselective α-functionalization of aldehydes has attracted great interest over the years. In this context, α-branched aldehydes are especially challenging substrates because of reactivity and selectivity issues. Firstly, the transient trisubstituted enamines and enolates resulting upon treatment with an aminocatalyst or a base, respectively, would exhibit attenuated reactivity; secondly, mixtures of E- and Z-configured enamines/enolates may be formed; and third, effective face-discrimination on such trisubstituted sp2 carbon intermediates by the incoming electrophilic reagent is not trivial. Despite these issues, in the last 15 years, several catalytic approaches for the α-functionalization of prostereogenic α-branched aldehydes that proceed in useful yields and diastereo- and enantioselectivity have been uncovered. Developments include both organocatalytic and metal-catalyzed approaches as well as dual catalysis strategies for forging new carbon–carbon and carbon–heteroatom (C-O, N, S, F, Cl, Br, …) bond formation at Cα of the starting aldehyde. In this review, some key early contributions to the field are presented, but focus is on the most recent methods, mainly covering the literature from year 2014 onward.

Regio- and Enantioselective Allylic Cyanomethylation by Synergistic Rhodium and Silane Catalysis

Rh/silane-cocatalyzed regio- and enantioselctive allylic cyanomethylation with inert acetonitrile directly has been developed. Addition of a catalytic amount neutral silane reagent as an acetonitrile anion carrier is essential for the success of this reaction. The synthesis of mono- and bis-allylation products can be switched by adjusting the size of substituents on the silane, ligands, and temperature. Chiral homoallylic nitriles could be synthesized in above 20:1 branch/linear ratio, up to 98% yield and >99% ee.

Accessing Chiral Pyrrolodiketopiperazines under Organocatalytic Conditions

The production of chiral pyrrolodiketopiperazines under organocatalytic conditions demonstrates the capacity of bicyclic acylpyrrol lactims to perform as pronucleophiles in direct carbon-carbon bond forming reactions. The good performance of ureidoaminal-derived Brønsted bases in the Michael addition to nitroolefins affords these heterocyclic scaffolds with high skeleton diversity.

Amine-Rich Carbon Dots as Novel Nano-Aminocatalytic Platforms in Organic Synthesis

The development of novel and effective metal-free catalytic systems, which can drive value-added organic transformations in environmentally benign solvents (for instance, water), is highly desirable. Moreover, these new catalysts need to be harmless, easy-to-prepare, and potentially recyclable. In this context, amine-rich carbon dots (CDs) have recently emerged as promising nano-catalytic platforms. These nitrogen-doped nanoparticles, which show dimensions smaller than 10 nm, generally consist of carbon cores that are surrounded by shells containing numerous amino groups. In recent years, organic chemists have used these surface amines to guide the design of several synthetic methodologies under mild operative conditions. This Concept highlights the recent advances in the synthesis of amine-rich carbon dots and their applications in organic catalysis, including forward-looking opportunities within this research field.

Bifunctional Iminophosphorane-Catalyzed Enantioselective Sulfa-Michael Addition to Unactivated α,β-Unsaturated Amides

The first metal-free catalytic intermolecular enantioselective Michael addition to unactivated α,β-unsaturated amides is described. Consistently high enantiomeric excesses and yields were obtained over a wide range of alkyl thiol pronucleophiles and electrophiles under mild reaction conditions, enabled by a novel squaramide-based bifunctional iminophosphorane catalyst. Low catalyst loadings (2.0 mol %) were achieved on a decagram scale, demonstrating the scalability of the reaction. Computational analysis revealed the origin of the high enantiofacial selectivity via analysis of relevant transition structures and provided substantial support for specific noncovalent activation of the carbonyl group of the α,β-unsaturated amide by the catalyst.

The design of C2-chiral bases with dual basic sites predicted to exceed the range of superbasicity: a computational study

A set of C2-chiral bases with bis-protonation sites (1–9) on dimethyl-binaphthalene derivatives have been explored using the B3LYP/6-311+G(d,p) level and the proton affinities calculated are in the range of superbases for both the sites.

Recent advances in the organocatalytic applications of cyclopropene- and cyclopropenium-based small molecules

The development of novel small molecule-based catalysts for organic transformations has increased noticeably in the last two decades. A very recent addition to this particular research area is cyclopropene- and cyclopropenium-based catalysts. At one point in time, particularly in the mid-20^th century, much attention was focused on the structural aspects and physical properties of cyclopropene-based compounds. However, a paradigm shift was observed in the late 20^th century, and the focus shifted to the synthetic utility of these compounds. In fact, a wide range of cyclopropene derivatives have been found serving as valuable synthons for the construction of carbocycles, heterocycles and other useful organic compounds. In the last few years, the catalytic applications of cyclopropene/cyclopropenium-based compounds have been uncovered and many synthetic protocols have been developed using cyclopropene-based compounds as organocatalysts. Therefore, the main objective of this review is to highlight recent developments in the catalytic applications of cyclopropene-based small molecules in different areas of organocatalysis such as phase-transfer catalysis (PTC), Brønsted base catalysis, hydrogen-bond donor catalysis, nucleophilic carbene catalysis, and electrophotocatalysis developed within the past two decades.

Stable Singlet Carbenes as Organic Superbases

A simple experimental procedure for scaling carbene Brønsted basicity is described. The results highlight the strong basicity of pyrazol-4-ylidenes, a type of mesoionic carbene, also named cyclic-bentallenes (CBA). They are more basic (pK_aH >42.7 in acetonitrile) than the popular proazaphosphatrane Verkade bases, and even the Schwesinger phosphazene superbase P₄ (^t Bu). The basicity of these compounds can readily be tuned, and they are accessible in multigram quantities. These results open new avenues for carbon centered superbases.

Synthesis of β-Hydroxy α-Amino Acids Through Brønsted Base-Catalyzed syn-Selective Direct Aldol Reaction of Schiff Bases of Glycine o-Nitroanilide

Here we report the highly enantio- and syn-selective synthesis of β-hydroxy α-amino acids from glycine imine derivatives under Brønsted base (BB) catalysis. The key of this approach is the use of benzophenone-derived imine of glycine o-nitroanilide as a pronucleophile, where the o-nitroanilide framework provides an efficient hydrogen-bonding platform that accounts for nucleophile reactivity and diastereoselectivity.

Hydroxyl-group-activated azomethine ylides in organocatalytic H-bond-assisted 1,3-dipolar cycloadditions and beyond

1,3-Dipolar cycloaddition constitutes a powerful means for the synthesis of five-membered heterocycles. Recently, the potential of this field of chemistry has been expanded by the employment of organocatalytic activation strategies. One group of substrates, namely imines derived from salicylaldehydes, is particularly useful. Additional activation via intramolecular H-bonding interactions offered by the presence of an ortho-hydroxyl phenolic group in their structure results in increased reactivity of these reactants. Furthermore, it can be utilized in subsequent reactions creating chemical and stereochemical diversity. This minireview provides a summary of the recent progress in this field of organocatalysis and indicates other important applications of hydroxyl-group-activated azomethine ylides in asymmetric organocatalysis.

Probing α-Amino Aldehydes as Weakly Acidic Pronucleophiles: Direct Access to Quaternary α-Amino Aldehydes by an Enantioselective Michael Addition Catalyzed by Brønsted Bases

The high tendency of α-amino aldehydes to undergo 1,2-additions and their relatively low stability under basic conditions have largely prevented their use as pronucleophiles in the realm of asymmetric catalysis, particularly for the production of quaternary α-amino aldehydes. Herein, it is demonstrated that the chemistry of α-amino aldehydes may be expanded beyond these limits by documenting the first direct α-alkylation of α-branched α-amino aldehydes with nitroolefins. The reaction produces densely functionalized products bearing up to two, quaternary and tertiary, vicinal stereocenters with high diastereo- and enantioselectivity. DFT modeling leads to the proposal that intramolecular hydrogen bonding between the NH group and the carbonyl oxygen atom in the starting α-amino aldehyde is key for reaction stereocontrol.

Enantioselective Protonation: Hydrophosphinylation of 1,1-Vinyl Azaheterocycle N-Oxides Catalyzed by Chiral Bis(guanidino)iminophosphorane Organosuperbase

Enantioselective protonation by hydrophosphinylation of diarylphosphine oxides with 2-vinyl azaheterocycle N-oxide derivatives was demonstrated using chiral bis(guanidino)iminophosphorane as the higher-order organosuperbase catalyst. It was confirmed by several control experiments that a chiral weak conjugate acid of the chiral bis(guanidino)iminophosphorane, instead of achiral diarylphosphine oxides, directly functioned as the proton source to afford the corresponding product in a highly enantioselective manner in most cases. Enantioselective protonation by a weak conjugate acid generated from the higher-order organosuperbase would broaden the scope of enantioselective reaction systems because of utilization of a range of less acidic pronucleophiles. This method is highlighted by the valuable synthesis of a series of chiral P,N-ligands for chiral metal complexes through the reduction of phosphine oxide and N-oxide units of the corresponding product without loss of enantiomeric purity.

Brønsted Base-Catalyzed Formal Reductive [3+2] Annulation of 4,4,4-Trifluorocrotonate and α-Iminoketones

A formal reductive [3+2] annulation of 4,4,4-trifluorocrotonate and α-iminoketones was developed under Brønsted base catalysis. A single phosphazene base efficiently catalyzes the one-pot tandem reaction involving two mechanistically different elementary processes, namely the chemoselective reduction of an imine moiety of α-iminoketones with thiols as the reductant and the subsequent intermolecular Michael addition of an enolate of α-aminoketones concomitant with lactam formation. This operationally simple method provides β-trifluoromethyl-substituted γ-lactams with a tetrasubstituted carbon as a single diastereomer.

N,N-Diacylaminals as Emerging Tools in Synthesis: From Peptidomimetics to Asymmetric Catalysis

N,N-Diacylaminals are flexible molecular scaffolds that have commonly been utilized as amide surrogates in peptidomimetics. The singularities of this motif as an N-acyl imine equivalent and as hydrogen-bond donor have recently opened new synthetic opportunities, especially in the field of asymmetric catalysis. This concept article highlights this diverse synthetic potential and provides the elements necessary for further developments.

Bifunctional Iminophosphorane Superbase Catalysis: Applications in Organic Synthesis

To improve the field of catalysis, there is a substantial and growing need for novel high-performance catalysts providing new reactivity. To date, however, the set of reactions that can be reliably performed to prepare chiral compounds in largely one enantiomeric form using chiral catalysts still represents a small fraction of the toolkit of known transformations. In this context, chiral Brønsted bases have played an expanding role in catalyzing enantioselective reactions between various carbon- and heteroatom-centered acids and a host of electrophilic reagents. This Account describes our recent efforts developing and applying a new family of chiral Brønsted bases incorporating an H-bond donor moiety and a strongly basic iminophosphorane, which we have named BIMPs (Bifunctional IMinoPhosphoranes), as efficient catalysts for reactions currently out of reach of more widespread tertiary amine centered bifunctional catalysts. The iminophosphorane Brønsted base is easily generated by the Staudinger reaction of a chiral organoazide and commercially available phosphine, which allows easy modification of the catalyst structure and fine-tuning of the iminophosphorane pK_BH+. We have demonstrated that BIMP catalysts can efficiently promote the enantioselective addition of nitromethane to low reactivity N-diphenylphosphinoyl (DPP)-protected imines of ketones (ketimines) to access valuable chiral diamine and α-quaternary amino acid building blocks, and later extended this methodology to phosphite nucleophiles. Subsequently, the reaction scope was expanded to include the Michael addition of high pK_a alkyl thiols to α-substituted acrylate esters, β-substituted α,β-unsaturated esters, and alkenyl benzimidazoles as well as the challenging direct aldol addition of aryl ketones to α-fluorinated ketones. Finally, BIMP catalysts were shown to be used in key steps in the synthesis of complex alkaloid natural products (-)-nakadomarin A and (-)-himalensine A, as well as in polymer synthesis. In most cases, the predictable nature of the BIMP promoted reactions was demonstrated by multigram scale-up while employing low catalyst loadings (down to 0.05 mol%). Furthermore, it was shown that BIMP catalysts can be easily immobilized onto a solid support in one-step for increased catalyst recycling and flow chemistry applications. Alongside our own work, this Account also includes elegant work by Johnson and co-workers utilizing the BIMP catalyst system, when alternative catalysts proved suboptimal.

BIMP-Catalyzed 1,3-Prototropic Shift for the Highly Enantioselective Synthesis of Conjugated Cyclohexenones

A bifunctional iminophosphorane (BIMP)-catalysed enantioselective synthesis of α,β-unsaturated cyclohexenones through a facially selective 1,3-prototropic shift of β,γ-unsaturated prochiral isomers, under mild reaction conditions and in short reaction times, on a range of structurally diverse substrates, is reported. α,β-Unsaturated cyclohexenone products primed for downstream derivatisation were obtained in high yields (up to 99 %) and consistently high enantioselectivity (up to 99 % ee). Computational studies into the reaction mechanism and origins of enantioselectivity, including multivariate linear regression of TS energy, were carried out and the obtained data were found to be in good agreement with experimental findings.

Synthesis of diarylalkanes through an intramolecular/intermolecular addition sequence by auto-tandem catalysis with strong Brønsted base

An auto-tandem catalysis with a strong Brønsted base enabled the synthesis of diarylalkanes containing a benzofuran moiety. Potassium tert-butoxide efficiently catalyzed both the intramolecular cyclization of less acidic ortho-alkynylaryl benzyl ethers and the following intermolecular addition of diarylmethanes to styrenes, demonstrating the high potential of the catalysis in organic synthesis.

Doubly vinylogous and doubly rearomative functionalization of 2-alkyl-3-furfurals

The manuscript describes a straightforward functionalization of 2-alkyl-3-furfurals via simple aminocatalytic conjugate addition. The reaction proceeds through the formation of dearomatized dienamine-like intermediate that undergoes 1,6-addition to 4-alkylidene-2,6-dialkylcyclohexa-2,5-dienones. This process can be described as doubly rearomative as it proceeds with the re-formation of both furan and phenyl aromatic moieties. Target products have been obtained in a highly stereoselective manner, providing an interesting example of 2-alkyl-3-furfural functionalization via doubly vinylogous Michael addition. The mechanism of the reaction has been studied by means of computational methods.