Computational Exploration of Concerted and Zwitterionic Mechanisms of Diels-Alder Reactions between 1,2,3-Triazines and Enamines and Acceleration by Hydrogen-Bonding Solvents

Phthalazine as a Diene in Diels-Alder Reactions With P- and As-Containing Anionic Dienophiles: Comparison of Possible Reaction Channels

Phthalazine can behave as a diene in Diels-Alder (DA) cycloadditions, typically at the pyridazine ring, however, its application is somewhat limited because these reactions usually require harsh conditions or sophisticated catalysts. As an unconventional example, phthalazine was reported to undergo cycloaddition with the [PCO]- anion without any catalyst. In this computational study, we scrutinise the mechanism of the DA reactions between phthalazine and the so far known [ECX]- (E: P, As; X: O, S, Se) anions as dienophiles. In principle, the attack of an [ECX]- anion may occur at two different sites of phthalazine, either at the benzene or the pyridazine ring, and both of these possible reaction channels were juxtaposed on the basis of energetic aspects. In all of the investigated cases, the analysis of the energy profiles reveals a clear regioselectivity that favours the attack at the pyridazine ring. As a result, so far unprecedented 2-pnictanaphth-3-olate analogues seem achievable as final products. Comparing the characteristics of these pathways allowed us to clarify the source of this regioselectivity: The pyridazine ring of phthalazine exhibits lower aromaticity than the benzene subring; therefore, in the DA step, the former ring shows a higher affinity toward a dienophile than the latter, leading to lower activation barriers. To further map the electronic and structural features of the cycloaddition steps, the local interactions evolving in the transition states were analysed and compared using global and local descriptors. In most aspects, the characteristics of both pathways were found to be rather similar, in contrast to the markedly differing activation barriers on the two routes.

Denitrogenative dismantling of heteroaromatics by nucleophilic substitution reactions with diazomethyl compounds

Nucleophiles from deprotonation of diazomethyl compounds having diverse electron withdrawing groups react with 4-carboxylato-1,2,3-triazines at the 6-position to extrude dinitrogen and produce diazovinylketoesters compounds with five or six linear contiguous sp2-hybridized carbons, whereas these same nucleophiles react with 4-carboxylato-1,2,3-triazine 1-oxides, also at the 6-position, to form pyrazolines with the expulsion of nitrous oxide and cyanocarboxylate. This disparity is due to the significant difference in reactivity of the nucleophilic addition products.

Computational Studies of Reactions of 1,2,4,5-Tetrazines with Enamines in MeOH and HFIP

The reaction between 1,2,4,5-tetrazines and alkenes in polar solvents proceeds through a Diels-Alder cycloaddition along the C-C axis (C3/C6 cycloaddition) of the tetrazine, followed by dinitrogen loss. By contrast, the reactions of 1,2,4,5-tetrazines with enamines in hexafluoroisopropanol (HFIP) give 1,2,4-triazine products stemming from a formal Diels-Alder addition across the N-N axis (N1/N4 cycloaddition). We explored the mechanism of this interesting solvent effect through DFT calculations in detail and revealed a novel reaction pathway characterized by C-N bond formation, deprotonation, and a 3,3-sigmatropic rearrangement. The participation of an HFIP molecule was found to be crucial to the N1/N4 selectivity over C3/C6 due to the more favored initial C-N bond formation than C-C bond formation.

Diazotization of o-Aminoamidoximes for the Preparation of Energetic 6,5,6-Fused 1,2,3-Triazine-3-oxides

Two 6,5,6-fused 1,2,3-triazine-3-oxides (4 and 6) were designed and synthesized via the reaction of o-aminoamidoximes with sodium nitrite. In addition, the ring-opening products (5, 7, and 8) derived from 1,2,3-triazine-3-oxides were isolated and characterized. A comprehensive exploration of the reaction mechanism governing the ring-opening process was performed through a combination of theoretical and experimental studies. Notably, compound 4 exhibited commendable detonation properties and low sensitivity, demonstrating its promising potential as an energetic material.

Site Reversal in Nucleophilic Addition to 1,2,3-Triazine 1-Oxides

One of the most important reactions of 1,2,3-triazines with a dienophile is inverse electron demand Diels-Alder (IEDDA) cycloaddition, which occurs through nucleophilic addition to the triazine followed by N2 loss and cyclization to generate a heterocycle. The site of addition is either at the 4- or 6-position of the symmetrically substituted triazine core. Although specific examples of the addition of nucleophiles to triazines are known, a comprehensive understanding has not been reported, and the preferred site for nucleophilic addition is unknown and unexplored. With access to unsymmetrical 1,2,3-triazine-1-oxides and their deoxygenated 1,2,3-triazine compounds, we report C-, N-, H-, O-, and S-nucleophilic additions on 1,2,3-triazine and 1,2,3-triazine-1-oxide frameworks where the 4- and 6-positions could be differentiated. In the IEDDA cycloadditions using C- and N-nucleophiles, the site of addition is at C-6 for both heterocyclic systems, but product formation with 1,2,3-triazine-1-oxides is faster. Other N-nucleophile reactions with triazine 1-oxides show addition at either the 4- or 6-position of the triazine 1-oxide ring, but nucleophilic attack only occurs at the 6-position on the triazine. Hydride from NaBH4 undergoes addition at the 6-position on the triazine and the triazine 1-oxide core. Alkoxides show a high nucleophilic selectivity for the 4-position of the triazine 1-oxide. Thiophenoxide, cysteine, and glutathione undergo nucleophilic addition on the triazine core at the 6-position, while addition occurs at the 4-position of the triazine 1-oxide. These nucleophilic additions proceed under mild reaction conditions and show high functional group tolerance. Computational studies clarified the roles of the nucleophilic addition and nitrogen extrusion steps and the influence of steric and electronic factors in determining the outcomes of the reactions with different nucleophiles.

Ab Initio Metadynamics Simulations of Hexafluoroisopropanol Solvent Effects: Synergistic Role of Solvent H-Bonding Networks and Solvent-Solute C-H/π Interactions

The solvent effects in Friedel-Crafts cycloalkylation of epoxides and Cope rearrangement of aldimines were investigated by using ab initio molecular dynamics simulations. Explicit molecular treatments were applied for both reactants and solvents. The reaction mechanisms were elucidated via free energy calculations based on metadynamics simulations. The results reveal that both reactions proceed in a concerted fashion. Key solvent-substrate interactions are identified from the structures of transition states with explicit solvent molecules. The remarkable promotion effect of hexafluoroisopropanol solvent is ascribed to the synergistic effect of H-bonding networks and C-H/π interactions with substrates.

Inverse Electron Demand Diels-Alder-Type Heterocycle Syntheses with 1,2,3-Triazine 1-Oxides: Expanded Versatility

1,2,3-Triazine 1-oxides are remarkably effective substrates for inverse electron demand Diels-Alder reactions. Formed from vinyldiazoacetates via reaction with tert-butyl nitrite, these stable heterocyclic compounds undergo clean nucleophilic addition with amidines to form pyrimidines, with β-ketocarbonyl compounds and related nitrile derivatives to form polysubstituted pyridines and with 3/5-aminopyrazoles to form pyrazolo[1,5-a]pyrimidines, in high yield. These practical reactions are rapid at room temperature, are base catalyzed, and offer a diversity of structural modifications.

Acyclic and Heterocyclic Azadiene Diels-Alder Reactions Promoted by Perfluoroalcohol Solvent Hydrogen Bonding: Comprehensive Examination of Scope

Herein, the first use of perfluoroalcohol H-bonding in accelerating acyclic azadiene inverse electron demand cycloaddition reactions is described, and its use in the promotion of heterocyclic azadiene cycloaddition reactions is generalized through examination of a complete range of azadienes. The scope of dienophiles was comprehensively explored; relative reactivity trends and solvent compatibilities were established with respect to the dienophile as well as azadiene; H-bonding solvent effects that lead to rate enhancements, yield improvements, and their impact on regioselectivity and mode of cycloaddition are defined; new viable diene/dienophile reaction partners in the cycloaddition reactions are disclosed; and key comparison rate constants are reported. The perfluoroalcohol effectiveness at accelerating an inverse electron demand Diels-Alder cycloaddition is directly correlated with its H-bond potential (pKa). Not only are the reactions of electron-rich dienophiles accelerated but those of strained and even unactivated alkenes and alkynes are improved, including representative bioorthogonal click reactions.

Understanding chemistry: from "heuristic (soft) explanations and reasoning by analogy" to "quantum chemistry"

"Soft theories," i.e., "heuristic models based on reasoning by analogy" largely drove chemistry understanding for 150 years or more. But soft theories have their limitations and with the expansion of chemistry in the mid-20th century, more and more inexplicable (by soft theory) experimental results were being obtained. In the past 50 years, quantum chemistry, most often in the guise of applied theoretical chemistry including computational chemistry, has provided (a) the underlying "hard evidence" for many soft theories and (b) the explanations for chemical phenomena that were unavailable by soft theories. In this publication, we define "hard theories" as "theories derived from quantum chemistry." Both soft and hard theories can be qualitative and quantitative, and the "Houk quadrant" is proposed as a helpful categorization tool. Furthermore, the language of soft theories is often used appropriately to describe quantum chemical results. A valid and useful way of doing science is the appropriate use and application of both soft and hard theories along with the best nomenclature available for successful communication of results and ideas.

HFIP in Organic Synthesis

1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) is a polar, strongly hydrogen bond-donating solvent that has found numerous uses in organic synthesis due to its ability to stabilize ionic species, transfer protons, and engage in a range of other intermolecular interactions. The use of this solvent has exponentially increased in the past decade and has become a solvent of choice in some areas, such as C-H functionalization chemistry. In this review, following a brief history of HFIP in organic synthesis and an overview of its physical properties, literature examples of organic reactions using HFIP as a solvent or an additive are presented, emphasizing the effect of solvent of each reaction.

Mechanistic Insights into the Reaction of Amidines with 1,2,3-Triazines and 1,2,3,5-Tetrazines

1,2,3-Triazines and 1,2,3,5-tetrazines react rapidly, efficiently, and selectively with amidines to form pyrimidines/1,3,5-triazines, exhibiting an orthogonal reactivity with 1,2,4,5-tetrazine-based conjugation chemistry. Whereas the mechanism of the reaction of the isomeric 1,2,4-triazines and 1,2,4,5-tetrazines with alkenes is well understood, the mechanism of the 1,2,3-triazine/1,2,3,5-tetrazine-amidine reaction as well as its intrinsic reactivity remains underexplored. By using ¹⁵N-labeling, kinetic investigations, and kinetic isotope effect studies, complemented by extensive computational investigations, we show that this reaction proceeds through an addition/N₂ elimination/cyclization pathway, rather than the generally expected concerted or stepwise Diels-Alder/retro Diels-Alder sequence. The rate-limiting step in this transformation is the initial nucleophilic attack of an amidine on azine C4, with a subsequent energetically favored N₂ elimination step compared with a disfavored stepwise formation of a Diels-Alder cycloadduct. The proposed reaction mechanism is in agreement with experimental and computational results, which explains the observed reactivity of 1,2,3-triazines and 1,2,3,5-tetrazines with amidines.

Computational insight into the mechanism and stereoselectivity of cycloaddition between donor-acceptor spirocyclopropane and aldehyde catalyzed by Brønsted acid TsOH

The mechanism and diastereoselectivity of the cycloaddition reaction between D-A spirocyclopropane and aldehydes, catalyzed by para-toluenesulfonic acid (TsOH) in dichloromethane to produce 2,5-disubstituted tetrahydrofuran-type lignans, have been investigated by density functional theory (DFT) at the M06-2X/6-311+G(d,p)//B3LYP-D3/6-31G(d,p) level combined with the solvation SMD model. Our calculations show that the entire reaction process includes three stages: the activation of the D-A cyclopropane by Brønsted acid, TsOH, the nucleophilic attack of the aldehyde on the spirocyclopropane, and the formation of the final product, 2,5-disubstituted tetrahydrofuran. It was concluded from the conceptual density functional theory (CDFT) reactivity index analysis that aldehydes with electron-rich substituents are more nucleophilic and more favorable for the reaction to proceed. Furthermore, based on the analyses of energetics as well as the noncovalent interaction (NCI) and reduced density gradient (RDG) in the key transition states, the origin of stereoselectivity was revealed to be determined thermodynamically rather than kinetically. The present work explains the experimental phenomenon well, and provides useful theoretical information for the future design of similar reactions.

Accessing a regiospecific isomer and a metastable polymorph through crystal engineering and solid-state reaction

We describe a solid-state Diels–Alder reaction where crystal engineering was used to design a reaction site yielding one regioisomer. Reaction was followed with SCXRD, compared to solution synthesis and rationalised using computational modelling.

Predicting Dinitrogen Activation via Transition-Metal-Involved [4+2] Cycloaddition Reaction

As the strongest triple bond in nature, the N≡N triple bond activation has always been a challenging project in chemistry. On the other hand, since the award of the Nobel Prize in Chemistry in 1950, the Diels-Alder reaction has served as a powerful and widely applied tool in the synthesis of natural products and new materials. However, the application of the Diels-Alder reaction to dinitrogen activation remains less developed. Here we first demonstrate that a transition-metal-involved [4+2] Diels-Alder cycloaddition reaction could be used to activate dinitrogen without an additional reductant by density functional theory calculations. Further study reveals that such a dinitrogen activation by 1-metalla-1,3-dienes screened out from a series of transition metal complexes (38 species) according to the effects of metal center, ligand, and substituents can become favorable both thermodynamically (with an exergonicity of 28.2 kcal mol^-1 ) and kinetically (with an activation energy as low as 13.8 kcal mol^-1 ). Our findings highlight an important application of the Diels-Alder reaction in dinitrogen activation, inviting experimental chemists' verification.

Electric field-catalyzed single-molecule Diels-Alder reaction dynamics

Precise time trajectories and detailed reaction pathways of the Diels-Alder reaction were directly observed using accurate single-molecule detection on an in situ label-free single-molecule electrical detection platform. This study demonstrates the well-accepted concerted mechanism and clarifies the role of charge transfer complexes with endo or exo configurations on the reaction path. An unprecedented stepwise pathway was verified at high temperatures in a high-voltage electric field. Experiments and theoretical results revealed an electric field-catalyzed mechanism that shows the presence of a zwitterionic intermediate with one bond formation and variation of concerted and stepwise reactions by the strength of the electric field, thus establishing a previously unidentified approach for mechanistic control by electric field catalysis.

Theoretical investigation of the effect of hydrogen bonding on the stereoselectivity of the Diels–Alder reaction

Here, we report the computational prediction of high exo selectivities in a series of Diels–Alder reactions with H-bonding interaction.

Selective N1/N4 1,4-Cycloaddition of 1,2,4,5-Tetrazines Enabled by Solvent Hydrogen Bonding

An unprecedented 1,4-cycloaddition (vs 3,6-cycloaddition) of 1,2,4,5-tetrazines is described with preformed or in situ generated aryl-conjugated enamines promoted by the solvent hydrogen bonding of hexafluoroisopropanol (HFIP) that is conducted under mild reaction conditions (0.1 M HFIP, 25 °C, 12 h). The reaction constitutes a formal [4 + 2] cycloaddition across the two nitrogen atoms (N1/N4) of the 1,2,4,5-tetrazine followed by a formal retro [4 + 2] cycloaddition loss of a nitrile and aromatization to generate a 1,2,4-triazine derivative. The factors that impact the remarkable change in the reaction mode, optimization of reaction parameters, the scope and simplification of its implementation through in situ enamine generation from aldehydes and ketones, the reaction scope for 3,6-bis(thiomethyl)-1,2,4,5-tetrazine, a survey of participating 1,2,4,5-tetrazines, and key mechanistic insights into this reaction are detailed. Given its simplicity and breath, the study establishes a novel method for the simple and efficient one-step synthesis of 1,2,4-triazines under mild conditions from readily accessible starting materials. Whereas alternative protic solvents (e.g., MeOH vs HFIP) provide products of the conventional 3,6-cycoladdition, the enhanced hydrogen bonding capability of HFIP uniquely results in promotion of the unprecedented formal 1,4-cycloaddition. As such, the studies represent an example of not just an enhancement in the rate or efficiency of a heterocyclic azadiene cycloaddition by hydrogen bonding catalysis but also the first to alter the mode (N1/N4 vs C3/C6) of cycloaddition.

Silylium ion mediated 2+2 cycloaddition leads to 4+2 Diels-Alder reaction products

The mechanism of silver(I) and copper(I) catalyzed cycloaddition between 1,2-diazines and siloxy alkynes remains controversial. Here we explore the mechanism of this reaction with density functional theory. Our calculations show that the reaction takes place through a metal (Ag⁺, Cu⁺) catalyzed [2+2] cycloaddition pathway and the migration of a silylium ion [triisopropylsilyl ion (TIPS⁺)] further controls the reconstruction of four-member ring to give the final product. The lower barrier of this silylium ion mediated [2+2] cycloaddition mechanism (SMC) indicates that well-controlled [2+2] cycloaddition can obtain some poorly-accessible IEDDA (inverse-electron demand Diels-Alder reaction) products. Strong interaction of d¹⁰ metals (Ag⁺, Cu⁺) and alkenes activates the high acidity silylium ion (TIPS⁺) in situ. This п-acid (Ag⁺, Cu⁺) and hard acid (TIPS⁺) exchange scheme will be instructive in silylium ion chemistry. Our calculations not only provide a scheme to design IEDDA catalysts but also imply a concise way to synthesise 1,2-dinitrogen substituted cyclooctatetraenes (1,2-NCOTs).

Rolf Huisgen's Classic Studies of Cyclic Triene Diels-Alder Reactions Elaborated by Modern Computational Analysis

Rolf Huisgen explored the Diels-Alder reactions of 1,3,5-cycloheptatriene (CHT) and cyclooctatetraene (COT) with the dienophiles maleic anhydride and 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD) to determine the kinetics and mechanisms of various electrocyclizations and Diels-Alder reactions. These reactions have been examined with density functional theory. Modern computational chemistry has provided information not previously available by experiment. Transition states for all the reactions have been identified, and their Gibbs energies are used to explain the experimental reactivities. Zwitterionic intermediates were not found in the [4+2] cycloadditions of both CHT or COT with PTAD and are thus not involved in these reactions. [2+2+2] cycloadditions, as an alternative path to the Diels-Alder products, are highly disfavored. Rapid double nitrogen inversion was found for the cycloaddition products with PTAD.

Synthesis, Characterization, and Cycloaddition Reactivity of a Monocyclic Aromatic 1,2,3,5-Tetrazine

Herein we disclose the synthesis and full characterization of the first monocyclic aromatic 1,2,3,5-tetrazine, 4,6-diphenyl-1,2,3,5-tetrazine. Initial studies of its cycloaddition reactivity, mode, regioselectivity, and scope illustrate that it participates as the 4π-component of well-behaved inverse electron demand Diels-Alder reactions where it preferentially reacts with electron-rich or strained dienophiles. It was found to exhibit an intrinsic reactivity comparable to that of the isomeric 3,6-diphenyl-1,2,4,5-tetrazine, display a single mode of cycloaddition with reaction only across C4/N1 (no N2/N5 cycloaddition observed), proceed with a predictable regioselectivity (dienophile most electron-rich atom attaches to C4), and manifest additional reactivity complementary to the isomeric 1,2,4,5-tetrazines. It not only exhibits a remarkable cycloaddition reactivity, surprisingly good stability (e.g., stable to chromatography, long-term storage, presence of H₂O even as reaction co-solvent), and broad cycloaddition scope, but it also displays powerful orthogonal reactivity with the 1,2,4,5-tetrazines. Whereas the latter reacts at extraordinary cycloaddition rates with strained dienophiles (tetrazine ligation), the new and isomeric 1,2,3,5-tetrazine displays similarly remarkable cycloaddition rates and efficiencies with amidines (1,2,3,5-tetrazine/amidine ligation). The crossover reactivities (1,2,4,5-tetrazines with amidines and 1,2,3,5-tetrazines with strained dienophiles) are sufficiently low to indicate they may be capable of use concurrently without competitive reactions.

Reversibility and reactivity in an acid catalyzed cyclocondensation to give furanochromanes - a reaction at the 'oxonium-Prins' vs. 'ortho-quinone methide cycloaddition' mechanistic nexus

Herein we report a combined experimental and computational investigation of the acid catalyzed cyclocondensation reaction between styrenyl homoallylic alcohols and salicylaldehyde to form furanochromanes. We disclose a previously unreported isomerisation of the 'unnatural' trans-fused products to the diastereomeric 'natural' cis-fused congeners. Notwithstanding the appeal of assuming this corresponds to endo to exo isomerisation of Diels-Alder (D-A) adducts via concerted retro-cycloaddition/cycloaddition reactions of an in situ generated ortho-quinone methide with the styrenyl alkene, our combined Hammett/DFT study reveals a stepwise Prins-like process via discrete benzylic carbocation intermediates for all but the most electron deficient styrenes. As these reactions fortuitously lie at the intersection of these two mechanistic manifolds, it allows us to propose an experimentally determined indicative ρ + value of ca. -3 as marking this nexus between a stepwise Prins-type pathway and a concerted cycloaddition reaction. This value should prove useful for categorising other reactions formally involving 'ortho-quinomethides', without the need for the extensive computation performed here. Logical optimisation of the reaction based upon the mechanistic insight led to the use of HFIP as an additive which enables exclusive formation of 'natural' cis-fused products with a ∼100-fold reaction rate increase and improved scope.

An Amine Group Transfer Reaction Driven by Aromaticity

A stereoselective domino inverse electron-demand Diels-Alder/amine group transfer reaction catalyzed by a bidentate Lewis acid provides 1-amino-1,2-dihydronaphthalenes, a core structure in many bioactive compounds. A concerted mechanism is proposed based on experimental studies as well as DFT computations demonstrating a new general reactivity scheme. The broad scope of the reaction was evaluated by variation of all three starting compounds, phthalazines, aldehydes, and amines. Scalability was demonstrated by a gram scale reaction without diminished yield.

Insights into the N-Heterocyclic Carbene (NHC)-Catalyzed Oxidative γ-C(sp3)-H Deprotonation of Alkylenals and Cascade [4 + 2] Cycloaddition with Alkenylisoxazoles

The N-heterocyclic carbene (NHC)-catalyzed oxidative C-H deprotonations have attracted increasing attention; however, the general mechanism regarding this kind of oxidative organocatalysis remains unclear. In this paper, the competing mechanisms and origin of the stereoselectivity of the NHC-catalyzed oxidative γ-C(sp³)-H deprotonation of alkylenals and cascade [4 + 2] cycloaddition with alkenylisoxazoles were systematically investigated for the first time using density functional theory (DFT). The computed results indicate that the oxidation of the Breslow intermediate by 3,3',5,5'-tetra- tert-butyl diphenoquinone (DQ) via a hydride transfer to oxygen (HTO) pathway is the most favorable among the four competing pathways. In addition, the analyses demonstrate that oxidant DQ plays a double role, i.e., strengthening the acidity of the hydrogen of the γ-carbon of alkylenal and forming π···π interactions with conjugated C═C bonds to promote the γ-C(sp³)-H deprotonation. The NHC catalyst acts as a Lewis base, and the hydrogen-bond network between the NHC and the substrate formed in the key Michael addition step is responsible for the origin of the stereoselectivity. Further DFT calculations reveal that the nonpolar solvent can stabilize the nonpolar R isomer but destabilize the polar S isomer for the stereoselectivity-determining transition states, thus improving the stereoselectivity.

Mechanistic studies of 1,3-dipolar cycloadditions of bicyclic thioisomünchnones with alkenes. A computational rationale focused on donor-acceptor interactions

This paper describes a mechanistic study, with the interplay of experiment and theory, on the cycloadditions of a bicyclic mesoionic 1,3-dipole versus a series of representative symmetrical (1-phenyl-1H-pyrrole-2,5-dione and dimethyl maleate) and asymmetrical [(E)-(2-nitrovinyl)benzene, acrylonitrile, and but-3-en-2-one] olefinic dipolarophiles. These results allow a comparative analysis with monocyclic dipoles and open further avenues to structurally diversified heteroatom-rich rings. The unichiral version of the bicyclic dipole leads to adducts containing up to five chiral centers, whose formation proceeds with high levels of facial stereoinduction in reactions involving bulky dipolarophiles. The second and largest part of this study provides a theoretical interrogation on the pericyclic mechanism with DFT-methods [M06-2X/6-311++G(d,p)]. In order to get further mechanistic insights, we have also explored charge transfers between reaction partners using NBO analysis, which satisfactorily justifies the stereochemical outcome.