The Origin of the Halogen Effect on Reactivity and Reversibility of Diels–Alder Cycloadditions Involving Furan

Efficient synthesis of fully renewable, furfural-derived building blocks via formal Diels-Alder cycloaddition of atypical addends

Diels-Alder (DA) cycloaddition of furanics is emerging as a key transformation in circular chemistry, providing access to highly versatile, biobased platform molecules. Further development of this technology into viable industrial applications faces major challenges, a notorious one being the lack of reactivity of the most readily available furans, i.e. the furfural derivatives. Herein we describe the remarkably-facile intramolecular DA reaction of allyl acetals of different furfurals to efficiently afford formal DA adducts with the atypical, unreactive dienophile allyl alcohol. Our methodology gives access to unprecedented oxanorbornene derivatives in high chemo-, regio- and stereoselectivity, which can be readily diversified into valuable products. This offers the potential of scalable production of renewable chemical building blocks from cheap, bioderived platform molecules.

Asymmetric Organocatalyzed Cascade Reactions─Merging the pseudo-Halogen and Halogen Effect with Dienamine Catalysis

The combination of asymmetric organocatalysis with the (pseudo)-halogen effect enables the formation of chiral norcarane scaffolds in high yields and selectivities (up to 92% yield, >99% ee, and >95:5 d.r.). This was achieved by reacting (pseudo)-halogenated 3-vinyl chromones with in situ generated chiral dienamines in an inverse electron demand [4 + 2] cycloaddition followed by an intramolecular S_N2 reaction. These scaffolds could easily undergo photoinduced rearrangements or lactonization to form intricate chiral ring structures.

Intramolecular Diels-Alder Reactions of α-Bromostyrene-Functionalized Unsaturated Carboxamides

Intramolecular cycloaddition reactions of α-bromostyrene-functionalized amides of monomethyl fumarate were investigated. The reaction of the amides with Et₃N in toluene at 110 °C gave 1,4-dihydronaphthalenes. The 1,4-dihydronaphthalenes may be produced via the intramolecular Diels-Alder reaction, proton transfer, and dehydrobromination by a base, along with C═C bond isomerization by proton transfer. The reaction of amide derivatives with halogen on a benzene ring and alkali metal carbonates in toluene at 110 °C gave naphthalene derivatives directly. Dehydrogenation of various 1,4-dihydronaphthalenes with cesium or rubidium carbonate in toluene at 110 °C gave naphthalene derivatives. The retardation by TEMPO, acceleration by air for some substrates, and density functional theory calculations suggest a radical mechanism caused by intervention of molecular oxygen.

The Interplay between Kinetics and Thermodynamics in Furan Diels-Alder Chemistry for Sustainable Chemicals Production

Biomass‐derived furanic platform molecules have emerged as promising building blocks for renewable chemicals and functional materials. To this aim, the Diels–Alder (DA) cycloaddition stands out as a versatile strategy to convert these renewable resources in highly atom‐efficient ways. Despite nearly a century worth of examples of furan DA chemistry, clear structure–reactivity–stability relationships are still to be established. Detailed understanding of the intricate interplay between kinetics and thermodynamics in these very particular [4+2] cycloadditions is essential to push further development and truly expand the scope beyond the ubiquitous addend combinations of electron‐rich furans and electron‐deficient olefins. Herein, we provide pertinent examples of DA chemistry, taken from various fields, to highlight trends, establish correlations and answer open questions in the field with the aim to support future efforts in the sustainable chemicals and materials production.

Machine Learning to Predict Diels-Alder Reaction Barriers from the Reactant State Electron Density

Reaction barriers are key to our understanding of chemical reactivity and catalysis. Certain reactions are so seminal in chemistry that countless variants, with or without catalysts, have been studied, and their barriers have been computed or measured experimentally. This wealth of data represents a perfect opportunity to leverage machine learning models, which could quickly predict barriers without explicit calculations or measurement. Here, we show that the topological descriptors of the quantum mechanical charge density in the reactant state constitute a set that is both rigorous and continuous and can be used effectively for the prediction of reaction barrier energies to a high degree of accuracy. We demonstrate this on the Diels-Alder reaction, highly important in biology and medicinal chemistry, and as such, studied extensively. This reaction exhibits a range of barriers as large as 270 kJ/mol. While we trained our single-objective supervised (labeled) regression algorithms on simpler Diels-Alder reactions in solution, they predict reaction barriers also in significantly more complicated contexts, such a Diels-Alder reaction catalyzed by an artificial enzyme and its evolved variants, in agreement with experimental changes in k_cat. We expect this tool to apply broadly to a variety of reactions in solution or in the presence of a catalyst, for screening and circumventing heavily involved computations or experiments.

Does the reaction of nitrone derivatives with allenoates proceed by an initial (3 + 2) cycloaddition or O-Nucleophilic addition? A quantum chemical investigation

The question of whether the reaction between nitrone derivatives and allenoates proceeds with initial (3 + 2) cycloaddition (32CA) or the oxygen-nucleophilic (O-nucleophilic) addition has been theoretically investigated using density functional theory (DFT) at the M06-2X/6-311G(d,p) level of theory. In all of the reactions considered, the initial 32CA route is kinetically and thermodynamically preferred over the initial O-nucleophilic addition. Increasing the steric bulk of the substituents on the reactants bridges the kinetic and thermodynamic gap between the initial 32CA and O-nucleophilic addition but the 32CA is still energetically preferred, showing that with increasing steric bulk, the O-nucleophilic addition may become competitive with the 32CA route. Within the 32CA reactions, analysis of electrophilic (P_k⁺) and nucleophilic (P_k^-) Parr functions at the various reaction centers in the allenoate indicates that the three-atom-components (TACs) chemo-selectively add across the olefinic bond bearing the ester functionality (COOMe) with the largest Mulliken spin density coefficients and this observation agrees with the energetic trends and experimental outcomes. The global electron density transfer (GEDT) analysis reveals a high polar character associated with the 32CA reaction of methylbuta-2,3-dienoate with N-methyl-C-phenylnitrone while that of the 32CA reaction of methylpenta-2,3-dienoate with N-cyclohexenylnitrone has a low polar character.

A DFT study of the double (3 + 2) cycloaddition of nitrile oxides and allenoates for the formation of spirobiisoxazolines

The molecular mechanism of the double (3 + 2) cycloaddition (32CA) reaction between nitrile oxides and allenoates has been studied using density functional theory at the M06-2X/6-311G (d,p) level of theory. In the first 32CA, the nitrile oxide adds chemo- and regio-selectively to the C-C double bond of the allenoate closest to the carboxylate group followed by a subsequent regioselective addition to the olefinic bond of the isoxazoline intermediate. The rate constant for the preferred pathway (formation of 4-methylene-2-isoxazoline intermediate) in the reaction of ethyl substituted allenoate and mesitonitrile oxide is 5.3 × 10² s^-1 in THF which is about 13 times faster than the closest competing step (formation of its regioisomer 5-methylene-2-isoxazoline intermediate) which has a rate constant of 4.4 × 10¹ s^-1. Strong electron-donating groups (EDGs) and electron-withdrawing groups (EWGs) decrease activation barriers and hence increase the reaction rate. Also, the dimerization of nitrile oxide to form furaxon is found to be kinetically unfavored.

Exploring the chemo-, regio-, and stereoselectivities of the (3 + 2) cycloaddition reaction of 5,5-dimethyl-3-methylene-2-pyrrolidinone with C,N-diarylnitrones and nitrile oxide derivatives: a DFT study

The (3 + 2) cycloaddition (32CA) reaction is an efficient method for the synthesis of many biologically active heterocyclic compounds, but there are several regio- and stereochemical issues that must be fully understood to exploit the full utility of its synthetic power. We herein explored the chemo-, regio-, and stereoselectivities of the 32CA reaction of 5,5-dimethyl-3-methylene-2-pyrrolidinone (B1) to C,N-diarylnitrones (B2), and nitrile oxide derivatives (B3) with DFT at the M06/6-311G(d,p) level of theory. The reactions occur via an asynchronous one-step mechanism, with the chemoselective addition of the C,N-diarylnitrones, and nitrile oxide derivatives across the olefinic bond of 5,5-dimethyl-3-methylene-2-pyrrolidinone being the most preferred kinetically and thermodynamically. The regio- and stereoselectivities of the reactions are affected by the electronic and steric nature of substituents on B2 but they are not affected by the electronic and steric nature of substituents on B3. The C,N-nitrones and the nitrile oxide derivatives add across the atomic centers with the largest atomic spin densities on 5,5-dimethyl-3-methylene-2-pyrrolidinone as seen through the local electrophilic ([Formula: see text]) and nucleophilic ([Formula: see text]) Parr functions of the various reaction centers. Results from the global electron density transfer (GEDT) reveal the low polar nature of the reactions.

Investigating the site-, regio-, and stereo-selectivities of the reactions between organic azide and 7-heteronorbornadiene: a DFT mechanistic study

The site-, regio-, and stereo-selectivities of the title reactions have been studied using density functional theory (DFT) at the M06/6-311G(d,p) level of theory. The effects of substituents on both the three-atom component (TAC) and norbornadiene derivatives have been investigated with a focus on the site-selectivity. The reaction of benzylazide with (1S,4R)-2-tosyl-7-oxabicyclo[2.2.1]hepta-2,5-diene and (1R,4S)-2-bromo-3-tosyl-7-oxabicyclo[2.2.1]hepta-2,5-diene proceeds via addition across the substituted olefinic bond of the two norbornadiene derivatives. Substituents on the TAC do not affect the selectivity of the reaction while substituents on the norbornadiene significantly affect the selectivity of the reaction. Benzylazide preferentially adds across the substituted olefinic bond of the norbornadiene derivative when strong electron-withdrawing group (EWGs) and electron-releasing group (ERGs) substituents are on the norbornadiene while weak ERGs and EWGs on the norbornadiene significantly decreases the site-selectivity such that addition across either double is no longer favored over the other. The formation of exo-cycloadducts is generally favored over the endo-cycloadducts. The reaction of benzylazide and norbornadiene derivatives is a highly irreversible exergonic reaction. The direction of electron density flux is dependent on the nature of the substituent on the reactants. Global reactivity indices and Parr function calculations are in good agreement with the activation barriers and the selectivity of the reactions.

A Direct Organocatalytic Enantioselective Route to Functionalized trans-Diels-Alder Products Having the Norcarane Scaffold

An enantioselective methodology to construct trans-Diels-Alder scaffolds by organocatalysis with excellent selectivity, high yield and up to five contiguous stereocenters is presented. The reaction concept integrates the halogen effect and a novel discovered pseudo-halogen effect to direct an endo-selective, secondary-amine catalyzed Diels-Alder reaction allowing for the subsequent formation of trans-Diels-Alder cycloadducts featuring the norcarene scaffold. The methodology relies on the reaction between an in situ generated trienamine and an α-brominated or α-pseudo-halogenated enone to form a fleeting cis-Diels-Alder intermediate. The endo-transition state-enhanced by the (pseudo-)halogen effect-sets the stereochemistry that allows for a subsequent S_N 2-like reaction at a tertiary center to obtain the trans-Diels-Alder scaffold. The mechanism was investigated and supported by experimental results as well as computational studies.

Structural characterisation of natural products by means of quantum chemical calculations of NMR parameters: new insights

In this review, we focus in all aspects of NMR simulation of natural products, from the fundamentals to the new computational toolboxes available, combining advanced quantum chemical calculations with upstream data processing and machine learning.

The origin of the regiospecificity of acrolein dimerization

The NCI at the transition state are the key factor controlling the regiospecificity in the acrolein dimerization.

(3 + 2) cycloaddition reaction of 7-isopropylidenebenzonorbornadiene and diazomethane derivatives: A theoretical study

The ability to synthesize targeted molecules hinges on detailed mechanistic insight of the reaction. The 1,3-dipolar cycloaddition reaction between diazomethane derivatives and 7-isopropylidenebenzonorbornadiene have been extensively studied using density functional theory (DFT) at the M06-2X/6-311G(d,p) level of theory in order to delineate the peri-, regio-, and stereo-selectivities of the reaction. The diazomethane is shown to periselectively add across the endocyclic olefinic bond of the 7-isopropylidenebenzonorbornadiene and stereoselectively in the exo fashion, yielding the exo-cycloadduct as the major product, with a rate constant of 3.83 × 10⁴ s^-1. The endo approach of this periselective path is the closest competing pathway with a rate constant of 8.78 × 10¹ s^-1. Neither electron-donating groups (R = methyl, ethyl, amine, cyclopropyl) nor electron-withdrawing groups (R = cyano, nitro, carbonyl) on the diazomethane alters the peri- and stereo-selectivity of the reaction. However, the substituents do have an effect on whether the addition follow normal or inverse electron demand mechanisms. EDGs favor a normal electron demand mechanism while EWGs favor an inverse electron demand 1,3-dipolar cycloaddition reaction. While EDGs-substituted diazomethane derivatives behave as nucleophiles in reactions with 7-isopropylidenebenzonorbornadiene, EWGs-substituted diazomethane derivatives behave as electrophiles. The 1,3-dipole adds across the dipolarophile via a concerted asynchronous mechanism, but a stepwise diradical mechanism has been ruled out. The selectivities observed in the title reaction are kinetically controlled. Analysis of the nucleophilic Parr function (P_K^-) at the different reaction sites in the dipolarophile indicates that the diazomethane adds across the atomic centers with highest NBO and Mulliken atomic spin densities.

Investigating the regio-, stereo-, and enantio-selectivities of the 1,3-dipolar cycloaddition reaction of C-cyclopropyl-N-phenylnitrone derivatives and benzylidenecyclopropane derivatives: A DFT study

The biomedical importance of spirocyclopropane isoxazolidine derivatives is widely known. The 1,3-dipolar cycloaddition (1,3-DC) of C-cyclopropyl-N-phenylnitrone derivative and benzylidenecyclopropane derivatives leading to the formation of 5- and 4-spirocyclopropane isoxazolidines derivatives have been studied using density functional theory (DFT) at M06-2X/6-311G (d,p) level of theory. An extensive exploration of the potential energy surface shows that the 1,3-dipole adds across the dipolarophile via an asynchronous concerted mechanism. While electron-donating groups (EDGs) on the benzylidenecyclopropane favor the formation of the 4-spirocyclopropane isomer, electron-withdrawing groups (EWGs) favor the reaction channels that furnish the 5-spirocyclopropane isoxazolidine isomer. Both EWDs and EDGs on the 1,3-dipole favor the formation of the 5-spirocyclopropane isoxazolidine isomer. Irrespective of the electronic nature of substituents on the C-cyclopropyl-N-phenylnitrone, the reaction channels that regioselectively lead to the formation of the 5-spirocyclopropane isoxazolidine isomer are favored. In all reactions considered, the channels that selectively lead to the formation of the cis-diastereoisomers proceed with lower activation barriers than the trans-diastereoisomers. In all cases, the observed selectivities in the title reaction are kinetically controlled.

DFT mechanistic studies on the regio-, stereo-, and enantio-selectivity of 1,3 dipolar cycloadditions of quinolinium imides with olefins, maleimides, and benzynes for the synthesis of fused N,N'-heterocycles

This work investigated computationally the regio-, stereo-, and enantio-selectivity of the reactions of azomethine imines with olefins, maleimides, and benzynes, important reactions towards the synthesis of heteropolycyclic, N,N'-fused, spirocyclic systems, which serve as building blocks for the synthesis of many pharmaceuticals, agrochemicals, and biologically active compounds. The results show that the thermally controlled diastereoselective [3 + 2] cycloaddition reaction between quinolinium imide and methyl acrylate provides two regio-isomers: 1,4-regioisomer (N-C1, C-C2) and 1,3-regioisomer (N-C2, C-C1). The 1,4-regioisomer has cis and trans-stereoisomers while the 1,3-regioisomer has R-enantiomer and S-enantiomer, and the barriers for the formation of these isomers are 5.1, 19.1, 10.7, and 10.5 kcal/mol, respectively. The reaction between quinolinium imide and maleimide leads to the formation of two stereoisomers; cis-isomer and trans-isomer in which the cis-isomer is kinetically and thermodynamically favoured by 6.1 kcal/mol and 8.7 kcal/mol, respectively. The reaction between quinolinium imide and benzyne also leads to the formation of two stereoisomers through one transition state, with a barrier of 3.0 kcal/mol. Global electrophilicity index calculations show that the dipole acts as a good electrophile in the reaction and decreasing or increasing electrophilicity index has no correlation with the activation barriers. Graphical abstractPathways of the 1,3 dipolar cycloadditions of quinolinium imides with olefins, maleimides, and benzynes for the synthesis of fused N,N'-heterocycles.

Peri-, Chemo-, Regio-, Stereo- and Enantio-Selectivities of 1,3-dipolar cycloaddition reaction of C,N-Disubstituted nitrones with disubstituted 4-methylene-1,3-oxazol-5(4H)- one: A quantum mechanical study

The peri-, chemo-, regio-, stereo- and enantio-selectivities of 1,3-dipolar cycloaddition reaction of C,N-disubstituted nitrones with disubstituted 4-methylene-1,3-oxazol-5(4H)-one have been studied using density functional theory (DFT) at the M06-2X/6-311G (d,p) level of theory. The 1,3-dipole preferentially adds chemo-selectively across the olefinic bond in a (3 + 2) fashion forming the corresponding spirocycloadduct. The titled reaction occurs with poor enantio- and stereo-selectivities, but a high degree of regio-selectivity is observed for the addition of the 1,3-dipole across the dipolarophile. Electron-withdrawing groups on the dipolarophile significantly reduce the activation barriers while electron-donating groups on the dipolarophile increase the activation barriers. Analysis of the HOMO and LUMO energies of the two reacting species indicates that the 1,3-dipole reacts as a nucleophile while the dipolarophile reacts as the electrophile. Investigation of the electrophilic Parr function (P_K⁺) at the various reaction centers in the dipolarophile indicates that the 1,3-dipole selectively adds across the atomic species with the largest electrophilic Mulliken and NBO atomic spin densities which is in accordance with the energetic trends observed.

Hydrosilylation-Promoted Furan Diels-Alder Cycloadditions with Stereoselectivity Controlled by the Silyl Group

Herein we describe an unprecedented B(C₆F₅)₃-catalyzed cascade reaction of N-allyl-N-furfurylamides involving an initial intramolecular furan Diels-Alder reaction and subsequent ether cleavage. The reaction has a broad substrate scope, even tolerating a trialkyl-substituted olefin as the dienophile, which has not previously been observed with conventional furan Diels-Alder reactions. In addition, the relative configuration of the product can be controlled by the choice of the silyl group: reactions involving Et₃SiH and ⁱPr₃SiH gave different diastereomers. Control experiments and the computational studies revealed that the steric bulk of the silyl group plays a key role in determining the reaction pathway and thus the relative configuration of the product.

Mechanistic studies on tandem cascade [4 + 2]/ [3 + 2] cycloaddition of 1,3,4-oxadiazoles with olefins

The mechanism of the reaction of 1,3,4-oxadiazoles with alkenes (ethylene) and cycloalkenes (cyclobutene, cyclopentene, cyclohexene and cycloocene) have been studied computationally at the DFT M06-2X/6-311G* level. The reaction is found to proceed via a concerted [4 + 2] addition followed by nitrogen extrusion and then [3 + 2] addition in a tandem cascade fashion, which in the case of cycloalkenes leads to exo-fused or endo-fused subframes, the exo of which is kinetically and thermodynamically favored. The [4 + 2] step is the rate-determining step of the reaction. CF₃ as a substituent on the 1,3,4-oxadiazole decreases the activation barriers of the rate-determining step, while CO₂Me on the oxadiazole increases the activation barriers of the rate-determining step, markedly in the case of the reaction with cyclopentene and only marginally in the reactions with ethylene. Increasing temperature decreases the barrier of the rate-determining step and stability of the products but increases the rate of the nitrogen extrusion step. The low barriers of the second and third steps of the reaction compared to the first step means that the intermediates will not be isolated in the reaction, confirming the experimental observations of earlier workers. Based on calculated activation barriers, the reactivity of the various cycloalkenes considered in this study follows the order: cyclooctene > cyclopentene > cyclohexene > cyclobutene which is consistent with the trends in product yields obtained in earlier experimental studies.

1, 3-Dipolar cycloaddition reactions of selected 1,3-dipoles with 7-isopropylidenenorbornadiene and follow-up thermolytic cleavage: A computational study

The mechanism, regio-, stereo-, and enantio-selectivities of the 1,3-dipolar cycloaddition reactions of 7-isopropylidenenorbornadiene (DENBD) with nitrones and azides to form pharmaceutically relevant isoxazolidine and triazole analogues have been studied computationally at the M06/6-31G(d), 6-31G(d,p), 6-311G(d,p), 6-311++G(d,p) and M06-2X/6-31G(d) levels of theory. In the reactions of DENBD with phenyl nitrones, the cycloaddition steps have low activation barriers, with the highest being 16 kcal/mol; and the Diels-Alder cycloreversion steps have generally high barriers, with the lowest being 20 kcal/mol, suggesting that the isolable products in these reactions are the bicyclic isoxazolidine cycloadducts and not the thermolytic products. This is in contrast to the reactions of DENBD with phenyl azide where the isolable products are predicted to be the thermolytic products since the Diels-Alder cycloreversion steps had relatively lower activation barriers. Electron-donating substituents on the dipolarophile substrate favour attack of the nitrone on the least hindered side of the DENBD substrate while electron-withdrawing substituents on the dipolarophile substrate favour attack on the more hindered side of the DENBD, indicating that site-selectivity is affected by nature of substituents. Global reactivity indices calculations are in good agreement with the activation barriers obtained. Analysis of the electrophilic (P_K⁺) and nucleophilic (P_K^-) Parr functions at the reactive centres reveal that the cycloaddition occurs between atoms with the largest Mulliken and NBO atomic spin densities which agrees well with the energetic trends and the experimental product outcomes.

Quantum chemical studies on the mechanistic aspects of tandem sequential cycloaddition reactions of cyclooctatetraene with ester and nitrones

The mechanisms of the tandem sequential [4 + 2]/[3 + 2] and [3 + 2]/[4 + 2] cycloaddition sequences involving an ester, cyclooctatetraene (COTE), and cyclic and acyclic nitrones for the formation of a diverse range of isoxazolidine derivatives and other synthetic precursors are reported. A thorough exploration of the PES has characterized several regio-, stereo- and enantio-selective mechanistic channels involved in these reactions. A perturbation molecular orbital (PMO) analysis been employed to rationalize the results. It has also been found that the initial electrocyclic ring closure of the COTE is the rate-determining step in the tandem sequential [4 + 2]/[3 + 2] addition sequence. The thermolytic breakdown of the tandem adducts to subsequent monocyclic, bicyclic and tricyclic adducts occurs generally with very high activation barriers making it an inconvenient synthetic approach. The different reactivity of all the three double bonds present in the dipolarophile is reported. Finally, the mechanistic possibilities of [3 + 2]/[4 + 2] addition sequences involving the same reaction components in the case of cyclic and acyclic nitrones are explored extensively. The results suggest a novel and convenient routes for obtaining products of high selectivity with less energetic requirements. In some instances, new cycloadducts hitherto unreported are obtained.

Computational studies on [4 + 2] / [3 + 2] tandem sequential cycloaddition reactions of functionalized acetylenes with cyclopentadiene and diazoalkane for the formation of norbornene pyrazolines

The mechanistic pathways for the sequential tandem [4 + 2] / [3 + 2] versus [3 + 2] / [4 + 2] cycloaddition reaction of functionalized acetylenes with cyclopentadiene and dimethyl diazopropane for the formation of norbornene pyrazolines, employed in the synthesis of pharmaceutically relevant compounds, have been studied computationally with DFT at the M06-2X/6-31G(d) and M06-2X/6-31G(d,p) levels of theory. We have established that, in the reaction of the parent (unsubstituted) acetylene with cyclopentadiene and dimethyl diazopropane, the order of the tandem addition has no substantial effects in product outcomes. The same product is obtained provided the reaction components remain the same for both [4 + 2]/[3 + 2] and [3 + 2]/[4 + 2] tandem addition sequences. The results indicate that the [4 + 2] Diels-Alder addition step is the rate-determining step irrespective of the addition order, while the 1,3-dipolar cycloaddition step has been found to generally proceed rapidly with very low activation energies. It has also been realized that the regio-, stereo-, and chemo-selectivities of the reaction are strictly dictated by the type of substituent on the parent acetylene. For substituted acetylenes, we conclude that the sequence of the tandem addition generally affects the type of isomeric product obtained. The [4 + 2]/[3 + 2] tandem addition sequence has been established to favor the exo stereo-selective isomer over the endo, whereas the [3 + 2]/[4 + 2] tandem addition sequence generally favors the endo product formation. Therefore, it is settled that the mechanistic route taken by any substrate in the [4 + 2]/[3 + 2] versus [3 + 2]/[4 + 2] sequential tandem cycloaddition is greatly affected by both electronic and steric factors. Electrophilicity indices calculations agree with the activation barriers obtained. Perturbation molecular orbital theory was employed to rationalize the results. Global reactivity indices calculations gave a good correlation with the activation energies. Graphical abstract The sequential tandem [4 + 2] / [3 + 2] versus [3 + 2] / [4 + 2] cycloaddition reaction of functionalized acetylenes with cyclopentadiene and dimethyl diazopropane for the formation of norbornene pyrazolines.

Origins of halogen effects in bioorthogonal sydnone cycloadditions

Halogen substituents increase sydnone cycloaddition reactivities substantially. Fluoro-sydnones are superior to bromo- and chloro-sydnones, and can achieve extremely high second-order rate constants with strained alkynes. Computational studies have revealed the fluorine substituent increases the reactivity of sydnone mainly by lowering its distortion energy.

Origins of the Endo and Exo Selectivities in Cyclopropenone, Iminocyclopropene, and Triafulvene Diels-Alder Cycloadditions

The endo and exo stereoselectivities of Diels-Alder reactions of cyclopropenone, iminocyclopropene, and substituted triafulvenes with butadiene were rationalized using density functional theory calculations. When cyclopropenone is the dienophile, there is a 1.8 kcal/mol preference for the exo cycloaddition with butadiene, while the reaction of 3-difluoromethylene triafulvene with butadiene favors the endo cycloaddition by 2.8 kcal/mol. The influence of charge transfer and secondary orbital interactions on the stereoselectivity of Diels-Alder reactions involving triafulvenes and heteroanalogs is discussed. The predicted stereoselectivity correlates with both the charge and highest occupied molecular orbital (HOMO) coefficient at the C3 carbon of the triafulvene motif.

Intramolecular Nitrofuran Diels-Alder Reactions: Extremely Substituent-Tolerant Cycloadditions via Asynchronous Transition States

Nitrofurans undergo intramolecular Diels-Alder reactions with tethered electron-poor dienophiles more rapidly and in higher yield than non-nitrated furans. Computational studies indicate that increased stabilization of a partial positive charge on the nitro-substituted carbon in both transition state and product is the driving force for these reactions. Frontier molecular orbital energy differences indicate a switch from normal to inverse electron demand upon nitration. There does not appear to be a contribution from any differences in aromatic stabilization energy between furans and nitrofurans. Calculations show that the nitrofuran reactions proceed via a highly asynchronous transition state allowing easier bond formation between two sterically hindered carbons.

Photoassisted diversity-oriented synthesis: accessing 2,6-epoxyazocane (oxamorphan) cores

The modular synthesis of photoprecursors and their photoinduced cyclization into substituted 1-benzazocanes of two distinct topologies is described. The key step producing an extended polyheterocyclic system involves the photogeneration of azaxylylenes and their subsequent intramolecular cycloaddition with furan-containing pendants tethered either via the aniline nitrogen or through the carbonyl group containing arm. The primary photoproducts-secondary or tertiary anilines which are not acylated at the nitrogen atom-undergo facile acid-catalyzed or spontaneous ring-opening-ring-closing rearrangement to yield fused polyheterocyclic structures possessing a 2,6-epoxyazocane (or oxamorphan) core.