Exo Selectivity Research Articles

Interception and Synthetic Application of Diradical and Diene Forms of Dual-Nature Azabicyclic o-Quinodimethanes Generated by 6π-Azaelectrocyclization.

We demonstrate that 2-alkenylarylaldimines and ketimines undergo thermal 6π-azaelectrocyclization to generate a wide range of azabicyclic o-quinodimethanes (o-QDMs). These o-QDMs exist as a hybrid of a diene and a benzylic diradical. The diradical nature was confirmed by their ability to undergo dimerization and react with H-atom donor, 2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) and O2. In addition, the interception of the diradicaloid o-QDMs by H-atom transfer was used to synthesize five tetrahydroisoquinoline alkaloids and related bioactive molecules. The diene form can undergo [4+2] cycloaddition reactions with different dienophiles to generate bridged azabicycles in high endo:exo selectivity. The azabicyclic o-QDMs can be generated for [4+2] cycloaddition from a wide range of electronically and sterically varied 2-alkenylarylimines, including mono, di, tri and tetrasubstituted alkenes, and imines derived from arylamine, alkylamine (1°, 2°, 3°), benzylamine, benzylsulfonamide and Boc-amine.

Interception and Synthetic Application of Diradical and Diene Forms of Dual‐Nature Azabicyclic o‐Quinodimethanes Generated by 6π‐Azaelectrocyclization

AbstractWe demonstrate that 2‐alkenylarylaldimines and ketimines undergo thermal 6π‐azaelectrocyclization to generate a wide range of azabicyclic o‐quinodimethanes (o‐QDMs). These o‐QDMs exist as a hybrid of a diene and a benzylic diradical. The diradical nature was confirmed by their ability to undergo dimerization and react with H‐atom donor, 2,2,6,6‐tetramethylpiperidin‐1‐yl)oxyl (TEMPO) and O2. In addition, the interception of the diradicaloid o‐QDMs by H‐atom transfer was used to synthesize five tetrahydroisoquinoline alkaloids and related bioactive molecules. The diene form can undergo [4+2] cycloaddition reactions with different dienophiles to generate bridged azabicycles in high endo:exo selectivity. The azabicyclic o‐QDMs can be generated for [4+2] cycloaddition from a wide range of electronically and sterically varied 2‐alkenylarylimines, including mono, di, tri and tetrasubstituted alkenes, and imines derived from arylamine, alkylamine (1°, 2°, 3°), benzylamine, benzylsulfonamide and Boc‐amine.

In DFT We Trust: Exhaustive Exploration of 1,3‐Dipolar Cycloadditions Between Nitrones and Levoglucosenone Exposes a Curious Case of Conformational Dynamics

AbstractAn experimental and computational study (including DFT calculations and distortion/interaction analysis) was conducted to assess the effect of the nitrone structure in the outcome of dipolar 1,3‐cycloadditions with levoglucosenone, a biomass derived chiral enone. While B3LYP/6‐31G* (the most popular method for modeling these reactions according to our literature search) provides qualitatively good results, large outliers were found for some systems versus experimental data. An exhaustive exploration of other levels allowed us to determine the most appropriate ones to predict simultaneously reactivity and selectivity. The systematically predicted high exo selectivity by the majority of the levels led us to reconsider our initial assignment for the reaction with the nitrone derived from piperidine, which resulted in the discovery of an interesting case of conformational dynamics.

Gold(I)-Catalyzed Substitution-Controlled Syntheses of Spiro[indoline-3,3'-pyrrolidine] and Spiro[indoline-3,3'-piperidine] Derivatives.

Spiro[indoline-3,3'-pyrrolidine] and spiro[indoline-3,3'-piperidine] derivatives were synthesized in a substitution-controlled manner under the catalysis of cationic gold(I) species in the presence of Hantzsch ester (HEH). The optimal reaction condition was determined by screening, and the functional group tolerances of these two pathways were examined by readily synthetic substrates. The endo and exo selectivities of these cyclizations were elucidated by density functional theory calculations, and a plausible mechanism for these transformations was proposed.

Inducing high exo selectivity in Diels–Alder reaction by dimethylborane substituent: a DFT study

In this work, the role of Lewis acid–base (LAB) interaction on the stereoselectivity of the Diels–Alder (DA) reaction has been studied by DFT in gas and solution (dichloromethane) phases. The calculations were performed at the B3LYP/6-311G++ (d, p) level. Two different series of DA reactions were investigated: (1)—three mono-substituted cyclopentadienes + dimethyl(vinyl)borane; (2)—five α,β-unsaturated carbonyl compounds + cyclopenta-2,4-dien-1-yldimethylborane. The reacting diene and dienophile pairs were chosen to restrict LAB interaction to the exo reaction pathway. It was found that in some of the examined cases, the favorable LAB interaction is so strong that it can lead to a completely exo-selective DA reaction. Furthermore, a novel multistep synthetic method was hypothesized for preparing exo cycloadduct with near 100% stereoselectivity. Our results can open up new avenues toward the rational design of exo-selective DA reactions for synthesizing novel bioorganic compounds.

Highly exo selective, photochemically promoted cyclization of iodoallene derivatives

AbstractA photochemically promoted intramolecular cyclization of aryl‐, vinyl‐, and alkyliodo allenes has been developed. The optimal conditions employed [Ir(ppy)2(dtbbpy)]PF6 (1 mol%) as catalyst affording products with high exo selectivity in moderate to good yields. Chiral substrates showed diastereoselectivity of up to 95/5 favoring trans product.

Understanding the catalysis by bis-selenonium cations as bidentate chalcogen bond donors

The mode of activation and catalysis of bidentate bis-selenonium cations have been computationally explored in detail by means of Density Functional Theory calculations. To this end, the parent Diels–Alder cycloaddition reaction between cyclohexadiene and methyl vinyl ketone is compared to the analogous reaction mediated by selenonium cations. It is found that, although the catalyst induces a significant stabilization of the LUMO of the dienophile, the observed acceleration of the process is not due to an enhancement of the orbital interactions between the reactants but to a remarkable reduction of both the steric (Pauli) repulsion between them and the destabilizing strain energy. Issues such as the origin of the endo:exo selectivity of the transformation and the influence of the nature of the tether connecting both selenium centers on the activity of the catalysts are also analyzed in detail.

Asymmetric Total Synthesis of Phomarol

Asymmetric Total Synthesis of Phomarol

Study and optimization of Diels-Alder reaction of piperine in aqueous ionic solutions using Gn.HCl as a catalyst

Study and optimization of Diels-Alder reaction of piperine in aqueous ionic solutions using Gn.HCl as a catalyst

Isolation, characterization and semi-synthesis of natural products dimeric amide alkaloids

Isolation, characterization and semi-synthesis of natural products dimeric amide alkaloids

Bifunctional Hydrogen Bond Donor-Catalyzed Diels-Alder Reactions: Origin of Stereoselectivity and Rate Enhancement.

The selectivity and rate enhancement of bifunctional hydrogen bond donor‐catalyzed Diels–Alder reactions between cyclopentadiene and acrolein were quantum chemically studied using density functional theory in combination with coupled‐cluster theory. (Thio)ureas render the studied Diels–Alder cycloaddition reactions exo selective and induce a significant acceleration of this process by lowering the reaction barrier by up to 7 kcal mol−1. Our activation strain and Kohn–Sham molecular orbital analyses uncover that these organocatalysts enhance the Diels–Alder reactivity by reducing the Pauli repulsion between the closed‐shell filled π‐orbitals of the diene and dienophile, by polarizing the π‐orbitals away from the reactive center and not by making the orbital interactions between the reactants stronger. In addition, we establish that the unprecedented exo selectivity of the hydrogen bond donor‐catalyzed Diels–Alder reactions is directly related to the larger degree of asynchronicity along this reaction pathway, which is manifested in a relief of destabilizing activation strain and Pauli repulsion.

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.

Endo-1-Phenylborneol as a novel, alternative chiral auxiliary for the aza-Diels-Alder reaction

We report the synthesis and preliminary evaluation of (1S,2S,4R)-7,7-dimethyl-1-phenylbicyclo[2.2.1]heptan-2-ol 1a, which was obtained in 3 steps from inexpensive starting materials, as a chiral auxiliary. The potential for asymmetric induction was investigated by carrying out aza-Diels-Alder reactions with cyclopentadiene and imine derivatives of 1a from (R)-(+)-1-phenylethylamine, (S)-(−)-1-phenylethylamine and benzylamine. The results showed marked exo selectivity and good diastereoisomeric excess when 1a was combined with (R)-(+)-1-phenylethylamine. These results are comparable with those reported using (−)-8-phenylmenthol, suggesting that 1a can represent an economically viable alternative to current chiral auxiliaries.

Synthesis of pyrroloindolines through formal [3 + 2]-cycloaddition of indoles with chiral N-2-acetamidoacrylyl oxazolidinones

Chiral N-2-acetamidoacrylyl oxazolidinones were produced and reacted with indoles under Lewis acid conditions to generate hexahydropyrrolo[2,3-b]indole products in a formal [3 + 2] cycloaddition process. Optimal conditions included the use of tin(IV) chloride in methylene chloride at 0 °C. Pyrroloindoline products were obtained from various indoles with shorter reaction times (12 hr) up to 91% yield with high, >20:1 exo selectivity. A mechanism involving reversible conjugate addition followed by an enamine lone-pair-iminium capture, tautomerization, and tin-enolate protonation accounts for the selectivity. The method enables selective applications to pyrroloindoline targets and further refinement with chiral catalysts.

Copper‐Catalyzed Alkylarylation of Unactivated Alkenes: Synthesis of 3‐Alkyl Indolines from N‐Allyl Anilines and Alkanes

A rare example of C(sp3 )-H functionalization of simple alkanes with unactivated alkenes is presented. In the presence of a copper salt and di-tert-butyl peroxide (DTBP), N-allyl anilines underwent exo-selective alkylation/cyclization cascade with unactivated alkenic bonds as radical acceptors and simple alkanes as radical precursors, providing a direct access to 3-alkyl indolines. The present protocol features simple operation, broad substrate scope and great exo selectivity.

Benzylarylation of N-Allyl Anilines: Synthesis of Benzylated Indolines.

An unprecedented benzylic C-H functionalization of methyl arenes across unactivated alkenes is presented. In the presence of MnCl2·4H2O and di- tert-butyl peroxide, N-allyl anilines underwent benzylation/cyclization cascade to give benzylated indolines, which are a previously unmet synthetic goal. This protocol features simple operation, broad substrate scope, and great exo selectivity.

Enantioselective Gold(I)-Catalyzed Heterocyclization-Intermolecular Exo [4 + 3]-Cycloaddition Reactions for the Synthesis of Chiral Oxa-Bridged Benzocycloheptanes.

The highly exo- and enantioselective gold-catalyzed tandem heterocyclization/[4 + 3] cycloaddition of 2-(1-alkynyl)-2-alken-1-ones and 1,3-diphenylisobenzofuran was implemented by utilizing Ming-Phos, which provides a facile access to chiral seven-membered oxa-bridged rings in 80-98% yield with high exo selectivity ( exo/ endo up to 50:1) and up to 97% ee.

Exo/endo Selectivity Control in Diels–Alder Reactions of Geminal Bis(silyl) Dienes: Theoretical and Experimental Studies

The factors controlling the reactivity and exo/ endo selectivity of Diels-Alder reactions of geminal bis(silyl) dienes catalyzed by AlEt2Cl are studied at the B3LYP-D3(BJ)(SMD,CH2Cl2)/6-31++G**//B3LYP-D3(BJ)(SMD,CH2Cl2)/6-31+G* theoretical level. The reaction proceeds via a two-stage one-step mechanism, and the AlEt2Cl as a Lewis acid catalyst enhances the electrophilicity of the carbonyl compound by coordination, consequently accelerating a cycloaddition reaction with a low energy barrier. A geminal bis(silyl) group of the diene and an α-substituent in α,β-unsaturated carbonyl compounds adjust the interaction energy (Δ Eint) as well as the deformation energy (Δ Estrain) of the diene and dienophile, affecting the barrier height and the diastereochemical outcome accordingly. The steric repulsion between the geminal bis(silyl) group and Al(III) catalyst increases the Pauli repulsion energy (Δ EPauli) and strain energy of dienophile fragment (Δ Estrain(dienophile-LA)) in the endo pathway, ensuring the exo selectivity. The introduction of a halogen atom (Cl or Br) or methyl group at the α-position of α,β-unsaturated carbonyl compounds increases the deformation energy of the diene fragment. Meanwhile, the noncovalent interactions (that is, dispersion and electrostatic interaction) stabilize the endo transition state, leading to predominant endo products. The theoretical predictions of the exo/ endo selectivity for Diels-Alder reactions of the substituted α,β-unsaturated carbonyl compound with Cl or Br atoms by the DFT method are also well confirmed by experiment.

Stereoinformation Relay: Coupling Diastereoselectivity of a Thermal Diels‐Alder Reaction with the Photochemical Ring‐Closure of Diarylethenes

AbstractControlling the stereochemistry, ideally in a reversible manner using external stimuli, is of great importance and appeal for biomedical and materials applications. By incorporating a Diels‐Alder adduct into one of the heteroaromatic rings of a diarylethene, we were able to transfer the exo selectivity of the thermal Diels‐Alder reaction upon photocyclization, leading to the preferred formation of one ring‐closed exo diastereomer in a diastereomeric ratio of 90 : 10. The structures of both ring‐closed exo isomers were assigned based on 2D NMR spectroscopy and DFT calculations. As indicated by both experimental and computational results, the conformational equilibrium of the Diels‐Alder adducts was kinetically trapped in the ring‐closed diarylethenes, leading to successful transfer of stereoinformation. The combination of thermal cycloaddition and photochemical cyclization steps and their associated diastereoselectivity provides ample opportunities to remotely control the transfer of stereoinformation and materials properties with light.

Effect of an α-Methyl Substituent on the Dienophile on Diels-Alder endo:exo Selectivity.

A detailed computational study of the Diels‐Alder reaction of cyclopentadiene with acrylonitrile, methylacrylate and their α‐methylated counterparts methacrylonitrile and methyl methacrylate at the M06‐2X(PCM)/TZVP level of theory has been performed. We want to understand the excellent exo‐selectivities observed experimentally due the presence of this substituent. To this end, analysis of the reaction coordinate by means of activation strain model of chemical reactivity (ASM‐distortion interaction model) including solvation effects and NBO second order perturbation energy have been carried out.