Dual Activation of Aromatic Diels–Alder Reactions

The unusually fast Diels–Alder reactions of [5]cyclophanes were analyzed by DFT at the BLYP-D3(BJ)/TZ2P level of theory. The computations were guided by an integrated activation-strain and Kohn–Sham molecular orbital analysis. It is revealed why both [5]metacyclophane and [5]paracyclophane exhibit a significant rate enhancement compared to their planar benzene analogue. The activation strain analyses revealed that the enhanced reactivity originates from 1) predistortion of the aromatic core resulting in a reduced activation strain of the aromatic diene, and/or 2) enhanced interaction with the dienophile through a distortion-controlled lowering of the HOMO–LUMO gap within the diene. Both of these physical mechanisms and thus the rate of Diels–Alder cycloaddition can be tuned through different modes of geometrical distortion (meta versus para bridging) and by heteroatom substitution in the aromatic ring. Judicious choice of the bridge and heteroatom in the aromatic core enables effective tuning of the aromatic Diels–Alder reactivity to achieve activation barriers as low as 2 kcal mol⁻¹, which is an impressive 35 kcal mol⁻¹ lower than that of benzene.     

Kekulé-based Valence Bond Model. II. Diels-Alder Reactivity of Polycyclic Aromatic Hydrocarbons

ＩｎｔｒｏｄｕｃｔｉｏｎＩｔｉｓｗｅｌｌｋｎｏｗｎｔｈａｔｍｏｌｅｃｕｌａｒｏｒｂｉｔａｌ (ＭＯ)ｔｈｅｏｒｙｈａｓｐｌａｙｅｄａｎｉｍｐｏｒｔａｎｔｒｏｌｅｉｎｕｎｄｅｒｓｔａｎｄｉｎｇｖａｒｉｏｕｓｃｈｅｍｉｃａｌｒｅａｃｔｉｏｎｓｏｆｐｏｌｙｃｙｃｌｉｃａｒｏｍａｔｉｃｈｙｄｒｏｃａｒｂｏｎｓ .1Ｅｓｐｅｃｉａｌｌｙ ,ｔｈｅＤｉｅｌｓ Ａｌｄｅｒｒｅａｃｔｉｏｎｓｏｆｍａｎｙｐｏｌｙｃｙｃｌｉｃｂｅｎｚｅｎｏｉｄｈｙｄｒｏｃａｒｂｏｎｓｗｉｔｈｍａｌｅｉｃａｎｈｙｄｒｉｄｅｈａｖｅｂｅｅｎｓ…     

Diels–Alder Reactions During the Biosynthesis of Sorbicillinoids

The sorbicillinoids are a class of biologically active and structurally diverse fungal polyketides arising from sorbicillin. Through co-expression of sorA, sorB, sorC, and sorD from Trichoderma reesei QM6a, the biosynthetic pathway to epoxysorbicillinol and dimeric sorbicillinoids, which resemble Diels–Alder-like and Michael-addition-like products, was reconstituted in Aspergillus oryzae NSAR1. Expression and feeding experiments demonstrated the crucial requirement of the flavin-dependent monooxygenase SorD for the formation of dimeric sorbicillinoids, hybrid sorbicillinoids, and epoxysorbicillinol in vivo. In contrast to prior reports, SorD catalyses neither the oxidation of 2′,3′-dihydrosorbicillin to sorbicillin nor the oxidation of sorbicillinol to oxosorbicillinol. This is the first report that both the intermolecular Diels–Alder and Michael dimerization reactions, as well as the epoxidation of sorbicillinol are catalysed in vivo by SorD.     

Singlet Biradical Versus Triplet Biradical/Zwitterion Characteristics in Isomers of C6−C5−C6−C7−C6-Fused Pentacyclic Aromatic Hydrocarbons Revealed through Reactivity Patterns

Among various polycyclic aromatic hydrocarbons, C₆−C₅−C₆−C₇−C₆ fused pentacyclic aromatic hydrocarbons have the unique potential to adopt quinonoid, zwitterion, singlet, or triplet biradical electronic configurations. Two such hybrid structures between pentacene and azulene were synthesized and their ground state electronic configurations were deduced from the reactivity patterns they exhibit respectively. Compound 6 , where the radicaloid carbons are linked through a para-phenylene, forms a head-to-head dimer like a singlet biradical. In contrast, isomer 7 , where the para-linkage was switched to meta, reacts readily with oxygen which resembles the reactivity of a triplet state. The oxidized intermediate(s) then undergoes rearrangement to furnish the C₆−C₅−C₆−C₆−C₆ ring contraction product 13 . Cation 14 , the protonated form of 7 , was synthesized, which implies 7 also reacts like a zwitterion. It was revealed the oxidative rearrangement takes place even with mesityl dibenzotropylium cation despite its perceived aromaticity. DFT calculations confirm the most stable forms of 6 and 7 are singlet and triplet diradical, which is consistent with the observed reactivity of respective molecules.     

Analysis of Polycyclic Aromatic Hydrocarbons Formed During Off-Line Pyrolysis of Ergosterol by GC–MS–SIM

This work describes the preparation and characterization of two stationary phases based on the immobilization of poly(methyltetradecylsiloxane) onto doubly titanized silica particles. Chromatographic tests indicate that these metallized silica-based stationary phases present similar chromatographic performance for apolar and polar compounds to those obtained for their bare silica-based counterparts. Spectroscopic characterizations indicate high residual silanol quantities on titanized silica-based stationary phases; however, this fact does not contribute to a high asymmetry factor for basic compounds as is usually found with titania-based stationary phases. These stationary phases may be useful for analysis of pharmaceuticals.     

Redox Modulation of the Reactivity and Regioselectivity in Diels–Alder Reaction of Metallofullerene La@C82 with Cyclopentadiene

Modulation of the reactivity of metallofullerenes is critical for production of metallofullerene derivatives with desired properties and functionalities. In this study, we investigate the effects of reduction and oxidation on the reactivity and regioselectivity in Diels–Alder reaction of metallofullerene La@C₈₂ by means of density functional theory calculations. Because of the enhanced electron-deficiency characteristic upon oxidation, the oxidized metallofullerene exhibits higher thermodynamic and kinetic reactivity as compared with neutral La@C₈₂. The regioselectivity in the reaction of La@C₈₂ with cylcopentadiene is remarkably changed after oxidation of the metallofullerene, which is explained in terms of the changes in the geometrical structure and the electronic structure of the metallofullerene. Quantitative analysis based on the activation-strain model demonstrates that the low activation energy barrier for the reaction of the cation La@C₈₂⁺ with cyclopentadiene originates from small strain energy and large interaction energy between the reactants. Energy decomposition analysis on the transition states of the reactions reveals that the exchange-repulsion interaction energy is one of the critical factors that determine the kinetic reactivity of the metallofullerene. This study not only provides new theoretical insights on how to modulate the reactivity of metallofullerenes, but also offers guideline for future experimental synthesis of new metallofullerene derivatives.     

Stereoselective synthesis of α-methylenecyclopentenones via a Diels–Alder/retro-Diels–Alder protocol

A new procedure for the stereoselective synthesis of cross-conjugated dienones is reported. This method makes use of the Diels–Alder adduct of anthracene and dimethyl fumarate, a precursor to a spirocyclopent-2-enone anthracene adduct as the key intermediate. The addition of propyllithium or octyllithium to the key intermediate followed by a retro-Diels–Alder reaction furnished α-methylenecyclopentenones bearing a γ-propyl or γ-octyl side chain, respectively, in moderate yields and as single geometric isomers.     

Reactivity of 2-methylene-1,3-dicarbonyl compounds: catalytic enantioselective Diels–Alder reaction

The catalytic enantioselective Diels–Alder reaction of 1,1-dicarbonylethenes 3 with cyclopentadiene in the presence of Ti-TADDOLs, Mg–Ph-box and Mg–Ph-mox complexes was investigated. Although both exo- and enantioselectivity with Ti-TADDOL catalysts were poor, they were much improved using Mg–Ph-box or Mg–Ph-mox complexes as chiral catalysts. Thus, 3 was an efficient two-point binding dienophile and the non-C₂-symmetric Ph-mox 8 could be used as a chiral ligand.     

Diels–Alder reactions of directly C3-dieneylated chlorophyll derivatives

The [4π?+?2π] cycloaddition of methyl 3-(1,3-butadien-1/2-yl)pyropheophorbides-a with tetracyanoethylene, dimethyl acetylenedicarboxylate, and naphthoquinone gave the corresponding Diels–Alder reaction adducts. The trans-1-substituted 1,3-butadiene was more reactive than its regioisomeric 2-substituent. The oxidation of some cycloadducts gave C3-arylated chlorophyll derivatives, whose ortho-substitution blue-shifted the corresponding Qy absorption maxima in dichloromethane because of the steric repulsion between the aryl and chlorin π-planes.     

A Nanospherical Metal–Organic Framework UiO-66 for Solid-Phase Microextraction of Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic hydrocarbons are a low content, but highly toxic organic pollutant in the environment. Because of their great threat to the health and survival of humans and other creatures, it is necessary to have a very simple and convenient as well as highly sensitive method to detect them. Based on previous studies, a nanospherical metal–organic framework UiO-66 was re-applied as the solid-phase microextraction coating and the extraction performance of polycyclic aromatic hydrocarbons was investigated. Scanning electron microscope and X-ray diffraction were used to characterize the extraction material. The extraction conditions were optimized factor by factor, and the analytical method was established with broad linear ranges (0.03–50 and 0.10–50 µg L^?1), excellent correlation coefficients (0.9955–0.9993) and low limits of detection (0.01–0.03 µg L^?1). The analytical method was applied to detect polycyclic aromatic hydrocarbons in rainwater and lake water, and satisfactory results were obtained. Compared with previous reports, this method had wider linear range, lower limit of detection and longer service life.     

Determination of Aromatic Hydrocarbons,Phenols, and Polycyclic Aromatic Hydrocarbons in Water by Stir bar Sorptive Extraction and Gas Chromatography–Mass Spectrometry

The paper presents procedures for stir bar sorptive extraction for selected priority substances that include polycyclic aromatic hydrocarbons, phenols, and benzene and its aromatic homologs. Three extraction and instrumental analysis procedures were developed for the determination of 16 compounds. The developed solid-phase extraction methods using a movable element had high recoveries above 70%. Extraction methods were optimized by selecting an appropriate time and temperature of the process, the sample pH, and stirring speed. The detection limits for the polycyclic aromatic hydrocarbons ranged from 0.33?µg/dm³ for naphthalene to 2.22?µg/dm³ for phenanthrene. Slightly higher limits of quantification were obtained for phenols: from 9.21?µg/dm³ for phenol to 0.51?µg/dm³ for 2,4,6-trimethylphenol. The highest limit of quantification for the tested compounds was obtained for benzene, with a value of 36.6?µg/dm³, while for toluene and ethylbenzene, the values did not exceed 2.7?µg/dm³.     

Proline-catalyzed asymmetric Diels–Alder reactions of an o-quinodimethane

Catalytic asymmetric Diels–Alder reaction of α-amino-o-quinodimethane with α,β-unsaturated aldehydes was achieved with high diastereo- and enantioselectivities in the presence of l-proline, which acts as a promoter to generate the quinodimethane from the corresponding precursor as well as a chiral catalyst for the enantioselective Diels–Alder reaction.     

C−N Coupling Reactions on Graphene with Aromatic Macromolecules and the Spatial Conformation of Grafted Macromolecules

The fabrication of advanced graphene-based nanocomposites with high-performance polymers requires covalent modification of graphene with aromatic macromolecules. Herein, C−N coupling reactions between fluorinated graphene (FG) and aromatic polyamides containing the benzimidazole moiety are successfully achieved. The optimized conditions are presented based on the nucleophilic behavior of the C−N coupling reaction on graphene. Different from the C−N coupling reaction between two small aromatic molecules, the conformation of grafted aromatic polyamide after reaction changes from torsional to paralleled alignment on graphene with the molecular length increment. Non-covalent interactions between graphene and aromatic polyamides result in this conformational change owing to the extended π systems of graphene and aromatic polyamides, and the synergistic effect of covalent and non-covalent interactions is put forward. As a consequence, graphene dispersibility is greatly enhanced in the solution of aromatic polyamide.     

Hetero Diels–Alder Cycloaddition of Indene for the Formal Synthesis of Onychnine

A highly regio‐ and stereoselective hetero Diels–Alder cycloaddition of indene with N‐sulfonyl‐1‐aza‐1,3‐butadiene was achieved. Subsequent transformation of the 5H‐indeno [l,2‐b]pyridine via elimination and reduction provides a new route to azafluorenone (e.g., 1‐methyl‐4‐azafluorene) for the synthesis of onychnine.     

Construction of functionalized B/C/D ring system of C19-diterpenoid alkaloids via intramolecular Diels–Alder reaction followed by Wagner–Meerwein rearrangement

A concise strategy for construction of B/C/D ring system of C₁₉-diterpenoid alkaloids was described, which features a highly diastereoselective intramolecular Diels–Alder reaction of masked o-benzoquinone and an efficient Wagner–Meerwein 1,2-shift rearrangement. The functional group transformations of rearrangement product 19 demonstrate a potential for stereoselective installation of oxygen functions on D-ring of many type of C₁₉-diterpenoid alkaloids.     

Tandem intramolecular Diels–Alder/retro-Diels–Alder cycloaddition of 2H-chromen-2-one as dienes with the expulsion of CO_2

To study the intramolecular Diels-Alder cycloadditon of 2H-chromen-2-one as a diene, a series of chiral N-allyl-N-benzylamides that contain a 2H-chromen-2-one moiety were designed for the synthesis of benzo[f]isoindol-1-ones via an intramolecular Diels-Alder and a subsequent retro-Diels-Alder cycloaddition with the expulsion of CO₂. Both the yield (80%-89%) and absolute stereocontrol of the tandem reaction were high when an electron-withdrawing group was attached to the dienophile. The double bond in the styrene substructure remained in the products could be further derivatized by dihydroxylation.     

Contraction of the cyclohexene ring in the Diels–Alder adduct of levoglucosenone with piperylene

Diels–Alder adducts of levoglucosenone with piperylene were subjected to allylic oxidation–epoxidation, followed by successive functionalization of the α- and β-positions, and transformations of the resulting 1,2-epoxy derivatives under the action of boron trifluoride–diethyl ether complex and bases were studied. The most efficient was the Wagner–Meerwein rearrangement of the α-hydroxy derivative with formation of a fused cyclopentane in 39% yield.     

On the Reactivity of Phosphaalumenes towards C−C Multiple Bonds

Heterocycles containing group 13 and 15 elements such as borazines are an integral part of organic, biomedical and materials chemistry. Surprisingly, heterocycles containing P and Al are rare. We have now utilized phosphaalumenes in reactions with alkynes, alkenes and conjugated double bond systems. With sterically demanding alkynes 1,2-phosphaalumetes were afforded, whereas the reaction with HCCH or HCCSiMe₃ gave 1,4-phosphaaluminabarrelenes. Using styrene saturated 1,2-phosphaalumates were formed, which reacted further with additional styrene to give different regio-isomers of 1,4-aluminaphosphorinanes. Using ethylene, a 1,4-aluminaphosphorinane is obtained, while with 1,3-butadiene a bicyclic system containing an aluminacyclopentane and a phosphirane unit was synthesized. The experimental work is supported by theoretical studies to shed light on the mechanism governing the formation of these heterocycles.