Diels–Alder Reactions of 1,2‐Azaborines

Diels–Alder reactions employing 1,2‐azaborine heterocycles as 1,3‐dienes are reported. Carbocyclic compounds with high stereochemical and functional complexity are produced, as exemplified by the straightforward two‐step synthesis of an amino allyl boronic ester bearing four contiguous stereocenters as a single diastereomer. Whereas electron‐deficient dienophiles undergo irreversible Diels–Alder reactions, a reversible Diels–Alder reaction with the less electron‐deficient methyl acrylate is observed. Both the N and the B substituent of the 1,2‐azaborine exert significant influence on the [4+2] cycloaddition reactivity as well as the aromatic character of the heterocycle. The experimentally determined thermodynamic parameters of the reversible Diels–Alder reaction between 1,2‐azaborines and methyl acrylate correlate with aromaticity trends and place 1,2‐azaborines approximately between furan and thiophene on the aromaticity scale.     

Efficient Synthesis of 1,3‐Dithiol‐2‐one Derivatives via 4,5‐Bis(dibromomethyl)‐1,3‐dithiol‐2‐one

A series of 1,3‐dithiol‐2‐one derivatives via [4 + 2] Diels–Alder cycloaddition reaction of 4,5‐bis(dibromomethyl)‐1,3‐dithiol‐2‐one with vinyl‐substituted compounds have been synthesized. Structures of all the newly synthesized compounds are well supported by spectral data such as ¹H‐NMR, MS, and elemental analysis. The structures of IVf and IVg have been analyzed by X‐ray crystallography.     

Pressure‐Induced Diels–Alder Reactions in C6H6‐C6F6 Cocrystal towards Graphane Structure

Pressure‐induced polymerization (PIP) of aromatics is a novel method for constructing sp³‐carbon frameworks, and nanothreads with diamond‐like structures were synthesized by compressing benzene and its derivatives. Here by compressing a benzene‐hexafluorobenzene cocrystal (CHCF), H‐F‐substituted graphane with a layered structure in the PIP product was identified. Based on the crystal structure determined from the in situ neutron diffraction and the intermediate products identified by gas chromatography‐mass spectrum, we found that at 20 GPa CHCF forms tilted columns with benzene and hexafluorobenzene stacked alternatively, and leads to a [4+2] polymer, which then transforms to short‐range ordered H‐F‐substituted graphane. The reaction process involves [4+2] Diels–Alder, retro‐Diels–Alder, and 1‐1′ coupling reactions, and the former is the key reaction in the PIP. These studies confirm the elemental reactions of PIP of CHCF for the first time, and provide insight into the PIP of aromatics.     

Investigation of Diels–Alder Reaction of 7‐Substituted 4‐Styrylcoumarins with Symmetrical Dienophiles Leading to 3,4‐Annulated Coumarins

The thermal [4 + 2] cycloaddition reaction of 7‐substituted 4‐styrylcoumarins with N‐phenylmaleimide and tetracyanoethylene in nitrobenzene under reflux conditions rapidly gives 3,4‐annulated coumarins as the Diels–Alder adducts. The position of the surviving double bond was determined on the basis of NMR and supported by energies of the possible structures. The effects of the 7‐substituent and the solvent on the reaction were studied.     

Bioinspired Intramolecular Diels–Alder Reaction: A Rapid Access to the Highly‐Strained Cyclopropane‐Fused Polycyclic Skeleton

A bioinsipred gold‐catalyzed tandem Diels–Alder/Diels–Alder reaction of an enynal and a 1,3‐diene, forming the highly‐strained benzotricyclo[3.2.1.0^2,7]octane skeleton, was reported. In contrast, a Diels–Alder/Friedel–Crafts tandem reaction occurred instead when silver salts were used as the catalyst. Although both reactions experienced the similar Diels–Alder reaction of a pyrylium intermediate with a 1,3‐diene, they have different reaction mechanisms. The former proceeded with a stepwise Diels–Alder reaction, while the latter one with a concerted one.     

Photochemistry of Tricyclo[5.2.2.02,6]undeca‐4,10‐dien‐8‐ones: An Efficient General Route to Substituted Linear Triquinanes from 2‐Methoxyphenols. Total Synthesis of (±)‐Δ9(12)‐Capnellene

An efficient and short entry to polyfunctionalized linear triquinanes from 2‐methoxyphenols is described by utilizing the following chemistry. The Diels–Alder reactions of masked o‐benzoquinones, derived from 2‐methoxyphenols, with cyclopentadiene afford tricyclo[5.2.2.0^2,6]undeca‐4,10‐dien‐8‐ones. Photochemical oxa‐di‐π‐methane (ODPM) rearrangements and 1,3‐acyl shifts of the Diels–Alder adducts are investigated. The ODPM‐rearranged products are further converted to linear triquinanes by using an O‐stannyl ketyl fragmentation. Application of this efficient strategy to the total synthesis of (±)‐Δ⁹⁽¹²⁾‐capnellene was accomplished from 2‐methoxy‐4‐methylphenol in nine steps with 20 % overall yield.     

Asymmetric Diels–Alder and Inverse‐Electron‐Demand Hetero‐Diels–Alder Reactions of β,γ‐Unsaturated α‐Ketoesters with Cyclopentadiene Catalyzed by N,N′‐Dioxide Copper(II) Complex

Highly enantioselective Diels–Alder (DA) and inverse‐electron‐demand hetero‐Diels–Alder (HDA) reactions of β,γ‐unsaturated α‐ketoesters with cyclopentadiene catalyzed by chiral N,N′‐dioxide–Cu(OTf)₂ (Tf=triflate) complexes have been developed. Quantitative conversion of β,γ‐unsaturated α‐ketoesters and excellent diastereoselectivities (up to 99:1) and enantioselectivities (up to >99 % ee) were observed for a broad range of substrates. Both aromatic and aliphatic β,γ‐unsaturated α‐ketoesters were found to be suitable substrates for the reactions. Moreover, the chemoselectivity of the DA and HDA adducts were improved by regulating the reaction temperature. Good to high chemoselectivity (up to 94 %) of the DA adducts were obtained at room temperature, and moderate chemoselectivity (up to 65 %) of the HDA adducts were achieved at low temperature. The reaction also featured mild reaction conditions, a simple procedure, and remarkably low catalyst loading (0.1–1.5 mol %). A strong positive nonlinear effect was observed.     

Preparation of a Porous polymer by a catalyst‐free diels‐alder reaction and its structural modification by post‐reaction

A microporous polymer is prepared by a catalyst‐free Diels–Alder reaction. A cyclopentadiene with both a diene and a dienophile functionality and a dienophilic maleimide are used for the Diels–Alder reaction. 1,3,5‐Tris(bromomethyl)‐2,4,6‐trimethylbenzene is reacted with sodium cyclopentadienide to produce the multicyclopentadiene‐functionalized monomer. A crosslinked polymer ( CDAP ) is obtained by the reaction of the cyclopentadiene monomer with N,N′‐1,4‐phenylenedimaleimide. The thermal dissociation of the cyclopentadiene dimeric unit and the subsequent Diels–Alder reaction with the maleimide group are investigated by the model reaction. We are able to restructure the crosslinked polymer network by taking advantage of the thermal reversibility of the Diels–Alder linkage. After the post thermal treatment, the BET surface area of the polymer ( CDAP‐T ) is greatly increased from 317 to 1038 m² g^?1. CDAP‐T is functionalized with pyrene by bromination with N‐bromosuccinimide and the subsequent substitution reaction with aminopyrene. The adsorption property of the pyrene‐functionalized polymer for an aromatic dye is investigated using malachite green. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3646–3653     

Indium Trichloride Catalyzed Diels–Alder Reaction: Synthesis of Novel 5‐Butyl‐11a‐aryl‐4a,5,11,11a‐tetrahydro‐11bH‐indolo[3,2‐c]quinoline‐1,4‐dione

Hetero Diels–Alder reaction of 3‐butyliminomethyl‐2‐aryl‐1H‐indoles (Schiff's base) 1 with p‐benzoquinone 2 affords six novel 5‐butyl‐11a‐aryl‐4a,5,11,11a‐tetrahydro‐11bH‐indolo[3,2‐c]quinoline‐1,4‐diones 3 in good yields. All the reactions proceeded with complete diastereoselectivity giving only one product in each case, which was characterized on the basis of its elemental analyses and spectral data (IR, ¹H NMR, and Mass).     

Diels–Alder Reactions of Novel (1′‐Arylallylidene)cyclopropanes with Heterodienophiles

Palladium‐catalyzed cross‐coupling of various aryl iodides with bicyclopropylidene provided isolable (1′‐arylallylidene)cyclopropanes, which reacted with a number of carbonyl compounds in the presence of Eu(fod)₃ under high pressure to furnish oxaspiro[2.5]octene derivatives in moderate to good yields (22–69 %). The reactions of the allylidenecyclopropanes with two azo compounds as dienophiles afforded diazaspiro[2.5]octenes in high yields (82 and 99 %) even at ambient pressure. When treated with nitrosobenzene, two of the allylidenecyclopropanes gave the Diels–Alder adducts in up to 83 and 40 % yield. 2,5‐Diiodo‐p‐xylene coupled twice with bicyclopropylidene, and the product underwent a twofold Diels–Alder reaction with nitrosobenzene to produce the bis(spirocyclopropaneoxazine) derivative in 88 % yield. This overall transformation can be brought about in a one‐pot, two‐step operation by addition of the nitrosoarene to the reaction mixture immediately after formation of the allylidenecyclopropanes to furnish various 5‐oxa‐4‐azaspiro[2.5]oct‐7‐ene derivatives in 22–77 % yield. The coupling of methyl bicyclopropylidenecarboxylate with 2,6‐dimethylphenyl iodide produced a mixture of very stable regioisomeric allylidenecyclopropane derivatives in 90 % yield. The reaction of this mixture with N‐phenyltriazolinedione gave a corresponding mixture of the spirocyclopropanated heterobicycles in 61 % yield.     

Asymmetric polymerization via cycloaddition reactions

The reaction of N‐phthaloyl‐L ‐leucine acid chloride (1) with isoeugenol (2) was carried out in chloroform, and novel optically active isoeugenol ester derivative 3 as a chiral monomer was obtained in high yield. Compound 3 was characterized by ¹H‐NMR, IR, and mass and elemental analysis and then was used for the preparation of model compound 5 and polymerization reactions. 4‐Phenyl‐1,2,4‐triazoline‐3,5‐dione, PhTD (4), was allowed to react with compound 3. The reaction is very fast and gives only one diastereomer of 5 via Diels–Alder and ene pathways in quantitative yield. In order to explain this diastereoselectivity, a nonconcerted two‐step mechanism involving benzylic cation (BC) and aziridinium (AI) have been proposed for the Diels–Alder and ene reactions, respectively. The polymerization reactions of novel monomer 3 with bis(triazolinedione)s [bis(p‐3,5‐dioxo‐1,2,4‐triazolin‐4‐ylphenyl)methane (8) and 1,6‐bis(3,5‐dioxo‐1,2,4‐triazolin‐4‐yl)hexane] (9)] were performed in N,N‐dimethylacetamide (DMAc) at room temperature. The reactions are exothermic, fast, and gave novel optically active polymers 10 and 11 via repetitive Diels–Alder–ene polyaddition reactions. These polymers have inherent viscosities in a range about 0.18–0.22 dL/g. Some physical properties and structural characterizations of these new polymers have been studied and are reported. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1211–1219, 1999     

Crystallographic characterization of a stable 7‐phosphanorbornadiene‐7‐oxide: 2,3‐benzo‐1,4,5,6,7‐pentaphenyl‐7‐phosphabicyclo[2.2.1] hepta‐2,5‐diene‐7‐oxide

The stable 7‐phosphanorbornadiene derivative, 2,3‐benzo‐1,4,5,6,7‐pentaphenyl‐7‐phosphabicyclo[2.2.1]hepta‐2,5‐diene‐7‐oxide ( 1 ) was synthesized in 45% yield via the Diels‐Alder reaction of pentaphenylphosphole oxide and benzyne. The reaction occurs specifically to give a single isomer, which was characterized by use of X‐ray crystallography and ³¹P NMR spectroscopy. © 2000 John Wiley & Sons, Inc. Heteroatom Chem 11:182–186, 2000     

Access to 1‐Phospha‐2‐azanorbornenes by Phospha‐aza‐Diels–Alder Reactions

The unprecedented phospha‐aza‐Diels–Alder reaction between an activated electron‐poor imine and 2H‐phospholes yields 1‐phospha‐2‐azanorbornenes in a highly chemoselective and moderately diastereoselective reaction. The intermediate 2H‐phospholes, which act as dienes, are formed in situ from the corresponding 1H‐phospholes. Theoretical calculations confirm that the phospha‐aza‐Diels–Alder reaction is of normal electron demand. The reactive P?N bond in 1‐phospha‐2‐azanorbornenes can be cleaved by nucleophiles leading to the formation of 2,3‐dihydrophospholes.     

The Diels–Alder Reaction of 4,6‐Dinitrobenzofuroxan with 1‐Trimethylsilyloxybuta‐1,3‐diene: A Case Example of a Stepwise Cycloaddition

The reaction of 4,6‐dinitrobenzofuroxan (DNBF) with 1‐trimethylsilyloxybuta‐1,3‐diene ( 8 ) is shown to afford a mixture of [2+4] diastereomeric cycloadducts ( 10 , 11 ) through stepwise addition–cyclization pathways. Zwitterionic intermediate σ‐adduct 9 , which is involved in the processes, has been successfully characterized by ¹H and ¹³C NMR spectroscopy and UV/visible spectrophotometry in acetonitrile. A kinetic study has been carried out in this solvent that revealed that the rate of formation of 9 nicely fits the three‐parameter equation log k=s(E+N) developed by Mayr to describe the feasibility of nucleophile–electrophile combinations. This significantly adds to the NMR spectroscopic evidence that the overall cycloadditions take place through a stepwise mechanism. The reaction has also been studied in dichloromethane and toluene. In these less polar solvents, the stability of 9 is not sufficient to allow direct characterization by spectroscopic methods, but a kinetic investigation supports the view that stepwise processes are still operating. An informative comparison of our reaction with previous interactions firmly identified as prototype stepwise cycloadditions is made on the basis of the global electrophilicity index, ω, defined by Parr within the density functional theory, and highlighted by Domingo et al. as a powerful tool for understanding Diels–Alder reactions.     

Quadruple click reactions for the synthesis of cysteine‐terminated linear multiblock copolymers

Synthesis of cysteine‐terminated linear polystyrene (PS)‐b‐poly(ε‐caprolactone) (PCL)‐b‐poly(methyl methacrylate) (PMMA)/or poly(tert‐butyl acrylate)(PtBA)‐b‐poly(ethylene glycol) (PEG) copolymers was carried out using sequential quadruple click reactions including thiol‐ene, copper‐catalyzed azide–alkyne cycloaddition (CuAAC), Diels–Alder, and nitroxide radical coupling (NRC) reactions. N‐acetyl‐L ‐cysteine methyl ester was first clicked with α‐allyl‐ω‐azide‐terminated PS via thiol‐ene reaction to create α‐cysteine‐ω‐azide‐terminated PS. Subsequent CuAAC reaction with PCL, followed by the introduction of the PMMA/or PtBA and PEG blocks via Diels–Alder and NRC, respectively, yielded final cysteine‐terminated multiblock copolymers. By ¹H NMR spectroscopy, the DP_ns of the blocks in the final multiblock copolymers were found to be close to those of the related polymer precursors, indicating that highly efficient click reactions occurred for polymer–polymer coupling. Successful quadruple click reactions were also confirmed by gel permeation chromatography. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Diels‐Alder reaction of (2,4,6‐trialkylphenyl)phospholes with N‐phenylmaleimide

2,4,6‐Trialkylphenylphospholes 3a (R=Me), 3b (R=i–Pr) and 3c (R=t–Bu), with increasing flattening at phosphorus and hence with increasing electron delocalisation, underwent the Diels‐Alder reaction with N‐phenylmaleimide to give predominantly cycloadducts 4a–c with the trialkylphenyl substituents anti to the phosphanorbornene double bond. With increasing aromaticity, the cycloaddition was slower. The stereostructure of the products ( 6 and 7 ) obtained after oxidation was confirmed by stereospecific ²J_PC NMR couplings and by an independent synthesis. © 2000 John Wiley & Sons, Inc. Heteroatom Chem 11:271–275, 2000     

Diels‐alder reaction of 1‐methyl‐3,6,8‐trinitro‐2‐quinolone

1‐Methyl‐3,6,8‐trinitro‐2‐quinolone (1) behaved as the dienophile in Diels‐Alder reactions with dienes. When cyclopentadiene was used, cycloadduct 4 was obtained, which was then aromatized on treatment with triethylamine. In the reaction of 1 with hydrazone of 2‐butenal, phenanthridine derivative 7 was produced.     

Enantioselective Nitroso‐Diels–Alder Reaction and Its Application for the Synthesis of (−)‐Peracetylated Conduramine A‐1

Cu^I‐catalyzed enantioselective nitroso‐Diels–Alder reactions (NDA reactions) of 2‐nitrosopyridine with various dienes are presented. The [CuPF₆(MeCN)₄]/Walphos‐CF₃ catalyst system is best suited to catalyze the NDA reaction of various dienes by using 2‐nitrosopyridine as a dienophile. In most of the cases studied, cycloadducts are obtained in quantitative yield with very good to excellent enantioselectivities. Based on DFT calculations, a model to explain the stereochemical outcome of the NDA reaction is presented. Finally, an efficient short synthesis of (?)‐peracetylated conduramine A‐1 by applying the enantioselective NDA reaction as a key step is described.     

New poly(oxyethylene) derivatives and their oligo analogues from Diels–Alder reactions of 5‐[methoxypoly(oxyethylene)]‐(3E)‐1, 3‐pentadiene and 5‐methoxyethoxy‐(3E)‐1,3‐pentadiene

Diels–Alder reactions of 5‐[methoxypoly(oxyethylene)]‐(3E)‐1,3‐pentadiene ( 1a ) with maleic anhydride, diethyl acetylenedicarboxylate (DADC), and acrolein were investigated for the synthesis of new poly(ethylene glycol) derivatives. To facilitate the characterization of the derivatives, Diels–Alder reactions of 5‐methoxyethoxy‐(3E)‐1,3‐pentadiene ( 1b ) with the aforementioned dienophiles were also studied. The reaction of o‐toluidine with the cycloaddition product from maleic anhydride and 1b resulted in the corresponding amide products. The reactions of 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone with cycloadducts derived from 1a and 1b with DADC resulted in the aromatization of the corresponding products. An NMR analysis of the adducts obtained from 1a and acrolein in water and from 1b and acrolein in water/acetonitrile (4:1 v/v) indicated a mixture of endo and exo, with the endo concentration being approximately 80%. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1895–1902, 2005     

How Alkali Cations Catalyze Aromatic Diels‐Alder Reactions

We have quantum chemically studied alkali cation‐catalyzed aromatic Diels‐Alder reactions between benzene and acetylene forming barrelene using relativistic, dispersion‐corrected density functional theory. The alkali cation‐catalyzed aromatic Diels‐Alder reactions are accelerated by up to 5 orders of magnitude relative to the uncatalyzed reaction and the reaction barrier increases along the series Li⁺ < Na⁺ < K⁺ < Rb⁺ < Cs⁺ < none. Our detailed activation strain and molecular‐orbital bonding analyses reveal that the alkali cations lower the aromatic Diels‐Alder reaction barrier by reducing the Pauli repulsion between the closed‐shell filled orbitals of the dienophile and the aromatic diene. We argue that such Pauli mechanism behind Lewis‐acid catalysis is a more general phenomenon. Also, our results may be of direct importance for a more complete understanding of the network of competing mechanisms towards the formation of polycyclic aromatic hydrocarbons (PAHs) in an astrochemical context.