Why a proximity-induced Diels-Alder reaction is so fast

Unlike normal Diels-Alder reactions of acyclic alkadienes with alkenes, the vinylbicyclo[2.2.2]octene employed in the Baran total synthesis of vinigrol undergoes a quantitative Diels-Alder reaction with a tethered alkene at room temperature. Density functional theory calculations reveal that this unprecedented reactivity originates from a combination of preorganization, diene strain, and tether stabilization.     

Kinetic studies of a fast, reversible alkene radical cation cyclization reaction

The radical cation formed by mesylate heterolysis from the 1,1-dimethyl-7,7-diphenyl-2-mesyloxy-6-heptenyl radical was studied in several solvents. Computational results suggest that the initially formed acyclic radical cation is a resonance hybrid with partial positive charge in both double bonds of 1,1-diphenyl-7-methyl-1,6-octadiene (10). Thiophenol trapping was used as the competing reaction for kinetic determinations. The acyclic radical cation rapidly equilibrates with a cyclic distonic radical cation, and thiophenol trapping gives acyclic product 10 and cyclic products, mainly trans-1-(diphenylmethyl)-2-(1-methylethenyl)cyclopentane (11). The rate constants for cyclization at ambient temperature were k = (0.5-2) x 10(10)(s-1), and those for ring opening were k = (1.5-9) x 10(10)(s-1). Laser flash photolysis studies in several solvents show relatively slow processes (k = (2.5-260) x 10(5)(s-1) that involve rate-limiting trapping reactions for the equilibrating radical cations. In mixtures of fluoroalcohols RfCH2OH in trifluoromethylbenzene, variable-temperature studies display small, and in one case a negative, activation energies, requiring equilibration reactions prior to the rate-limiting processes. Fast equilibration of acyclic and cyclic radical cations implies that product ratios can be controlled by the populations of the acyclic and cyclic species and relative rate constants for trapping each.     

Synthesis and Diels-Alder reaction of a sapphyrin derivative

Sapphyrins participate in Diels-Alder reactions with pentacene affording novel barrelene-fused sapphyrins. The new compounds were synthesized using traditional heating and microwave irradiation conditions. The experiments carried out under microwave irradiation proved cleaner, affording only the monoadduct and in higher yields.     

Dehydrogenative Diels-Alder reaction

The dehydrogenative cycloaddition of dieneynes, which possess a diene in the form of a styrene moiety and a dienophile in the form of an alkyne moiety, produces naphthalene derivatives when heated. It was found that a key requirement of this process is the presence of a silyl group attached to the alkyne moiety, which forces a dehydrogenation reaction to occur.     

Hydrocarbon ladder aromatics from a Diels-Alder reaction

The Diels-Alder homopolycycloaddition of 2,5-diphenyl-3,4-(5,6-acenaphthylenylene)-cyclopentadienone (VIII) affords a low molecular weight soluble ladder polymer having reduced specific viscosities between 0.17 and 0.25 dl/g in benzene and an insoluble fraction of higher molecular weight. The ladder polymers exhibited a major TGA break at 450°C in an air atmosphere and lost approximately 30% of their weight at 700°C in a nitrogen atmosphere.     

Acceleration of a Diels-Alder reaction in an ultracentrifuge

The reaction rate for the Diels-Alder reaction between maleic anhydride and furan is increased when performed in an ultracentrifuge.     

Asymmetric intramolecular Diels-Alder reaction of a sulphoximine-substituted triene

The synthesis and thermally-induced intramolecular Diels-Alder (IMDA) reaction of (S)_s-(+)-S-[(1E,7E)-1,7,9-decatrienyl]-S-phenyl-N-(p-tolylsulphonyl)sulphoximine 4 is described. A model is proposed for the cis-selectivity of the cycloaddition based on previous results from the reactions of sulphonyl-substituted trienes. The stereochemistry of the major cis-fused cycloadduct is determined by single-crystal X-ray analysis of that obtained from reaction of racemic 4.     

Tandem Pauson-Khand reaction and Diels-Alder reaction for access to polycycles in a one-pot reaction

Starting from easily available cyclic alkenes, enynes, and dienophiles, a tandem intermolecular Pauson-Khand reaction and Diels-Alder reaction yields polycyclic compounds in high yields.     

Intramolecular Diels-Alder reaction of selenoaldehydes

Intramolecular Dieis-Alder reaction of selenoaldehydes which were generated from bis(trimethylsilyl) selenide and dienals gave the corresponding bicyclic adducts.     

The change of aromaticity along a Diels-Alder reaction path

[reaction: see text] Quinodimethanes are highly reactive toward dienophiles since Diels-Alder cycloaddition results in an aromatic product. Density functional-based (13)C, (1)H NMR, NICS, and MO-NICS calculations indicate that the increase of aromatic character of the developing benzenoid ring along the reaction path is especially pronounced after the transition state is reached, even though the number of pi orbitals decreases. The forming aliphatic ring exhibits large ring current effects during the reaction.     

Highly stereoselective hydroxy-directed Diels-Alder reaction

The successful stereocontrol of the Diels-Alder reaction of semicyclic dienes possessing a secondary and tertiary allylic magnesium alkoxide alcohol functionality and activated dienophiles such as methyl acrylate, methacrolein, acrolein, and N-phenylmaleimide is described.     

Synchronism in the Diels-Alder reaction

Defined as the formation of two new bonds of equal length at the same time, synchronism has been shown to be not possible for the Diels-Alder reaction when the products are not of strict C_s symmetry. This has been done in two steps. First, the SCF -MO transition-state structures for the cyclization reactions of ethylene with cyclopentadiene and furan have been calculated. The activation energies (and heat of reaction) with the STO -3G, 3-21G, and 4-31G basis sets are as follows: with cyclopentadiene 35.0 (?80.7), 30.0 (?32.0), 38.4 (?19.9) kcal/mol; with furan 33.4 (?66.4), 34.6 (?16.5), 42.1 (?8.8) kcal/mol. Then, using a bond-order-bond-length relationship, a value for the tendency toward asynchronism has been calculated for substituted reactants where the two new bonds are arbitrarily held equal. This tendency has been shown in all cases which break strict C_s symmetry. Further, inspection of the atomic-orbital coefficients showed that the substituents cannot be considered as mere perturbations on the pi system in many cases.     

An unusual intramolecular Diels-Alder reaction

The diene-ester (2) on heating undergoes an intramolecular Diels-Alder reaction via the intermediate (3). The β-alkyl furan moiety in 3 reacts as the dienophile adding to the cyclohexadiene unit to give 4.     

Synthesis of pisiferol revisited; control of stereochemistry in an intramolecular Diels-Alder reaction

Thermolysis of 4(X=H) gives 6a and 6b(X=H) in a ratio of 1:4 whereas 4 (X=SO₂Ph) gives more of the trans-ring junction product 6a(X=SO₂Ph) suitable for the synthesis of pisiferol [ratio 6a : 6b(X=SO₂Ph) = 1.5:1].     

Supramolecular self-assembly of dendronized polymers: reversible control of the polymer architectures through acid-base reactions

Acid-base switchable supramolecular dendronized polyacetylenes (DPAs) with increasing steric bulk on going from generation one [G1] to three [G3], were constructed using multiple self-assembly processes between Fréchet-type [G1]-[G3]-dendritic dialkylammonium salts and a dibenzo[24]crown-8-containing polymer. The formation of the supramolecular systems is acid-base switchable to either an ON (rodlike dendronized polymers) or an OFF (flexible polymers) state. Thus, by controlling the superstructures of the supramolecular polymers with the [G1]-[G3] dendrons, it is possible to induce conformational changes within the polymer backbones. The supramolecular dendronized polymers, as well as their threading-dethreading properties, were characterized by (1)H NMR and UV absorption spectroscopies, gel permeation chromatography (GPC) and light scattering (LS). Independent measures of molecular weight (GPC, LS) indicate that DPAs behave as increasingly rigid macromolecules with each generation in solution. Molecular dynamics simulations of each DPA suggest that the lengths of the polymer backbones increase accordingly. Atomic force microscopy of the [G3]-dendronized polystyrene (DPS), as well as the DPAs, reveal surface morphologies indicative of aggregated superstructures.     

Diels-Alder reaction of eight-membered cyclic siloxydienes

Eight-membered cyclic siloxydienes, 2-(tert-butyldimethylsiloxy)-1-methyl-5-oxacycloocta-1,3-diene and 2-(tert-butyldimethylsiloxy)-5-oxacycloocta-1,3-diene, were prepared from δ-valerolactone, and their Diels-Alder reactions with various dienophiles are reported.     

由Diels-Alder反应合成四氢咔唑

采用W.E.Noland等人提出的咔唑合成路线,以吲哚、醛或酮、丁烯二酰亚胺为原料,在酸催化下经缩合和Diels-Alder反应而形成四氢咔唑。本研究采用乙酸乙酯为溶剂,对甲苯磺酸为催化剂,提高了反应速度和产物质量。共合成11个新的四氢咔唑化合物,进行了^1HNMR和IR测试及元素分析,确认了其结构。     

Hydrophobic effects in a simple Diels-Alder reaction in water

The endo/exo ratio for a simple Diels-Alder reaction carried out in water has been used to argue that hydrophobic effects can dominate the geometries of the transition states.     

Theoretical study of the Diels-Alder reaction

The cycloaddition of ethylene to butadiene has been studied by theab initio LCAO-SCF-MO method of Roothaan using STO-3G and 7s-3p basis sets. The potential energy hypersurface of the supersystem formed by the reactants has been calculated in order to determine the reaction path. It was found that, during the approach of the partners, the planes of the molecules form an angle around 70 °. The activated complex has a geometry which prefigures the half-chair conformation of cyclohexene and exhibits no biradical character. Our theoretical results are in reasonable agreement with the corresponding experimental ones.