Aromatic enynes can be transformed into arylnaphthalenes or benzofulvenes depending on the reaction conditions. Under gold(I) catalysis, exclusive or major 6‐endo‐dig cyclization took place leading to arylnaphthalenes. However, a catalytic system based on palladium iodide/1,3‐bis(diphenylphosphino)propane, in the presence of cesium carbonate as a base was necessary to furnish exclusively 5‐exo‐dig cyclization pattern, regardless of the electronic effects of the substituents. In the latter transformation, a mechanistic study (kinetic isotopic effect, density functional theory) involving a C H activation is suggested for the exclusive formation of benzofulvenes.
An efficient synthesis of (E)‐5‐aryl(halo)methylenebicyclo[2.2.2]oct‐2‐enes is reported. Lewis acid‐promoted carbohalogenation of 4‐(3‐arylprop‐2‐ynyl)‐cyclohex‐2‐enols in dichloromethane proceeds rapidly to afford the exo‐methylene‐bridged bicycles in good yields. This method also provides an easy access to (E)‐5‐aryl(halo)methylenebicyclo[2.2.1]hept‐2‐enes from the five‐membered ring 2,6‐enynols. The reactions are procedurally simple and high yielding, producing the aryl(halo)methylene‐bridged bicycles in minutes under air and mild conditions.
Calcium‐catalyzed regioselective synthesis of oxindole‐derived 1,5‐enynes, followed by cycloisomerization, from readily accessible 3‐hydroxy‐3‐(alkynyl)indolin‐2‐ones and styrenes in one‐pot, under solvent‐free conditions is described. This method offers the synthesis of diverse molecules: phenanthridinones, 3‐(cyclopentenylidene)indolin‐2‐ones, and 3‐spirocyclic indolin‐2‐ones are obtained through cascade reactions including cross‐dehydrative‐coupling, [3,3]‐sigmatropic rearrangement, carbocyclization, isomerization, oxidative‐ring rearrangement, and Diels‐Alder cycloaddition. In addition, this method features atom‐ and step‐economy, broad substrate scope, and high yields.
Cycloisomerizations allow the generation of defined molecular complexity starting from structurally simple polyunsaturated starting materials. Iron-based catalysts are newcomers in this field of catalysis that is dominated by catalysts based mainly on palladium, ruthenium, and in particular rhodium. In this review we summarize the most important contributions in this field of catalysis. Apart from “classical” Lewis-acid activations catalysts that possess redox-active ligands are of particular interest since they open new mechanistic pathways and hence new cycloisomerization venues. 相似文献
The selective oxidation of 1- and 2-propanol by molecular oxygen over supported platinum catalysts was investigated in supercritical carbon dioxide as an environmentally benign and safe reaction medium. The reaction occurs exclusively to acetone or propionic aldehyde and propionic acid in a single-phase region at 100–190 bar and at a mild temperature (40 °C). Compared to conversions in aqueous solution, catalyst stability is significantly enhanced in supercritical carbon dioxide and depends on the oxygen concentration in the reaction medium. Thus, at least a fourfold higher substrate/catalyst ratio than with water as a solvent can be used. Platinum catalysts with nanoporous silica (MCM-41, silicalite-1) as a support are also active for the oxidation of 2-propanol in supercritical carbon dioxide. 相似文献
We recently developed a completely new type of Rh‐catalyzed [5+2] cycloaddition by using 3‐acyloxy‐1,4‐enyne (ACE) as the 5‐carbon building block. In this update, we show that ACE can undergo intramolecular [5+2] cycloaddition with either an alkene or an allene in the presence of an appropriate rhodium catalyst and ligands to afford bicyclic compounds with multiple stereogenic centers. In most cases, cis‐fused bicyclo[5.3.0]decadienes are prepared highly diastereoselectively.
Herein we describe a direct, multicomponent assembly of 1,5‐enynes. The titanocene‐catalyzed coupling of an aryl aldehyde, iodoalkyne, and allylsilane enables the convergent and rapid synthesis of this versatile architectural motif in good to excellent yields. 相似文献
1,6‐Diyne‐4‐en‐3‐ols with one terminal alkyne were applied as test substrates for a possible dual catalyzed cyclization. Instead of a dual catalysis cycle, naphthyl ketone derivatives were obtained as single products. The regioselectivity of the obtained products is unprecedented. Instead of the expected naphthyl ketones bearing the keto group in the α‐position, the keto group is positioned in the ß‐position of the naphthyl skeleton by a complex rearrangement of the starting materials. 相似文献
A gold(I)/Brønsted acid‐catalyzed cyclization of 2‐azidobenzaldehydes with 3‐aza‐1,6‐enynes has been developed for the synthesis of tetrahydrobenzo[b][1,8]naphthyridine derivatives. This protocol enabled the modular synthesis of tetracyclic heterocycles in one operation with water and nitrogen gas as the byproducts.
One class of potential Lewis acids that has received negligible attention as a catalyst is the carbocation. Here we show the potential of triarylmethylium ions as highly powerful Lewis acid catalysts for organic reactions. The Lewis acidity of the triarylmethylium ion can be easily tuned by variation of the electronic properties of the aromatic rings and the catalytic activity of the carbocation is shown to correlate directly to the level of stabilization of the empty pC‐orbital at the cationic carbon. The versatility of triarylmethylium ions as efficient Lewis acid catalysts for organic reactions is demonstrated in Diels–Alder, aza‐Diels–Alder, conjugate addition, halogenation, epoxide rearrangement and intramolecular hetro‐ene reactions.
The gold(I)‐catalyzed isomerization of readily available 1,8‐dien‐4‐ynes allows the rapid construction of a variety of synthetically useful bicyclo[4.3.0]nonenes by a stereoselective sequence involving a [4+2] annelation/nucleophilic addition process. 相似文献
N‐Protected β‐propargylamino acrylic esters with a push‐pull olefinic bond afforded good to high yields of dihydropyridines upon treatment with 5% tris(2‐furyl)phosphine‐gold(I) chloride/silver(I) tetrafluoroborate [(TFP)AuCl/AgBF4] in anhydrous benzene. Carbamate and sulfonyl groups were employed for nitrogen protection. On a model enyne, the p‐methoxybenzenesulfonyl (MBS) group was found to be a better protective group than tosyl in terms of cyclization yield, and also the yield of elimination to the corresponding 2,3,4‐trisubstituted pyridines. Boc‐protected dihydropyridines underwent partial deprotection/oxidation under the cyclization conditions, which enabled a more straightforward, one‐pot preparation of the corresponding pyridines. In another application, an appropriately substituted derivative protected as a stable methoxycarbamate was subjected to catalytic hydrogenation affording the known precursor of paroxetine. The chemoselectivity of enyne cyclization (dihydropyridine vs. pyrrole) is governed, among other factors, by C‐3 substitution. Dihydropyridines were obtained as sole products regardless of the catalyst/conditions when C‐3 was unsubstituted.
The Brønsted acidic resorcin[4]arene hexamer can be applied as an effective catalyst in the dehydrative cyclization and subsequent rearrangement of unsaturated tertiary alcohols. This is the first report on catalyzing such a reaction with a Brønsted acid. Scope and limitations of this cyclopentene‐forming reaction sequence are presented. Furthermore, substrate‐selective conversion as well as competitive inhibition are described and provide evidence that the reactions proceed within the cavity of the self‐assembled structure. Additionally, a cyclobutanone‐forming intramolecular hydride transfer of an encapsulated cyclopropyl acetate is reported.
