32 research outputs found
Borane-ctalyzed stereoselective CâH insertion, cyclopropanation, and ring-opening reactions
Lewis acidic boranes have been shown to be effective metal-free catalysts for highly selective reactions of donor-acceptor diazo compounds to a range of substrates. The reactions of α-aryl α-diazoesters with nitrogen heterocycles indole or pyrrole selectively generate C3 and C2 CâH insertion products, respectively, in good to excellent yields even when using unprotected indoles. Alternatively, benzofuran, indene, and alkene substrates give exclusively cyclopropanated products with α-aryl α-diazoesters, whereas the reactions with furans lead to ring-opening. Comprehensive theoretical calculations have been used to explain the differing reactivities and high selectivities of these reactions. Overall, this work demonstrates the selective metal-free catalytic reactions of α-aryl α-diazoesters with (hetero)cycles and alkenes. This simple, mild reaction protocol represents an alternative to the commonly used precious metal systems and may provide future applications in the generation of biologically active compounds
Tris(pentafluorphenyl)boran-katalysierte Erzeugung von Carbenium-Ionen und autokatalytische Pyrazol-Synthese â eine theoretische und experimentelle Studie
In den letzten Jahren hat sich die metallfreie organische Synthese unter Verwendung von Triarylboranen als Katalysatoren zu einem weit verbreiteten Forschungsgebiet entwickelt. Hier berichten wir ĂŒber eine umfassende theoretische und experimentelle Studie fĂŒr die hochselektive Synthese von N-substituierten Pyrazolen durch die Erzeugung von Carbenium-Ionen aus der Reaktion zwischen Arylestern und Vinyldiazoacetaten in Gegenwart von katalytischem Tris(pentafluorphenyl)boran [B(C6F5)3]. DFT-Studien zum Reaktionsmechanismus zeigen, dass die In-situ-Generierung einer Carbenium-Spezies als Autokatalysator fungiert, welcher die regiospezifische Bildung von N-substituierten Pyrazolen in guter bis hervorragender Ausbeute (bis zu 81â%) auslöst
Tris(pentafluorophenyl)borane-catalyzed carbenium ion generation and autocatalytic pyrazole synthesis-a computational and experimental study
In recent years, metal-free organic synthesis using triarylboranes as catalysts has become a prevalent research area. Herein we report a comprehensive computational and experimental study for the highly selective synthesis of N-substituted pyrazoles through the generation of carbenium species from the reaction between aryl esters and vinyl diazoacetates in the presence of catalytic tris(pentafluorophenyl)borane [B(C6F5)3]. DFT studies were undertaken to illuminate the reaction mechanism revealing that the in situ generation of a carbenium species acts as an autocatalyst to prompt the regiospecific formation of N-substituted pyrazoles in good to excellent yields (up to 81â%)
Dual Gold-Catalyzed Cycloaromatization of Unconjugated (E)-Enediynes
A synthesis of novel unconjugated (E)âenediynes from allenyl amino alcohols is reported and their goldâcatalyzed cascade cycloaromatization to a broad range of enantioenriched substituted isoindolinones developed. Experimental and computational studies support the reaction proceeding via a dualâgold Ï,Ïâactivation mode involving a key goldâvinylidene and allenylâgold containing intermediate
Understanding the Influence of Donor-Acceptor Diazo Compounds on the Catalyst Efficiency of B(C6F5)3 Towards Carbene Formation
Diazo compounds have been largely used as carbene precursors for carbene transfer reactions in a variety of functionalization reactions. However, the ease of carbene generation from the corresponding diazo compounds depends upon the electron donating/withdrawing substituents either side of the diazo functionality. These groups strongly impact the ease of N2 release. Recently, tris(pentafluorophenyl)borane [B(C6F5)3] has been shown to be an alternative transition metal-free catalyst for carbene transfer reactions. Herein, a density functional theory (DFT) study on the generation of carbene species from α-aryl α-diazocarbonyl compounds using catalytic amounts of B(C6F5)3 is reported. The significant finding is that the efficiency of the catalyst depends directly on the nature of the substituents on both the aryl ring and the carbonyl group of the substrate. In some cases, the boron catalyst has negligible effect on the ease of the carbene formation, while in other cases there is a dramatic reduction in the activation energy of the reaction. This direct dependence is not commonly observed in catalysis and this finding opens the way for intelligent design of this and other similar catalytic reactions