Metal-Free Carbocyclization of Homoallylic Silyl Ethers Leading to Cyclopropanes and Cyclobutanes

© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, WeinheimWe have developed a Hosomi-Sakurai type carbocyclization of homoallylic silyl ethers in reaction with silyl nucleophiles, catalyzed by Lewis acidic silylium salt. It offers cyclopropane and cyclobutane products with high efficiency and selectivity. A range of silyl nucleophiles could be engaged in this transformation to give small-sized carbocycles incorporating allyl, allenyl, carbonyl, indole or thioether groups. Diastereoselectivity in the cyclobutane formation was observed to be dependent on the steric bulkiness of incoming nucleophiles.11sciescopu

Borane-Catalyzed Ring-Opening and Ring-Closing Cascades of Furans Leading to Silicon-Functionalized Synthetic Intermediates

The conversion of renewable biomass resources to synthetically valuable chemicals is highly desirable, but remains a formidable challenge in regards to the substrate scope and reaction conditions. Here we present the development of tris(pentafluorophenyl)borane–catalysed conversion of furans via ring-opening and closing cascade processes to afford siliconfunctionalized synthetic chemicals under transition metal-free conditions. The furan ring-opening with hydrosilanes is highly efficient (TON up to 2,000) and atom-economical without forming any byproduct to give rise to a-silyloxy-(Z)-alkenyl silanes. Additional equivalents of silane smoothly induce a subsequent B(C6F5)3-catalysed cyclization of initially formed olefinic silane compounds to produce anti-(2-alkyl)cyclopropyl silanes, another versatile synthon being potentially applicable in the synthesis of natural products and pharmacophores. (c) The Author(s) 201611sciescopu

Copper(I)-Catalyzed Regio- and Chemoselective Single and Double Addition of Nucleophilic Silicon to Propargylic Chlorides and Phosphates

Copper(I)-catalyzed propargylic substitution of linear precursors with (Me₂PhSi)₂Zn predominantly yields the γ isomer independent of the propargylic leaving group. The thus formed allenylic silane reacts regioselectively with another equivalent of (Me₂PhSi)₂Zn, yielding a bifunctional building block with allylic and vinylic silicon groups. The reaction rates of both steps are well-balanced for chloride (γ:α ≥ 99:1) where the propargylic displacement occurs quantitatively prior to the addition step. Substitutions of α-branched propargylic phosphates are also reported

Metal‐Free Carbocyclization of Homoallylic Silyl Ethers Leading to Cyclopropanes and Cyclobutanes

© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, WeinheimWe have developed a Hosomi-Sakurai type carbocyclization of homoallylic silyl ethers in reaction with silyl nucleophiles, catalyzed by Lewis acidic silylium salt. It offers cyclopropane and cyclobutane products with high efficiency and selectivity. A range of silyl nucleophiles could be engaged in this transformation to give small-sized carbocycles incorporating allyl, allenyl, carbonyl, indole or thioether groups. Diastereoselectivity in the cyclobutane formation was observed to be dependent on the steric bulkiness of incoming nucleophiles.11sciescopu

Borane catalysed ring opening and closing cascades of furans leading to silicon functionalized synthetic intermediates

The conversion of renewable biomass resources to synthetically valuable chemicals is highly desirable, but remains a formidable challenge in regards to the substrate scope and reaction conditions. Here we present the development of tris(pentafluorophenyl)borane-catalysed conversion of furans via ring-opening and closing cascade processes to afford silicon-functionalized synthetic chemicals under transition metal-free conditions. The furan ring-opening with hydrosilanes is highly efficient (TON up to 2,000) and atom-economical without forming any byproduct to give rise to α-silyloxy-(Z)-alkenyl silanes. Additional equivalents of silane smoothly induce a subsequent B(C6 F5)3 -catalysed cyclization of initially formed olefinic silane compounds to produce anti-(2-alkyl)cyclopropyl silanes, another versatile synthon being potentially applicable in the synthesis of natural products and pharmacophores. © The Author(s) 2016113121sciescopu

Synthesis of Axially Chiral Biaryls through Sulfoxide-Directed Asymmetric Mild CH Activation and Dynamic Kinetic Resolution

International audienc

Tandem Cooperative Friedel-Crafts Reaction of Aldehydes with Electron Deficient Arenes Through Catalyst-Activation via Hydrogen Bonding Network

Since its discovery in 1877, the Friedel-Crafts alkylation reaction has been the method of choice to prepare various aryl hydrocarbons. Recent developments for this reaction have resulted in the synthesis of these compounds in one pot process with various metal as well as metal free protocols. However, the alkylation of common feedstock aldehydes using electron-deficient arenes and also with two different arene nucleophiles are quite challenging and scantily explored. Herein, we provide a solution to these problems by a new concept, “catalyst activation” accomplished by increasing the Brønsted acidity of p-toluenesulfonic acid (pTSA) through strong hydrogen bonding with hexafluoroisopropanol (HFIP). The real-time NMR titration, as well as computational studies, reveal multiple roles of HFIP in increasing the Brønsted acidity of para-toluene sulphonic acid (pTSA) and stabilization of the transition states formed during the electrophilic aromatic substitution. The developed process has a great potential for industrial application reflected in the synthesis of various bio-active natural products like arundine, tartarinoid C, and several other bioactive molecules. Also, the used HFIP was recovered in a gram-scale synthesis making this protocol highly cost-effective and conducive to industrial production

Disrotatory Ring-Opening of Furans Gives Stereocontrol

The ring-opening of 2-methylfuran and 2,3-dihydro-5-methylfuran catalyzed by the Lewis acid catalyst tris(pentafluorophenyl)borane in the presence of hydrosilanes was studied using quantum chemical methods. In a previous study, it was suggested that the stereoselective formation of the product is due to a nucleophilic vinylic substitution (SNV) during the reaction. Our calculations show that the pathway involving the SNV reaction is energetically not accessible. Instead, the intramolecular C-O bond cleavage is found to be much more favorable in energy for the ring opening reaction. The experimentally observed excellent stereoselectivity toward the Z-isomer product originates from an intrinsic preference of the furan ring to couple the C-O bond cleavage with a disrotatory motion of the oxygen and carbon fragments. This stereoselective feature is naturally programmed into the furan ring manifold and should be generally exploitable for engineering stereoselective ring-opening processes of bioderived furans. © 2019 American Chemical Society11sciescopu

Enantiopure Sulfoxides: Efficient Chiral Directing Group for Stereoselective C‒H Bond Activation: Towards the Control of Axial Chirality

International audienc