398 research outputs found
(3+3)-Annulation of Carbonyl Ylides with Donor-Acceptor Cyclopropanes: Synergistic Dirhodium(II) and Lewis Acid Catalysis
The first (3+3)-annulation process of donor-acceptor cyclopropanes using synergistic catalysis is reported. The Rh2 (OAc)4 -catalyzed decomposition of diazo carbonyl compounds generated carbonyl ylides inâ
situ. These 1,3-dipoles were converted with donor-acceptor cyclopropanes, activated by Lewis acid catalysis, to afford multiply substituted pyran scaffolds in high yield and diastereoselectivity. Extensive optimization studies enabled access to 9-oxabicyclo[3.3.1]nonan-2-one and 10-oxabicyclo[4.3.1]decen-2-ol cores, exploiting solvent effects on intermediate reactivity
Ring-Opening 1,3-Aminochalcogenation of Donor-Acceptor Cyclopropanes: A Three-Component Approach
A 1,3-aminothiolation was realized by reacting 2-substituted cyclopropane 1,1-dicarboxylates with sulfonamides and N-(arylthio)succinimides. Under Sn(OTf)2 catalysis the transformation proceeded smoothly to the corresponding ring-opened products bearing the sulfonamide in the 1-position next to the donor and the arylthio residue in the 3-position next to the acceptor. The procedure was extended to the corresponding selenium analogues by employing N-(phenylseleno)succinimides as an electrophilic selenium source
Ring-Opening Regio-, Diastereo-, and Enantioselective 1,3-Chlorochalcogenation of Cyclopropyl Carbaldehydes
meso-Cyclopropyl carbaldehydes are treated in the presence of an organocatalyst with sulfenyl and selenyl chlorides to afford 1,3-chlorochalcogenated products. The transformation is achieved by a merged iminium-enamine activation. The enantioselective desymmetrization reaction, leading to three adjacent stereocenters, furnished the target products in complete regioselectivity and moderate to high diastereo- and enantioselectivities (d.r. up to 15:1 and e.r. up to 93:7)
Ketenedithioacetals as Surrogates for the Formal Insertion of Ketenes into DonorâAcceptor Cyclopropanes
Reactivity Studies of Donor-Acceptor Cyclopropanes: Is there a Relation to Structural and Electronic Properties?
The kinetics of (3+2) cycloaddition reactions of 18 different donor-acceptor cyclopropanes with the same aldehyde were studied by inâ
situ NMR spectroscopy. Increasing the electron density of the donor residue accelerates the reaction by a factor of up to 50 compared to the standard system (donor group=phenyl), whereas electron-withdrawing substituents slow down the reaction by a factor up to 660. This behavior is in agreement with the Hammett substituent parameter Ï. The obtained rate constants from the (3+2) cycloadditions correlate well with data from additionally studied (3+n) cycloadditions with a nitrone (n=3) and an isobenzofuran (n=4). A comparison of the kinetic data with the bond lengths in the cyclopropane (obtained by X-ray diffraction and computation), or the 1 H and 13 Câ
NMR shifts, revealed no correlation. However, the computed relaxed force constants of donor-acceptor cyclopropanes proved to be a good indicator for the reactivity of the three-membered ring
Transforming Dyes into Fluorophores:Exciton-Induced Emission with Chain-like Oligo-BODIPY Superstructures
Herein we present a systematic study demonstrating to which extent exciton formation can amplify fluorescence based on a series of ethylene-bridged oligo-BODIPYs. A set of non- and weakly fluorescent BODIPY motifs was selected and transformed into discrete, chain-like oligomers by linkage via a flexible ethylene tether. The prepared superstructures constitute excitonically active entities with non-conjugated, Coulomb-coupled oscillators. The non-radiative deactivation channels of Internal Conversion (IC), also combined with an upstream reductive Photoelectron Transfer (rPET) and Intersystem Crossing (ISC) were addressed at the monomeric state and the evolution of fluorescence and (non-)radiative decay rates studied along the oligomeric series. We demonstrate that a âmaskedâ fluorescence can be fully reactivated irrespective of the imposed conformational rigidity. This work challenges the paradigm that a collective fluorescence enhancement is limited to sterically induced motional restrictions
Ferrocenyl-substituted tetrahydrothiophenes via formal [3 + 2]-cycloaddition reactions of ferrocenyl thioketones with donor-acceptor cyclopropanes
Ferrocenyl thioketones reacted with donor-acceptor cyclopropanes in dichloromethane at room temperature in the presence of catalytic amounts of Sc(OTf)3 yielding tetrahydrothiophene derivatives, products of formal [3 + 2]-cycloaddition reactions, in moderate to high yields. In all studied cases, dimethyl 2-arylcyclopropane dicarboxylates reacted with the corresponding aryl ferrocenyl thioketones in a completely diastereoselective manner to form single products in which (C-2)-Ar and (C-5)-ferrocenyl groups were oriented in a cis-fashion. In contrast, the same cyclopropanes underwent reaction with alkyl ferrocenyl thioketones to form nearly equal amounts of both diastereoisomeric tetrahydrothiophenes. A low selectivity was also observed in the reaction of a 2-phthalimide-derived cyclopropane with ferrocenyl phenyl thioketone
Functionalization of Sydnones with DonorâAcceptor Cyclopropanes, Cyclobutanes, and Michael Acceptors
We present a Lewis acid catalyzed nucleophilic ring-opening of donor-acceptor cyclopropanes and -butanes by sydnones, utilizing their respective 1,3- and 1,4-reactivity. The same conditions can be applied for the addition of sydnones to Michael acceptors. We propose a Friedel-Crafts like mechanism. The reaction provides a rare, low-temperature, transition metal-free, and functional group tolerant protocol for the late-stage functionalization of these mesoionic compounds of emerging importance in catalysis and bio-orthogonal chemistry
- âŠ