Cyclopropane

About: Cyclopropane is a research topic. Over the lifetime, 6463 publications have been published within this topic receiving 84838 citations. The topic is also known as: cyclopropanum & trimethylene.

Intramolecular reactions of alkynes with furans and electron rich arenes catalyzed by PtCl2: the role of platinum carbenes as intermediates.

Abstract: 5-(2-Furyl)-1-alkynes react, with PtCl2 as catalyst, to give phenols. On the basis of DFT calculations, a cyclopropyl platinacarbene complex was found as the key intermediate in the process. The cyclopropane and dihydrofuran rings of this intermediate open to form a carbonyl compound, which reacts with the platinum carbene to form an oxepin, which is in equilibrium with an arene oxide. When the reaction is carried out in the presence of water, dicarbonyl compounds are obtained, which support the proposed mechanism. Other cyclizations of alkynes with furans or electron-rich arenes give products of apparent Friedel−Crafts-type reactions, although these processes could also proceed by pathways involving the formation of cyclopropyl platinum carbenes.

Selective activation of carbon–carbon bonds next to a carbonyl group

Abstract: ORGANOMETALLIC complexes are used to effect a wide range of catalytic transformations in organic synthesis, such as the activation of C–H bonds1,2. Carbon–carbon bonds, however, are generally unreactive towards transition metals under homogeneous conditions. C–C bond activation by a process of oxidative addition to soluble transition-metal complexes has been limited mostly to stoichiometric (not catalytic) reactions1,3–7,18, to highly strained substrates such as cyclopropane and cubane1,8–11or to chelating ketones19. Here we present a synthetically useful process of selective C–C bond activation in which the C–C bond adjacent to a carbonyl group is opened by insertion of a soluble rhodium(I) complex. The resulting organometallic intermediate can be transformed to a variety of products in a way that regenerates the rhodium complex. We anticipate that this catalytic scheme will have considerable utility in organic synthesis.

Enantiospecific Sn(II)- and Sn(IV)-Catalyzed Cycloadditions of Aldehydes and Donor−Acceptor Cyclopropanes

Abstract: A cycloaddition strategy for the synthesis of highly enantioenriched 2,5-cis-disubstituted tetrahydrofurans has been developed. In the presence of catalytic Sn(OTf)2 or SnCl4, a range of aldehydes will undergo formal [3 + 2] cycloadditions with a scalemic donor−acceptor cyclopropane to form optically active heterocycles. Mechanistic studies support an unusual SN2 attack by the aldehyde on the activated cyclopropane. Through this mechanism, stereochemical information contained in the cyclopropane is effectively transferred to the tetrahydrofuran products.

Optical and Photoelectron Spectra of Small Rings. III. The Saturated Three‐Membered Rings

Abstract: The electronic states of cyclopropane, ethylene oxide, ethylenimine, and diaziridine were investigated using gas‐phase and condensed‐phase vacuum ultraviolet spectroscopy, photoelectron spectroscopy, and Gaussian orbital self‐consistent field calculations. Correlation of the gas‐phase and condensed‐phase optical spectra of cyclopropane with the first band in its photoelectron spectrum confirms the presence of Rydberg transitions in the optical spectrum involving excitation of an electron from the 3e′ sigma level. The valence shell spectrum of cyclopropane is complex but is dominated by two very strong 1A1′ → 1E′, σ → σ*, excitations. The Rydberg spectrum of ethylene oxide is reassigned to include the two absorptions in the 55 000–65 000‐cm−1 region, all bands originating from the 2b2π orbital, while its valence shell spectrum is closely related to that of cyclopropane. Similar but less conclusive results were obtained for ethylenimine. Virtual orbital calculations of σ → σ* excitation energies gave result...

Extended Hückel Theory. IV. Carbonium Ions

Abstract: The conformations, relative stabilities, and electronic distribution of a sample of the more important carbonium ions and positively charged hypothetical transition states are examined. The species studied include the methyl, ethyl, isopropyl, tertiary butyl, and higher alkyl carbonium ions; protonated ethylene, acetylene, benzene, cyclopropane; the cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl carbinyl, allyl, and benzyl cations; the carbonium ions based on norbornane, norbornene, norbornadiene. Significant charge delocalization for a classical carbonium ion geometry is observed—the extent of this phenomenon is wider than anticipated. For the alkyl carbonium ions it is shown that the order of stabilities may be obtained from a calculation in which the polarity of the C–H bond is C—–H+. Protonated ethylene and acetylene show local minima for a symmetrical complex, but with rearrangement to an unsymmetrical cation favored. Protonated cyclopropane prefers an unsymmetrical three‐center bonded structure, protonated benzene stabilizes in the familiar benzenium. The orientation of the empty carbonium p orbital with respect to other π‐type orbitals determines the conformation in cyclopropyl carbinyl, benzyl, and allyl. The peculiar nature of the cyclopropane electron distribution is studied. The carbonium ions based on the bicyclo[2.2.1]‐heptane structure show some nonclassical features; confirming experimental conclusions, the unusual 7‐norbornadienyl cation is calculated to prefer an unsymmetrical geometry. Difficulties in applying the extended Huckel theory to charged species make some of the conclusions from the calculations less certain.