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1.
Helmut Schwarz 《Angewandte Chemie (International ed. in English)》1981,20(12):991-1003
Pyramidal cations are discussed with reference to their role as the connecting link between organic and inorganic chemistry. The electronic structure of these ions is treated with respect to their physical and chemical properties, namely charge distribution, geometry, and quenching reactions with nucleophiles. The chemistry in the gas phase of certain carbenium ions, in particular the scrambling of carbon atoms, is readily explicable by invoking transition states or intermediates of pyramidal structure. Moreover, the behavior of unimolecular processes can be understood in terms of transition states in which a hydrogen molecule is positioned as a “side-on” or an “end-on” ligand. 相似文献
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George A. Olah 《Angewandte Chemie (International ed. in English)》1973,12(3):173-212
The general concept of carbocations encompasses all cations of carbon containing compounds which can be differentiated into two distinct classes: trivalent (“classical”) carbenium ions and pentacoordinated (“nonclassical”) carbonium ions. In this paper the preparation and structural study (by NMR, IR, Raman and ESCA spectroscopy) of stable carbenium ions and carbonium ions are discussed. As is well known, trivalent carbenium ions play an important role in electrophilic reactions of π- and n-donor systems. Similarly, pentacoordinated carbonium ions are the key to electrophilic reactions of σ-donor systems (single bonds). The ability of single bonds to act as σ-donor lies in their ability to form carbonium ions via triangular two electron, three-center bond formation. Consequently there seems to be no principle difference between the electrophilic reactions of π- and σ-bonds except that the former react more easily, even with weak electrophiles, whereas the latter necessitate more severe conditions. The role of carbocations in electrophilic reactions of π- and σ-donor systems is discussed. 相似文献
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Viability of Nonclassical Carbocations Proposed as Intermediates in the Biosynthesis of Atiserene,Beyerene, Kaurene,and Trachylobane Diterpenes
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The results of quantum‐chemical calculations aimed at assessing the viability of nonclassical carbocations proposed to be involved in the biosynthesis of atiserne, beyerene, kaurene, and trachylobane diterpenes are presented. While the proposed edge‐protonated structure is much lower in energy than the proposed face‐protonated structure, neither is predicted to be a viable intermediate energetically. 相似文献
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G. K. Surya Prakash Tarik N. Rawdah George A. Olah 《Angewandte Chemie (International ed. in English)》1983,22(5):390-401
Methods developed for the generation and study of stable carbomonocations can also be employed for similar investigations on carbodications. Trivalent (classical) as well as hexacoordinate (non-classical) carbodications studied under long-lived ion conditions are reviewed with special emphasis on their structural features, and investigated primarily by 1H- and 13C-NMR spectroscopy. 相似文献
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Ronald E. Leone Paul von R. Schleyer 《Angewandte Chemie (International ed. in English)》1970,9(11):860-890
Carbonium ions are called “degenerate” when they rearrange through finite energy barriers to give products whose gross structures are identical with those of the starting ions. In such degenerate rearrangements (“automerizations”), individual atoms and groups become mixed but the overall structure is conserved. Automerizations can be observed by noting the dispersal of isotopic labels, the loss of optical activity, or the temperature dependence of NMR spectra. In certain cases, a combination of well-known rearrangement processes results in complete mixing of all atoms, C or H. The 2-norbornyl and 9-barbaralyl cations are examples of fully degenerate systems; conditions can be found where both give but single-line NMR spectra. 相似文献
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Michael Hanack 《Angewandte Chemie (International ed. in English)》1978,17(5):333-341
Compelling evidence for the intermediacy of vinyl cations has accumulated in recent years. Vinyl cations can be generated by electrophilic additions to alkynes and allenes, and on participation of such functions in solvolysis reactions. Heterolysis of vinyl compounds likewise affords vinyl cations. Syntheses of substituted indenes, cyclobutanones, and cyclopropyl ketones illustrate preparative applications. 相似文献
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Initiation and propagation rate constants of carbocationic and carbanionic polymerizations can be predicted by the correlation equation log k20 °C = s(N + E), where E characterizes the electrophilicity of carbocations and electron-deficient alkenes, and N characterizes the nucleophilicity of carbanions and electron-rich alkenes. Since the nucleophile-specific slope parameter s is generally close to 1, it can be neglected in a first approximation, and the two-dimensional representation in Figure 3 illustrates the gradual change from carbanionic to carbocationic polymerizations with Hall's “initiation by bond-formation” as the link connecting the two ranges. The value of model studies for understanding ionic polymerizations is illustrated. 相似文献
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Dr. Natasza Sprutta Monika Wełnic Michał J. Białek Prof. Tadeusz Lis Dr. Ludmiła Szterenberg Prof. Lechosław Latos‐Grażyński 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(20):6974-6980
In the search for tricarbaporphyrinoids, a three‐component acid‐catalyzed condensation of azulene, 2,5‐bis[(p‐tolyl)hydroxymethyl]thiophene, and an aryl aldehyde has been elaborated, affording the appropriate thiatriazuliporphyrinogens. The subsequent oxidation yielded a rare example of a macrocyclic organic tetracation, which can be readily and reversibly converted into macrocyclic tri‐ and dicarbocations by addition of one or two hydroxides bound at the meso position(s). Further insight into the influence of carbocation formation on the geometry, electronic structure, and magnetic manifestation in 1H NMR spectroscopy has been obtained by using density functional theory calculations. The charge distribution was evaluated by mapping electron density surfaces with electrostatic potential (ESP). 相似文献
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In this study, an old concept of anchimeric assistance is viewed from a different angle. Primary cations with two different heteroatomic substituents in the α-position to the cationic carbon atom CHXY–CH2+ (X, Y = Me2N, MeO, Me3Si, Me2P, MeS, MeS, Br) can be stabilized by the migration of either the X or Y group to the cation center. In each case, the migration can be either complete, resulting in the transfer of the migrating group to the adjacent carbon atom and the formation of a secondary carbocation stabilized by the remaining heteroatom, or incomplete, leading to an anchimerically assisted iranium ion. For all combinations of the above groups, these transformations have been studied by theoretical analysis at the MP2/aug-cc-pVTZ level and were shown to occur depending on the ability of anchimeric assistance by X and Y, as well as the conformation of the starting primary carbocation. In the conformers of α-amino cations with the p-orbital, C–N bond and the nitrogen lone pair in one plane, the Me2N group migrates to the cationic center to give aziranium ions. Otherwise, the second heteroatom is shifted to give iminium ions, without or with very slight anchimeric assistance. In the α-methoxy cations, the MeO group can be shifted to the cationic center to give the O-anchimerically assisted ions as local minima, the global minima being the ions anchimerically assisted by another heteroatom. The electropositive silicon tends to migrate towards the cationic center, but with the formation of a π-complex of the Me3Si cation with the C=C bond rather than a Si-anchimerically assisted cation. The phosphorus atom can either fully migrate to the cationic center (X = P, Y = S, Se) or form anchimerically stabilized phosphiranium ions (X = P, Y = O, Si, Br). The order of the anchimeric assistance for the heaviest atoms decreases in the order Se >> S > Br. 相似文献
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Jeong Hwan Koh 《Tetrahedron》2004,60(34):7405-7410
A catalytic oxidative polycyclization reaction initiated by the carbocyclization of 1,5-dienes with Pd(II) is reported. Trapping of a putative carbocation with suitable functional groups (phenols, alkenes, alcohols, sulfonamide), or rearrangement protocols (Pinacol) yields poly-cyclic products in good yields and in excellent diastereoselectivities. Turnover of the intermediate Pd-C bond is via β-H elimination. 相似文献
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Alessandro Volonterio 《Tetrahedron letters》2005,46(50):8723-8726
An alkylation/cyclization sequence, with both steps mediated by the ortho-N-methylformamido-phenylsulfinyl function, provided two new C-C bonds and an efficient entry to 1-aryl-tetralins and 4-aryl-benzopyrans. Scope and limits of the process have been studied in detail. 相似文献
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Cristina Dubceac Dr. Alexander S. Filatov Dr. Alexander V. Zabula Prof. Andrey Yu. Rogachev Prof. Marina A. Petrukhina 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(41):14268-14279
A detailed structural overview of a family of bowl‐shaped polycyclic aromatic carbocations of the type [C20H10R]+ with different R functionalities tethered to the interior surface of corannulene (C20H10) is provided. Changing the identity of the surface‐bound groups through alkyl chains spanning from one to four carbon atoms and incorporating a different degree of halogenation has led to the fine tuning of the bowl structures and properties. The deformation of the corannulene core upon functionalization has been revealed based on X‐ray crystallographic analysis and compared for the series of cations with R=CH3, CH2Cl, CHCl2, CCl3, CH2CH3, CH2CH2Cl, and CH2CH2Br. The resulting carbocations have been isolated with several metal‐based counterions, varying in size and coordinating abilities ([AlCl4]?, [AlBr4]?, [(SnCl)(GaCl4)2]?, and [Al(OC(CF3)3)4]?). A variety of aggregation patterns in the solid state has been revealed based on different intermolecular interactions ranging from cation–anion to π–π stacking and to halogen???π interactions. For the [C20H10CH2Cl]+ ion crystallized with several different counterions, the conformation of the R group attached to the central five‐membered ring of corannulene moiety was found to depend on the solid‐state environment defined by the identity of anions. Solution NMR and UV/Vis investigations have been used to complement the X‐ray diffraction studies for this series of corannulene‐based cations and to demonstrate their different association patterns with the solvent molecules. 相似文献
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Quantum chemical calculations indicate that the kinetic stability of an earlier proposed elegant planar ‘triangular’ benzene dication 1a is very low. The kinetic stability of its methylated derivative 2a is even lower. 相似文献
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L.Yu. UkhinA.R. Akopova A.V. BicherovL.G. Kuzmina A.S. Morkovnik G.S. Borodkin 《Tetrahedron letters》2011,52(42):5444-5447
A convenient method for the preparation of functionalized derivatives of isobenzofuran-1(3H)-ones and isoindol-1(3H)-ones by reactions of their carbocations with Bunte salts is described. The reactions proceed by means of electrophilic attack on the S(II)-atom of the Bunte salts. 相似文献
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Contrary to widely held opinion, for many reactions in organic and organometallic chemistry it is possible to define nucleophilicity and electrophilicity parameters that are independent of the reaction partners. This phenomenon, discovered by Ritchie during the early 1970s for reactions of highly stabilized carbenium and diazonium ions with n-nucleophiles, also occurs with reactions of carbenium ions with aliphatic and aromatic π-electron systems and in hydride transfer reactions. With the aid of the scales of nucleophilicity and electrophilicity set out here, which extend over eighteen orders of magnitude, forecasts can be made about the feasibility and rate of a given CC bond formation, ionic reduction, or diazo coupling. Linkage with the reactivity scales of Ritchie and Sweigart/Kane-Maguire enables a unified treatment of a large number of polar reactions. 相似文献