Vertical stabilization of cations by neighboring .sigma. bonds. General considerations

Abstract

A theory for stabilization of carbonium ions or other cations, by delocalization of neighboring bonds, is described. Such delocalization is available without changing the reactant geometry and is termed "vertical stabilization." The stabilizing influence is contrasted to the bridged-ion theory of neighboring group participation, in which the neighboring group moves toward the reaction center as the transition state is approached, and to frangomeric acceleration, in which the neighboring group moves away from the reaction center as the transition state is approached. The effects of structural changes on the magnitude of (vertical) -conjugation are discussed. In addition, further evidence is offered against significant effects of C-H hyperconjugation on bond lengths. Delocalization of bonds was first discussed in detail by Mulliken6 in his treatment of carbon-hydrogen hyperconjugation as an explanation of certain properties (1) Supported by the Air Force Office of Scientific Research, Grants AFOSR-69-1639 and AFOSR-69-1639A.(2) The effects ofconjugation on singlet neutral molecules, on free radicals, and on carbanions will be considered in subsequent papers.(3) The major part of these ideas was presented at the 155th National Meeting of the American Chemical Society, San Francisco, Calif., April 1968, p 39. Previous discussions ofconjugation in cations are found in ref 4a-g.(4) (a) W.

Darstellung und Eigenschaften von und Reaktionen mit metallhaltigen Heterocyclen, XLVII [1]. Einschiebungsreaktionen von phasentransferkatalytisch in situ erzeugten Dihalocarbenen in P-haltige Manganacycloalkane – Struktur und chemisches Verhalten der Reaktionsprodukte / Preparation and Properties of and Reactions with Metal-Containing Heterocycles, XLVII [1]. Insertion Reactions of Phase-Transfer-Catalytically in situ Generated Dihalocarbenes into P-Containing Manganacycloalkanes – Structure and Chemical

“…Wie aus spektroskopischen und röntgenographischen Untersuchungen hervorgeht, wird das Dihalocarben bei la, b unter Bildung von 2a, a', b, b' ausschließlich in die ß-C-H-Funktion eingeschoben. Dieses Resultat läßt sich mit der auch bei Metallacyclen und Alkylverbindungen des Siliciums, Germaniums und Zinns diskutierten, dirigierenden Wirkung des Metalls [22,23] für die Carbenspezies erklären. Die unterschiedlichen Ausbeuten von 2a, a', b, b' bzw.…”

“…unvollstän-dige Reaktion von la, auch unter verschärften Bedingungen, dürfte auf konformelle Gründe [4] zurückzuführen sein. Für eine optimale M-C-C-o-JIÜberlappung [22] ist eine M-C-C-W-trans-Anordnung [23] In den ^P^Hj-NMR-Spektren von 2a, a', b, b' wird die für Phosphamanganacycloalkane mit ungerader Ringgliederzahl auftretende Tieffeldverschiebung [1], welche auch an anderen P-haltigen Heterocyclen der Übergangsmetalle [26] Zur besseren Auflösung der in den 'H-NMRSpektren von 2a, a', b, b' auftretenden komplizierten Multiplettstrukturen wurde am Beispiel von 2 a eine Hochfeldmessung (400 MHz) durchgeführt (vgl. Abb.…”

Carbocation

“…The interaction between the methyl group orbitals and the empty ρ orbital of the cationic center is shown in Fig. Although it has been argued to the contrary on the basis of experimental data (Traylor et al, 1971), computations indicate that consider able structural modification toward nonclassical or charge delocalized structures may accompany hyperconjugation (Lathan et al, 1971;Radom et al, 1972a). The net stabilization is accompanied by charge derealization from one of the C -H bonding orbitals to the empty ρ orbital.…”

Functionalization of Endohedral Metallofullerenes with Reactive Silicon and Germanium Compounds

“…In this context, derivatization of fullerenes using silicon and germanium atoms is an attractive method for modifying the electronic properties of fullerenes because of the electron-donating effects of silyl and germyl groups [ 16 , 17 , 18 , 19 ]. We found that the reactions of empty fullerenes with reactive silicon and germanium compounds afforded the corresponding fullerene derivatives [ 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 ].…”