Abstract Bei der Thermolyse von zwölf β‐Aryl‐perisovaleriansäure‐tert.‐butylestern I in verschiedenen Lösungsmitteln entstanden Gemische substituierter tert.‐Butyl‐und Isobutyl‐benzole mit 3‐Aryl‐ und 1‐Aryl‐isobutenen‐(1), die gaschromatographisch analysiert wurden. Mit Hilfe einer kinetischen Konkurrenzmethode ließen sich aus den Produktverhältnissen relative Wanderungsgeschwindigkeiten der Arylreste bei dieser Radikalumlagerung ermitteln. Die Ergebnisse und ihr Vergleich mit Lit.‐Daten 3,4) fordern keine Nachbargruppenbeteiligung der Arylreste bei der Decarboxylierung der β‐Aryl‐isovaleryloxy‐Radikale.
Eine Reihe 1,2-Dialkyl-1,2-diphenylethane 3 wurde durch Dimerisationsreaktionen dargestellt. Teilweise wurden die reinen Diastereomeren isoliert und ihre Konfigurationen aufgeklärt. Der thermische Zerfall der Kohlenwasserstoffe 3 wurde präparativ und kinetisch untersucht, er erfolgt fast ausschließlich durch primäre Spaltung der zentralen CC-Bindung. Die Zerfallskonstanten steigen semiquantitativ parallel mit den sterischen Substitutionskonstanten E
Substituent Effects on the CC Bond Strength, 16. — Thermal Stability and Enthalpies of Formation of β‐Dicarbonyl Compounds. — Stabilisation Enthalpies of α,α′‐Diketoalkyl Radicals The geminal interactions of the two carbonyl groups in seven β‐diketones of the type 8–9 and 11–13 were estimated from their enthalpies of formation, which were deduced from their heats of combustion and enthalpies of vaporization. The radical stabilisation enthalpies RSE of the α,α′‐diketoalkyl radicals 9–13 were obtained from the enthalpies of activation Δ H ≠ of thermal CC bond cleavage of eight model compounds of the type 9–13 by their comparison with Δ H ≠ of comparable hydrocarbons of similar strain. For non‐cyclic α,α′‐diacyl‐alkyl radicals and six‐membered cyclic ones RSE = 54.8 ± 1.3 kJ mol −1 was determined, which indicates an additive stabilisation by two carbonyl groups.
Abstract The radical mono‐ions of three azoalkanes in which the azo group is connected to the polycyclic alkane moieties at the bridgehead C‐atoms, i. e . 1,1′‐azonorbornane ( 1 ), 1,1′‐azotwistane ( 2 ), and 1,1′‐azobi‐cyclo[3.2.1]octane ( 3 ), were studied in fluid solution by ESR spectroscopy. According to the ESR parameters and MO models, the radical cations of 1 – 3 should be considered as σ radicals, whereas the corresponding radical anions are π radicals. INDO calculations point to a remarkable dependence of the l4 N‐coupling constants on the geometry at the N‐atoms in the radical cations of aliphatic azo compounds.
Thermolabile Hydrocarbons, 31. – Stereoselective Formation and Cleavage of the Dimers of the 1‐(5,6,7,8‐Tetrahydro‐1‐naphthyl)neopentyl Radical The 1,2‐diaryl‐1,2‐di‐ tert ‐butylethanes meso ‐ and DL‐ 6 are synthesised by partial catalytic hydrogenation of the parent dinaphthylethanes 5 . The crystal structure of meso ‐ 6 was obtained experimentally and calculated by molecular mechanics methods (MM2). It is shown, that 1‐(5,6,7,8‐tetrahydro‐1‐naphthyl)neopentyl radical 8 forms its dimers 6 with high stereoselectivity, e.g. DL‐ 6 : meso ‐ 6 = 45 (at ‐20°C) and 7.07 (at 100°C). The selectivity was measured over a range of 300 K by using several radical precursors. The difference of the enthalpy of activation has been derived for the two dimerisation reactions: δ H † dim (DL – meso ) = –2.8 ± 0.2 kcal/mol. The cleavage of DL‐ 6 and of meso ‐ 6 into 8 was measured kinetically, and the enthalpies of activation δ H † dis = 46.2 ± 0.6 (DL‐ 6 ) and 52.6 ± 1.3 kcal/mol ( meso ‐ 6 ) and the entropies of activation δ† dis = 8.4 ± 0.6 (DL‐ 6 ) and 20.0 ± 2.5 e.u. ( meso ‐ 6 ) have been obtained. A complete thermodynamic cycle is constructed by using the calculated (MM2) heats of formation δ H ° f = –27.6 (DL‐ 6 ) and –30.0 kcal/mol ( meso ‐ 6 ). Thus, the diastereomer (DL‐ 6 ), which is formed preferentially, appears to be the thermodynamically and kinetically less stable one. It turns out, that the high stereoselectivity of the dimerisation of 8 , compared to the parent 1‐phenyl‐neopentyl radical (2a), is mainly caused by the steric repulsions between the approaching radicals.
11 α-Aryloxy-peressigsäure-tert.-butylester und 4 α-Arylmercapto-peressigsäure-tert.-butylester wurden synthetisiert und durch Spektren und Elementaranalysen charakterisiert. Die Thermolyse dieser Peroxide erfolgt durch homolytische Fragmentierung. Ihre Geschwindigkeit gehorcht in beiden Reihen der Hammett-Beziehung mit σ = −1.1 bzw. −1.3 und unterliegt somit einem polaren Effekt. Organic Peroxides, IX. Preparation and Thermolysis of tert-Butyl α-Aryloxy- and α-Arylthioperacetates Eleven tert-butyl α-aryloxyperacetates and four tert-butyl α-arylthioperacetates were prepared and characterized by spectra and analyses. The products and rates of their thermolyses are explained by a homolytic fragmentation mechanism. The rates are predominantly influenced by a polar effect. This is supported by the observation of Hammett relations in both series (σ = −1.1 and −1.3).
Abstract Bei der Darstellung von Neophylmagnesiumchlorid bilden sich 1–6% Isobutylbenzol (II), 3‐Phenyl‐isobuten‐(1) (III) und β.β‐Dimethyl‐styrol (IV). Es wird gezeigt, daß diese Kohlenwasserstoffe durch Umlagerung der intermediär auftretenden β‐Phenyl‐isobutyl‐Radikale (V) in Phenyl‐tert. – butyl‐Radikale (VI) und deren Disproportionierung entstehen. Die umgelagerten Radikale nehmen auch an der Dimerenbildung teil. Das normale Grignard‐Produkt wird nicht oder nur unwesentlich durch Isomere verunreinigt.
