Organische Peroxide, XIX. Zur Frage der Stabilität und Reaktivität von Kohlenstoffradikalen

Abstract

Aryl-, Alkyl-, Cycloalkyl-, Polycycloakyl-und Benzyl-Radikale wurden aus PerGureestern 1 oder (Ary1azo)triphenylmethan durch Thermolyse in CC14/BrCC13 erzeugt; ihre Selektivitat bei der konkurrierenden Halogenubertragung von BrCCl, und CC14 wurde durch gaschromatographische Produktanalyse bestimmt. Die Konkurrenzkonstanten r sinken in der Reihe Benzyl-> Alkyl-> Aryl-> Briickenkopf-Radikal, wofur das Selektivitiitsprinzip verantwortlich gemacht wird.Sterische und polare Effekte besinflussen die Konkurrenzkonstanten ebenfalls deutlich. Es besteht keine einfache Beziehung zwischen der Bildungsgeschwindigkeit der Radikale aus den Perestern und ihrer Selektivitat. -Der Anteil an Kafigrekombination bei der Thermolyse von Cycloalkanpercarbonsaureestern mit 5 -12 Ringgliedern sinkt parallel mit zunehmender Masse dcr beteiligten Radikale. Organic Peroxides, XIX') Relations between Stability and Selectivity of CarbonAryl, alkyl, cycloalkyl. polycycloalkyl, and benzyl radicals were generated by thermolysis of peresters 1 or (ary1azo)triphenylmethanes in CC14/BrCC13. The selectivity of the competing halogen transfer from BrCCl, and C C 4 was determined by gaschromatography of the products. The selectivity relationship lo) is made responsible for the decreasing of the competition constants r in the series benzyl > alkyl > aryl > bridgehead radicals. In addition steric and polar effects determine the size of the competition constants. No relationship is recognized between the rate of radical production from peresters 1 and their selectivity. -The yields of cage recombination product in the thermolysis of tert-butyl cycloalkanepercarboxylates (n = 5 -12) in CC14 decrease regularly with increasing mass of the cycloalkyl radicals. I) Diese Aussage ist in der Polanyi-Evans-Beziehung zwischen Aktivierungsenergie und Reaktionsenthalpie enthalten. M. G. Evans und M . Polmlyi, Trans. Faraday SOC. 34. 1 1 (1938); J . A. Kerr, Chem. Rev. 66,465 (1966). ' ) '*) R. S. Davidson, Quart. Rev. 1%7, 249. -6b) A . A . Zavitsas und J . A . Pinto, J. Amer. Chem. Soc. 94, 7390 (1972), und zit. Lit. -") W A . Pryor, W H . Davis und J . P . Stanley, ebenda 95, 4754 (1973). -6d) J . A. Howard und J . H . B. Chenier, ebenda 95,3054 (1973). -R. C. Neumann und R. P. Pankratz, ebenda 95, 8372 (1973). -" I J . P. Engstrom und J . C . Du Bose, J. Org. Chem. 38, 3817 (1973). -6g)

Reductive and Brominative Termination of Alkenol Cyclization in Aerobic Cobalt-Catalyzed Reactions

“…The proposed mechanistic scheme furthermore allows us to explain the unsatisfactory yield of acceptor-substituted bromocyclization product 3c (Table , entry 10). Br-atom trapping of cyclized intermediates requires homolytic displacement of •CCl 3 from BrCCl 3 by carbon radicals. , A Hammett correlation suggests that partial negative charge develops at CCl 3 in the transition state as the Br-atom is transferred from BrCCl 3 onto a positively polarized carbon radical center. The radical that is left in the course of cyclization of 1c is expected for reasons of electron-withdrawing capabilities of the CO 2 Me-substituent to react notably slower with BrCCl 3 , due to a marked lowering of its SOMO energy and thus reduced ability to serve as an electron donor according to the polar transition state model.…”