Organische Peroxide, XXI. Zum Käfigeffekt bei der Peresterfragmentierung

Abstract

Die Ausbeute an substituierten Benzyl-terr-butyllthern 3ai, den Produkten der Kafigrekombination bei der Thermolyse substituierter Phenylperessigsaure-tert-butylester 1 ai in n-Octan wird durch Substituenten in o-und p-Stellung nur geringfugig beeinfluBt. Bei Solvensvariation steigt die Ausbeute an 3 mit der Viskositiit des Solvens, aber weitgehend unabhangig von dessen Natur entsprechend den von Noves und Koenig abgeleiteten Beziehungen an. Auch bei der Thermolyse der Phenylperessigsaure-tert-alkylester 4a -f andert sich der Anteil der Kafigrekombination kaum. Die Perester 4b -e zerfallen konkurrierend homolytisch und heterolytisch. Der Anteil der beiden Zerfallswege lien sich durch kinetische Radikalabfangversuche und quantitative Produktanalysen bestimmen. Elektronische und sterische Effekte haben in den untersuchten Beispielen keinen wesentlichen EinfluC auf die Radikalrekombinationsrate. Organic Peroxides, XXI')The Cage Effect in Homolytic Perester Fragmentations2)The yields of substituted benzyl tert-butyl ethers 3a -i obtained as products of cage recombination in the thermoiysis of substituted tert-butyl phenylperacetates 1 a -i in n-octane depend only little on the substitution pattern of the phenyl ring. The influence of solvent variation on the yields of 3 is determined mainly by solvent viscosity according to the equations of Noyes and Koenig, and only to a minor degree by the nature of the solvent. The proportion of cage recombination in the homolytic fragmentation of tert-alkyl phenylperacetates 4a -f is also quite independent of the tert-alkyl group. The peresters 4be decompose competitively in homolytic and heterolytic fashion. Therefore radical counting experiments and quantitative product analyses were required for obtaining a quantitative estimate of the cage effect. The rate ofcage recombination of tert-alkoxy-benzyl radical pairs proved to be quite independent of steric and electronic effects.Zu den schnellsten Reaktionen organischer Radikale zahlt ihre Dimerisierung, deren Geschwindigkeit in Losung oft durch die Geschwindigkeit der Diffusion gesteuert wird 4. 5 ) . Wahrend in neuerer Zeit absolute Geschwindigkeitskonstanten der Reaktionen zweier Radikale rniteinander in groI3erer Zahl bekannt wurden 4 * ' ), diskutierte man den EinfluB d e r Struktur auf die Dimerisierung lange ausschlieDlich anhand der Konkurrenz von ') XX. Mitteil.: H. Lanqhals und C. Riichardr, Chem. Ber. 108, 2156 (1975. 2' Aus der Dissertation M. Grundmeier, Univ. Munster 1972.

The effects of microwave power and dielectric properties on the microwave-assisted decomposition kinetics of AIBN in n-butanol

“…This means that 19 cmol mol À1 of the initiator radicals are lost in solvent cage recombinations and only 81 cmol mol À1 of AIBN molecules are efficient in free-radical production under these reaction conditions [23]. Solvent cage effects have been observed not only in the decomposition of azoalkanes, but also in the thermolysis of other compounds, such as peroxides [23,24], which generate two radicals simultaneously. So, hemmed AIBN molecules and radicals by solvent molecules cannot absorb MW irradiation due to the cage effect of the solvent due to large amount of microwave energy absorbed and converted to thermal energy by solvent molecules.…”

Preparation of Benzylic α,α-Difluoronitriles, -tetrazoles, and -sulfonates via Electrophilic Fluorination

“…Nitriles 6 , 9 , 11 , 13 , 15 , 17 , 21 , and 23 and other starting materials and reagents were purchased from Aldrich Chemical Co. (Milwaukee, WI) unless stated otherwise. Nitrile 19 was prepared according to a literature procedure . Methyl, ethyl, propyl, and neopentyl ( 30 ) esters of (phenyl)methanesulfonate were prepared by literature procedures .…”

“…Nitrile 19 was prepared according to a literature procedure. 19 Methyl, ethyl, propyl, and neopentyl (30) esters of (phenyl)methanesulfonate were prepared by literature procedures. 20 All NMRs were run in CDCl3 unless stated otherwise.…”

“…Column chromatography (silica, 8:2 hexane/CH2Cl2, Rf ) 0.5) yielded pure 7 as a white solid in 50% yield using t-BuLi as base: mp 33-34 °C; 1 H NMR δ 8.22 (1 H, s), 7.91-8.04 (3 H, m), 7.63-7.70 (3 H, m); 13 C NMR δ 134.77,132.11,129.62,128.86,128.59,128.21,127.89,127.53,126.20 (t,J ) 8). Column chromatography (silica, 8:2 hexane/CH2Cl2, Rf ) 0.4) yielded pure 8 as a white solid in 60% yield using t-BuLi as base: mp 54-56 °C; 1 H NMR δ 8.03 (1 H, d, J ) 11.7 Hz), 7.89-7.96 (3 H, m), 7.55-7.63 (3 H, m), 6.23 (1 H, d, J ) 46.9 Hz, CHF); 19 F NMR δ -91.06 (d, J ) 47.3 Hz); 13 C NMR δ 134.15 (d, J ) 2.2 Hz), 132.59 (d),129.51,128.45 (d,J ) 1.5 Hz),128.43 (d,J ) 20.5 Hz),127.86,127.81,127.14,123.52 (d,J ) 2.9 Hz), 115.27 (d, J ) 33.7 Hz), 80.31 (d, J ) 181.6 Hz); MS m/z (relative intensity) 185 (100), 158 (29); HRMS calcd for C12H8NF 185.0641, found 185.0641.…”

“…2,2-Difluoro-2-(4-methoxyphenyl)acetonitrile ( 16). Column chromatography (silica, 6.5:3.5 hexane/CH2Cl2, Rf ) 0.5) yielded pure 16 as a colorless oil in 34% yield using t-BuLi as base: 1 H NMR δ 7.60 (2 H, d, J ) 8.8 Hz), 7.01 (2 H, d, J ) 8.8 Hz), 3.88 (3 H, s, CH3O); 19 20). Column chromatography (silica, 6.5:3.5 hexane/CH2Cl2, Rf ) 0.5) yielded pure 20 as a colorless oil in 47% yield using t-BuLi as base: m),bm),m); 13 C NMR δ 142.65,139.34,131.84 (t,J ) 25.3 Hz),131.24,129.73,129.07,128.87 (t,J ) 7.3 Hz),128.28,127.20,123.87 (t, J ) 5.2 Hz), 112.57 (t, J ) 48.0 Hz), 108.85 (t, J ) 243.5 Hz); MS m/z (relative intensity) 229 (100), 152 (15); HRMS calcd for C14H9NF2 229.0703, found 229.0694.…”