Rate constants for abstraction of bromine from bromotrichloromethane by butyl, cyclopropylmethyl, and phenyl radicals in solution

Abstract

LUKOSE MATHEW and JOHN WARKENTIN. Can. J. Chem. 66, l l (1988).The radical chain decomposition of cyclopropylmethyl (I-hydroxy-1-mcthylethy1)-diazcnc ((CH3)2C(OH)N=NCHI--<1 1) at 253-341 K in hexafluorobenzene or in dichloromethane solution containing brornotrichloromethane affords cyclopropylmethyl bromide, 4-bromo-I-butene, I-bromo-5,5,5-trichloro-2-pentene, and 3,5-dibromo-I, I, I-trichloropcntane from the cyclopropylmethyl portion of 1. Other major products are nitrogen, acetone, and chloroform. The rate constant for formation of cyclopropylmethyl bromide by attack of cyclopropylmethyl free radicals from 1 at bromine of BrCCI, (kyy) was calculated from the product composition using the known rate constant for rearrangement of cyclopropylmethyl radicals to 3-buten-I-yl radicals. At 25"C, kTF = 6.5 X lo8 M-I s-I and the temperature dependence is given by log (ksm/M-I s-) = ( 10.6 5 0.3) -(2.4 C 0.4)/0, where 0 = 2.3R~kcal/mol-l. Non-chain decompositionof (CH3),C(OH)N=N-R(2, R = Bu, and3, R = Ph) in the presence of excess 1 ,I ,3,3-tetramethylisoindolin-2-yloxyl (4) and bromotrichloromethane afforded BuBr and PhBr, respectively, in yields determined by the relative concentrations of 4 and BrCCI3. Rate constants for coupling ( k c ) of Bu. and Ph. with 4 were assumed to be proportional to rate constants for diffusion controlled reactions, kd, which were estimated from measured viscosities. Values of k ! : and k; : , calculated from kc and product yields for reactions at 80°C, are 0.26 X lo9 and 1.55 X lo9 M-' s-l, respectively. The relative radical reactivities toward BrCC13 at 80°C are Ph, 6; cpm, 5; Bu, 1.LUKOSE MATHEW et JOHN WARKENTIN. Can. J. Chem. 66, 1 1 (1988).La dCcomposition radicalaire en chaine du cyclopropylmCthy1 (hydroxy-l methyl-1 Cthyl) diazkne ((CH3)2C(OH)N= NCH2CH(CH2),, I), a des tempkratures allant de 253 2 341 K, dans des solutions d'hexafluorobenzkne ou de dichloromCthane contenant du bromotrichloromtthane, conduit au bromure de cyclopropylmCthyle, au bromo-4 butkne-I, au bromo-l trichloro-5,5,5 pentkne-2 et au dibromo-3,5 trichloro-l ,I , l pentane qui dtrivent tous de la portion cyclopropylmCthyle du compost 1. Les autres produits importants sont l'azote, I'acCtone et le chloroforme. On a calculi2 la constante de vitesse pour la formation du bromure de cyclopropylmtthyle par attaque des radicaux libres cyclopropylmCthyles, provenant du compost 1, au niveau du brome du BrCC13 (kTF); pour ce faire, on afait appel a lacomposition des produits-et on a utilisC la constante de vitesse connue pour la transposition des radicaux cyclopropylmCthyles en radicaux butkne-3 yles. A 25"C, kgm = 6.5 X lo8 M-' s-I et lavariation avec la tempkratureest donnte parl'iquation suivante : log (kxm/M-I s-I) = (10,6 C 0,3) -(2,4 -C 0,4)/0, dans laquelle 0 = 2,3R~kcal/rnol-'. Les dCcompositions des composCs (CH3)?C(OH)N=N-R (2, R = Bu, et 3, R = Ph) par un processus qui n'implique pas de chaines, en presence d'un excks de tCtramCthyl-1,1,3,3 isoindolinyl-2 oxyl (4) et de bromotrichloromCthane, fournissent respect...

Targeted Fluorination with the Fluoride Ion by Manganese‐Catalyzed Decarboxylation

“…Since the rate constant for bromine transfer from BrCCl 3 to alkyl radicals is known to be about 10 8 m À1 s À1 , [22] the 1:2 fluorination/bromination (1/1 b) ratio corresponds to a nanosecond radical lifetime, which is comparable to the rate of the manganese porphyrin catalyzed C À H fluorination reaction. The major product was the alkyl bromide 1 b.…”

“…As shown in Scheme 2, a variety of functional groups, including heterocycle, amide, imide, ester, ketone, ether, nitrile, halogen, and even alkene and alkyne, are well tolerated. [18] While the present method efficiently fluorinated benzylic and aryloxy carboxylic acids, tertiary, secondary, and primary acids were less reactive (22)(23)(24)(25). Molecules containing strongly electron-rich aromatic rings, which are challenging substrates for Selectfluor-based decarboxylative fluorination methods because of the competing aryl fluorination, [16d] were readily fluorinated without any ring fluorination (11)(12)(13).…”

Enantioselective cyanation of benzylic C–H bonds via copper-catalyzed radical relay

“…When the reaction was performed in the presence of BrCCl 3 (2 equivalents), a mixture of the benzylic nitrile and bromide products was obtained (44% and 24% yields, respectively). This product ratio, together with precedent for efficient trapping of organic radicals by BrCCl 3 (36), implicates very rapid trapping of organic radicals by a chiral L*Cu II (CN) species under the reaction conditions.…”

Distinguishing Biomimetic Oxidations from Oxidations Mediated by Freely Diffusing Radicals

“…The rate constant for the alkyl radical/CCl 3 Br reaction is approximately 1.4 x 10 8 M~'s -1 . 1, 58 If we assume that, on average, there is one CC^Br molecule present in the cage wall then bromination could, in principle, compete with alcohol formation if the effective rate constant for hydroxyl group rebound was ca. (27.6/18) x 1.4 x 10 8 s _1 .…”