Kinetics of the thermal isomerization of 1,1,2,2‐tetramethylcyclopropane

Abstract

Reaction rates for the structural isomerization of 1,1,2,2-tetramethylcyclopropane to 2,4-dimethyl-2-pentene have been measured over a wide temperature range, 672-750 K in a static reactor and 1000-1120 K in a single-pulse shock tube. The combined data from the two temperature regions give Arrhenius parameters E a = 64.7 (±0.5) kcal/mol and log 10 (A, s −1 ) = 15.47 (±0.13). These values lie at the upper end of the ranges of E a and log A values (62.2-64.7 kcal/mol and 14.82-15.55, respectively) obtained from three previous experimental studies, each of which covered a narrower temperature range. The previously noted trend toward lower E a values for structural isomerization of methylcyclopropanes as methyl substitution increases extends only through the dimethylcyclopropanes (1,1-and 1,2-); E a then appears to increase with further methyl substitution. In contrast, the pre-exponential factors for isomerization of cyclopropane and all of the methylcyclopropanes through tetramethylcyclopropane lie within ±0.3 of log 10 (A, s −1 ) = 15.2 and show no particular trend with increasing substitution. C 2006

Molecular Rearrangements: Part 2. Other Reactions

“…DFT calculations revealed for these reactions low barriers to cyclization that proceed through stepwise or non-synchronous yet concerted reaction mechanisms, thus not permitting the viable trapping of carbene intermediates. 146 The Arrhenius parameters for the structural isomerization of 1,1,2,2tetramethylcyclopropane to 2,4-dimethylpent-2-ene, 147 and of 1,1,2-trimethylcyclopropane to three isomeric methylpentenes and two dimethylbutenes, 148 have been measured over a wide temperature range (670-1120 • C); E a and logA values were higher than previously reported from experimental work. gem-Dihalogenospiropentanes (171) in the presence of alkyllithium reagents at −55 • C, in contrast to the common reactivity leading to allenes (174), undergo a skeletal rearrangement leading to cyclobutenes (172), and their homocoupling dimers (173) in the case of R 2 = H (Scheme 47).…”

Kinetics of the thermal isomerization of 1,1,2‐trimethylcyclopropane

“…static-reactor study in this laboratory covering the temperature range 656-1120 K [11], yielded an activation energy of 64.7 (±0.5) kcal/mol for isomerization of 1,1,2,2-tetramethylcyclopropane to 2,4-dimethyl-2-pentene over the full temperature range of the study, a value very close to that for isomerization of unsubstituted cyclopropane and more than 5 kcal/mol higher than predicted by the early thermochemical calculations [7]. In light of the previous studies of the isomerizations of methylcyclopropanes summarized above, further study of the isomerization of tri-MCP seems warranted.…”

Syntheses of three stable isotope‐labeled ethylcyclopropanes

“…To take up the mechanistic uncertainties continuing to shroud these fragmentation reactions, screening them from intellectual or active research attention, we in collaboration with Lewis and his colleagues 8 have committed to take up the issues they raise through studying the thermal reactions of isotopically labeled ethylcyclopropanes under single‐pulse shock‐tube reactor conditions so as to obviate complications associated with wall effects. The present work reports the syntheses of three isotopomers of 1 : carbon‐13 methyl‐labeled ethylcyclopropane ( 1‐ 13 C ), the d 3 ‐methyl‐labeled analog ( 1‐ d 3 ), and 2,2‐ d 2 ‐1‐ethylcyclopropane ( 1‐ d 2 ).…”