Kinetics of Thermal Gas-Phase Isomerizations and Fragmentations of cis- and trans-1-(E)-Propenyl-2-methylcyclobutanes at 275 °C

Abstract

Kinetic studies of the thermal isomerization and fragmentation reactions exhibited by cis- and trans-1-(E)-propenyl-2-methylcyclobutanes at 275 degrees C in the gas phase have provided first-order rate constants for cis,trans interconversions of the cyclobutanes, 1,3-carbon migrations leading to 3,4- and 3,6-dimethylcyclohexenes, isomerizations providing directly and indirectly four acyclic dienes, and fragmentations to ethylene, propene, and mixtures of pentadienes and hexadienes. Both cis and trans isomers of 1-(E)-propenyl-2-methylcyclobutane form trans-3,4-dimethylcyclohexene faster than they are converted to cis-3,4-dimethylcyclohexene; the trans reactant gives rise to cis-3,6-dimethylcyclohexene in preference to its trans isomer, while the cis starting material gives neither at measurable rates; both form the relatively minor product 1,6-(Z)-octadiene. The rate constants derived from 35 kinetic runs starting with four distinct 1-(E)-propenyl-2-methylcyclobutane samples are consistent to within narrow error limits. The stereomutations, isomerizations, and fragmentations of the 1-(E)-propenyl-2-methylcyclobutanes are interpreted in terms of competitive processes involving conformationally flexible short-lived 2-(E)-octene-4,7-diyl and 3-methyl-5-(E)-heptene-1,4-diyl diradicals.

Kinetic study of the thermal rearrangement of cis- and trans-2-pinanol

“…[37,38]. The stepwise mechanism first proposed by Burwell in 1952 is therefore the prevailing and generally accepted mechanism [39][40][41].…”

10 Reaction mechanisms : Part (iii) Pericyclic reactions

“…A comparison of [1,5] and [1,7] chlorine and hydrogen shifts was reported by Okajima and Imafuku. 42 The authors were intrigued by the room-temperature 1 H NMR spectrum of 1,7,7-trichlorocycloheptatriene, which shows only three signals, indicative of a rapid rearrangement reaction.…”

“…42 The authors were intrigued by the room-temperature 1 H NMR spectrum of 1,7,7-trichlorocycloheptatriene, which shows only three signals, indicative of a rapid rearrangement reaction. This reaction could occur either via a direct [1,7] chlorine migration or via two sequential [1,5] chlorine migrations. The B3LYP/6-311G** computational study revealed that the concerted [1,7] suprafacial process, while formally forbidden, is lower energy than the [1,5] shift alternative.…”

“…For example, two studies were published on vinylcyclobutane derivatives, one focusing on the thermal stereomutations and cycloreversions, 4 and the other on the [1,3] sigmatropic reactions. 5 The strength of the interplay between experiment and theory is demonstrated in studies of the intramolecular [4ϩ2] cycloaddition of cyclobutadiene with tethered alkenes, 6 in the use of isotope effects to determine the mechanism of an electron-transfer catalyzed Diels-Alder reaction, 7 and in the degenerate interconversions of bicyclo[3.1.0]hex-2-ene. 8,9…”

Total Syntheses of Amphidinolides B, G, and H

“…The DIBAL‐H reduction of 17 , followed by oxidation and addition of MeLi furnished the corresponding secondary alcohol, which was oxidatively transformed into the ketone 7 . The carboxylic acid 5 , the final component needed in the convergent approach, was prepared from 4‐penten‐1‐ol by a PCC oxidation,18 Wittig olefination, and hydrolysis sequence.…”