The [C{sub 6}H{sub 10}]{sup {sm{underscore}bullet}+} hypersurface: The parent radical cation Diels-Alder reaction

Journal Article · Wed Jul 21 04:00:00 EDT 1999 · Journal of the American Chemical Society

DOI:https://doi.org/10.1021/ja983338q· OSTI ID:20000100

Hoffmann, M; Schaefer, III, H F

Various possible reaction pathways between ethene and butadiene radical cation (cis- and trans-), have been investigated at different levels of theory up to UCCSD(T)/DZP/UMP2(fc)/DZP and with density functional theory at B3LYP/DZP. A stepwise addition involving open chain intermediates and leading to the Diels-Alder product, the cyclohexene radical cation, was found to have a total activation barrier {Delta}G{sup 298{ne}} = 6.3 kcal mol{sup {minus}1} and a change in free Gibbs energy, {Delta}G{sup 298}, of {minus}33.5 kcal mol{sup {minus}1}. On the E{degree} potential energy surface, all transition states are lower in energy than separated ethene and butadiene, the exothermicity {Delta}E = -45.6 kcal mol{sup {minus}1}. A more direct path could be characterized as stepwise with one intermediate only at the SCF level but not at electron-correlated levels and hence might actually be a concerted strongly asynchronous addition with a very small or no activation barrier (UCCSD(T)/DZP/UHF/6-31G* gives a {Delta}G{sup 298{ne}} of 0.8 kcal mol{sup {minus}1}). The critical step for another alternative, the cyclobutanation-vinylcyclobutane/cyclohexene rearrangement, is a 1,3-alkyl shift which involves a barrier ({Delta}G{sup 298{ne}}) only 1.7 kcal mol{sup {minus}1} higher than that of stop use addition for both cis-, and trans-butadiene radical cation. However, from the (ethene and trans-butadiene) reactions, ring expansion of the vinylcyclobutane radical cation intermediate, to a methylene cyclopentane radical cation, requires an activation only 1.3 kcal mol{sup {minus}1} larger than for (trans-butadiene radical). While cis/trans isomerization of free butadiene radical cation requires a high activation (24.9 kcal mol{sup {minus}1}), a reaction sequence involving addition of ethene (to stepwise give an open chain intermediate and vinyl cyclobutane radical cation) has a barrier of only 3.5 kcal mol{sup {minus}1} ({Delta}G{sup 298{ne}}). This sequence also makes ethene and butadiene radical cations to exchange terminal methylene groups.

• Research Organization: Univ. of Georgia, Athens (US)
• Sponsoring Organization: US Department of Energy; Deutsche Akademische Austauschdienst
• DOE Contract Number: FG02-97ER14748
• OSTI ID: 20000100
• Journal Information: Journal of the American Chemical Society, Journal Name: Journal of the American Chemical Society Journal Issue: 28 Vol. 121; ISSN JACSAT; ISSN 0002-7863
• Country of Publication: United States
• Language: English

Related Subjects

40 CHEMISTRY
ALKENES
BUTADIENE
CATIONS
DIELS-ALDER REACTION
ISOMERS
RADICALS
STEREOCHEMISTRY
SYNTHESIS