The [C<sub>6</sub>H<sub>10</sub>]<sup>•+</sup> Hypersurface: the Parent Radical Cation Diels−Alder
Reaction
Matthias Hofmann
Henry F. Schaefer
10.1021/ja983338q.s001
https://acs.figshare.com/articles/journal_contribution/The_C_sub_6_sub_H_sub_10_sub_sup_sup_Hypersurface_the_Parent_Radical_Cation_Diels_Alder_Reaction/3672855
Various possible reaction pathways between ethene, <b>1</b>, and butadiene radical cation (<i>cis</i>-, <b>2</b>, <i>trans</i>-,
<b>11</b>) 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, <b>6</b>, (path A) was found to have a total activation barrier
Δ<i>G</i><sup>298≠</sup> = 6.3 kcal mol<sup>-1</sup> and a change in free Gibbs energy, Δ<i>G</i><sup>298</sup>, of −33.5 kcal mol<sup>-1</sup>. On the <i>E</i>° potential
energy surface, all transition states are lower in energy than separated <b>1</b> + <b>2</b>, the exothermicity
Δ<i>E</i> = −45.6 kcal mol<sup>-1</sup>. A more direct path B 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 Δ<i>G</i><sup>298≠</sup>
of 0.8 kcal mol<sup>-1</sup>). The critical step for another alternative, the cyclobutanation−vinylcyclobutane/cyclohexene
rearrangement, is a 1,3-alkyl shift which involves a barrier (Δ<i>G</i><sup>298≠</sup>) only 1.7 kcal mol<sup>-1</sup> higher than that of
path A for both <i>cis</i>-, <b>2</b>, (path C) and <i>trans</i>-butadiene radical cation, <b>11</b> (path D). However, from the <b>1</b> + <b>11</b>
reactions, ring expansion of the vinylcyclobutane radical cation intermediate, <b>14</b>, to a methylene cyclopentane
radical cation, <b>16</b>, (path E) requires an activation only 1.3 kcal mol<sup>-1</sup> larger than for path D. While cis/trans
isomerization of free butadiene radical cation requires a high activation (24.9 kcal mol<sup>-1</sup>), a reaction sequence
involving addition of ethene (to stepwise give an open chain intermediate <b>13</b> and vinyl cyclobutane radical
cation, <b>10</b>) has a barrier of only 3.5 kcal mol<sup>-1</sup> (Δ<i>G</i><sup>298≠</sup>). This sequence also makes ethene and butadiene
radical cations to exchange terminal methylene groups.
1999-07-03 00:00:00
exothermicity Δ E
activation
1.7 kcal mol
ethene
SCF
barrier
1.3 kcal mol
C 6 H 10
Δ G 298
3.5 kcal mol
3LYP
butadiene
cation
6.3 kcal mol
exchange terminal methylene groups
0.8 kcal mol
UCCSD
path D