Heat of Hydrogenation of 1,5-Dehydroquadricyclane. A Computational and Experimental Study of a Highly Pyramidalized Alkene
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- 作者：Rebecca L. Hoenigman ; Shuji Kato ; Veronica M. Bierbaum ; and Weston Thatcher Borden
- 刊名：Journal of the American Chemical Society
- 出版年：2005
- 出版时间：December 21, 2005
- 年：2005
- 卷：127
- 期：50
- 页码：17772 - 17777
- 全文大小：83K
- 年卷期：v.127,no.50(December 21, 2005)
- ISSN：1520-5126
文摘
The radical anion of the highly pyramidalized alkene 1,5-dehydroquadricyclane (1) was generatedin the gas phase from the Squires reaction of 1,5-bis(trimethylsilyl)quadricyclane with F-/F2. The electronbinding energy and proton affinity of 1
mages/entities/bull.gif">- were determined by bracketing experiments to be 0.6 ± 0.1 eVand 386 ± 5 kcal/mol, respectively. These values are in good agreement with values predicted by densityfunctional theory (B3LYP/6-31+G*) and ab initio (CASPT2/6-31+G*) calculations. The experimental heatof hydrogenation of 1, obtained from a thermochemical cycle, was found to be 91 ± 9 kcal/mol. This valueof mages/gifchars/Delta.gif" BORDER=0 >HH2 leads to values of 67 ± 9 kcal/mol for the olefin strain energy (OSE) of 1, 172 ± 9 kcal/mol for itsheat of formation, and 23 ± 9 kcal/mol for its mages/gifchars/pi.gif" BORDER=0 > bond dissociation enthalpy. Since the retro-Diels-Alderreaction of neutral 1 is computed to be highly exothermic, the finding that 1mages/entities/bull.gif">- apparently does not undergoa retro-Diels-Alder reaction is of particular interest. The B3LYP/6-31+G* optimized geometry of 1 suggeststhat the bonding in this alkene is partially delocalized, presumably because the highly pyramidalized doublebond in 1 interacts with the distal cyclopropane bonds in a manner that eventually leads to a retro-Diels-Alder reaction. The good agreement of the B3LYP and (2/2)CASPT2 values for the heat of hydrogenationand OSE of 1 with the experimentally derived values provides indirect evidence for the correctness of theB3LYP prediction that the equilibrium geometry of 1 lies part way along the reaction coordinate to thetransition structure for the retro-Diels-Alder reaction.