Ab Initio Computational Study of the Allenyl Cope Rearrangement of syn-7-Allenylnorbornene

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• 作者：James A. Duncan ; Joseph K. Azar ; J. Callan Beathe ; Scott R. Kennedy ; and Carolyn M. Wulf
• 刊名：Journal of the American Chemical Society
• 出版年：1999
• 出版时间：December 29, 1999
• 年：1999
• 卷：121
• 期：51
• 页码：12029 - 12034
• 全文大小：289K
• 年卷期：v.121,no.51(December 29, 1999)
• ISSN：1520-5126

文摘

Results of (8,8)CASPT2/6-31G*//(8,8)CASSCF/6-31G* level calculations on the potential surfacefor the conformationally restricted allenyl Cope rearrangement of syn-7-allenylnorbornene (10) to triene 11are reported. The rearrangement is found to involve two separate transition structures 13 and 15, the former2.1 kcal/mol higher in enthalpy than the other, that both lead to a common diradical intermediate (12). Theseresults differ substantially from those previously obtained for the allenyl Cope rearrangement of 1,2,6-heptatriene(1) to 3-methylene-1,5-hexadiene (2),⁷ which has been shown to involve a single transition structure that eitherproceeds to diradical 3 or bypasses 3 to form 2 directly. The terminal methylene group of 10 is also shown torotate in only one direction when passing through the lower-energy transition structure 13, but appears torotate freely in either direction when passing through 15. This finding is shown to be remarkably consistentwith the 90% stereoselectivity observed in the thermal Cope rearrangements of dimethyl allenylnorbornenederivatives racemic-7a and racemic-7b.¹⁰ Furthermore, direct participation of the terminal allenyl es/gifchars/pi.gif" BORDER=0 >-bond isobserved in the 10es/entities/rarr.gif">11 rearrangement but not in the 1es/entities/rarr.gif">2 rearrangement. This difference is evidenced by acomparison of the computed bond lengths and the calculated active space molecular orbitals in the two transitionstructures 13 and 15 verses transition structure 17, the latter involved in the 1es/entities/rarr.gif">2/3 pathways. Consideringsuch evidence, it may be concluded that the particular restriction in conformational mobility afforded the1,2,6-heptatriene moiety in 10 appears to force the participation of the terminal allenyl es/gifchars/pi.gif" BORDER=0 >-bond, resulting in anaugmented Cope process.