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DFT calculations on the allenyl Cope rearrangement of syn ‐7‐allenylnorbornene: comparison with results obtained from CASSCF calculations
Author(s) -
Duncan James A.,
Spong Marie C.
Publication year - 2005
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.871
Subject(s) - diradical , chemistry , cope rearrangement , transition state , density functional theory , computational chemistry , stereochemistry , atomic physics , physics , organic chemistry , excited state , singlet state , catalysis
The conformationally restricted allenyl Cope rearrangement of syn ‐7‐allenylnorbornene ( 10 ) to racemic‐triene 11 was studied computationally using density functional theory (DFT) methods and the results were compared with those already calculated for this rearrangement at the (8,8)CASPT2/6–31G*//(8,8)CASSCF/6–31G* level of theory. The CASSCF level calculations had shown that the rearrangement involves two separate transition structures 12 and 13 that both lead to common diradical intermediate 14 . These findings are consistent with the 90% stereoselectivity observed in the thermal Cope rearrangements of dimethyl allenylnorbornene derivatives racemic 7a and 7b . In this study, we found that CASSCF optimized structures 10 – 12 and 15 could be successfully re‐optimized at the UB3LYP/6–31G* level to structures of similar geometries and relative enthalpies. However, the CASSCF transition structure corresponding to 13 did not optimize to a UB3LYP version of 13 , but rather to the UB3LYP‐optimized structure corresponding to transition structure 12 . Despite taking several approaches, an optimized UB3LYP/6–31G* version of 13 was not found on the UB3LYP potential energy surface. Hence the UB3LYP results are not only at variance with the CASSCF results but also with the above mentioned experimental results. The 10 → 11 rearrangement was also studied using UBLYP and UBPW91 functionals in addition to UB3LYP and each of the three functionals predicted a different mechanism, none of them consistent with the experimental or CASSCF results. Copyright © 2004 John Wiley & Sons, Ltd.