Proceedings - Interdisciplinary Center for Nanotoxicity

Conference on Current Trends in Computational Chemistry 2009
Mechanisms of 1,2‐Cycloadditions of 1,3‐Imidazol‐2‐ylidene
to 4‐Substituted Phenylethenes
Fillmore Freeman* and Tieuphuong Dinh Le
Department of Chemistry, University of California, Irvine, Irvine, California 92697
The mechanisms of 1,2‐cycloaddition reactions of singlet imidazol‐2‐ylidene (S‐1) to
phenylethene (styrene) and 4‐substituted phenylethenes to form derivatives of
spirocyclopropanes have been studied using CBS‐QB3 and also using the B3LYP, PBE1PBE, and
MP2 methods with the 6‐311+G(d,p) basis set. The effects of levels of theory and the effects of
4‐substituted electron attracting substituents (4X = CH3, NH2, N(CH3)2), OH, OCH3), and of
electron withdrawing groups (4X = CF3, CN, NO2, F, Cl) on the mechanisms leading to the
respective unsymmetrical cyclic three‐center transition states were determined. The activation
energies and enthalpies of reactions are compared with the highest occupied molecular orbital
(HOMO) energies, lowest unoccupied molecular orbital (LUMO) energies, and ionization
potentials of the 4‐substituted phenylethenes. The cycloaddition mechanism may be described
as concerted with asynchronous bond formation to the ethenyl carbons.
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