Thermodynamic and strain effects in the competition between 5-exo-dig and 6-endo-dig cyclizations of vinyl and aryl radicals |
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Authors: | Alabugin Igor V Manoharan Mariappan |
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Affiliation: | Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, USA. alabugin@chem.fsu.edu |
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Abstract: | Electronic and structural factors controlling the competition between 5-exo-dig and 6-endo-dig cyclizations of sp2-radicals were analyzed using a combination of available experimental data and computation. Although the stereoelectronically favored 5-exo pathways usually has the lower activation energy, formation of a new aromatic ring not only makes the 6-endo process favorable thermodynamically in conjugated systems but also lowers its activation barrier to the extent where the 5-exo/6-endo selectivity is controlled by subtle factors such as the different sensitivity of the two pathways to strain effects in polycyclic systems. In particular, the stronger sensitivity of the 5-exo pathway to strain leads to a crossover in selectivity. The 6-endo cyclization is kinetically favored in smaller (and strained) cycles, whereas the 5-exo cyclization has lower barriers in the larger rings. |
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