期刊
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
卷 127, 期 36, 页码 12583-12594出版社
AMER CHEMICAL SOC
DOI: 10.1021/ja052677y
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Electronic and structural factors controlling the competition between 5-exo-dig and 6-endo-dig cyclizations of sp(2)-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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