Thermodynamic and strain effects in the competition between 5-exo-dig and 6-endo-dig cyclizations of vinyl and aryl radicals.
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.
Alabugin
Igor V
IV
Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, USA. alabugin@chem.fsu.edu
Manoharan
Mariappan
M
D000480:Alkynes / Q000737:chemistry*; D018360:Crystallography, X-Ray; D003500:Cyclization; D005609:Free Radicals / Q000737:chemistry; D008956:Models, Chemical; D008958:Models, Molecular; D015394:Molecular Structure; D013816:Thermodynamics; D014753:Vinyl Compounds / Q000737:chemistry*
D000480:Alkynes; D005609:Free Radicals; D014753:Vinyl Compounds