<?xml version="1.0" encoding="UTF-8"?><xml><records><record><source-app name="Biblio" version="7.x">Drupal-Biblio</source-app><ref-type>17</ref-type><contributors><authors><author><style face="normal" font="default" size="100%">Chatgilialoglu, Chryssostomos</style></author><author><style face="normal" font="default" size="100%">Ferreri, Carla</style></author><author><style face="normal" font="default" size="100%">Guerra, Maurizio</style></author><author><style face="normal" font="default" size="100%">Timokhin, Vitaliy</style></author><author><style face="normal" font="default" size="100%">Froudakis, Georgios</style></author><author><style face="normal" font="default" size="100%">Gimisis, Thanasis</style></author></authors></contributors><titles><title><style face="normal" font="default" size="100%">{5- Endo-trig Radical Cyclizations: Disfavored or Favored Processes?}</style></title><secondary-title><style face="normal" font="default" size="100%">J. Am. Chem. Soc.</style></secondary-title></titles><dates><year><style  face="normal" font="default" size="100%">2002</style></year><pub-dates><date><style  face="normal" font="default" size="100%">sep</style></date></pub-dates></dates><urls><web-urls><url><style face="normal" font="default" size="100%">http://pubs.acs.org/doi/abs/10.1021/ja0261731</style></url></web-urls></urls><number><style face="normal" font="default" size="100%">36</style></number><volume><style face="normal" font="default" size="100%">124</style></volume><pages><style face="normal" font="default" size="100%">10765–10772</style></pages><language><style face="normal" font="default" size="100%">eng</style></language><abstract><style face="normal" font="default" size="100%">Relative kinetic data were determined for the 5-endo-trig cyclization of radical 12 compared to hydrogen abstraction from (TMS) 3SiH in the temperature range of 344-430 K, which allows for the estimation of a rate constant of 2 × 10 4 s -1 at 298 K with an activation energy of ca. 9 kcal/mol for the cyclization process. The 5-endo-trig cyclization of a variety of radicals that afford five-membered nitrogen-containing heterocycles was addressed computationally at the UB3LYP/6-31G* level. The 5-endo vs 4-exo mode of cyclication and the effect of delocalization of the unpaired electron in the transition state were investigated. Because the ring formed during cyclization contains five sp 2 centers, electrocyclization via a pentadienyl-like resonance form was also considered. For comparison, similar calculations were performed for 4-penten-1-yl and related radicals. The factors that affect the activation energies of homolytic 5-endo-trig cyclization were determined. In the absence of steric or conformational effects, the endo cyclization to form the five-membered ring was strongly favored over exo cyclization to form the four-membered ring not only on thermodynamic grounds but also kinetically. When a substituent on the double bond was able to delocalize the unpaired electron in the transition state of the 4-exo path, the two modes of cyclization became kinetically comparable. These results have an important bearing on the generalization of the Baldwin-Beckwith rules, which classified the 5-endo-trig radical cyclization as a &quot;disfavored&quot; process.</style></abstract></record></records></xml>