Kinetics, cycloaromatization

Kinetic and DFT studies have found that the rapid cycloaromatization of 2,3-diethynyl-l-methoxybenzene compared with 1,2-diethynylbenzene can be attributed to intramolecular hydrogen abstraction by the p-bcnzyne diradical intermediate from the OMe substituent. This has been shown to render the cyclization effectively irreversible by leading to a more stable diradical, and may have significant implications for the action of therapeutic enediynes.57 In general, the Bergman cycloaromatizations of 1-substituted 2,3-diethynylbenzenes have been found to be highly sensitive to the nature of the ortho-substituent, but the measurements and even the relative trends... 

IV-Aryl and iV-alkyl substituted imidazole-fused enediynes 785 undergo thermally promoted Bergman cycloaroma tization to the benzimidazoles 786 at 145 °C (Scheme 194). Kinetic data indicate that iV-aryl substitution enhances the rate by up to sevenfold <2004TL3621>. 4,5-Bis-(alkyn-l-yl)imidazoles also undergo photoinduced Bergman cycloaromatization <2005TL1373>. 

Mohamed, R. K., Peterson, P. W., Alabugin, I. V. (2013). Concerted Reactions That Produce Diradicals and Zwitterions Electronic, Steric, Conformational, and Kinetic Control of Cycloaromatization Processes. Chemical Reviews, http //pubs.acs.org/doi/pdf/10.1021/cr4000682... 

Concerted reactions that produce diradicals and zwitterions Electronic, steric, conformational, and kinetic control of cycloaromatization processes 13CRV7089. 

The location of a substituent in the orf/ o-position to the yne branch of the benzoannelated enediynes affects the reaction kinetics of Bergman cyclization [234-236]. In all cases, there is an acceleration of the cycloaromatization process that is associated with the stabilization of the transition state through the interaction of acetylene orbitals with the nearest substituent (Scheme 3.11) [234],... 

Zeidan, T.A., Kovalenko, S.V., Manoharan, M. and Alabugin, I.V. (2006) Ortho effect in the Bergman cyclization comparison of experimental approaches and dissection of cycloaromatization kinetics. Journal of Organic Chemistry, 71(3), 962-975. 

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