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Radical 4‐exo Cyclizations via Template Catalysis |
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| Authors: | Prof. Dr. Andreas Gansäuer Karsten Knebel Christian Kube Dr. Maurice van Gastel Asli Cangönül Prof. Dr. Kim Daasbjerg Tim Hangele Michael Hülsen Prof. Dr. Michael Dolg Dr. Joachim Friedrich |
| Affiliation: | 1. Kekulé‐Institut für Organische Chemie und Biochemie, Universit?t Bonn, Gerhard Domagk Str. 1, 53121 Bonn (Germany), Fax: (+49)?228 73‐5683;2. Institut für Physikalische und Theoretische Chemie, Universit?t Bonn, Wegeler Str. 12, 53115 Bonn (Germany);3. Department of Chemistry, University of Aarhus, Langelandsgade140, 8000 Aarhus C (Denmark);4. Institut für Theoretische Chemie, Universit?t zu K?ln, Greinstr. 4, 50939 K?ln (Germany);5. Institut für Chemie, Technische Universit?t Chemnitz, Strasse der Nationen 62, 09111 Chemnitz (Germany), Fax: (+49)?371‐531839415 |
| Abstract: | The mechanism of catalytic 4‐exo cyclizations without gem‐dialkyl substitution was investigated by a comparison of cyclic voltammetry, EPR, and computational studies with previously published synthetic results. The most active catalyst is a super‐unsaturated 13‐electron titanocene(III) complex that is formed by supramolecular activation through hydrogen bonding. The template catalyst binds radicals via a two‐point binding that is mandatory for the success of the 4‐exo cyclization. The computational investigations revealed that formation of the observed trans‐cyclobutane product is not possible from the most stable substrate radical. Instead, the most stable product is formed with the lowest energy of activation from a disfavored substrate in a Curtin–Hammett related scenario. |
| Keywords: | cyclic voltammetry cyclobutane radicals titanium |