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Radical 4‐ exo Cyclizations via Template Catalysis
Author(s) -
Gansäuer Andreas,
Knebel Karsten,
Kube Christian,
van Gastel Maurice,
Cangönül Asli,
Daasbjerg Kim,
Hangele Tim,
Hülsen Michael,
Dolg Michael,
Friedrich Joachim
Publication year - 2012
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201102959
Subject(s) - catalysis , chemistry , radical , supramolecular chemistry , electron paramagnetic resonance , substrate (aquarium) , cyclobutane , cyclic voltammetry , photochemistry , stereochemistry , computational chemistry , combinatorial chemistry , molecule , organic chemistry , electrochemistry , ring (chemistry) , physics , oceanography , electrode , nuclear magnetic resonance , geology
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.