Strain‐Accelerated Formation of Chiral,Optically Active Buta‐1,3‐dienes |
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| Authors: | Dr Melanie Chiu Boris H Tchitchanov Daniel Zimmerli Italo A Sanhueza Prof?Dr Franziska Schoenebeck Dr Nils Trapp Dr W Bernd Schweizer Prof?Dr François Diederich |
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| Affiliation: | 1. Laboratorium für Organische Chemie, ETH Zurich, Vladimir‐Prelog‐Weg 3, 8093 Zurich (Switzerland);2. Discovery Technologies, Bldg 92/5.64C, F. Hoffmann‐La Roche Ltd. 4070 Basel (Switzerland);3. Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, 52074 Aachen (Germany) |
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| Abstract: | The formal 2+2] cycloaddition–retroelectrocyclization (CA–RE) reactions between tetracyanoethylene (TCNE) and strained, electron‐rich dibenzo‐fused cyclooctynes were studied. The effect of ring strain on the reaction kinetics was quantified, revealing that the rates of cycloaddition using strained, cyclic alkynes are up to 5500 times greater at 298 K than those of reactions using unstrained alkynes. Cyclobutene reaction intermediates, as well as buta‐1,3‐diene products, were isolated and their structures were studied crystallographically. Isolation of a rare example of a chiral buta‐1,3‐diene that is optically active and configurationally stable at room temperature is reported. Computational studies on the enantiomerization pathway of the buta‐1,3‐diene products showed that the eight‐membered ring inverts via a boat conformer in a ring‐flip mechanism. In agreement with computed values, experimentally measured activation barriers of racemization in these compounds were found to be up to 26 kcal mol?1. |
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| Keywords: | chirality click chemistry cycloadditions kinetics strained molecules |
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