Zur Bildung von Vinylcyclopropan‐ und Cyclopentenderivaten aus alkenylsubstituierten Chromcarben‐Komplexen: Konkurrenz von formalen [2 + 1]‐ und [3 + 2]‐Cycloadditionen

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Abstract
Formation of Vinylcyclopropane and Cyclopentene Derivatives from Alkenyl‐Substituted Chromium Carbene Complexes: Competition between Formal [2 + 1] and [3 + 2] Cycloadditions.The alkenyl‐substituted chromium carbene complexes 1 – 4 were synthesized following the method of Aumann which starts from the corresponding aldehydes. At 80°C the styryl‐substituted complex 1 and electron‐deficient olefins smoothly provide vinylcyclopropane derivatives 6,8, 10, and 12 in good yields. On the other hand, methyl acrylate and the anisyl‐ and furyl‐substituted vinylcarbene complexes 2 and 3 give mixtures of the expected vinylcyclopropanes and of cyclopentene derivatives. The pyrrolyl‐substituted complex 4 and methyl acrylate exclusively afford the cyclopentenes 24a/b which are the result of a hitherto unprecedented formal [3 + 2] cycloaddition of the vinylcarbene complex to the olefin. However, there are strong arguments suggesting the corresponding vinylcyclopropanes as intermediates. The very smooth vinylcyclopropane→cyclopentene rearrangement probably occurs via zwitterionic intermediates that are wellstabilized by donor and acceptor substituents. Hydrolysis of 24a/b to the cyclopentanone derivatives 25a/b demonstrates that this new route to five‐membered carbocycles should have synthetic potential.