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Synthesis 2010(13): 2213-2222  
DOI: 10.1055/s-0029-1218799
PAPER
© Georg Thieme Verlag Stuttgart ˙ New York

Classical Versus Nonstatistical Behavior in the C²-C6/Ene Cyclization of Enyne-Allenes: Intramolecular Kinetic Isotope Effects and Radical Clock Reactions

Chandrasekhar Vavilalaa,†, Jan W. Batsb, Michael Schmittel*a
a FB 8 (Chemie-Biologie), Universität Siegen, Adolf-Reichwein-Str., 57068 Siegen, Germany
b Institut für Organische Chemie und Chemische Biologie, Johann Wolfgang Goethe-Universität, Max-von-Laue-Str. 7, 60438 Frankfurt am Main, Germany
Fax: +49(271)7403270; e-Mail: schmittel@chemie.uni-siegen.de;
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Publication History

Received 15 April 2010
Publication Date:
27 May 2010 (online)

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Abstract

The effects of aryl groups on the mechanism of the C²-C6/ene cyclization of enyne-allenes were studied by means of radical clock openings and intramolecular kinetic isotope effects. Upon attachment of a single aryl group at either the alkyne or the allene terminus, the thermal reaction proceeds by a stepwise mechanism that shows significant nonstatistical dynamic effects. Despite this, we were able to intercept the intermediate diradical intramolecularly by using the ultrafast diphenylcyclopropyl radical clock reaction. When aryl groups were present at both the alkyne and allene termini, the intramolecular kinetic isotope effects were consistent with a classical stepwise mechanism. The present study thus demonstrates the shift in reaction mechanism from a nonstatistical stepwise mechanism to a classical stepwise behavior, depending on the substituents.

Key words

ene reactions - allenes - alkynes - radical reactions - ring closure - kinetic isotope effects

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