ORGANIC
LETTERS
2013
Vol. 15, No. 11
2708–2711
Scope and Limitations of 1,3,5-Hexatriene
Derivatives in Regioselective
Cobalt-Catalyzed Reactions
Anastasia Schmidt and Gerhard Hilt*
€
Fachbereich Chemie, Philipps-Universitat Marburg, Hans-Meerwein-Strasse,
35043 Marburg, Germany
Received April 12, 2013
ABSTRACT
Applications of 1,3,5-hexatriene derivatives in atom-economic cobalt-catalyzed transformations, such as the DielsꢀAlder reaction with alkynes,
the 1,4-hydrovinylation reaction with terminal alkenes, and the 1,4-hydrohexatrienylation reaction, are investigated. In all cases, regioselective
transformations were found to generate cyclic derivatives such as stilbenes or acyclic products with a high control of the double bond geometry in
the skipped trienes derived from the 1,4-hydrovinylation process or the tetraenes generated in the so far unprecedented 1,4-hydrohexatrienylation
reaction.
Low-valent cobalt complexes are excellent catalysts for
the reaction of unsaturated educts, such as alkenes, 1,3-dienes,
allenes, alkynes, enynes, diynes, and many other unsatura-
ted starting materials.1 Generally, cyclic products are gener-
ated by cobalt-catalyzed [2 þ 2], [4 þ 2], [6 þ 2], [4 þ 2 þ 2],
and other cycloaddition reactions via cobalt-initiated
insertion reactions and reductive eliminations as key steps in
the reaction mechanisms. On the other hand, acyclic prod-
ucts are obtained when a competing β-hydride elimination
is possible, mostly outrunning the alternative reductive
elimination processes of a cycloaddition. In these cases,
cobalt-catalyzed 1,2- or 1,4-hydrovinylation,2 Alderꢀene
reactions,3 or alternatively the reductive codimerization
reactions4 in the presence of a proton source are encountered.
Herein, we report our investigation of 1,3,5-hexatriene
derivatives as starting materials in several cobalt-catalyzed
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10.1021/ol401015e
Published on Web 05/10/2013
2013 American Chemical Society