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In summary, we have developed a novel and efficient protocol
for the Mn-catalyzed dehydrogenative alkylation or α-olefination of
alkyl-N-heteroaromatics by alcohols.
A broad range of (E)-
disubstituted alkenes have been synthesized. Ten of the 38
synthesized examples are novel compounds. The olefination
reaction is catalyzed efficiently by a PN5P-pincer complex (triazine
backbone) with a low catalyst loading of 0.5 mol%. The presence of
a catalytic amount of base is crucial. Co and Fe complexes stabilized
by the same ligands as the most active Mn catalyst are nearly
inactive in our dehydrogenative alkylation reaction. Existing
synthesis protocols for olefination reactions are divers. Classic
carbonyl compound based olefination reactions such as Wittig,[16]
Horner-Wadsworth-Emmons,[17]
Peterson,[18]
Julia[19]
or
McMurry[20] are important methodologies but require expensive pre-
functionalization and partially the involvement of toxic reagents.
Catalytic transformations like Heck reaction[21] or olefin
metathesis[22] are very important too, but employ olefins already as
starting materials. In addition, the Heck reaction, which forms very
similar products compared to our olefination, proceeds via Aryl-X
activation with X being, for instance Cl, Br as well as I and, thus,
such functionalities are difficult to tolerate. We could demonstrate
the tolerance of all of them in our olefination reaction.
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Acknowledgements
We thank Anna-Maria Dietel for her excellent help in the laboratory
and the Deutsche Forschungsgemeinschaft (KE-756/31-1), the
Shandong Provincial Natural Science Foundation of China (Nos.
ZR2017MB029, ZR2017BB060), and the Alexander von Humboldt
Foundation for financial support.
Conflict of interest
The authors declare no conflict of interest.
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Keywords: alcohols · alkyl-N-heteroaromatics · dehydrogenation ·
manganese · olefination
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