Journal
CHEMISTRY-A EUROPEAN JOURNAL
Volume 25, Issue 66, Pages 15189-15197Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/chem.201903910
Keywords
alkenylation; homogeneous catalysis; reaction mechanisms; regioselectivity; rhenium
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Funding
- MEXT, Japan [18H03911]
- Grants-in-Aid for Scientific Research [18H03911] Funding Source: KAKEN
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A (mu-aryloxo)rhenium complex was isolated and confirmed as a key precatalyst for rhenium-catalyzed ortho-alkenylation (C-alkenylation) of unprotected phenols with alkynes. The reaction exclusively provided ortho-alkenylphenols; the formation of para or multiply alkenylated phenols and hydrophenoxylation (O-alkenylation) products was not observed. Several mechanistic experiments excluded a classical Friedel-Crafts-type mechanism, leading to the proposed phenolic hydroxyl group assisted electrophilic alkenylation as the most plausible reaction mechanism. For this purpose, the use of rhenium, a metal between the early and late transition metals in the periodic table, was key for the activation of both the soft carbon-carbon triple bond of the alkyne and the hard oxygen atom of the phenol, at the same time. ortho-Selective alkenylation with allenes also provided the corresponding adducts with a substitution pattern different from that obtained by the addition reaction with alkynes.
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