10.1002/asia.201901778
Chemistry - An Asian Journal
COMMUNICATION
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standard conditions (Z/E = 90:10
33:67). Substituting methyl
by cyclopropyl on the double bond led only to a very minor erosion
of selectivity (Z-4b and Z-4v, Z/E = 95:5 and 93:7, respectively).
However, the installation of cyclopentyl or cyclohexyl led to a
remarkable decrease in selectivity (Z-4w and Z-4x, Z/E = 33:67
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of the plane and reducing conjugation. Moreover, Stern-Volmer
fluorescence quenching experiments of fac-Ir(ppy)3 with
substrates E-1a, E-4a and E-4b were performed (Scheme 4c and
Figure S1-S3). According to Hammond’s mechanistic analyses,[24]
together with the study of control experiments and mechanistic
probes, the Stern-Volmer analysis indicated that the isomerization
reaction mechanism consistent with triplet energy transfer was
operational. Interestingly, from the slope of the line generated by
plotting of I°/I vs concentration of the quencher, and comparison
with the Z/E selectivities reported in Scheme 2 and Scheme 3 (Z-
4b 91%, Z-1a 87%, Z-4a 78%), shows that selectivity correlate
with quenching efficiency. Additional support for the biradical
intermediate is reflected in the increasing Z/E ratio when
improving the stabilizing auxiliary groups.
In conclusion, we have established an efficient and
operationally simple protocol to access thermodynamically less
stable cis-alkenes through a photochemical isomerization. Many
cis-cinnamyl ethers and cinnamyl alcohols, which are not readily
accessible via classic methods, can thus be achieved in
synthetically useful ratios and yields. This reaction is remarkably
clean and tolerates a wide range of functional groups with regard
to both the cinnamyl ethers and cinnamyl alcohols. In addition, Z-
configured magnolol and honokiol derivatives, which possess
potential biological activity, could be easily obtained in good yields
and selectivity by this straightforward photocatalysis
isomerization.
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Acknowledgements
We acknowledge financial support from the National Natural
Science Foundation of China (21602144), and the Science and
Technology Program of Sichuan Province (2018JY0485).
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Keywords: Allylic alcohols • Cinnamyl ethers • Isomerization •
Iridium • Photocatalysis
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