1798
Ahmed H. Aboo et al. / Chinese Journal of Catalysis 40 (2019) 1795–1799
both electron-donating and withdrawing substituents on the
styryl side are tolerated (Table 1, entries 1–22). Of particular
note is that chalcones having 2-sustituted or 2,6-disubstituted
styryl units were all reduced with high yields (Table 1, entries
6, 9, 10, 16, 20), considering that the hydride addition would
take place at the β position of the C=C double bond (see below).
The rhodacycle A-catalyzed reduction with MeOH also
works for styryl methyl ketones (Table 1, entries 23–30). Again
the transfer hydrogenation is highly chemoselective, only af-
fording the saturated ketones. As in the case of chalcones, the
reduction tolerates both electron-donating and withdrawing
substituents on the styryl side (Table 1, entire 23, 24, 28, 29)
and substrates bearing 2,6-disubstituted styryl units are
equally viable (Table 1, entries 25–27).
We also examined the reduction of aliphatic methyl vinyl
ketones with A in MeOH. As can be seen from Table 1, these
ketones including examples of cyclohexenone and cyclohep-
tenone were also reduced, affording the corresponding satu-
rated ketones in high yields (Table 1, entries 31–36). Notably, a
-disubstituted methyl vinyl ketone was reduced with no diffi-
culty (Table 1, entry 31).
gen-bonds with the ketone oxygen, rendering the C=C bond
more electrophilic and thereby facilitating the hydride transfer.
Such ligand-facilitated reduction has been well documented;
however, the ligand-substrate interaction is usually confined to
a smaller ring [11,12,36]. Finally, protonation of E with MeOH
affords the saturated ketone while regenerating B.
4. Conclusions
This report shows that under the catalysis of the rhodacycle
A, methanol can be used as an effective hydrogen source for the
highly
chemoselective
transfer
hydrogenation
of
α,β-unsaturated ketones under mild conditions. Catalyzed by A,
a wider variety of chalcones, styryl methyl ketones and vinyl
methyl ketones, including sterically demanding ones, were
reduced to the saturated ketones in refluxing methanol in a
short reaction time. Using safe, cheap and easily available
methanol as both reductant and solvent, the protocol has the
potential to be practically useful.
Acknowledgments
3.3. Mechanistic considerations
We thank the Higher Committee for Education Development
in Iraq for financial support (AHB) and the Commonwealth
Scholarships Commission in the UK for a Split-Site PhD Schol-
arship (RB).
On the basis of our previous study [26], a plausible reaction
mechanism for the A-catalyzed chemoselective reduction of
unsaturated ketones with methanol is proposed in Scheme 2.
Under the basic reaction conditions employed, the alkoxide
complex B is formed from the rhodacycle A in MeOH. Then the
complex B undergoes hydrogen elimination, giving rise to the
hydride complex C while releasing formaldehyde, support for
which was provided in our study of reduction of aldehydes
with methanol [26]. The next step sees the hydride in complex
C being transferred to the β position of the C=C double bond,
affording the species E. The hydride transfer may proceed via
an intermediate D, in which the ligand hydroxyl group hydro-
Conflict of interest
The authors declare no conflict of interest.
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