10.1002/chem.202101003
Chemistry - A European Journal
FULL PAPER
easy synthesis of the catalyst and from the absence of support
modification. As far as we know, this is the first immobilization of
salen derivatives on carbon surfaces by π-π interactions, and its
success opens the way for many extensions, given the wide
applicability of these complexes in asymmetric catalysis. Work is
in progress to further extend this procedure to different
asymmetric catalytic transformations with other pyrene-tagged
salen complexes, both in batch and in flow conditions.
[a] substrate, 0.8 mmol, [5 M] in DCM, rt, 24 h, washing with Et2O. [b] determined
by GC with dodecane as internal standard (see Supporting Information). [c]
determined by GC (see Supporting Information). [d] isolated yield
A
first run was thus devoted to the transformation of
benzaldehyde 1b, resulting in the isolation of the corresponding
dihydropyranone 2b with a good yield and an enantioselectivity
value of 64 % ee, even slightly better than that obtained under
homogeneous conditions (Table 5, run 1 and Supporting
Information for comparison). After washing with diethyl ether, the
recovered catalyst was then engaged in the transformation of
cyclohexane-carboxaldehyde 1a (Table 5, run 2).
Acknowledgements
The subsequent recycling procedure involved consecutively two
other aromatic aldehydes (1c-d, runs 3-4). If the heterogeneous
catalysis turned out to be slightly less effective in terms of activity
compared to the results obtained under homogeneous conditions
(Table 5 and Supporting Information), similar enantioselectivity
values could nevertheless be obtained. The products were easily
recovered without being contaminated by traces of
dihydropyranones prepared in the previous runs.
We acknowledge Université Paris Saclay, CNRS, Ministère de
l’Europe et des Affaires Etrangères (bourse d’excellence “Eiffel”)
and Charm3At LabEx (ANR-11-LABEX-0039) for financial
support. This project has been jointly funded with the support of
the National Council for Scientific Research in Lebanon CNRS-L
and Lebanese University. The authors are also grateful to Drs.
Philippe Dauban and Olivier David for fruitful discussions.
Finally, recovered Unsym-Cr-A@rGO after the 4th run was
reassigned to the HDA of benzaldehyde (run 5), interestingly
offering improved activity and the same enantioselectivity for the
target product 2b. Recycling was continued again with the other
three substrates (1a, 1c and 1d) while maintaining its efficiency
(runs 6-8), as clear evidence of the stability of the procedure.
Spent Unsym-Cr-A@rGO was also subjected to XPS-analyses
and the Cr to C ratio was compared to that determined for the
fresh supported catalyst (see Supporting Information). Within the
limits of precision of the analysis, no leaching of active species
could be determined; with regard to the stability of the catalytic
values, no degradation of the chromium complex has occurred,
obviously. In addition, Unsym-Cr-A@rGO was stirred for 18 h in
DCM and the resulting solution was collected after filtration
without decreasing the temperature. No catalytic activity was
detected after adding substrates to this solution, proving that there
is no release of the active species during the heterogeneous
catalysis reaction.
Conflict of interest
The authors declare no conflict of interest.
Keywords: Asymmetric catalysis • Cr-salen complexes •
Supported catalysts • reduced Graphene Oxide • Recycling
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