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lecule of aldehyde to afford 2 equiv. of acid III. The carbox-
ylic anion then attacks substrate 2 to generate ester 4 in the
presence of base. It is clear that oxygen is necessary for this
oxidative esterification reaction of aldehydes with alkyl ha-
lides or alkyl 4-methylbenzenesulfonate, and that the con-
figuration of the alkyl halides was completely inversed.
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Conclusions
We have developed an efficient NHC-catalyzed oxidative
esterification reaction of aldehydes with alkyl halides and
4-methylbenzenesulfonates through the umpolung of alde-
hydes. It is worth noting that the configuration of the alkyl
halide or 4-methylbenzenesulfonate was completely in-
versed, and that the presence of oxygen was crucial for this
transformation. In addition, we have developed an efficient,
tandem oxidation/esterification reaction of alcohol in the
presence of MnO2. This methodology provides a rare exam-
ple of the reaction of the Breslow intermediate to sp3-car-
bon centered electrophiles.
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Experimental Section
General Procedure for NHC-Catalyzed Oxidative Esterification Re-
action of Aldehydes: To a solution of N-heterocyclic carbene pre-
cursor 3d (14 mg, 0.04 mmol), DBU (62 μL, 0.4 mmol), and alde-
[6]
hyde (0.4 mmol) in anhydrous THF (4 mL), compound
2
(1.2 mmol) was added under an oxygen atomosphere. The mixture
was then stirred at 50 °C until the reaction was complete. After
cooling to ambient temperature, the reaction was concentrated in
vacuo and the crude product was purified by column chromatog-
raphy to give ester 4.
Procedure for NHC-Catalyzed Tandem Oxidation/Esterification: To
a solution of (4-nitrophenyl)methanol (31 mg, 0.2 mmol), N-het-
erocyclic carbene precursor 3d (6.8 mg, 0.02 mmol), DBU (31 μL,
0.2 mmol), and MnO2 (261 mg, 3 mmol) in anhydrous THF
(2 mL), 1-bromoheptane (88 μL, 0.6 mmol) was added under an
oxygen atomosphere. The mixture was then stirred at 50 °C for
24 h. After cooling to ambient temperature, the reaction was con-
centrated in vacuo and the crude product was purified by column
chromatography to give the desired product 4a in 76% yield.
[7]
[8]
Supporting Information (see footnote on the first page of this arti-
cle): Experimental procedures, full characterization, NMR and
HPLC spectra.
Acknowledgments
This work was supported by the National Natural Science Founda-
tion of China (NSFC) (grant number 20702022), the International
Cooperation Project of Gansu Province (grant numbers
1011WCGA170 and lzujbky-2009-79).
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Eur. J. Org. Chem. 2011, 6527–6531