Chemistry - A European Journal
10.1002/chem.201701865
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-1 -1
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molecules as in the present cases, we believe that the S 2 type
H
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transition states should be greatly slowed down by the steric
effects between decatungstate anion and the target C-H bond.
The observed lack of reactivity in these cases is probably related
to a difficult approach of the apical oxygens of decatungstate,
recently proposed to be responsible for the H-abstraction
step,[
15]
to the reacting site. All the results shown here
convinced us that a synergistic strategy based on polar and
steric effects provides a promising approach for the realization of
site-selective C-H functionalization by radicals, and importantly
the strategy would be much more promising for highly
substituted molecules rather than simple molecules.
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Experimental Section
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Typical Procedure for the C-H Alkylation of Esters, Lactones, and
Ketones
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A magnetic stirring bar, TBADT (tetrabutylammonium decatungstate, 66.4
mg, 0.02 mmol, 4 mol%), tert-butyl 4-methylpentanoate (1m, 861.3 mg, 5
mmol) and acetonitrile (1 mL) were added to the Pyrex tube. The reaction
mixture was then degassed by three freeze-pump-thaw cycles. The
resultant solution was purged with argon, and dimethyl maleate (2a, 70.5
mg, 0.5 mmol) was added to the flask. The reaction mixture was
irradiated using a Xe lamp (300 W) under stirring for 24 h. After the
reaction, the solvent was evaporated under reduced pressure. The
residue was purified by flash chromatography on silica gel
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1,2,5-tricarboxylate (3a, 111.3 mg, 72%).
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1
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This work was supported by a Grant-in-Aid for Scientific
Research (A) (no. 26248031) from the JSPS.
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Keywords: Site-selective C-H functionalization • Photocatalysis
•
Radical • Polar effects• Steric effects
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[2]
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