Direct acyl radical formation of linear aldehydes (RCH2-CHO) and subsequent hydroacylation with electron-deficient olefins can be effected with various types of metal and nonmetal catalysts/reagents. In marked contrast, however, no successful reports on the use of branched aldehydes have been made thus far because of their strong tendency of generating alkyl radicals through the facile decarbonylation of acyl radicals. Here, use of a hypervalent iodine(III) catalyst under visible light photolysis allows a mild way of generating acyl radicals from various branched aldehydes, thereby giving the corresponding hydroacylated products almost exclusively. Another characteristic feature of this approach is the catalytic use of hypervalent iodine(III) reagent, which is a rare example on the generation of radicals in hypervalent iodine chemistry.
Scheme 2. Proposed pathway for the formation of iodanyl radical. Scheme 3. Oxidation of methyl valerate with 2 and 3 d.Scheme 4. Hypervalent iodine(III) reagents 3. [12] naph = naphthalene.
The mechanism of the C-H activation of aldehydes and the succeeding acylation of an alkene using a hypervalent iodine reagent is investigated by theoretical calculations. In contrast to the initial proposed mechanism, the present calculations show that the hypervalent iodine is the initiator of the radical reaction. The formation of acyl radical is rate-determining, and the resulting radical acts as the chain carrier. The kinetic isotope effect (KIE) of deuterated aldehyde, as well as other experimental observations, can now be rationalized from the newly proposed mechanism.
A practical approach for the oxidation of unactivated C sp3 À H bonds by an ortho-nitrophenyl-substituted hypervalent iodineA C H T U N G T R E N N U N G (III) reagent is presented. The nitro group coordinates to the adjacent iodine center through dipolar interaction, which leads to a single substitution of the acetate ligand by tert-butyl hydroperoxide. As a result, a strong iodanyl radical is formed, which can activate the inert C sp3 ÀH bonds in a highly efficient manner. The reagent is stable in the presence of moisture, and can be used for oxidation reactions on semipreparative scale.
A Diels–Alder reaction in a fluorous biphasic system was accelerated by a hexameric capsule of resorcinarene bearing fluorous feet. The reaction takes place predominantly within the capsule, which can be recovered as a fluorous solution and recycled after simple decantation.
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