Abstract
Methods for direct functionalization of C-H bonds mediated by N-oxyl radicals constitute a powerful tool in modern organic synthesis. While several N-oxyl radicals have been developed to date, the lack of structural diversity for these species has hampered further progress in this field. Here we designed a novel class of N-oxyl radicals based on N-hydroxybenzimidazole, and applied them to the direct C-H functionalization reactions. The flexibly modifiable features of these structures enabled facile tuning of their catalytic performance. Moreover, with these organoradicals, we have developed a metal-free approach for the synthesis of acyl fluorides via direct C-H fluorination of aldehydes under mild conditions.
Highlights
The direct functionalization of C–H bonds has been recognized as an innovative approach to realize step- and atomeconomical synthesis of various functionalized molecules
We initially carried out density functional theory (DFT) calculations in order to estimate the bond dissociation energies (BDEs) values of the O–H bonds in our designed NHBI derivatives (1a–f) (Fig. 2).[12]
Some of synthesized NHBI derivatives have the similar or even higher BDEs compared to that of NHPI, which has been widely used as an efficient catalyst for several hydrogen atom abstraction reactions.[6]
Summary
The direct functionalization of C–H bonds has been recognized as an innovative approach to realize step- and atomeconomical synthesis of various functionalized molecules. Considerable research efforts have been devoted to the development of a variety of C–H functionalization reactions based on transition-metal-catalyzed or photoredox-promoted approaches.[1,2] On the other hand, transition-metal-free and non-photolytic direct C–H functionalization reactions still remain a signi cant challenge,[3] they will provide new synthetic strategies with complementary reactivity and selectivity.[4] In this context, N-oxyl radicals have attracted much attention as promising organoradical catalysts for these transformations.[5] As represented by N-hydroxyphthalimide (NHPI), several N-hydroxy compounds have been used to generate active N-oxyl radicals in situ, which selectively abstract hydrogen atoms from C–H bonds of organic molecules.
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