Zincative Functionalization of C–H Bonds Using Lithium Amide Zincate Bases
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Carbon–hydrogen bond functionalization is a highly desirable transformation as C–H bonds are plentiful in feedstock chemicals and the direct introduction of valuable functional groups improves atom economy and efficiency. However, field of direct C–H functionalization still has many challenges including regioselectivity, directing group limitations, and often harsh reaction conditions with toxic and rare transition metal catalysts. Deprotonative zincation has been demonstrated as a useful method to achieve C–H functionalization.
The direct C–H amination of a wide scope of arenes and heteroarenes has been achieved by copper-catalyzed electrophilic amination with O-benzoyl hydroxylamines. Key to the expanded scope of substrates is the use of a lithium zincate base, Li[ZnEt2TMP] which can coordinate to a variety of common functionalities to direct zincation. The mild reaction conditions and compatibility with functional groups such as esters, nitriles and halides make this method orthogonal to many existing strategies.
Additionally, a novel lithium zincate base with inexpensive dicyclohexylamine has been developed and shown to be capable of efficient and regioselective zincation with an extensive scope of directing groups. Aryl and heteroaryl zincates can undergo direct electrophilic silylation and borylation without the need for any transition metal catalyst.
α-Functionalization of substituted amides and esters has been achieved by utilizing a tribasic lithium zincate for direct allylation and copper-catalyzed arylation and vinylation with aryl iodides and vinyl iodides. In summary, lithium zincate bases have been demonstrated to be broadly useful for diverse and regioselective C–H functionalization to introduce valuable complexity from simple starting materials.
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