The
synthesis of aromatic ketones by chromium-catalyzed Kumada
arylation of secondary amides with organomagnesium reagents is described.
This reaction was enabled by using low-cost chromium(III) salt as
a precatalyst, combined with trimethylsilyl chloride as an additive,
and presents a rare example of catalytic transformation of secondary
amides to ketones at room temperature. It was shown that catalytically
active low-valent chromium species might be responsible for the amide–ketone
exchange by mechanism involving the activation of benzimidate intermediate.
An easily synthesized and accessible N,O-bidentate auxiliary has been developed for selective C-H activation under palladium catalysis. The novel auxiliary showed its first powerful application in C-H functionalization of remote positions. Both C(sp(2))-H and C(sp(3))-H bonds at δ- and ε-positions were effectively activated, thus giving tetrahydroquinolines, benzomorpholines, pyrrolidines, and indolines in moderate to excellent yields by palladium-catalyzed intramolecular C-H amination.
The first Pd-catalyzed regioselective γ-carbonylation of oxalyl amide protected aliphatic amines with carbon monoxide leading to synthesis of pyrrolidones has been developed.
Palladium-catalyzed direct ortho-silylation of oxalyl amide-protected phenylmethanamine and phenethylamine with commercially available disilanes is reported. Germanylation products were also produced under the same reaction conditions. This protocol tolerated oxalyl amide-protected aliphatic amines, which gave γ-C-H silylation products.
The discovery of NHCs (NHC = N‐heterocyclic carbenes) as ancillary ligands in transition‐metal‐catalysis ranks as one of the most important developments in synthesis and catalysis. It is now well‐recognized that the strong σ‐donating properties of NHCs along with the ease of scaffold modification and a steric shielding of the N‐wingtip substituents around the metal center enable dramatic improvements in catalytic processes, including the discovery of reactions that are not possible using other ancillary ligands. In this context, although the classical NHCs based on imidazolylidene and imidazolinylidene ring systems are now well‐established, recently tremendous progress has been made in the development and catalytic applications of BIAN‐NHC (BIAN = bis(imino)acenaphthene) class of ligands. The enhanced reactivity of BIAN‐NHCs is a direct result of the combination of electronic and steric properties that collectively allow for a major expansion of the scope of catalytic processes that can be accomplished using NHCs. BIAN‐NHC ligands take advantage of (1) the stronger σ‐donation, (2) lower lying LUMO orbitals, (3) the presence of an extended π‐system, (4) the rigid backbone that pushes the N‐wingtip substituents closer to the metal center by buttressing effect, thus resulting in a significantly improved control of the catalytic center and enhanced air‐stability of BIAN‐NHC‐metal complexes at low oxidation state. Acenaphthoquinone as a precursor enables facile scaffold modification, including for the first time the high yielding synthesis of unsymmetrical NHCs with unique catalytic properties. Overall, this results in a highly attractive, easily accessible class of ligands that bring major advances and emerge as a leading practical alternative to classical NHCs in various aspects of catalysis, cross‐coupling and C−H activation endeavors.
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