Although
pyridine derivatives are versatile supporting ligands
in catalysis, the development of their chiral versions has been relatively
limited. Herein, we report the design, synthesis, and proof-of-concept
application of a structurally tunable chiral pyridine framework featuring
an annulated compact ring system. Using an N,B-bidentate ligand skeleton
containing the chiral pyridine moiety, we have developed an enantioselective
iridium-catalyzed desymmetrizing C–H borylation reaction of
diaryl(2-pyridyl)methane compounds with up to 96% ee and 93% yield.
The resulting borylation products could be readily transformed into
various chiral tri(hetero)arylmethane compounds. Density functional
theory investigations revealed that the two chair conformations of
the flexible ketal motif both favored the enantiomer that was consistent
with experimental results. This work has thus introduced an effective
and tunable chiral pyridine ligand framework that may be used in many
catalytic asymmetric transformations.
A new class of chiral 2,2'-bipyridine ligands, SBpy, featuring minimized short-range steric hindrance and structural tunability was rationally designed and developed, and the effectiveness was demonstrated in the first highly enantioselective Ni-catalyzed addition of aryl halides to aldehydes. In comparison with known approaches using preformed aryl metallic reagents, this reaction is more step-economical and functional group tolerant. The reaction mechanism and a model of stereocontrol were proposed based on experimental and computational results.
To achieve a safe, efficient, and sustainable (even fully automated) production for the continuous‐flow hydrogenation reactions, which is among the most often used reactions in chemical synthesis, new catalyst types and immobilization methods as well as flow reactors and technologies have been developed over the last years; in addition, these approaches have been combined with new and transformational technologies in other fields such as artificial intelligence. Thus, attention from academic and industry practitioners has increasingly focused on improving the performance of hydrogenation in flow mode by reducing the reaction times, increasing selectivities, and achieve safe operation. This Minireview aims to summarize the most recent research results on this topic with focus on the advantages, current limitations, and future directions of flow chemistry.
The first nickel‐catalyzed highly enantioselective reductive Ullmann coupling of ortho‐chlorinated aryl aldehyde was achieved under mild reaction conditions with a chiral 2,2′‐bipyridine ligand (+)‐DTB‐SBpy, thus providing axially chiral biphenyl or binaphthyl dials with up to 99 % yield and 99.5:0.5 er. The versatility of the products as common synthetic intermediates for diverse axially chiral ligands, catalysts, and functional molecules was demonstrated by short‐step transformations. This protocol also allowed the concise and highly enantioselective formal total synthesis of biologically active natural products (+)‐kotanin, (−)‐isoschizandrin and (+)‐gossypol.
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