Rhodium-catalyzed asymmetric arylation (RCAA) reactions
provide
one of the most straightforward and powerful ways to introduce aryl
fragments in an enantioselective manner. The discovery of novel chiral
ligands and catalytic systems is a major focus in generating optical
chiralities for RCAA reactions. In the past decade, the chelating
functionalities in ligands have been significantly expanded from traditional
phosphorus to interesting diene, bissulfoxide, and their hybrids.
Herein we highlight the research on these distinct families of chiral
ligands and describe their applications in the RCAA of arylmetals
to activated alkenes, aldehydes, ketones and imines, and RCAA-tandem
reactions.
The first Cu-catalyzed asymmetric borylative cyclization of cyclohexadienone-containing 1,6-enynes is achieved through a tandem process: selective β-borylation of propargylic ether and subsequent conjugate addition to cyclohexadienone. The reaction proceeds with excellent regioselectivity and enantioselectivity to afford an optically pure cis-hydrobenzofuran framework bearing alkenylboronate and enone substructures. Furthermore, the resulting bicyclic products could be converted to bridged and tricyclic ring structures. This method extends the realm of Cu-catalyzed asymmetric tandem reactions using bis(pinacolato)diboron (B2pin2).
Two tunable arylative cyclizations of cyclohexadienone-containing 1,6-enynes are reported via rhodium(III)-catalyzed C-H activation of O-substituted N-hydroxybenzamides. The use of different O substituents, i.e. O-Piv and O-Me, on the directing group allows the formation of either tetracyclic isoquinolones through an Ⓝ-Michael addition process or hydrobenzofurans through a Ⓒ-Michael addition process. Mechanistic investigations of these two cascade reactions clearly indicated that the C-H bond cleavage process was involved in the turnover-limiting step. Furthermore, the cyclization products could be subjected to various transformations for elaborating the pharmaceutically and synthetically valuable potential. This is the first example of a rhodium(III)-catalyzed arylative cyclization reaction of 1,6-enynes, and the results extend the application realm of Cp*Rh(III)-catalyzed C-H activation cascade reactions.
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