Abstract
Pd-catalyzed asymmetric Suzuki–Miyaura couplings of 3-methyl-2-bromophenylamides, 3-methyl-2-bromo-1-nitrobenzene and 1-naphthaleneboronic acids have been successfully developed and the corresponding axially chiral biaryl compounds were obtained in very high yields (up to 99%) with good enantioselectivities (up to 88% ee) under mild conditions. The chiral-bridged biphenyl monophosphine ligands developed by our group exhibit significant superiority to the naphthyl counterpart MOP in both reactivity and enantioselectivity control. The large steric hindrance from π-conjugated ortho-substituents of the bromobenzene substrates and the Pd···O interaction between carbonyl and palladium seem essential to achieve high enantioselectivity.
Highlights
Chiral molecules have received much attention from chemists because of their widespread appearance in biologically active compounds [1-4] such as vancomycin [5] and korupensamine A [6] and as useful chiral ligands in asymmetric catalysis
Different strategies with various metals and phosphine ligands had been successfully employed for the efficient synthesis of this scaffold [7-22], like Hiyama [23,24], Negishi [25,26]
4-substituted or unsubstituted 1-naphthaleneboronic acids provided the corresponding products in high yields with good ee values (5g, 98% yield, 75% ee; 5h, 97% yield, 83% ee; 5i, 98% yield, 85% ee) [39,62]. All these results show that the large steric hindrance from π-conjugated ortho-substituents of the bromobenzenes and the O···Pd interaction work together for the acquisition of high enantioselectivity
Summary
Chiral molecules have received much attention from chemists because of their widespread appearance in biologically active compounds [1-4] such as vancomycin [5] and korupensamine A [6] and as useful chiral ligands in asymmetric catalysis. Different strategies with various metals and phosphine ligands had been successfully employed for the efficient synthesis of this scaffold [7-22], like Hiyama [23,24], Negishi [25,26]. In these synthetic strategies, the reaction system of palladium with chiral phosphine ligands was studied fruitfully by Cammidge [37,38], Buchwald [13,28,39], Tang [40-43] and other groups [43-59]. For the asymmetric formation of large steric systems such as sterically demanding biaryls still remain limitations [24], especially on how to obtain the large steric axially chiral biaryl.
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