Development of Dinuclear Vanadium Catalysts for Enantioselective Coupling of 2-Naphthols via a Dual Activation Mechanism

Abstract

Catalytic asymmetric reactions are among the most powerful synthetic methods to obtain optically active compounds.1,2) Recent challenges focus on the development of asymmetric catalysts for carbon-carbon bond-forming reactions with high activity and broad substrate generality, which lead to practical, efficient, and environmentally benign chemical syntheses. Dual activation systems for substrates such as nucleophiles and electrophiles lead to enhanced reaction rates and more specific control of the transition structure with respect to the catalyst's asymmetric environment. Dual activation catalyses can be classified into three categories ( Fig. 1): Type 1) dual activation using two different kinds of catalysts [3][4][5][6][7][8][9][10][11][12] ; Type 2) conjugated-type dual activation with a functional group such as a phosphate which has both acidic and basic sites in one functionality [13][14][15] ; and Type 3) dual activation by two catalytic sites in a single catalyst. For efficient dual activation of the substrates, a balance between these two functionalities in the catalyst is required. For example, in an acid-base type catalyst the selfquenching reaction of acidic and basic moieties on the catalyst can lead to its inactivation.We postulated that a chiral complex possessing two identical metal centers in a single molecule, which could activate two substrates simultaneously in a homolytic coupling reaction of 2-naphthol molecules, would enhance the reaction rate with high enantioselectivity. To achieve the new dual activation catalysis in an oxidative coupling reaction, we designed the catalyst (R a ,S,S)-1 42) bearing two active sites attached to a binaphthyl skeleton, taking advantage of the activation entropy (Fig. 2). Activation entropy strongly con- Development of Dinuclear Vanadium Catalysts for Enantioselective Coupling of 2-Naphthols via a Dual Activation MechanismShinobu TAKIZAWA This review describes our recent efforts in the development of chiral dinuclear vanadium complexes which work as dual activation catalysts for oxidative coupling of 2-naphthols. A chiral dinuclear vanadium(IV) complex (R a ,S,S)-1a possessing (S)-tert-leucine moieties at the 3,3 -positions of the (R)-binaphthyl skeleton was developed, and found to promote oxidative coupling of 2-naphthol to afford (S)-1,1 -bi-2-naphthol (BINOL) with 91% ee. To verify the dual activation mechanism, mononuclear vanadium(IV) complex (S)-10 was also prepared. Kinetic analysis revealed that the reaction rate of oxidative coupling of 2-naphthol promoted by (R a ,S,S)-1a is 48.3 times faster than that of (S)-10. In the coupling reaction, the two vanadium metals in the chiral complex activate two molecules of 2-naphthol simultaneously achieving a high reaction rate with high enantiocontrol. Since the dinuclear vanadium(IV) complex was found to be readily oxidized to afford a corresponding vanadium(V) species during preparation in air, a new synthetic procedure using VOCl 3 and a convenient one-pot procedure using VOSO 4 under O 2 have been applie...

Comparison of Oxidative Aromatic Coupling and the Scholl Reaction

“…Many interesting copper complexes for the oxidative coupling of naphthols were developed by Kozlowski and co‐workers, and and have successfully been applied in the total syntheses of numerous chiral, natural binaphthyl derivatives with high enantioselectivity 65e. 67…”

Chemical Synthesis of Complex Molecules Using Nanoparticle Catalysis

“…The products of the AgNP-catalyzed oxidative couplings were highly regioselective and substrate-dependent, with α- ( 27 ) and β- ( 29 ) naphthols affording exclusive 2,2′- ( 28 ) and 3,3′- ( 30 ) binaphthols, respectively, under the same conditions. The formation of binaphthol 30 differed in the regioselectivity for oxidative coupling of 29 using homogeneous catalysis. − These intriguing observations implied subtle mechanistic variations related to specific phenol starting materials, which warranted further investigation. In addition to the above notable features, the novel AgNP-catalyzed reactions showcased low catalyst loading and mild reaction conditions.…”

Oxidative aromatische Kupplung und Scholl‐Reaktion im Vergleich

“…Viele interessante Kupferkomplexe für die oxidative Kupplung von Naphtholen wurden von Kozlowski und Mitarbeitern entwickelt und erfolgreich in der Totalsynthese zahlreicher chiraler, natürlicher Binaphthylderivate mit hohen Enantioselektivitäten eingesetzt 65e. 67…”