Structurally Defined Catalysts for Enantioselective Oxidative Coupling Reactions

Abstract

+Pure binaphthol enantiomers and their derivatives are important chiral ligands for a wide range of asymmetric syntheses.[1] The facile conversion of binaphthol into 2,2'-(diphenylphosphanyl)-1,1'-binaphthyl (BINAP) is particularly noteworthy because BINAP greatly expanded the synthetic utilities of binaphthol in asymmetric catalysis.[2] In light of the wide-ranging and important applications of these compounds, catalytic asymmetric preparation of chiral binaphthols and improvements in the enantioselectivity of the associated catalysts have continued to attract the attention of many researchers. [3] Existing catalysts for the asymmetric preparation of chiral binaphthols include copper complexes of chiral amines or their derivatives [4] and chiral V IV O complexes with either one or two stereogenic metal centers.[5] Recently, dimeric copper(ii) complexes [4b,e,f] or dinuclear V IV O complexes [5d,e] have been shown to be better catalysts with a high enantioselectivity. In the above reports, the catalytic metal complexes were prepared in situ by reacting a chiral ligand with a desired metal ion (Cu I , Cu II , or V IV O). Generally, this method produces at least three species in solution: metal-free ligand, mononuclear, and dinuclear complexes. However, it is not easy to identify the active species that contribute to the overall reaction rate. Furthermore, the X-ray crystal structures of the above catalysts have never been determined, and this has restricted mechanistic investigations aimed at further improving the catalytic enantioselectivity and robustness. Given the success of Jacobsen's salen catalysts [6] in a broad range of asymmetric catalytic reactions, we decided to investigate whether coupled salen complexes would catalyze reactions that require two catalytic centers. Herein we describe the syntheses and crystal structures of a family of dicopper catalysts for the asymmetric oxidative coupling of 2-naphthol.The reaction of (1R,2R)-1,2-diaminocyclohexane (R,R-DACH) with 2,6-diformylphenol or its derivatives in the presence of Cu II gave yellow-green solids of 1 a-c (Scheme 1), Scheme 1. Structures of Schiff base dicopper(ii) complexes 1 a-c, macrocyclic ligands 2 a-c, and polyamino dicopper(ii) complexes 3 a-c.

Catalytic Asymmetric Ring‐Opening Reaction of meso‐Epoxides with Aryl Selenols and Thiols Catalyzed by a Heterobimetallic Gallium‐Titanium‐Salen Complex

“…It turned out, however, that Ga-Ti-14 gave poor results (entry 15). The last modification of the ligand structure was the change to the macrocycle Schiff-base ligand (15), [19] with the expectation that this multidentate N 4 O 2 -coordination compartment could bind the two metals in a planar mode. Unfortunately, the enantioselectivity obtained was still far from satisfying (entry 16).…”

Selective Synthesis of [2+2] Macrocyclic Schiff Bases from Chiral 1,4‐Diamines

“…In recent years, asymmetric catalysis based on complexes of macrocyclic Schiff bases has attracted much attention [1][2][3] parallel to the tremendous developments in Jacobsen's metallosalen catalysts for a variety of asymmetric reactions. [4][5][6] In particular, macrocyclic systems having two or more redox active centers in close proximity within an enzyme-like chiral cavity, capable of cooperative interactions, are of especial interest.…”

“…A case in point is the report on asymmetric oxidative coupling of naphthol using the dinuclear copper(II) macrocyclic complexes. 3 However, highly selective synthesis of the desired macrocyclic compounds sometimes remains problematic since competitive reactions may occur and result in several kinds of oligomeric compounds. In general, several techniques such as templateassisted 2,7-10 or preorganization-assisted syntheses combined with the high dilution method 11,12 are employed for the preparation of desired macrocycles.…”

Lewis-Acid-Assisted Highly Selective Oxidative Cross-Coupling Polymerization with Copper Catalysts

“…The oxidative coupling reaction includes a radical coupling step, 1,[18][19][20] in which the radical species generated by the oneelectron oxidation participate in the coupling process. On the other hand, a Lewis acid catalyst, such as the trifluoromethanesulfonate salts of ytterbium and scandium [Yb(OTf) 3 and Sc(OTf) 3 ], is often used as an effective tool for selectivity control during the radical reaction, especially, the radical addition process.…”