Enantioselective Vanadium-Catalyzed Oxidative Coupling: Development and Mechanistic Insights

Abstract

The evolution of a more reactive chiral vanadium catalyst for enantioselective oxidative coupling of phenols is reported ultimately resulting in a simple monomeric vanadium species combined with a Brønsted or Lewis acid additive. The resultant vanadium complex is found to effect the asymmetric oxidative ortho—ortho coupling of simple phenols and 2-hydroxycarbazoles with good to excellent levels of enantioselectivity. Experimental and quantum mechanical studies of the mechanism indicate that the additives aggregate the vanadium monomers. In addition, a singlet to triplet crossover is implicated prior to carbon-carbon bond formation. The two lowest energy diastereomeric transition states leading to the enantiomeric products differ substantially with the path to the minor enantiomer involving greater torsional strain between the two phenol moieties.

Synthetic Applications of Oxidative Aromatic Coupling—From Biphenols to Nanographenes

“…Recently, Egami and Katsuki developed chiral iron(salan) complexes (Scheme ), which afforded unsubstituted BINOL and 3,3′‐disubstitued derivatives with an enantiomeric excess of 64 % and up to 97 %, respectively . Progress in the asymmetric synthesis of biaryls has been briefly summarized in recent articles . The most important recent contributions are presented below.…”

“…Although numerous chiral catalysts have been developed for the asymmetric synthesis of BINOLs by oxidative coupling, the enantioselective preparation of chiral biphenyl derivatives has remained challenging until very recently. Kozlowski and co‐workers addressed this issue and performed an extensive catalyst optimization study using dinuclear vanadium catalysts as a starting point . It turned out that nitro‐substituted mononuclear vanadium complex 109 actually provided the best results, especially in the presence of acetic acid or lithium chloride as additives (Scheme ).…”

“…[89] Progress in the asymmetric synthesis of biaryls has been briefly summarized in recent articles. [47,88,90,91] Themost important recent contributions are presented below.…”

“…Kozlowski and co-workers addressed this issue and performed an extensive catalyst optimization study using dinuclear vanadium catalysts as astarting point. [91,97] It turned out that nitro-substituted mononuclear vanadium complex 109 actually provided the best results,especially in the presence of acetic acid or lithium chloride as additives (Scheme 17). A series of biphenols of type 108 were obtained in good yields and moderate ee values.…”

Benzimidazole bearing Pd–PEPPSI complexes catalyzed direct C2‐arylation/heteroarylation ofN‐substituted benzimidazoles

“…We prepared nine different N,N 0diaralkylbenzimidazolium Pd-PEPPSI complexes (1-9) from a cheap and readily available benzimidazole nucleus following a simple and high-yielding route. Initially Nalkylations were carried out on benzimidazole by utilizing the report of H. Kang and co-workers [28] to acquire various benzimidazolium salts (5a-i) with similar and dissimilar N,N 0 -aralkyl groups (bearing both electron donating methyl, isopropyl and electron-withdrawing -F and -NO 2 substituents on phenyl rings), which were then converted to corresponding Pd-PEPPSI complexes (1)(2)(3)(4)(5)(6)(7)(8)(9) upon reaction with 3-chloropyridine in the presence of K 2 CO 3 in moderate to good yields (Scheme 2).…”