Atroposelective Synthesis of C–C Axially Chiral Compounds via Mono- and Dinuclear Vanadium Catalysis

Abstract

Conspectus Axially chiral compounds with rotationally constrained σ-bonds that exhibit atropisomerism are lucrative synthetic targets because of their ubiquity in functional materials and natural products. The metal complex-catalyzed enantioselective fabrication of axially chiral scaffolds has been widely investigated, and thus far, considerable progress has been made. Over the past two decades, we have developed a highly efficient strategy for constructing axially chiral biarenol derivatives using chiral mono- and dinuclear vanadium complexes. These complexes are readily prepared from vanadium­(IV) salts and Schiff base ligands (generated from the condensation of (S)-tert-leucine and di- or monoformyl-(R)-1,1′-bi-2-naphthol (BINOL) derivatives) under O2 and act as highly active catalysts for highly stereoselective C–C bond formation. In particular, the vanadium complex-catalyzed enantioselective oxidative coupling of 2-naphthols 1 under oxygen or in air, which is a green oxidant, affords the desired axially chiral molecules in high yields and high stereoselectivity (up to quantitative yield and 97% ee), along with water as the sole coproduct. This coupling reaction tolerated various functional groups (such as halogens, alkoxys, and boryls) and avoided overoxidation of coupling products. The key feature of dinuclear vanadium­(V) catalysts such as (R a,S,S)-5a is an outstanding mode of the homocoupling reaction, in which a single molecule of the catalyst activates two molecules of the starting material (e.g., 2-naphthols) simultaneously. With this “dual activation” mechanism, the oxidative coupling promoted by the dinuclear catalyst proceeds in an intramolecular manner. The homocoupling rate using 5 mol % of the dinuclear vanadium­(V) complex (R a ,S,S)-5a was measured to be 111 times faster than that of the mononuclear vanadium­(IV) complex (S)-4a bearing a half motif of the dinuclear vanadium complex. In the case of the heterocoupling reaction utilizing two different kinds of arenol derivatives, only a starting arenol having lower oxidation potential seems to be activated by the mononuclear vanadium complex. The reaction rate of the heterocoupling using either mono- or dinuclear vanadium complexes showed no difference to give the coupling product in high yields but with a different enantioselective manner; chiral mononuclear vanadium­(V) complexes showed better enantioselectivites than that of the dinuclear vanadium­(V) complexes. A competing heterocoupling study and a linear correlation between the ee of the mononucaler vanadium catalyst and ee of the heterocoupling suggested that the heterocoupling involves an intermolecular radical-anion coupling pathway. In this Account, we summarize the recent advances in vanadium-catalyzed coupling reactions that produced important chiral molecules, such as biresorcinols, polycyclic biphenols, oxa[9]­helicenes, bihydroxycarbazoles, and C 1-symmetrical biarenols, and their coupling reaction mechanisms. By pursuing vanadium catalysis, we believe numerous additional transform...

Asymmetric Cross Couplings of Trifluoromethyl Radical to Vinylarenes with N‐Hydroxy‐oxazinediones and Subsequent Aerobic Oxidative Homocoupling of 2‐Naphthols Catalysed by Chiral Vanadyl Complexes

“…The predominant ( P )‐form enantio‐control ( P / M ‐ 25 , 96/4) in the homo‐coupling of N ‐(benzoxy)‐3‐hydroxy‐2‐naphthamide ( 24 , N−OH−Np‐amide) catalysed by 2 g can be rationalised by a similar S H 2 mechanism between vanadyl‐bound 3OH−Np‐amide (R=H) and the sandwiched 3‐oxy‐Np‐amide radical or anion [41a,b] with a favourable π‐π‐π interaction (Figure 10A). On the other hand, the direct S H 2 between vanadyl‐bound chiral 3OH−Np‐amide (R=CH 2 CF 3 ) and chiral 3oxy‐Np‐amide radical or anion seems to be hampered by the adverse steric repulsion between CH 2 CF 3 group and the vanadyl‐bound N ‐ tert ‐leucinate template.…”

Recent advances in iron-catalysed coupling reactions for the construction of the C(sp2)–C(sp2) bond

“…Biaryl compounds have garnered significant research interest over the past decades due to their widespread presence in numerous natural products, pesticides, dyes, and commonly employed ligands. 4 The Ullmann reaction, discovered in the early 20th century, 5 has spurred extensive investigation into methods for synthesizing biaryl compounds, particularly transition-metal-catalyzed cross-coupling reactions between organohalides and organometallic reagents. 1,1′-Bi-2-naphthol (BINOL) serves as a notable example of a versatile structural motif and ligand substructure in asymmetric catalysis.…”

Advances in the Asymmetric Synthesis of BINOL Derivatives

“…In this work, published by Zhang et al [72] (Scheme 10), 6,6'-disubstituted (R)-BINOLs (1) were obtained with yields of up to 89% and excellent enantioselectivities (up to 96% ee). The reaction was accompanied by Mass Spectroscopy, and identification of a peak corresponding to the complex J allowed the authors to propose a mechanism pathway through the transition state K. Continuing work involving multifunctional chiral catalysis via double activation, Takizawa's group [73] developed complexes A-C (Scheme 11)-from VOSO4 and Schiff base ligands generated via condensation of (S)-tert-leucine and 3,3 '-formyl-(R)-BINOLwhich have been successfully applied in the synthesis of (R)-and (S)-BINOL (1) with yields between 46 and 76%, in addition to enantiomeric excesses of up to 91%.…”