Highly enantioselective oxidative couplings of 2-naphthols catalyzed by chiral bimetallic oxovanadium complexes with either oxygen or air as oxidant.
The chiral bimetallic oxovanadium complexes have been designed for the enantioselective oxidative coupling of 2-naphthols bearing various substituents at C6 and/or C7. The chirality transferring from the amino acid to the axis of the biphenyl in oxovanadium complexes 2 was found to occur with the use of UV and CD spectra and DFT calculation. The homo-coupling reaction with oxygen as the oxidant was promoted by 5 mol % of an oxovanadium complex derived from L-isoleucine and achiral biphenol to afford binaphthols in nearly quantitative yields with high enantioselectivities of up to 98% ee. An oxovanadium complex derived from L-isoleucine and H8-binaphthol is highly efficient at catalyzing the air-oxidized coupling of 2-naphthols with excellent enantioselectivities of up to 97% ee. 51V NMR study shows that the oxovanadium complexes have two vanadium(V) species. Kinetic studies, the cross-coupling reaction, and HRMS spectral studies on the reaction have been carried out and illustrate that two vanadium(V) species are both involved in catalysis and that the coupling reaction undergoes a radical-radical mechanism in an intramolecular manner. Quantum mechanical calculations rationalize the importance of the cooperative effects of the axial chirality matching S-amino acids on the stereocontrol of the oxidative coupling reaction. The application of the transformation in the preparation of chiral ligands and conjugated polymers confirms the importance of the current process in organic synthesis.
Guo
Qi-Xiang
QX
Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, China.
Wu
Zhi-Jun
ZJ
Luo
Zhi-Bin
ZB
Liu
Quan-Zhong
QZ
Ye
Jian-Liang
JL
Luo
Shi-Wei
SW
Cun
Lin-Feng
LF
Gong
Liu-Zhu
LZ
D000388:Air; D002384:Catalysis; D020650:Combinatorial Chemistry Techniques; D008956:Models, Chemical; D015394:Molecular Structure; D009284:Naphthols / Q000138:chemical synthesis* / Q000737:chemistry*; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D016877:Oxidants / Q000737:chemistry*; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D013237:Stereoisomerism; D014638:Vanadates / Q000737:chemistry*
D009284:Naphthols; D009942:Organometallic Compounds; D016877:Oxidants; C075743:oxovanadium IV; D014638:Vanadates; D010100:Oxygen