One‐pot synthesis of [2+2]‐helicate‐like macrocycle and 2+4‐μ4‐oxo tetranuclear open frame complexes : Chiroptical properties and asymmetric oxidative coupling of 2‐naphthols

Abstract

Synthesis of binuclear Cu(II) terminally closed [2+2]‐ double‐stranded helicate‐like macrocycles 1, 1′,1″, 2, 2′, 2″ and 2+4‐μ4‐oxo tetranuclear open frame complexes 3, 3′, 3″, 4, 4′, 4″ are established. Adapting one‐pot self‐assembly technique from simple three components systems: 1,1′‐binaphthyl‐2,2′‐diamine, 4‐methyl‐2,6‐diformyl phenol and cupric salts, the helicate‐like [2+2]‐ macrocyclic complexes 1–1″, 2–2″ and 2+4‐μ4‐oxo tetranuclear complexes 3–3″, 4–4″ were obtained by appropriately altering the reaction condition such as temperature and subcomponent ratio. Density Functional Theory (DFT) calculations were carried out for understanding the structural geometries, intermediates involved in the diverse formation of [2+2] and 2+4 frameworks. The single crystal X‐ray structures obtained for 1′, 2 and 3 confirms the self‐assembly process in line with DFT. This detailed analysis tempted us to derive a plausible mechanism for this long standing challenge in the synthesis of such macrocycles using 1,1′‐binaphthyl‐2,2′‐diamine (BNDA) and aromatic aldehyde. The chiroptical properties of enantiopure complexes and their catalytic applications in asymmetric oxidative coupling of 2‐naphthol to chiral 1,1’‐Bi‐2‐naphthol (BINOL) achieved in good yield and ee were discussed.