A Highly Selective Synthesis of 1,1′-Bi-2-naphthol by Oxidative Coupling of Naphthol on Mesoporous Fe,Cu/MCM-41 Aluminosilicates

The oxidative coupling of 2-naphtol to 2,2′-dihydroxy-1,1′-binaphthyl (binaphthol) by air or oxygen has been carried out in the presence of Cu2+- and Fe3+-doped MCM-41 aluminosilicate as catalyst. Fe-exchanged MCM-41 was found to be a very efficient catalyst; excellent mass balances (> 95%) with almost total conversion and selectivity to binaphthol were achieved. The same reaction has also been carried out on Cu2+– and Fe3+–Y zeolites. Taking into account the relative dimensions of binaphthol and the catalyst pores, molecular modeling predicts that binaphthol can be accommodated inside the zeolite Y supercages (1.3 nm), but it cannot diffuse outside the zeolite cavities through the smaller pore apertures (0.74 nm). This prediction has been confirmed by dissolving a Y zeolite after the reaction, whereby unextractable binaphthol entrapped within the cavities was recovered. Variable amounts of two secondary by-products have also been detected, and their structure assigned to (2,8′);(8,2′)-dioxo-1,1′-binaphthyl and bisnaphthofuran based on analytical and spectroscopic data. Their percentage is particularly high when alumina-supported CuSO4 is used as the catalyst.