Asymmetric Oxidative Coupling of 2-Naphthol to BINOL

We have found that the novel chiral self-dimerizahon of V complexes produces an active unsaturated structure with a new chiral conformation on a Si02 surface, and provides highly enanhoselective achve sites for the asymmetric oxidative coupling of 2-naphthol [35, 36]. Table 10.3 summarizes catalyhc activities, selectivities and enantiomeric excess (e.e.) of homogeneous and heterogeneous V catalysts for the oxidative coupling of 2-naphthol. The homogeneous reactions on a representative L-leucine based V monomer did not exhibit the catalyhc activity. Only in the presence of chlorotrimethylsilane as an acidic promoter did the reachon proceed, but conversion was as low as 15% at 293 K, selectivity to BINOL and e.e. to (P)-BINOL were quite low at 73% and 8%, respectively (Table 10.3). Furthermore, loss of activity occurred and no reachon proceeded after 9 days. 

Catalyst-1 igand Temperature (K) Time (day) Solvent Conversion (%) Selectivity (%) e.e. (R) (%) 

On AI2O3, the selectivity toward BINOL was only 53% and not enantioselec-tive (-2%) in CHCI3. The Ti02-supported V complex did not produce BINOL (0% selectivity). Neither of these supported catalysts produced BINOL in toluene. 

V loading on Si02 including a L-leucine segment was varied in the V loading range 0.3-3.4wt% to maximize enantioselectivity for the coupling reaction (Table 

Notably, the enantioselectivity of the coupUng reaction is determined by the chiral conformation on the V center rather than the chirality of the Schiff-base ligands. The chirality of the ligands affects stericaUy the chiral self-dimerization 

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