Solvent and Temperature Effects on the Platinum‐Catalyzed Oxidative Coupling of 1‐Naphthols

Abstract

Using H2O2 as the oxidant, 1‐naphthols with electron‐donating groups at the 2‐ and 4‐positions couple oxidatively over a carbon‐supported platinum catalyst to 3,3′‐substituted 1,1′‐binaphthalenylidene‐4,4′‐diones and 4,4′‐substituted 2,2′‐binaphthalenylidene‐1,1′‐diones, respectively. The binaphthalenyl diols are the intermediates. The selectivity to individual products is influenced by the reaction temperature (room temp. or reflux) and by the solvent used. Under reflux, complete conversions are obtained within 40 min. At room temp. high diol yields can be obtained, e.g. 96 % from 2‐methyl‐1‐naphthol in MeOH. Under reflux the reaction proceeds always further to the diones (at least to some extent), and THF is a promising solvent for the selective one‐pot two‐step oxidation of 1‐naphthols to the diones (e.g. 81 % from 4‐methoxy‐1‐naphthol). In most other solvents (reflux) naphthoquinones are observed as byproducts. In an attempt to optimize the yield of menadione, 30.5 % was obtained in boiling MeNO2.

Oxidative Coupling of 2‐Methyl‐1‐naphthol: A Comparison between Bismuth‐Promoted Pt, Pd and Ag Catalysts

“…The explicit outcome of the reaction depends on the noble metal, the solvent used and the reaction temperature. The binaphthol is regarded as the intermediate to the binaphthone, while the formation of 4 is a parallel reaction competing with the formation of 2 ,…”

“…Up to 10% of 4 are produced in reactions showing 100% conversion. As earlier seen for OD Pt, under reflux a high selectivity for 2 is only observed in MeNO 2 , and in that solvent 3 is not found over Pt and Pd. In contrast, VD Ag shows an unexplained, but reproducibly high activity in refluxing MeNO 2 , and 3 is the main product.…”

“…At elevated temperature (reflux) the naphthoquinone 4 is observed as a side product (≤10%). The binaphthol is the intermediate to the binaphthone, while the formation of 4 is a parallel reaction competing with the formation of 2 ,. The only other byproduct is water.…”

“…The binaphthol is the intermediate to the binaphthone, while the formation of 4 is a parallel reaction competing with the formation of 2. [5,24] The only other byproduct is water. Depending on the solvent used, the catalyst productivity (CCC) increases in the order MeOH < MeNO 2 < MeCN (r.t.).…”

“…The binaphthol is regarded as the intermediate to the binaphthone, while the formation of 4 is a parallel reaction competing with the formation of 2. [5,24] Partial conversions of 1 are obtained at r.t., while total conversions are obtained under reflux, except for VD Ag in MeOH (Table 4, entry 6). The isolated yields are plotted in Figure 6.…”