Oxidative Coupling of 1‐Naphthols over Noble and Base Metal Catalysts

Abstract

Bismuth‐promoted platinum catalysts were tested for the oxidative coupling of 2‐ and 4‐substituted 1‐naphthols at different temperatures and ambient pressure. The principal final products are the 3,3′‐substituted 1,1′‐binaphthalenylidene‐4,4′‐diones and the 4,4′‐substituted 2,2′‐binaphthalenylidene‐1,1′‐diones, respectively. Hydrogen peroxide was used as the oxidant. Only naphthols with electron‐donating substituents reacted. The corresponding binaphthalenyl diols can be considered as reaction intermediates. Yields of up to 99 % were obtained from 2‐methyl‐1‐naphthol as the starting material within 20 minutes. Probably for steric reasons, the diol is the final product obtained from 2‐ethyl‐1‐naphthol. For 4‐methoxy‐1‐naphthol the outcome is determined by the reaction temperature. At 25 °C the expected 1,1′‐dione is the major product, whereas at 60 °C 1′‐hydroxy‐4′‐methoxy‐2,2′‐binaphthalenyl‐1,4‐dione is formed; the loss of one methoxy unit and the preservation of the hydroxy group can be explained by the competitive cleavage of one of the two OMe bonds at higher temperature. Unpromoted platinum and a range of other metallic catalysts, including gold and Raney nickel, were also found to be active. The products obtained are brightly colored solids that could be used as dyes. The method described is truly catalytic and environmentally benign. The potential of the technique justifies further research to expand on the applicability of this novel method.

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 ,…”

“…The CCCs are calculated based on our suggested reaction mechanism . One catalytic cycle is required for each, the formation of 2 and 4 , while two catalytic cycles are required for the formation of 3 .…”

“…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.…”

“…Binaphthols are also used as chirality‐inducing ligands . The oxidative cross‐coupling of naphthols, such as 1‐naphthol, 2‐methyl‐1‐naphthol, 1,3‐, 2,7‐ and 1,7‐dihydroxynaphthalene is the principle of hair dyeing . As a test molecule 2‐methyl‐1‐naphthol ( 1 , Scheme ) is of particular interest: apart from the coupled products ( 2 and 3 ), the direct oxidation product itself (2‐methyl‐1,4‐naphthoquinone, 4 ) is of great commercial importance; compound 4 is also known as menadione or vitamin K 3 .…”

“…The addition of 5% bismuth to 5%Pt/AC causes a significant improvement of the Pt dispersion, resulting in a remarkable increase in turn‐over frequency (TOF) . This novel system for naphthol coupling requires metallic‐state catalysts . Interestingly, over Au/O 2 systems and in non‐catalyzed reactions with molecular O 2 the selectivity switches from the formation of 2 and 3 (oxidative coupling) to the formation of 4 (oxidation) …”