During an exploratory study of acid-catalyzed reactions with model compounds relevant to coal-oil coprocessing, we observed that triflic acid induces the isomerization of 5,6,7,8-tetrahydro-l-naphthol to 5,6,7,8-tetrahydro-2-naphthol.

Surprisingly, this isomerization is accelerated dramatically by the presence of small amounts of polycyclic aromatic hydrocarbons or coal. The present paper presents our results and mechanistic rationalization of this strong-acid-catalyzed isomerization reaction.The present paper presents our results and mechanistic rationalization of this strong-acid-catalyzed isomerization reaction. Inthe course of our studies on the transalkylation reactions catalyzed by triflic acid, we found that reaction of 5,6,7,8-tetrahydro-l-naphthol with polycyclic alkylaromatics is accompanied by a reversible isomerization of 1 to 5,6,7,8-tetrahydro-2-naphthol. Under mild conditions, the interconversion of 1 and 2 is the only reaction observed. The equilibrium mixture obtained from either 1 or 2 contains roughly equal amounts of the two isomers. The isomerization reaction proceeds cleanly when either 1 or 2 is treated with triflic acid in refluxing cyclohexane. The rate of isomerization increases with the acid concentration.

As established for closely related cases, the reaction mechanism most likely involves ring carbon atom protonation,1,2-shift to a hydroxyspiroarenium ion intermediate, and another shift to the isomerized product. The mechanism illustrated is supported by literature data concerning preference for ring protonation over oxygen protonation in phenols and phenol ethers and by solvolysis studies conducted on arenes~lfonates~ in which spiroarenium compounds are formed as intermediates[1-3].

5,6,7,8-Tetrahydro-1-naphthol is a useful chemical intermediate, widely applied in organic synthesis and also used as a fuel additive. 5,6,7,8-Tetrahydro-1-naphthol can be produced through partial reduction of aromatic ring of 1-naphthol by various homogeneous or heterogeneous catalytic hydrogenation methods.It has been reported that regioselective hydrogenation of 1-naphthol over supported Pt and Pd catalysts for production of high-temperature jet fuel stabilizer. By selecting appropriate support and noble-metal species, catalytic hydrogenation of 1-naphthol can be tailored to produce more phenolic or aromatic ring hydrogenation product, namely 1,2,3,4-Tetrahydro-1-naphthol or 5,6,7,8-Tetrahydro-1-naphthol. Bimetallic catalyst with Pt/Pd weight ratio of 3 showed the highest yield of 5,6,7,8-Tetrahydro-1-naphthol of 51.8% at 150 C and 500 psi. reported a method for reduction of aromatic rings using a Raney Ni-Al alloy in dilute aqueous alkaline solution under mild condition.In this method, Ni-Al alloy is actually a reactant, reacting with dilute alkaline to produce hydrogen in situ for subsequent reduction of 1-naphthol.

In comparison with catalytic reaction over metal catalyst, this method consumes a larger amount of Ni-Al alloy per product molecule, restricting its application for industrial-scale production.reduction of 1-naphthol in dilute aqueous KOH using Ni-Al alloy gave 1-tetralone and 1,2,3,4-Tetrahydro-1-naphthol as major products, besides formation of minor 5,6,7,8-Tetrahydro-1-naphthol.reported hydrogenation of 1-naphthol over Ru catalysts in supercritical carbon dioxide solvent with 5,6,7,8-Tetrahydro-1-naphthol yield of 31% at 323 K and 130 bar. This process uses environmentally benign carbon dioxide as solvent, eliminating use of organic solvents and simplifying product separation.

However, the high equipment cost associated with high-pressure operation represents a drawback.In sum, previous work on synthesis of 5,6,7,8-Tetrahydro-1-naphthol suffers from several drawbacks such as high cost of noble-metal (Pt, Pd, and Ru) catalysts, low catalyst efficiency, and high equipment investment. Thus, efficient means to produce 5,6,7,8-Tetrahydro-1-naphthol are highly desired. We report herein selective hydrogenation of 1-naphthol over several Ni--and Pd-based catalysts such as supported Ni/Al2O3, Pd/C, and NiB/USY catalysts[4-6].

References

1.Dewar, M. J. S. In MoZecular Rearrangements, Part 1; de Mayo,P., Ed.; Wiley-Interscience: New York, 1963; Chapter 3, pp 302-303.

2.Birchall, T.; Bourns, A. N.; Gillespie, R. J.; Smith, P. J. Can. J.Chem. 1964,42, 1433.

3.Heck, R.; Winstein, S. J. Am. Chem. SOC. 1957, 79, 3105.

4.T. Tsukinoki, T. Kanda, G.B. Liu, H. Tsuzuki, M. Tashiro, Tetrahedron Lett. 41, 5865 (2000).

5.J.L. Zhang, Y. Wang, Chin. J. Org. Chem. 28, 723 (2008).

6.M. Shirai, C.V. Rode, E. Mine, A. Sasaki, O. Sato, N. Hiyoshi, Catal. Today 115, 248 (2006).