The Dehydrocyclization of Alkylnaphthalenes

The rate of dehydrocyclization increase with the number of aromatic rings in the molecule (19). The dehydrocyclization of alkylnaphthalenes can follow the same pathways as the cyclization of alkylbenzenes C5-dehydro-cyclization gives benzindans and benzindenes, while C6-dehydrocyclization yields anthracenes and phenanthrenes. In addition to these two pathways, a-substituted alkylnaphthalenes can cyclize to acenaphthenes and acenaphthylenes  

This reaction was first observed by Plate, Erivanskaya, and Khalima-Mansur over platinum-on-carbon and platinum-on-alumina catalysts (43-48). Platinum-on-carbon catalyzes this reaction between 310°C and 390°C (above which the catalyst is poisoned) (44). Over an acidic platinum-alumina catalyst containing 0.5 wt% platinum and 0.1 wt% sodium, 16.7% acenaphthenes and 1.5% acenaphthylene are obtained at 460°C and at 0.4 liquid hourly space velocity in hydrogen diluent. Conversions are considerably lower in helium. 

Kinetic studies at short residence times first suggested the following reaction sequence ethylnaphthalene dehydrogenates to vinylnaphthalene vinylnaphthalene dehydrocyclizes to acenaphthylene and finally acenaphthylene is hydrogenated to acenaphthene. However, further work by Isagulyants and co-workers, using 14C-labeled 1-vinylnaphthylene, shows that over platinum on alumina at 470°C, acenaphthene and acenaphthylene are formed from both 1-ethylnaphthalene and 1-vinylnaphthalene. Vinylnaphthalene dehydrocyclizes about three times faster than ethylnaphthalene. The vinylnaphthalene intermediate remains adsorbed on the catalyst surface during the reaction (48). 

1-Propylnaphthalene can give two types of products by C5-dehydrocycli-zation dehydrocyclization at the pen-carbon atom of naphthalene gives 1-methylacenaphthene and 1 -methyl-acenaphthylene, while dehydrocyclization involving the /1-carbon atom of naphthalene gives 4,5-benzindan and 4,5-benzindene  

Hydrogen has an important directing effect here. In the presence of hydrogen over 0.5 wt% platinum on y-alumina, Erivanskaya and co-workers found that the rate of pen-dehydrocyclization of 1-propenylnaphthalene is about four times faster than the rate of /f-C5-dehydrocyclization (49). However, if the catalyst, after the usual reduction, is treated with helium for three hours and the experiment is also carried out in helium, the rate of peri-dehydrocyclization decreases by about a factor of seven. The rate of /f-dehydrocyclization does not change significantly. The rates for both types of C5-dehydrocyclization increase with the acidity of the alumina (50, 51). 

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