2-Phenylpentane cyclization

Phenyl group, as hydrocarbon surface species, vibrational spectra, 42 228 Phenylindanyl cation NMR spectra, 42 146 Phenylnaphthalene, cyclization, 28 318 2-Phenylpentane cyclization, 28 299 isomerization, 30 67-68 reactions on different catalysts, 28 296,301 Phillips catalyst, 31 29... 

The primary products of the cyclization of 2-phenylpentane are cis- and /ra j-l,3-dimethylindans, 1,3-dimethylindene, and 1-methylnaphthalene ... 

In the cases of -butylbenzene and 2-phenylpentane, there is an additional difference (other than ring size) between cyclization to naphthalenes and alkylindans. The reaction forming naphthalenes involves the addition of a primary carbon atom to the aromatic ring, while formation of alkylindans (and alkylindenes) involves secondary carbon atoms. With n-pentylbenzene, however, secondary carbon atoms are involved in both five- and six-ring cyclizations. 

First-order cyclization rate constants n-Butylbenzene n-Pentylbenzene 2-Phenylpentane... 

The methyl group in the y-position of the side-chain interferes with cyclization. The rate of C5-cyclization of n-butylbenzene at 371°C over platinum-on-silica gel is 3.5 times higher than that of 2-phenylpentane (Table IV) (14). The difference in C6-cyclization is even larger. Now, the side-chain carbon atoms involved in the cyclizations of n-butylbenzene and 2-phenylpentane have identical natures (i.e., secondary in five-membered ring closure and primary in six-membered ring closure). The difference between the two molecules (the extra methyl group) is far removed from the two carbon atoms involved in the formation of the new bond ... 

The stability of the intermediate carbonium ion determines whether cyclization forms five- or six-membered rings. In the case of n-butylbenzene and 2-phenylpentane, acid-catalyzed six-ring cyclization would involve very unstable primary carbonium ions C6H5-CH2-CH2-CH2-CH2+ or C6H5-CH(CH3)-CH2-CH2-CH2 +. Therefore, only five-membered rings could be formed in acid-catalyzed cyclization from these two hydrocarbons. 

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