Naphthalenes, alkyl-substituted, hydrogenation

In the hydrogenation of 1-alkyl-substituted naphthalenes over Pd-C, the proportion of hydrogenation of the substituted ring increases with increasing bulkiness of the 1-alkyl groups, as shown in eq. 11.67.238 The effect of the substituents is most pronounced in 1-Z-butylnaphthalene, where the ring with f-butyl was hydrogenated... 

TH. J. Nieuwstad, P. Klapwijk, H. Van Bekkum. Hydrogenation of Alkyl-substituted Naphthalenes over Palladium. J. Catal. 29 404-411, 1973... 

AU of the results of substitution of various groups for hydrogen on benzene (CeH ) outhned above also apply to alkyl-substituted benzenes. They apply as well to other aromatic systems. In both cases, the symmetry of benzene (CeHe) has been perturbed and thus it is common for more than one product to be obtained. Consider the case of naphthalene (CioHs), the first of the class of polynuclear aromatic hydrocarbons. A samphng of some of the reactions of naphthalene (CioHg) is provided in Table 6.13 and, when comparing the reactions to those of Table 6.12, it is important to note that it is common to find simultaneous formation of more than one substitution product. 

Substitution. Substitution products retain the same nuclear configuration as naphthalene. They are formed by the substitution of one or more hydrogen atoms with other functional groups. Substituted naphthalenes of commercial importance have been obtained by sulfonation, sulfonation and alkah fusion, alkylation, nitration and reduction, and chlorination. 

Nieuwstad, Klapwijk, and van Bekkum (105) have added to the knowledge of aromatic hydrogenation by their study of the influence of alkyl substituents in the 1 and 2 positions of naphthalene on the rate. Tetrahydro-naphthalenes were the products of hydrogenation over palladium at 80°C. The selectivity of the reaction was also followed and expressed as the ratio of the rate constants for the saturation of the unsubstituted and substituted rings, respectively. Steric effects play an important role, and, beside steric hindrance by the bulky substituents, steric acceleration also has been observed, the latter being caused by a release of the strain between the 1-alkyl group and hydrogen in position 8. 

In one of the more frequently utilized Birch reactions (the reduction of alkyl/alkoxy-substituted naphthalenes), two reduction products are obtained as shown in Scheme 7.10. The acidity of the alcohol employed for protonation determines the ratio of these two products. For example, the ratio of the product hydrogenated in the substituted fused ring to the product hydrogenated in the unsubstituted fused ring was compared for methanol... 

An important case is the oxidation of alkenes or alkyl arenes containing at least one hydrogen in an a-position [70-77]. The reaction sequence that leads to a-substitution [Eq. (29)] for alkylbenzenes and -naphthalenes most likely includes Eqs. (1), (3), (7), and (11), whereas for alkylanthracenes there is evidence that substitution may take place as an addition-elimination process (see Sec. IV for details). Typical examples are the oxidation of cyclohexene to 3-acetylcyclohexene in AcOH [Eq. (30)] [76], the oxidation of 1,3,5-cycloheptatriene to 7-methoxy-l,3,5-cycloheptatriene in MeOH [74], and the two-... 

Partial hydrogenation of substituted naphthalene 1 finely divided Ni yields the phenyl ether 2 under slightly acidic conditions [equation (c)] and yields the alkyl ether 3 in neutral solvent [equation (d)] ... 

---