The Determination of Double-bond Character in Cyclic Systems. IV. Tetrahydronaphthalene. Steric Facilitation of Chelation¹

Abstract

Infrared spectral measurements have led to the conclusion that there is no sigtiificatit Mills-Sixon bond fixation in the aromatic ring of tetrahydronaphthalene. Z- 6,'i, and the corresponding methyl ketone and methyl ester are more strongly chelated than their 3,2-isomers, as shown by determinatioii of Av(C=O) values (which represent the lowering of the frequency of the C=O absorption hand observed in going from the noli-hydroxylated to the hydroxylated compounds). The average S Y ( C=O) values for the 3,2-disubstituted tetrabydroiiaphthalenes tire virtually identical with those determined earlier for the analogous o-disubstituted benzenes, the 5,6-disubstituted indans and the 4,5-disubstituted o-xylenes, showing that the ring bonds (bond B ) between the chelated groups all have 50% doublebond character. The larger S v ( c = O ) values for the 2,l-disubstituted tetral~~dronaphthalenes, the 5,+disubstituted indans and the 4,3-disubstituted o-xylenes are explained by assuming that the bulk of the adjacent inethylene groups (ill tetrahydronaphthalene and indan) or methyl groups (in o-xylene) pushes tlie carbonyl oxygen atom closer to the hydroxyl group than is the case with the first group of compounds. This effect (steric facilitation of chelation) affords a more satisfactory explanation of the higher Av(C=O) values than would he obtained by assuming that the ring hoiids (bond 4) bctween the chelated groups have greater than 50% double-bond character. Tlie concept of steric facilitation of chelation is shown to permit a refinement of the earlier interpretation of the Sv(C=O) values for naphthalene and phenanthrene. The steric effects of methyl groups and of alicyclic methylene groups are compared. N-Methylformanilide reacts with 2-~netli-oxy-5,6,7,8-tetrahydronaphthalene to produce tlie synimetrical methoxyaldeliyde. Depending on conditions, the haloforin reaction on 3-methoxy-5,6,7,8-tetrahydro-2-acetonaphthone produces either ~-chloro-3-meth0x~-~,6,7,Y-tetral1ydro-~-naphthoic acid or methyl 3-methoxy-5,6,7,8-tetrahydro-~-naplithoate. Syntheses of certain known t e t r n l i~d r o t i a p l~t l~a l~~~e s are improved, and proofs of structure are offered which are independent of earlier work. IntroductionEarlier work3 in these laboratories has shown that the strength of an intramolecular hydrogen bond in an aromatic system such as I varies with the amount of double-bond character exhibited by the ring bond between the carbon atoms bearing the chelated substituents. The strength of such a hydrogen bond is measured by Av(C=O), defined as the shift to lower frequency exhibited by the C=O absorption band (in the infrared) in going from 11 to the corresponding I. Thus, Av(C=O) has a characteristic value for each carbon-to-carbon bond in an aromatic ring. A linear relationship has been observed between Av(C=O) and Paul-

peri Interaction in Naphthalene Derivatives

“…More precise studies of hydrogen bonding in perisubstituted naphthalenes have been made utilizing infrared methods. The compounds studied include hydroxycarbonyl compounds (62,282,286,288), polyhydroxynaphthalenes (100,282,334), nitronaphthols (392), and nitronaphthylamines (111,178,251,260). In hydroxycarbonyl compounds, the infrared stretching frequencies of both hydroxyl and carbonyl groups are lowered if they participate in internal hydrogen bonding.…”

“…The corresponding figure for CCL-XXX -* CCLXXXII is only 54 cm-1. This is attributed to a buttressing of the 1-carbonyl group in CCLXXXI by pen-hydrogen leading to a stronger hydrogen bond; this effect is operative in CCLXXXII also but to a lesser extent, since in the former cases the bulkier -C(Y)=0 suffers buttressing, and in the latter only the hydroxyl group is buttressed (286,288). Similar findings have been made on orfAo-substituted 5,6,7,8-tetrahydro-l-and -2-naphthols (288).…”

“…This is attributed to a buttressing of the 1-carbonyl group in CCLXXXI by pen-hydrogen leading to a stronger hydrogen bond; this effect is operative in CCLXXXII also but to a lesser extent, since in the former cases the bulkier -C(Y)=0 suffers buttressing, and in the latter only the hydroxyl group is buttressed (286,288). Similar findings have been made on orfAo-substituted 5,6,7,8-tetrahydro-l-and -2-naphthols (288). From infrared studies, 5,6,7,8-tetrahydro-l-acetyl-2-naphthol CCLXXXIII CCLXXXIV is said to exist mostly in the chelated structure CCL-XXXIII (288).…”