The Chemistry of cycloalkene fused naphthalenes and anthracenes

A general two step synthesis of fused, ring naphthalenes is described. This synthetic route includes the Diels-Alder addition of benzyne to a 1,2-dimethylenecycloalkane or a 1-vinylcycloalkene, followed by aromatization utilizing 2,3-di-chloro-5,6-dicyano-l,4—benzoquinone (DDQ). This Diels-Alder approach is used to propare both isomers of the four-, five-, and six-membered fused ring naphthalenes. The Diels-Alder approach is also adopted to give the first syntheses of anthro[a]cyclobutene and anthro[b]cyclobutene, which represent the smallest fused ring anthracenes known to date. The chemistry of 1,4—dihydronaphtho[b]cyclobutene(52) has been explored. It is found that the cyclobutene carbon-carbon double bond of 52 is reactive toward cycloaddition as well as oxidation. The four-membered ring of 52 is found to undergo ring opening to give 1,2,3,4-tetrahydro-2,3-dimethylenenaphthalene (94), which also undergoes cycloaddition reactions. The apparent oxidation rate difference observed between 1,4-dihydronaphtho[b]cyclobutene and 1,4-dihydronaphtho[a]cyclobutene with DDQ let to an indepth kinetic study. For this kinetic study, a series of 1,4—dihydrobenzenes and naphthalenes has been prepared. The rate of oxidation of these dihydroaromatic compounds has been measured by following the disappearance of the uv absorption of DDQ at 3900 A, under pseudo-first-order conditions. The pseudo-first-order rate constants are calcuclated, as well as the second-order rate constants, and relative rate constants. From the relative rate constants of the 1dihydrobenzene series, it is observed that a definite relationship exists between the number of electron donating groups attached to the 1,4-dihydroaromatic nucleus and the relative rate of oxidation. This relationship is interpreted as evidence for a carbonium ion intermediate. Using the new evidence for a carbonium ion intermediate, the existing proposed mechanisms for DDQ oxidation are reexamined, and a new mechanism proposed. The evidence for a carbonium ion intermediate, as well as other rate data, led us to propose a two step mechanism. This mechanism involves: 1) abstraction of a hydride from the 1dihydroaromatic nucleus by DDQ followed by formation of a tight ion pair, and 2) loss of a proton resulting in formation of the aromatic compound. This mechanism is substantiated by the rate data obtained from the oxidation of the 1,4—dihydronaphthalene series.