Partially Hydrogenated [2.2]Paracyclophanes as Precursors in Polycyclic Hydrocarbon Chemistry

Abstract

The chemical behavior of the tetrahydro- and dihydro[2.2]paracyclophanes, 3 and 4, respectively, was investigated. In particular, 3 was subjected to carbene addition (methylenation with triethylaluminum/diiodomethane, dichloro- and dibromocarbene addition) and epoxidation with m-chloroperbenzoic acid. The resulting mono adducts 5a, 5b, and 5d were characterized by spectroscopic methods and by chemical transformations to 6, 7, and 10. Reaction of 3 with dehydrobenzene (8) furnished the ene-product 9. Similarly, the bis adducts 14a−c were formed, each in good to excellent yield, from the bis-olefin 4. When 14c was ring-opened by treatment with trifluoroacetic acid, the chiral diol 16 was produced, which was then oxidized to the diketone 18 and brominated to give the dibromide 19. When the latter was treated with potassium tert-butoxide in tetrahydrofuran, it produced the bridged benzobarrelene 21, the smallest and most highly strained representative of this class of hydrocarbons. On pyrolysis, 21 underwent a retro-Diels−Alder reaction to pyracene (20) and ethylene. The derivatives 14b and 16, which show polymorphism and crystallized in two macroscopically distinguishable crystal forms 16-I and 16-II, together with compounds 19 and 21 were amenable to X-ray crystal structure analysis. The structural properties of these compounds are discussed in detail.