The synthesis and structure of 5,6,6aα,6bβ,7,8,12bβ,12cα-octahydrodibenzo[a,i]biphenylene, the major photodimer from 1,2-dihydronaphthalene

Abstract

The photodimer 5,6,6aa,6bfl,7,8,12bfl, 12ca-octahydrodibenzo[a,i]biphenylene is obtained as the major product from the benzophenone-sensitized photolysis of 1,2-dihydronaphthalene. The preparation, purification, and spectral data of the compound are described. Crystals of the compound are orthorhombic, space group Pbca. The cell dimensions are: a = 14.638 (5), b = 11.09 (1), c = 17.536 (8) A (at -160°C).The structure was determined by direct methods and refined with least-squares calculations based on 2704 data collected at -160°C. The final R value is 0.047. The molecule has approximately twofold symmetry. The central cyclobutane ring is planar and has one elongated bond of 1.579 (2) A.

Photoadditions of aromatic compounds

“…The sensitized dimerization of indene is typical,101'102 the product (16) having headto-head regiochemistry and anti-trans stereochemistry.102 Similarly, 1,2-dihydronaphthalene affords a dimer with the same regiochemistry and stereochemistry as 16 on sensitized dimerization. 103 The structure and stereochemistry of dimer 16 were determined by conversion to ketone 17, followed by Baeyer-Villiger oxidation to the known dimer 18 of coumarin. 102 The latter is formed on sensitized dimerization of coumarin,104 and since the Baeyer-Villiger oxidation is stereospecific, the conversion of 16 to 18 shows that both dimers have the same geometry.…”

“…The 1,3 or meta addition is only observed with benzene. 182 The examples given below (89)(90)(91)(92)(93)(94)(95)(96)(97)(98)(99)(100)(101)(102)(103)(104) are additions occur in the substituted naphthalene ring and are almost exclusively 1,4. Structure 88b is based on X-ray crystal structure analysis,173 and the observed regiochemistry apparently rules out significant biradical or zwitterionic character in the transition state for addition.…”