Attempts to find a solution to the problem of atropisomer interconversion in 1,8-diarylnaphthalenes and 5,6-diarylacenaphthenes

Abstract

A series of sterically restricted 5,6-diarylacenaphthenes 5, 11, 12, 13 and 14 have been prepared via Suzuki cross-couplings of the appropriate boronic acids with 5,6-dibromoacenaphthene 3 in an attempt to prevent atropisomer interconversion in these systems. Attempts to further functionalise bis(p-methoxyphenyl) system 5 in the position ortho to the methyl ethers by Friedel–Crafts acylation or metallation were unsuccessful; however, two unexpected products were obtained. p,p′-Dimethoxybiphenyl 6 results from an unexpected rearrangement of 5 under strongly basic conditions and is dependent on the base used, whilst acylated derivative 7 results from a Friedel–Crafts acylation of the acenaphthene scaffold in the 3-position, rather than the desired functionalisation of the peri-aryl rings, presumably due to the difficulty in forming a tetrahedral intermediate. The oxygen functionality in 5 has been used, following methyl ether cleavage via diphenol 8 and allylation via9, to demonstrate the viability of a double Claisen rearrangement yielding 11 after acetylation. However, the broad ¹H NMR exhibited by 11 clearly showed that this system is not configurationally stable, hence steps were required to access more sterically demanding systems which would be configurationally stable. Molecular mechanics and semi-empirical simulations were carried out on related biaryl systems to determine if a single bulky substituent in the 3-position of the peri-aryl rings would be sufficient to prevent atropisomer interconversion. The modelling showed that the energies of the syn- and anti-atropisomeric forms, e.g. for 12–14, were surprisingly similar. With the objective of preparing conformationally stable molecules in this class in mind, 12–14 were prepared in remarkable yield for such a hindered system. In spite of extensive attempts to determine whether 13 was configurationally stable, enantiomeric separation could not be achieved. Unsuccessful attempts were thus made to detect the presence of stable atropisomeric forms of 13 through the synthesis of bis(benzyl ether) 19, in which the benzylic protons could act as enantiotopic reporters. In addition mandelate ester 20 was prepared and it was shown by ¹H NMR that a mixture of anti- and syn-diastereoisomers had been obtained. It was therefore concluded that steric groups in the 3-position of the peri-aryl rings cannot be used to prevent atropisomer interconversion in 1,8-diarylnaphthalenes and 5,6-diarylacenaphthenes. During attempts to access diphenols 18 and 24, other by-products were isolated, i.e.21 and 25 respectively, resulting from a steric strain-induced 1,2-aryl shift.