Synthesis and stereoisomerism of [n]cyclo-2,9-phenanthrenylene congeners possessing alternating E/Z- and R/S-biaryl linkages

Abstract

The synthesis and cyclostereoisomerism of [n]cyclo-2,9-phenanthrenylenes ([n]CPhen_2,9, n = 4, 6 and 8), possessing hybrid E/Z- and R/S-biaryl linkages, were elaborated. The dimer of a phenanthrene derivative was used as a starting material and underwent Ni-mediated Yamamoto-type coupling to afford [6]CPhen_2,9 as a major cyclic product, as well as [4]CPhen_2,9 and [8]CPhen_2,9 as minor products. The stereoisomers of [n]CPhen_2,9 were isolated and characterized, and the number of stereoisomers indicated that E/Z linkages did not provide any experimentally separable isomers, whereas the chirality in [n]CPhen_2,9 originated from the intrinsic axial chirality at constrained R/S linkages. Theoretical calculations predicted that the 2,2′-linkages in [n]CPhen_2,9 adopted a fixed Z- or E-configuration, which suggested a novel type of dynamics of atropisomerism in contrast to the reported rigid or flexible behavior. This study enriches our understanding of the stereochemical features of E/Z linkages in aromatic macrocycles.