Synthesis and Stereochemical Properties of “Extended” Biphenols Bridged by ortho ‐, meta ‐, and para ‐Phenylene Spacers

A series of isomeric biphenols based on para ‐ ( 1 ), meta ‐ ( 2 ), and ortho ‐ ( 3 ) terphenyl backbones was synthesized. Suzuki cross‐coupling methodology was employed for the construction of the terphenyl backbone of their methyl‐protected precursors (respectively 8 , 13 , and 16 ). Using K 2 CO 3 as the base, the best reaction conditions involved DMF at 100 °C as solvent. Anhydrous conditions greatly improved the yields of the sterically crowded systems (particularly 16 ). ortho ‐Terphenyls 3 and 16 exist as mixtures of syn / anti atropisomers in solution. Compound 16 crystallizes in the anti‐in form. DFT calculations at the B3LYP/6‐311+G(d,p) level indicate that for both compounds the anti‐in form is more stable than the syn form by 6.07 and 2.07 kJ mol –1 for 16 and 3 , respectively. These ΔΔ G 0 values are higher by the same factor (of two) than those obtained in solution at 240 K (3.17 and 0.93 kJ mol –1 ). The atropisomeric properties of 16 and 3 are finally discussed in the context of ortho ‐terphenyls in the literature. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)