Synthesis, conformational stability and molecular structure of 4-aryl- and 4,5-diaryl-1,8-bis(dimethylamino)naphthalenes.

4-Bromo- and 4,5-dibromo-1,8-bis(dimethylamino)naphthalenes were arylated with arylboronic acids under Suzuki reaction conditions to provide 4-aryl- and 4,5-diaryl-1,8-bis(dimethylamino)naphthalenes, respectively. The interaction of 4,5-dibromo-1,8-bis(dimethylamino)naphthalene with pyridin-3-ylboronic acid was accompanied by heterocyclization leading unexpectedly to the formation of ,,,-tetramethylacenaphtho[1,2-]pyridine-3,4-diamine. Dynamic H NMR experiments showed fast interconversion between and conformers of 4,5-diaryl-1,8-bis(dimethylamino)naphthalenes in CDCl solution at room temperature. The free energy of the rotational isomerization was determined to be ∼14.0 kcal mol for 4,5-di(-tolyl) and 4,5-di(naphthalen-2-yl) derivatives. X-ray analysis of 4,5-diaryl-1,8-bis(dimethylamino)naphthalenes revealed a high degree of structural deformation due to internal steric repulsions between both -dimethylamino and -aryl groups. In crystals, 4,5-di(naphthalen-1-yl)-1,8-bis(dimethylamino)naphthalene molecules exist exclusively in the most stable form, while for 4,5-di(naphthalen-2-yl) and 4,5-di(-tolyl) counterparts, only the -form is realized. The introduction of two -aryl substituents in the 1,8-bis(dimethylamino)naphthalene scaffold affected the basic properties, making the 4,5-diphenyl derivative 0.7 p units less basic. The protonation of 4,5-diaryl-1,8-bis(dimethylamino)naphthalenes leads to dramatic changes in their structures. Compared to the corresponding bases, the inter-nitrogen distance in these salts noticeably decreases whereas -aromatic rings move away from each other demonstrating the so-called "clothespin effect". This lowers the barriers of /-isomerization; as a result, protonated molecules with -tolyl and even -(naphthalen-2-yl) substituents exist in crystals as mixtures of rotamers.