Conformations of 1,3,3,5,7,7-Hexamethyl-1,5-diazacyclooctane and Its Bis-BH3 Adduct. Mono- and Bis-BH3 Adducts of Di-Tertiary Amines

A variable-temperature 1H- and 13C-NMR study revealed a conformational equilibrium for 1,3,3,5,7,7-hexamethyl-1,5-diazacyclooctane (4) having ΔG ⧧ = 8.8 ± 0.6 kcal/mol at 184 K. This activation barrier connects a major and a minor form of 4. Molecular mechanics calculations on 4 led to the conclusion that the major form is a set of twist-chair-chairs interconverting rapidly via the chair-chair and that the minor form is most likely a set of twist-boat-boats interconverting rapidly via the boat-boat. The proximity of the two nitrogen lone pairs in the major form of 4 made plausible the expectation that 4, as well as a related diamine with apposed nitrogens, 3,7-dimethyl-3,7-diazabicyclo[3.3.1]nonane (3), might bind a Lewis acid, namely BH3, using both lone pairs simultaneously and equally. This proved not to be the case: for 3 only the bis-BH3 adduct was found and for 4 the mono-BH3 adduct utilized only one nitrogen lone pair. The structure of the bis-BH3 adduct of 4 (12) was determined by X-ray crystallography to be a twist-boat-boat with BH3s cis. Molecular mechanics calculations on 12 were consistent with the solid state conformation found.