CONFORMATIONS OF 1,3,3,5,7,7-HEXAMETHYL-1,5-DIAZACYCLOOCTANE AND ITS BIS-BH3 ADDUCT - MONO-BH3 AND BIS-BH3 ADDUCTS OF DI-TERTIARY AMINES

A variable-temperature H-1- and C-13-NMR study revealed a conformation al equilibrium for 1,3,3,5,7,7-hexamethyl-1,5-diazacyclooctane (4) hav ing Delta G double dagger = 8.8 +/- 0.6 kcal/mol at 184 K. This activa tion barrier connects a major and a minor form of 4. Molecular mechani cs calculations on 4 led to the conclusion that the major form is a se t of twist-chair-chairs interconverting rapidly via the chair-chair an d that the minor form is most likely a set of twist-boat-boats interco nverting 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 mon o-BH3 adduct utilized only one nitrogen lone pair. The structure of th e bis-BH3 adduct of 4 (12) was determined by X-ray crystallography to be a twist-boat-boat with BH(3)s cis. Molecular mechanics calculations on 12 were consistent with the solid state conformation found.