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

Abstract

A variable-temperature (1)H- and (13)C-NMR study revealed a conformational equilibrium for 1,3,3,5,7,7-hexamethyl-1,5-diazacyclooctane (4) having DeltaG() = 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 BH(3), using both lone pairs simultaneously and equally. This proved not to be the case: for 3 only the bis-BH(3) adduct was found and for 4 the mono-BH(3) adduct utilized only one nitrogen lone pair. The structure of the bis-BH(3) 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.

Facile Preparation of 1,5-Diazacyclooctanes from Unsaturated Imines: Effects of the Hydroxyl Groups on [4+4] Dimerization

“…Because the reduced 1,5-diazacyclooctanes did not include diastereomeric sp 3 carbon centers, these two products were considered to be conformational isomers of the eight-membered ring. 8 Interestingly, the conformational isomers could be selectively prepared by using one reducing reagent over the other, and the isomers were sufficiently stable on silica gel to allow for detection.…”