Chemical Effects of Steric Strains. 24. 13C NMR Study of the Interaction of 9-Borabicyclononane with Amines of Increasing Steric Requirements

Article abstract of DOI:10.1021/jo01298a003

The 13C NMR spectra of the interaction of 9-borabicyclo<3.3.1>nonane (9-BBN) with two series of amines, involving regularly increasing steric requirements, were used to study the role of steric strains as a factor in the stability of addition compounds formed and their exchange with the amine.A set of pyridine bases, including pyridine and 2-methyl-, 2-ethyl-, 2-isopropyl-, and 2-tert-butylpyridines, and a set of aliphatic amines, including n-propylamine, isopropylamine, diethylamine, diisopropylamine, and triethylamine, with increasing steric requirements were selected for examination.Quinuclidine was also selected, as a base of relatively low steric requirements, for comparison with triethylamine, a base with very large steric requirements.The results reveal four types of behavior: formation of stable complexes with no observable exchange with excess amine; formation of stable complexes with rapid exchange of amine; formation of partially dissociated complexes with rapid exchange; and no detectable interaction of 9-BBN and amines of large steric requirements.In general, there is a regular progression along these four types of behavior with increasing steric requirements in both series of amines.Thus, triethylamine fails to show any interaction with 9-BBN, whereas quinuclidine forms a stable adduct which does not exchange with excess amine. 13C NMR provides a valuable tool for exploring the role of such steric effects in the formation and stability of molecular addition compounds.