Mixed boron trihalide adducts of amines: a multinuclear nuclear magnetic resonance study

Abstract

The ¹⁹F chemical shifts of mixed boron trihalide adducts of tertiary amines have a marked dependence on the steric effects of amine substituents, as estimated from the ‘cone angle’ of the amines. Base strength of the amine has little effect on adduct ¹⁹F shifts, but does have a pronounced effect on the rate of halogen redistribution. The ¹¹B and ¹³C chemical shifts and ¹¹B–¹⁹F coupling constants of the adducts are discussed. Amines NRR′R″ become chiral when complexed to Lewis acids, since complexation stops the rapid inversion process. Because of this the fluorines in R″R′RN·BF₂X (X = Cl, Br, or I) adducts are diastereotopic and magnetically non-equivalent, differing in chemical shift by up to 2 p.p.m. The four-adduct system of benzyl(ethyl)methylamine with BF_nI_3–n(n = 0–3) gives rise to the cation B(PhCH₂NMeEt)₂F₂⁺ which, because of the chirality of the co-ordinated amines, exists in meso and optically active forms distinguishable by ¹⁹F n.m.r. spectroscopy.