An Acyclic Trialkylamine Virtually Planar at Nitrogen. Some Chemical Consequences of Nitrogen Planarity

The synthesis of the exceedingly congested amine tris(1,3-dihydroxy-2-propyl)amine, 9, was achieved in 47− 51% overall yield. The nitrogen atom of 9 is virtually planar; it is 0.082 Å out of the plane defined by the three attached carbons. The corresponding out-of-plane measurement is 0.282 Å for triisopropylamine and ca. 0.4 Å for uncongested trialkylamines. The N−C bonds of 9 are quite short, despite the steric congestion. The conjugate acid of 9 (viz., 9H⁺) is very strong: pK_a = 3.08 (cf. Et₃NH⁺ pK_a = 10.7). Comparison with suitable model compounds suggests 9 is less basic than predicted by ca. 1.5 pK_a units. The structure of 9H⁺Cl⁻ was determined by X-ray crystallography. Here too, the nitrogen is severely flattened relative to ordinary ammonium cations. In 9H⁺Cl⁻, the proton on the nitrogen of 9H⁺ forms three intramolecular hydrogen bonds to hydroxyl groups, i.e., a so-called trifurcated hydrogen bond. The NH···O lengths in 9H⁺ are slightly shorter than comparable trifurcated hydrogen bonds. Cyclic voltammetry (CV) on 9 finds E_1/2^ox is 0.88 V, which is consistent with the inductive effect of the 1,3-dihydroxy-2-propyl groups attached to nitrogen. It is also observed that the electrochemical oxidation of 9 is reversible on the CV time scale. The ¹⁵N NMR chemical shift of the essentially planar nitrogen atom of 9 is discussed.