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[摘要]：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 angstrom out of the plane defined by the three attached carbons. The corresponding out-of-plane measurement is 0.282 angstrom for triisopropylamine and ca. 0.4 angstrom for unconuested 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 center dot center dot center dot 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 N-15 NMR chemical shift of the essentially planar nitrogen atom of 9 is discussed. |
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