Boron nitrogen—hydrogen bonds

As for the chiral ytterbium and scandium catalysts, the following structures were postulated. The unique structure shown in scheme 13 was indicated by 13C NMR and IR spectra. The most characteristic point of the catalysts was the existence of hydrogen bonds between the phenolic hydrogens of (R)-binaphthol and the nitrogens of the tertiary amines. The 13 C NMR spectra indicated these interactions, and the existence of the hydrogen bonds was confirmed by the IR spectra (Fritsch and Zundel 1981). The coordination form of these catalysts may be similar to that of the lanthanide(III)-water or -alcohol complex (for a review see Hart 1987). It is noted that the structure is quite different from those of conventional chiral Lewis acids based on aluminum (Maruoka and Yamamoto 1989, Bao et al. 1993), boron (Hattori and Yamamoto 1992), or titanium... 

The crystal structure of the boron triethanolamine complex B(OC2H4)gN shows that the boron atom is surrounded tetrahedrally, by 3 oxygen atoms (B—O 1.431, 1.432, and 1.475 A) and the nitrogen atom (B—N 1.693 A), Figure 12. There are no hydrogen bonds between the various molecules. 

In hydrogen cyanide, H —C=N, the values for the carbon-nitrogen bond are XE = 5.5 and AE = 0.5. The triple bond is thus stable, but weaker than the nitrogen-nitrogen triple bond. Correspondingly, hydrogen cyanide is also known in polymeric form. In the boron-nitrogen compounds, falls to 5.0,... 

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