Investigation of Dienophile−TiCl4 Complexation by Means of X-ray Absorption and 13C-NMR Spectroscopies

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Abstract
Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) techniques, in combination with 13C−NMR spectroscopy, have been used to study the complexation of methyl acrylate (1), N-acryloylbenzylamine (2), O-acryloyl-(R)-pantolactone (3), methyl N-acryloyl-(S)-prolinate (4), and methyl N-acryloyl-(S)-phenylalaninate (5) with excess TiCl4 in solution. The results obtained show that TiCl4 has a great tendency to coordinate with two ester ligands, but this tendency is not so marked with amides, which is related to the greater basicity of the latter. Complexation increases the Ti−Cl bond distance, in comparison with TiCl4, which is clearly shown by the EXAFS spectra. Chelate complexes are formed with bidentate ligands, but comparison between the EXAFS spectra, obtained with different TiCl4/dienophile ratios, shows that chelation is more difficult with methyl N-acryloyl-(S)-phenylalaninate (5).