Synthesis, structure and dynamic stereochemistry of (O → Si)-chelate N-(trifluorosilylmethyl)-[N-(S)-(1-phenylethyl)]acetamide and 1-(trifluorosilylmethyl)-2-oxoperhydroazepine: Retention of the O → Si coordination in the adduct with KF and 18-crown-6

The novel compounds, N-(trifluorosilylmethyl)-[N-(S)-(1-phenylethyl)]-acetamide (1a) and 1-(trifluorosilylmethyl)-2-oxoperhydroazepine (1b) have been prepared from the corresponding NH-compounds using ClCH2SiCl3/Et3N or ClCH2SiCl3/(Me3Si)2NH followed by methanolysis or hydrolysis of the reaction mixture in the presence of Lewis bases, and then BF3 etherate. Potassium-(18-crown-6)-(2-oxoperhydroazepinomethyl)tetrafluorosilicate (2) was synthesized by reaction of the trifluoride (1b) with KF in the presence of 18-crown-6. Using 19F, 29Si NMR and X-ray diffraction techniques it was established that the silicon atom is pentacoordinate in the trifluorides (1a, b) and hexacoordinate in the adduct 2. Thus the internal coordination of the O → Si bond present in the trifluoride (1b) is retained in the adduct 2.The stereochemical non-rigidity of the trifluorides (1a, b) and the N-(trifluorosilylmethyl)-N-methylacetamide (1c) was investigated using dynamic 19F NMR spectroscopy. The activation barriers for permutational isomerization are in the range 9.5–10 kcal mol−1. Lower values of ΔG# for permutation of trifluorides (1a–c) compared to the monofluorides with the coordination core OSiC3F together with small negative values for the activation entropy implies a non-dissociative mechanism. Quantum-chemical analysis suggests a mechanism involving a turnstile rotation.

Graphical abstractThis paper describes the synthesis and (O → Si)-chelate structure of the novel compounds N-(trifluorosilylmethyl)-[N-(S)-(1-phenylethyl)]acetamide (1a) and 1-(trifluorosilylmethyl)-2-oxoperhydroazepine (1b). It describes the expansion of the silicon coordination environment upon reaction of the trifluoride (1b) with KF in the presence of 18-crown-6 resulting in adduct 2 with retention of (O → Si)-chelation. Based on dynamic 19F NMR investigations and quantum-chemical calculations, the stereodynamical behavior of the above compounds and the N-(trifluorosilylmethyl)-N-methylacetamide (1c) are discussed.Figure optionsDownload full-size imageDownload as PowerPoint slide