Different spectroscopic methods were applied in situ as well as ex situ to elucidate the mode of action of the Lewis acids TiCl4 and trimethylsilyl trifluoromethanesulfonate (TMSOTf) in the formal [3+3] cyclization reaction of a 1,3-bis(silyloxy)-1,3-butadiene with fluorinated ketenacetales. Different products were formed depending on the type of Lewis acid employed, which can be explained by their completely different interaction with the ketenacetale. Results of attenuated total reflection infrared (ATR-IR) and UV/vis spectroscopic studies revealed that TMSOTf interacts with only one site of the ketenacetale, forming an ionic species. In contrast, TiCl4 forms a bischelate complex, which was confirmed by using X-ray crystal structure analysis. Both reaction pathways were studied by simultaneously applying in situ ATR-IR/UV/vis spectroscopy in a specifically developed reaction cell. During cooling and subsequent heating of the reaction mixture, formation and structural changes of different intermediates were observed, in particular in the case of the TMSOTf catalyzed reaction. Although the TiCl4 mediated reaction proceeds in only one step, the TMSOTf route comprises two reaction steps. In situ spectroscopic results revealed that the applied workup procedure influences product formation and distribution as well.
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