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Solid-state 13C-NMR Spectroscopy, 1. — Titanium Tetrachloride Complexes of Diketones: Their Importance for Hydride Reductions and Their Solid-State NMR Spectra1 The influence of titanium tetrachloride on the diastereoselectivity of hydride reductions of 1,2-, 1,3-, 1,4- and 1,5-diketones has been studied. The effect is especially pronounced in the case of 1,3-diketones, where the addition of titanium tetrachloride reverses the stereoselectivity. The assumption of a chelate of type 32 or 33 can be confirmed by several observations: β-hydroxy ketone 7 and β-benzyloxy ketone 9 show the same dependence on the stereochemical course as diketone 5a. This compound forms complex 34, whose conformation is known by X-ray structure determination. From the solid-state 13C-NMR spectra of the TiCl4 adducts it can be deduced that 7 forms chelate 42, which is close to the assumed intermediate 32. In the case of phenyl-substituted substrates solid-state 13C-NMR spectroscopy can also be used to differentiate between real chelates (coordination of TiCl4 at the oxygen atoms of the carbonyl groups) and π complexes. |
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