Silicon-29 NMR Spectra of tert-Butyldimethylsilyl and Trimethylsilyl Derivatives of Some Non-Rigid Diols

Abstract

29Si NMR spectra of trimethylsilyl (TMS) and tert-butyldimethylsilyl (TBDMS) derivatives of selected diols were measured under standardized conditions (i.e., in diluted CDCl3 solutions). Application of the recently reported correlation between the chemical shifts in TMS and TBDMS derivatives revealed considerable and systematic deviations which exceeded experimental errors and error estimates from the correlation. Two possible explanations of the deviations are considered: interaction between the two bulky substituent groups and invalidity of the reported correlation for simple hydroxy derivatives. An independent study of analogous derivatives of monohydroxy compounds has shown that the linear correlation holds but the slope and intercept are significantly different from those reported previously on the basis of a study of amino acid derivatives. The data obtained for the diol derivatives fit the new correlation very well and no indication of an interaction between the bulky TBDMS groups was noticed. However, deviations do occur in branched diol derivatives in which branching reduces accessibility of the oxygen atoms surface to associate with proton donors. The largest deviation was found when intramolecular hydrogen bond was formed.