CONFORMATIONAL-ANALYSIS .18. LANTHANIDE-INDUCED SHIFT (LIS) INVESTIGATION OF SOME 2-SUBSTITUTED 1,3-DIOXANES

A LIS analysis of 2-isopropyl-1,3-dioxane (1) and 2-methyl-2-phenyl-1,3-dioxane (2) is reported. The monodentate complexing model gives good agreement with the observed shifts in both compounds, whereas the bidentate model (lanthanum binding to both oxygens) does not give an acceptable solution. In 1 the lanthanide complexes almost exclusively (90%) with the axial lone pair of the oxygen atom. The four-site binding model gives excellent agreement with the observed shifts (R(x) 3.5%). In 2 a similar lanthanide binding is found, and the analysis of the LIS allows both the deduction of the molecular conformation (C-2-methyl equatorial, C-2-phenyl axial) and also the orientation of the phenyl group (perpendicular).