Nuclear Magnetic Resonance Experiments on Acetals. 35. Proton Magnetic Resonance Spectra and Quantitative Conformational Analysis of 4-Fluoromethylated-6-Me-1,3-dioxanes

Abstract

4-Substituted ([Formula: see text] and CH2F)-6-Me-1,3-dioxanes together with model derivatives (cis- and trans-4-CHnF3−n-6-t. Bu-1,3-dioxane, and 2-Me-4,6-disubstituted 1,3-dioxanes) were prepared and their p.m.r. spectra analyzed, using i.a. INDOR techniques. Special features which correlate configuration and p.m.r. data are discussed and especially the fluorine–hydrogen (long-range) couplings have been studied. From vicinal (ring) coupling data (using appropriate models), from the vicinal exo-couplings 3J(H4,CF3−nHn) and from absolute shift values, the equilibria were obtained in the conformational heterogenous trans-4-CHnF3−n-6-Me-1,3-dioxanes (n = 0 → 2). The results, according to the different methods, are mutually in excellent agreement, as is also the case for ΔG0(CF3), obtained approximately from dipole moment measurements. The anomeric effect is pronounced, and the conformer with the axial fluoromethylated substituent is the preferred one (CF3:76%; CF2H:62%) at room temperature in CS2. For CH2F the contrary seems to be the case (presumably for entropy reasons also). Finally ΔH0(chair–twist) was evaluated at 3.7 kcal/mol [Formula: see text] using 2J(H2) criteria in trans-4-CF3-6-t. Bu-1,3-dioxane.