¹⁷O and ¹³C NMR spectra of stable simple enols 1

¹⁷O NMR spectra of twelve stable simple enols 1 [Ar ₂C=C(R)OH, Ar = crowded aryl group, R = H (1c, Ar = Mes), Bu ^t, m- and p-substituted aryl, mesityl] were measured in CDCl ₃ and of six of them in [²H₆] DMSO. ¹³C spectra of all the enols were measured in both solvents. These are the first values measured for enols not strongly intramolecularly hydrogen bonded. The δ(¹⁷O) values in CDCl₃ are divided into three groups which parallel the steric bulk of R: (i) 70.5-74.9 when R = H; (ii) 84.1-87.4 when R = m- and p-substituted aryl; (iii) 93.0-98.5 ppm for R = Bu^t, mesityl (Mes). ΔδO = δ(¹⁷O) [[ ²H₆]DMSO] - δ(¹⁷O) [CDCl₃] = 7-10 for group (i), ca. 3 for group (ii) and -17 for trimesitylethenol 1l. IGLO calculations on CH₂=CHOH (4) give a shallow parabola for the δ(¹⁷O) vs. the C=C-O-H torsional angle θ plot, with maxima at θ = 0 and 180° (syn and anti planar conformations) and a minimum at θ = 90°. δ(¹³C-2) values show an approximate mirror-image change while δ(¹³C-1) changes slightly with θ. Calculation on the solvates 4·O=SH₂ with syn, anti and perpendicular conformations show trends similar to those for uncomplexed 4 with upfield shifts of δ(¹⁷O, ¹³C-2) and downfield shift of δ(¹³C-1, ¹H) in all conformations. When the calculations also included the effect of the reaction field, the δ(¹⁷O) values of the perpendicular and anti conformations differ little from those of uncomplexed 4. PM3 calculations for four enols including 1c and 1l also show a minimum at θ = 90° and the plot of Δ_fH for 1c and 1l vs. θ resemble the calculated energy (by MP2/6-311G**) vs. θ plot for 4. No correlation exists between the Ar-C=C torsion angles or λ_max of the enols with δ(¹⁷O), but the polar effect of the α-substituent affects the δ(¹⁷O) values. The solvent effect is ascribed to OH-[²H₆]DMSO hydrogen bonding, accompanied by a conformational change from syn-periplanar in CDCl₃ to mainly anti-clinal in [²H₆]DMSO. A lower association with DMSO for bulkier Rs and an accompanying change of θ towards 90° qualitatively account for the ΔδO values. Several correlations between the δ(¹⁷O), δ(¹H) and δ( ¹³C) shifts or of δ(¹³C) and Hammett's σ values were found.