STABLE SIMPLE ENOLS .35. O-17 AND C-13 NMR-SPECTRA OF STABLE SIMPLE ENOLS

O-17 NMR spectra of twelve stable simple enols 1 [Ar2C=C(R)OH, Ar = crowded aryl group, R = H (1c, Ar = Mes), Bu(t), m- and p-substituted aryl, mesityl] were measured in CDCl3 and of six of them in [H-2(6)] DMSO. C-13 spectra of all the enols were measured in both solvents. These are the first values measured for enols not strongly intramolecularly hydrogen bonded. The delta(O-17) 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). Delta delta O = delta(O-17) [[H-2(6)]DMSO] - delta(O-17) [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 delta(O-17) vs. the C=C-O-H torsional angle theta plot, with maxima at theta = 0 and 180 degrees (syn and anti planar conformations) and a minimum at theta = 90 degrees. delta(C-13-2) values show an approximate mirror-image change while delta(C-13-1) changes slightly with theta. 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 delta(O-17, C-13-2) and downfield shift of delta(C-13-1, H-1) in all conformations. When the calculations also included the effect of the reaction field, the 6(O-17) 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 theta = 90 degrees and the plot of Delta(f)H for 1c and 1l vs. theta resemble the calculated energy (by MP2/6-311G**) vs. theta plot for 4. No correlation exists between the Ar-C=C torsion angles or lambda(max) of the enols with delta(O-17), but the polar effect of the alpha-substituent affects the delta(O-17) values. The solvent effect is ascribed to OH-[H-2(6)]DMSO hydrogen bonding, accompanied by a conformational change from syn-periplanar in CDCl3 to mainly anti-clinal in [H-2(6)]DMSO. A lower association with DMSO for bulkier Rs and an accompanying change of theta towards 90 degrees-qualitatively account for the Delta delta O values. Several correlations between the delta(O-17), delta(H-1) and delta(C-13) shifts or of delta(C-13) and Hammett's sigma values were found.