Ditipyl ketene (tipyl = 2,4,6-triisopropylphenyl) (20), ditipylacetic acid (19), and 2,2-ditipyl bis(trimethylsilyl) acetal 28 were prepared as potential precursors for the enol of 19, i.e., 2,2-di...

Generation and Detection of a Relatively Persistent Carboxylic Acid Enol2,2-Bis(2‘,4‘,6‘-triisopropylphenyl)ethene-1,1-diol

17 O NMR spectra of twelve stable simple enois 1[Ar2CC(R)OH, Ar = crowded aryl group, R = H (1c, Ar = Mes), But, m- and p-substituted aryl, mesityl] were measured in CDCl3 and of six of them in [2H6] DMSO. 13C spectra of all the enois were measured in both solvents. These are the first values measured for enois 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 = But, 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 CH2CHOH (4) give a shallow parabola for the δ(17O)vs. the CC–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·OSH2 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 enois including 1c and 1l also show a minimum at θ= 90° and the plot of ΔfH for 1c and 1lvs. θ resemble the calculated energy (by MP2/6-311 G**)vs. θ plot for 4. No correlation exists between the Ar–CC torsion angles or λmax 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.

17O and13C NMR spectra of stable simple enols

The title compound, 3, is prepared in high yield starting from 2,4,6-tri-tert-butylaniline, 1. Reaction with formaldehyde provides N-methylene-2,4,6-tri-tert-butylaniline, 5; N-methylation then gives the immonium salt 6 which is reduced to 3. The thermodynamic dissociation constant in 50% ethanol at 25°C, pKa* = - 1.42, is 5.6 pK units lower than that of N,N-dimethylaniline. The effects contributing to this difference are evaluated. Protonated 3 is found to have a rotational barrier around CarN, of 63.6 kJ/mol, which is about 4.5 kJ/mol lower than that found for protonated N,N-dimethyl-2,4,6-trimethylaniline.

Mamoru Shimizu

Papers

Ester aus sterisch gehinderten grignard-verbindungen und “oxidierenden” aldehyden