Generation and detection of a relatively persistent carboxylic acid enol-2,2-bis(2',4',6'-triisopropylphenyl)ethene-1,1-diol

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-ditipylethene-1,l-diol (25). Protonation of the dianion of 19 or fluoride ion desilylation of 28 gave the radical Tip(2)C=C(OH)O-. (27), and only hydration of 20 in 42:5:3 DMF-d(7):THF-d(8):H2O or in THF-d(8):H2O gave solutions of greater than or equal to 98% of the enediol 25. H-1 and C-13 NMR spectra of 25 and of mixtures of 25 with its O-mono- and dideuteriated derivatives 25-D and 25-D-2 and linear delta(OH)[25] vs delta(OH) of 2,2-ditipyl-1-R-ethenols, R = Me, H (30, 31) correlations corroborated the structure and indicated that the two OH groups are identical on the NMR time scale. A propeller arrangement for the ditipylvinyl moiety and anti arrangements for the C=C-O-H moieties where the OH groups are solvated by the dipolar aprotic solvent (e.g.; DMF) are suggested for the conformation of 25. Although solid 25 could not be isolated, it is the longest-lived 1,1-enediol prepared so far in solution. The hydration rates of di(bulky)aryl ketenes decrease with the increased bulk of the aryl groups.