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-l,l-diol (25). Protonation of the dianion of 19 or fluoride ion desilylation of 28 gave the radical Tip₂C=C(OH)O^• (27), and only hydration of 20 in 42:5:3 DMF-d₇:THF-E₈:H₂O or in THF-d₈:H₂O gave solutions of > 98% of the enediol 25. ¹H and ¹³C NMR spectra of 25 and of mixtures of 25 with its O-mono- and dideuteriated derivatives 25-D and 25-D₂ and linear δ(OH)[25] vs δ(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.