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-1,1-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-d8: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.