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

Abstract

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 Tip 2 C C(OH)O • ( 27 ), and only hydration of 20 in 42:5:3 DMF- d 7:THF- d 8:H 2 O or in THF- d 8:H 2 O gave solutions of ≥ 98% of the enediol 25 . 1 H and 13 C NMR spectra of 25 and of mixtures of 25 with its O -mono- and dideuteriated derivatives 25-D and 25-D 2 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.