Stable Simple Enols. 5. Solvent Dependence of the Conformation of the OH Group in 2,2-Dimesitylethenol and Several 1,2,2-Triarylethenols

Silvio E. Biali, Zvi Rappoport

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Abstract

3J(HCOH) and δ(OH) values of 2,2-dimesitylethenol (Mes2C=CHOH, 6) and δ(OH) values of five stable triarylethenols were measured in 12–14 solvents and found to be strongly solvent dependent. 3J(HCOH) values for 6 at 293 K decreased from 14.1 Hz in CCl4, via 8.2 for CD3COCD3, to 4.6 in DMF-d7, and δ(OH) increased from 4.30 in C6D5CD3, via 7.65 in CD3COCD3, to 9.17 in DMF-d7. The δ(OH) values of all enols are linearly related. A use of a Karplus-type equation argues strongly that in nonpolar solvents where 3J(HCOH)'s are the highest known, the conformation of the OH group is syn-periplanar, 1a; i.e., the hydroxylic hydrogen is directed toward the cis-α-mesityl group. Extensive correlations with the Kamlet-Taft solvatochromic parameters show that the 3J and δ values are approximately linear with the hydrogen bond accepting ability parameter β. This and the 3J(HCOH)'s were interpreted as due to a rapid equilibrium on the NMR time scale between 1a and a solvent-associated anti-clinal conformer, which predominates in the hydrogen bond accepting solvents. The difference in δ(OH) values between the isotopomers 16-E and 16-Z [Mes*C(Mes)=C(OH)Mes (Mes* = 2,4,6-(CD3)3C6H2)] is 9.1 ± 1.5 ppb in nonpolar solvents, and 0 in three hydrogen bond accepting solvents, indicating that in the latter solvents the OH hydrogen is directed away from the cw-mesityl group. IR studies in CCl4 show a weak vOH band at 3580–3628 cm−1 which is ascribed to a free OH and a main vOH band at 3490–3528 cm-1 which indicates an intramolecular OH—π-(cis-β-mesityl) hydrogen bonding in 1a. Two OH absorptions, ascribed to intramolecularly bound OH and to a solvent-bound OH, were observed in PhBr and PhNO2, but only a wider OH band was observed at lower wavenumbers in MeCN and THF. Conformational syn-planar ⇄ anti-clinal equilibrium constants were calculated from the NMR and the IR data and were in satisfactory agreement. Addition of low concentrations of Me2SO-d6 to CCl4 reduces strongly 3J(HCOH) and increases strongly δ(OH) of 6, and analysis of the data in CCl4-Me2SO-d6 mixtures strongly suggests that the anti-clinal conformer is associated with only one Me2SO molecule. The conformational conclusions are compared with those for other enols.

Original languageAmerican English
Pages (from-to)5641-5653
Number of pages13
JournalJournal of the American Chemical Society
Volume106
Issue number19
DOIs
StatePublished - 1 Sep 1984

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