137144-24-2Relevant articles and documents
Substituted xanthenylidene enols. The importance of β-Ar-C=C conjugation in the stabilization of aryl-substituted enols
Rochlin, Elimelech,Rappoport, Zvi
, p. 230 - 241 (2007/10/02)
9-Xanthenecarboxaldehyde (7a) and methyl (7b), mesityl (7c), 2,4,6-triisopropylphenyl (Tip) (7d), and tert-butyl (7e) 9-xanthenyl ketones were prepared and found to be ketonic in CDCl3. In DMSO-d6,7a, 7c, 7d, and diphenylacetaldehyde 16 are in rapid equilibrium with the enols 15a, 15c, 15d, and 17, respectively. Keto ? enol equilibrium constants, Kenol, were measured in DMSO-d6 at various temperatures. The values at 294 K are 7a (101) > 16 (5.06) > 7d (1.1) > 7c (0.48). The ΔH° values are -2.0 to -4.4 kcal mol-1, and the ΔS° values are -9.1 to -11.8 cal mol-1 K-1. From the decrease of δ(OH) values on increasing the temperature, the association constants, Kassoc, of the enols with DMSO-d6 and the derived ΔHassoc and ΔSassoc values were obtained. The qualitative rates of formation of the enol acetates in DMSO-pyridine/Ac2O follow the Kenol values. X-ray diffraction data for 7b, 7c, and 18c - the acetate of 15c - gave the Ar - C=C and Mes - CO dihedral angles and showed that the xanthenyl moiety had a butterfly conformation. Several triarylethanones PhCH(Ar1)COAr2 (Ar1 = Ph, Ar2 = Mes, Tip; Ar1 = Ar2 = Mes) also isomerize in DMSO-d6 to the ketone-enol mixtures. The increase of Kenol by planarization of the β-aryl groups is reflected by the Kenol(7a)/Kenol(16) ratio of 20 at 294 K. Lower ratios for the α-aryl derivatives are Kenol(15c)/Kenol(Ph2CHCOMes) = 2 and 13.2 for the α-Tip analogues. These ratios were discussed in terms of ArC=C and ArC=O conjugation in the enols and ketones. β,β-Dimesityl substitution increased Kenol more than did β,β-diphenyl substitution due to steric effects. α-Alkyl substitution decreased Kenol strongly due to the higher stability of the ketones. The Kenol(DMSO-d6)/Kenol(H2O) ratio was 46 for 16 at 294 K. This first such ratio measured for a simple enol is ascribed to higher hydrogen bond acceptance by the DMSO-d6. Solvation of the enol is an important contributor to Kenol. The thermodynamic parameters resemble those for the nonsimple stable enol of acetylacetone rather than for the simple but much less stable enol, H2C=C(OH)Me.
