Substituent Effects on Carbocation Stability: The pKR for p-Quinone Methide
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- 作者：Maria M. Toteva ; Michael Moran ; Tina L. Amyes ; and John P. Richard
- 刊名：Journal of the American Chemical Society
- 出版年：2003
- 出版时间：July 23, 2003
- 年：2003
- 卷：125
- 期：29
- 页码：8814 - 8819
- 全文大小：104K
- 年卷期：v.125,no.29(July 23, 2003)
- ISSN：1520-5126
文摘
A value of kH = 1.5 × 10-3 M-1 s-1 has been determined for the generation of simple p-quinonemethide by the acid-catalyzed cleavage of 4-hydroxybenzyl alcohol in water at 25 
C and I = 1.0 (NaClO4).This was combined with ks = 5.8 × 106 s-1 for the reverse addition of solvent water to the 4-hydroxybenzylcarbocation [J. Am. Chem. Soc. 2002, 124, 6349-6356] to give pKR = -9.6 as the Lewis acidity constantof O-protonated p-quinone methide. Values of pKR = 2.3 for the Lewis acidity constant of neutral p-quinonemethide and pKadd = -7.6 for the overall addition of solvent water to p-quinone methide to form4-hydroxybenzyl alcohol are also reported. The thermodynamic driving force for transfer of the elementsof water from formaldehyde hydrate to p-quinone methide to form formaldehyde and p-(hydroxymethyl)phenol (4-hydroxybenzyl alcohol) is determined as 6 kcal/mol. This relatively small driving force representsthe balance between the much stronger chemical bonds to oxygen at the reactant formaldehyde hydratethan at the product p-(hydroxymethyl)phenol and the large stabilization of product arising from thearomatization that accompanies solvent addition to p-quinone methide. The Marcus intrinsic barrier fornucleophilic addition of solvent water to the "extended" carbonyl group at p-quinone methide is estimatedto be 4.5 kcal/mol larger than that for the addition of water to the simple carbonyl group of formaldehyde.O-Alkylation of p-quinone methide to give the 4-methoxybenzyl carbocation and of formaldehyde to give asimple oxocarbenium ion results in very little change in the relative Marcus intrinsic barriers for the additionof solvent water to these electrophiles.
C and I = 1.0 (NaClO4).This was combined with ks = 5.8 × 106 s-1 for the reverse addition of solvent water to the 4-hydroxybenzylcarbocation [J. Am. Chem. Soc. 2002, 124, 6349-6356] to give pKR = -9.6 as the Lewis acidity constantof O-protonated p-quinone methide. Values of pKR = 2.3 for the Lewis acidity constant of neutral p-quinonemethide and pKadd = -7.6 for the overall addition of solvent water to p-quinone methide to form4-hydroxybenzyl alcohol are also reported. The thermodynamic driving force for transfer of the elementsof water from formaldehyde hydrate to p-quinone methide to form formaldehyde and p-(hydroxymethyl)phenol (4-hydroxybenzyl alcohol) is determined as 6 kcal/mol. This relatively small driving force representsthe balance between the much stronger chemical bonds to oxygen at the reactant formaldehyde hydratethan at the product p-(hydroxymethyl)phenol and the large stabilization of product arising from thearomatization that accompanies solvent addition to p-quinone methide. The Marcus intrinsic barrier fornucleophilic addition of solvent water to the "extended" carbonyl group at p-quinone methide is estimatedto be 4.5 kcal/mol larger than that for the addition of water to the simple carbonyl group of formaldehyde.O-Alkylation of p-quinone methide to give the 4-methoxybenzyl carbocation and of formaldehyde to give asimple oxocarbenium ion results in very little change in the relative Marcus intrinsic barriers for the additionof solvent water to these electrophiles.