Mechanisms for the Uncatalyzed and Hydrogen Ion Catalyzed Reactions of a Simple Quinone Methide Withch Solvent and Halide Ions

John P. Richard

doi:10.1021/ja00012a031

Mechanisms for the Uncatalyzed and Hydrogen Ion Catalyzed Reactions of a Simple Quinone Methide Withch Solvent and Halide Ions

John P. Richard

Chemistry

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Abstract

The quinone methide 4-[bis(trifluoromethyl)methylene]cyclohexa-2,5-dienone (1) has been prepared in situ from the solvolysis of 4-MeOArC(CF₃)₂OTs in 50:50 (v/v) trifluoroethanol/water, and its reactions with solvent and halide ions have been studied. Rate constants for the uncatalyzed and hydrogen ion catalyzed reactions of solvent, CI⁻, Br⁻, and I⁻ with 1 to form the respective phenols 4-HOArC(CF₃)₂Y, equilibrium constants for the addition of HI and HBr, and solvent deuterium isotope effects are reported. The equilibrium constant for the addition of HI to 1 is 17-fold larger than for the addition of HBr. The reactions of 1 have been compared with the reactions of the O-methylated analogue 4-MeOArC(CF₃)₂⁺. Methylation of the carbonyl oxygen causes a ~ 10¹⁹-fold increase in K_eq for the addition of Br⁻ to the electrophile and a large decrease in the selectivity for the addition of solvent and halide ions. Rate constants for the capture of the highly unstable O-methylated carbocation by nucleophilic reagents do not obey the N₊ equation, which requires that carbocation selectivity be independent of electrophile reactivity. The solvent deuterium isotope effects on the uncatalyzed addition of water and bromide ion are (k_o)_HOH/(k_o)_DOD= 2.0 and (k_Br)_HOH/(k_Br)_DOD⁼ 1.0, respectively. The difference in the isotope effects on these two reactions may be explained by considering the changes in the interaction of solvent with the water and bromide ion nucleophiles that occur on moving to the reaction transition states. The solvent deuterium isotope effects on the hydrogen ion catalyzed addition of water and bromide ion are k_H/k_D = 0.68 and k_HBr/k_DBr ⁼ 1.0, respectively. The difference in the isotope effects on these reactions and several other experimental observations are consistent with a stepwise mechanism for the H⁺-catalyzed addition of solvent and a concerted mechanism for the H⁺-catalyzed reactions of halide ions in which protonation of the carbonyl oxygen and carbon-bromine bond formation occur in a single step.

Original language	English
Pages (from-to)	4588-4595
Number of pages	8
Journal	Journal of the American Chemical Society
Volume	113
Issue number	12
DOIs	https://doi.org/10.1021/ja00012a031
State	Published - Jun 1 1991

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10.1021/ja00012a031

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@article{5ecdecbbd2d446de93f7212d301f0195,

title = "Mechanisms for the Uncatalyzed and Hydrogen Ion Catalyzed Reactions of a Simple Quinone Methide Withch Solvent and Halide Ions",

abstract = "The quinone methide 4-[bis(trifluoromethyl)methylene]cyclohexa-2,5-dienone (1) has been prepared in situ from the solvolysis of 4-MeOArC(CF3)2OTs in 50:50 (v/v) trifluoroethanol/water, and its reactions with solvent and halide ions have been studied. Rate constants for the uncatalyzed and hydrogen ion catalyzed reactions of solvent, CI−, Br−, and I− with 1 to form the respective phenols 4-HOArC(CF3)2Y, equilibrium constants for the addition of HI and HBr, and solvent deuterium isotope effects are reported. The equilibrium constant for the addition of HI to 1 is 17-fold larger than for the addition of HBr. The reactions of 1 have been compared with the reactions of the O-methylated analogue 4-MeOArC(CF3)2+. Methylation of the carbonyl oxygen causes a \textasciitilde{} 1019-fold increase in Keq for the addition of Br− to the electrophile and a large decrease in the selectivity for the addition of solvent and halide ions. Rate constants for the capture of the highly unstable O-methylated carbocation by nucleophilic reagents do not obey the N+ equation, which requires that carbocation selectivity be independent of electrophile reactivity. The solvent deuterium isotope effects on the uncatalyzed addition of water and bromide ion are (ko)HOH/(ko)DOD= 2.0 and (kBr)HOH/(kBr)DOD= 1.0, respectively. The difference in the isotope effects on these two reactions may be explained by considering the changes in the interaction of solvent with the water and bromide ion nucleophiles that occur on moving to the reaction transition states. The solvent deuterium isotope effects on the hydrogen ion catalyzed addition of water and bromide ion are kH/kD = 0.68 and kHBr/kDBr = 1.0, respectively. The difference in the isotope effects on these reactions and several other experimental observations are consistent with a stepwise mechanism for the H+-catalyzed addition of solvent and a concerted mechanism for the H+-catalyzed reactions of halide ions in which protonation of the carbonyl oxygen and carbon-bromine bond formation occur in a single step.",

author = "Richard, \{John P.\}",

year = "1991",

month = jun,

day = "1",

doi = "10.1021/ja00012a031",

language = "English",

volume = "113",

pages = "4588--4595",

journal = "Journal of the American Chemical Society",

issn = "0002-7863",

publisher = "American Chemical Society",

number = "12",

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TY - JOUR

T1 - Mechanisms for the Uncatalyzed and Hydrogen Ion Catalyzed Reactions of a Simple Quinone Methide Withch Solvent and Halide Ions

AU - Richard, John P.

PY - 1991/6/1

Y1 - 1991/6/1

N2 - The quinone methide 4-[bis(trifluoromethyl)methylene]cyclohexa-2,5-dienone (1) has been prepared in situ from the solvolysis of 4-MeOArC(CF3)2OTs in 50:50 (v/v) trifluoroethanol/water, and its reactions with solvent and halide ions have been studied. Rate constants for the uncatalyzed and hydrogen ion catalyzed reactions of solvent, CI−, Br−, and I− with 1 to form the respective phenols 4-HOArC(CF3)2Y, equilibrium constants for the addition of HI and HBr, and solvent deuterium isotope effects are reported. The equilibrium constant for the addition of HI to 1 is 17-fold larger than for the addition of HBr. The reactions of 1 have been compared with the reactions of the O-methylated analogue 4-MeOArC(CF3)2+. Methylation of the carbonyl oxygen causes a ~ 1019-fold increase in Keq for the addition of Br− to the electrophile and a large decrease in the selectivity for the addition of solvent and halide ions. Rate constants for the capture of the highly unstable O-methylated carbocation by nucleophilic reagents do not obey the N+ equation, which requires that carbocation selectivity be independent of electrophile reactivity. The solvent deuterium isotope effects on the uncatalyzed addition of water and bromide ion are (ko)HOH/(ko)DOD= 2.0 and (kBr)HOH/(kBr)DOD= 1.0, respectively. The difference in the isotope effects on these two reactions may be explained by considering the changes in the interaction of solvent with the water and bromide ion nucleophiles that occur on moving to the reaction transition states. The solvent deuterium isotope effects on the hydrogen ion catalyzed addition of water and bromide ion are kH/kD = 0.68 and kHBr/kDBr = 1.0, respectively. The difference in the isotope effects on these reactions and several other experimental observations are consistent with a stepwise mechanism for the H+-catalyzed addition of solvent and a concerted mechanism for the H+-catalyzed reactions of halide ions in which protonation of the carbonyl oxygen and carbon-bromine bond formation occur in a single step.

AB - The quinone methide 4-[bis(trifluoromethyl)methylene]cyclohexa-2,5-dienone (1) has been prepared in situ from the solvolysis of 4-MeOArC(CF3)2OTs in 50:50 (v/v) trifluoroethanol/water, and its reactions with solvent and halide ions have been studied. Rate constants for the uncatalyzed and hydrogen ion catalyzed reactions of solvent, CI−, Br−, and I− with 1 to form the respective phenols 4-HOArC(CF3)2Y, equilibrium constants for the addition of HI and HBr, and solvent deuterium isotope effects are reported. The equilibrium constant for the addition of HI to 1 is 17-fold larger than for the addition of HBr. The reactions of 1 have been compared with the reactions of the O-methylated analogue 4-MeOArC(CF3)2+. Methylation of the carbonyl oxygen causes a ~ 1019-fold increase in Keq for the addition of Br− to the electrophile and a large decrease in the selectivity for the addition of solvent and halide ions. Rate constants for the capture of the highly unstable O-methylated carbocation by nucleophilic reagents do not obey the N+ equation, which requires that carbocation selectivity be independent of electrophile reactivity. The solvent deuterium isotope effects on the uncatalyzed addition of water and bromide ion are (ko)HOH/(ko)DOD= 2.0 and (kBr)HOH/(kBr)DOD= 1.0, respectively. The difference in the isotope effects on these two reactions may be explained by considering the changes in the interaction of solvent with the water and bromide ion nucleophiles that occur on moving to the reaction transition states. The solvent deuterium isotope effects on the hydrogen ion catalyzed addition of water and bromide ion are kH/kD = 0.68 and kHBr/kDBr = 1.0, respectively. The difference in the isotope effects on these reactions and several other experimental observations are consistent with a stepwise mechanism for the H+-catalyzed addition of solvent and a concerted mechanism for the H+-catalyzed reactions of halide ions in which protonation of the carbonyl oxygen and carbon-bromine bond formation occur in a single step.

UR - https://www.scopus.com/pages/publications/0001650376

U2 - 10.1021/ja00012a031

DO - 10.1021/ja00012a031

M3 - Article

AN - SCOPUS:0001650376

SN - 0002-7863

VL - 113

SP - 4588

EP - 4595

JO - Journal of the American Chemical Society

JF - Journal of the American Chemical Society

IS - 12

ER -

Mechanisms for the Uncatalyzed and Hydrogen Ion Catalyzed Reactions of a Simple Quinone Methide Withch Solvent and Halide Ions

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