Structure-Reactivity Relationships and Intrinsic Reaction Barriers for Nucleophile Additions to a Quinone Methide: A Strongly Resonance-Stabilized Carbocation
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- 作者：John P. Richard ; Maria M. Toteva ; and Juan Crugeiras
- 刊名：Journal of the American Chemical Society
- 出版年：2000
- 出版时间：March 1, 2000
- 年：2000
- 卷：122
- 期：8
- 页码：1664 - 1674
- 全文大小：187K
- 年卷期：v.122,no.8(March 1, 2000)
- ISSN：1520-5126
文摘
Second-order rate constants kNu (M-1 s-1) were determined for addition of a wide range ofnucleophiles to the simple quinone methide 4-[bis(trifluoromethyl)methylene]cyclohexa-2,5-dienone (1) to givethe nucleophile adduct 1-Nu in water. Equilibrium constants were determined for the overall addition of HBrand HI to 1 to give H-1-Nu, and the data were used to calculate equilibrium constants for the addition of Br-and I- to 1, and to estimate equilibrium constants for the addition of Cl- and AcO-. The values of log kNushow a linear correlation with the Ritchie nucleophilicity parameter N+ with a slope s = 0.92 ± 0.10 that isessentially the same as the electrophile-independent value of 1.0 for highly resonance-stabilized carbocations.Marcus intrinsic barriers 
of 12.4, 13.9, 15.4, and 19.8 kcal/mol are reported for the addition of I-, Br-, Cl-,and AcO- to 1, respectively. The thermodynamic barriers 
G
and intrinsic barriers for addition of Br-,Cl-, and AcO- to 1 are 8.4 ± 1.0 and 5.2 ± 0.2 kcal/mol larger, respectively, than the corresponding barriersfor addition of these nucleophiles to the triphenylmethyl carbocation. It is concluded that, by the criterion ofits chemical reactivity, 1 behaves as a highly resonance-stabilized carbocation. Values of N+ = 4.0, 2.2, 1.2and 0.60, respectively, are reported for I-, Br-, Cl-, and AcO-, which do not form stable adducts to Ritchieelectrophiles. The slope of 2.0 (r = 0.98) for the linear correlation between Ritchie (N+) and Swain-Scott (n)nucleophilicity parameters shows that there is substantially greater bonding between the nucleophile and carbonat the transition state for nucleophile addition to sp2-hybridized carbon than for addition to sp3-hybridizedcarbon. Azide ion and nucleophiles with a nonbonding electron pair(s) at atoms adjacent to the nucleophilicsite (
-effect nucleophiles) exhibit significant positive deviations from this correlation.
of 12.4, 13.9, 15.4, and 19.8 kcal/mol are reported for the addition of I-, Br-, Cl-,and AcO- to 1, respectively. The thermodynamic barriers G and intrinsic barriers for addition of Br-,Cl-, and AcO- to 1 are 8.4 ± 1.0 and 5.2 ± 0.2 kcal/mol larger, respectively, than the corresponding barriersfor addition of these nucleophiles to the triphenylmethyl carbocation. It is concluded that, by the criterion ofits chemical reactivity, 1 behaves as a highly resonance-stabilized carbocation. Values of N+ = 4.0, 2.2, 1.2and 0.60, respectively, are reported for I-, Br-, Cl-, and AcO-, which do not form stable adducts to Ritchieelectrophiles. The slope of 2.0 (r = 0.98) for the linear correlation between Ritchie (N+) and Swain-Scott (n)nucleophilicity parameters shows that there is substantially greater bonding between the nucleophile and carbonat the transition state for nucleophile addition to sp2-hybridized carbon than for addition to sp3-hybridizedcarbon. Azide ion and nucleophiles with a nonbonding electron pair(s) at atoms adjacent to the nucleophilicsite (-effect nucleophiles) exhibit significant positive deviations from this correlation.