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Structure−Reactivity Relationships and Intrinsic Reaction Barriers for Nucleophile Additions to a Quinone Methide:  A Strongly Resonance-Stabilized Carbocation

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journal contribution
posted on 10.02.2000, 00:00 by John P. Richard, Maria M. Toteva, Juan Crugeiras
Second-order rate constants kNu (M-1 s-1) were determined for addition of a wide range of nucleophiles to the simple quinone methide 4-[bis(trifluoromethyl)methylene]cyclohexa-2,5-dienone (1) to give the nucleophile adduct 1-Nu in water. Equilibrium constants were determined for the overall addition of HBr and 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 kNu show a linear correlation with the Ritchie nucleophilicity parameter N+ with a slope s = 0.92 ± 0.10 that is essentially the same as the electrophile-independent value of 1.0 for highly resonance-stabilized carbocations. Marcus intrinsic barriers Λ 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 ΔG° and intrinsic barriers Λ 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 barriers for addition of these nucleophiles to the triphenylmethyl carbocation. It is concluded that, by the criterion of its chemical reactivity, 1 behaves as a highly resonance-stabilized carbocation. Values of N+ = 4.0, 2.2, 1.2 and 0.60, respectively, are reported for I-, Br-, Cl-, and AcO-, which do not form stable adducts to Ritchie electrophiles. 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 carbon at the transition state for nucleophile addition to sp2-hybridized carbon than for addition to sp3-hybridized carbon. Azide ion and nucleophiles with a nonbonding electron pair(s) at atoms adjacent to the nucleophilic site (α-effect nucleophiles) exhibit significant positive deviations from this correlation.