Structure-Reactivity Relationships for Addition of Sulfur Nucleophiles to Electrophilic Carbon: Resonance, Polarization, and Steric/Electrostatic Effects
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- 作者：Maria M. Toteva and John P. Richard
- 刊名：Journal of the American Chemical Society
- 出版年：2000
- 出版时间：November 15, 2000
- 年：2000
- 卷：122
- 期：45
- 页码：11073 - 11083
- 全文大小：156K
- 年卷期：v.122,no.45(November 15, 2000)
- ISSN：1520-5126
文摘
Second-order rate constants kRS and kRSH (M-1 s-1) for the direct addition of substituted alkanethiolanions RS- and neutral thiols RSH and third-order rate constants (kRSH)H (M-2 s-1) for acid-catalyzed additionof RSH to the simple quinone methide 4-[bis(trifluoromethyl)methylene]cyclohexa-2,5-dienone (1) in waterare reported. Rate and equilibrium constants for the addition of Me2S and H+ to give H-1-SMe2+ were alsodetermined. The data for addition of RS- to 1 are correlated by the Br
nsted coefficient 
nuc = 0.11, whichis similar to that for addition of RS- to other highly resonance-stabilized carbocations. The rate constants foraddition of RS- to 1 are similar to those for addition of substituted alkyl alcohol ROH to the much moreelectrophilic 1-(4-methoxyphenyl)ethyl carbocation (5+). The larger value of nuc = 0.32 for addition of ROHto 5+ than nuc = 0.11 for addition of RS- to 1 shows that there are important differences in the reactioncoordinate profiles for these nucleophile addition reactions, which are discussed. The transition state for additionof RSH to 1 is stabilized by electron-donating alkyl groups R (nuc > 0.5) and by substitution of an electron-donating methyl group for hydrogen at RSH. By contrast, there is relatively little destabilization of the transitionstate for addition of Me2S to 1 from interactions between the developing cationic center at the bulky sulfurnucleophile and the electron-withdrawing 
-CF3 groups at 1. The results suggest that C-S bonding interactionsin the transition state for addition of Me2S to 1 develop at a relatively long distance and that product destabilizingsteric/electrostatic interactions become significant only at smaller C-S bond distances, after the transitionstate has been traversed on the reaction coordinate.
nsted coefficient nuc = 0.11, whichis similar to that for addition of RS- to other highly resonance-stabilized carbocations. The rate constants foraddition of RS- to 1 are similar to those for addition of substituted alkyl alcohol ROH to the much moreelectrophilic 1-(4-methoxyphenyl)ethyl carbocation (5+). The larger value of nuc = 0.32 for addition of ROHto 5+ than nuc = 0.11 for addition of RS- to 1 shows that there are important differences in the reactioncoordinate profiles for these nucleophile addition reactions, which are discussed. The transition state for additionof RSH to 1 is stabilized by electron-donating alkyl groups R (nuc > 0.5) and by substitution of an electron-donating methyl group for hydrogen at RSH. By contrast, there is relatively little destabilization of the transitionstate for addition of Me2S to 1 from interactions between the developing cationic center at the bulky sulfurnucleophile and the electron-withdrawing -CF3 groups at 1. The results suggest that C-S bonding interactionsin the transition state for addition of Me2S to 1 develop at a relatively long distance and that product destabilizingsteric/electrostatic interactions become significant only at smaller C-S bond distances, after the transitionstate has been traversed on the reaction coordinate.