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 Me
2S and H
+ to give
H-1-SMe2+ were alsodetermined. The data for addition of RS
- to
1 are correlated by the Br
![](/images/entities/oslash.gif)
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 Me
2S to
1 from interactions between the developing cationic center at the bulky sulfurnucleophile and the electron-withdrawing
![](/images/gifchars/alpha.gif)
-CF
3 groups at
1. The results suggest that C-S bonding interactionsin the transition state for addition of Me
2S 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.