The lack of scrambling with deuterium-labeled reactants, a nonlinear ethanol-trifluoroethanol plot, and rate acceleration by added thiourea are used to show that sulfonate esters of methoxyethanol (MeOCH2CH2OH) undergo solvent-assisted displacement in a variety of solvents; neighboring group participation by ether oxygen (RO-3 participation) does not occur. This conclusion is in accord with predictions of rate based on the Taft treatment of substituent effects. On the other hand, the branched derivative MeOCMe2CH2OBs reacts with concerted RO-3 participation to give completely rearranged product. Its solvolysis rate is insensitive to added thiourea, the Taft treatment predicts modest anchimeric assistance, and a linear ethanol-trifluoroethanol plot is observed. We discuss the implications of these results relative to the previously observed nonlinear plot for mustard chlorohydrin.In discussing intramolecular nucleophilic assistance by a neighboring group, Winstein found it useful to define solvolytic rate constants in mechanistic terms: kA being the rate constant for anchimerically assisted ionization, ks being the rate constant for nucleophilically assisted displacement by solvent, and kc being the rate constant for the idealized process that involves neither neighboring group nor solvent nucleophilic assistance. In their pioneering studies, Winstein and his co-workers found that oxygen in the five or six position of MeO(CH2)"OBs gave large solvolytic rate enhancements although oxygen in the three or four position slowed solvolysis.1•2 They concluded that participation to form fiveand sixmembered rings was strong, while participation to form threeand four-membered rings was weaker; the RO-3 system was described as a borderline ks-kA system.Recently, Eliel and co-workers have shown RO-4 participation to be absent from the system with = 4.3 However, in branched systems such as PhCH20-and MeOCMe2(CH2)"X ( = 1, 2), Winstein1 and Eliel3 have shown that RO-3 and RO-4 participation can be enhanced relative to direct displacement by solvent. Branching favors neighboring group participation by sterically slowing solvent displacement,4 by stabilizing the cyclic ions,5 and by the Thorpe-Ingold effect.6 Although the branched RO-3 substrate gives complete rearrangement, the RO-4 system gives a variable amount of rearrangement dependent on the number and position of methyl branches and medium effects. Neither study yielded compelling data to confirm significant anchimeric assistance by the neighboring oxygen atom.
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