Solvolysis neighboring group participation

Keywords SN2 and SN2 reactions Baldwin mles Oxirane opening Activation of a cyclopropane 1,2-rearrangement Ring contraction Ring expansion Solvolysis Neighboring group participation Oxirane rearrangement Classical and nonclassical carbocations 5-exo- vis-a-vis 5-endo cyclization Addition and elimination reactions... 

A classic example of neighboring-group participation involves the solvolysis of compounds in which an acetoxy substituent is present next to a carbon that is undergoing nucleophilic substitution. For example, the rates of solvolysis of the cis and trans isomers of 2-acetoxycyclohexyl p-toluenesulfonate differ by a factor of about 670, the trans compound being the more reactive one ... 

Brown-Hammett correlation of 1-aryl-1-cyclobutylethyl p-nitrobenzoate gave a p+ value of -3.94 which is more positive than that of a model compound, l-aryl-l,2-dimethylpropyl p-nitrobenzoate (p+ = -4.65), which indicates substantial neighboring group participation of the cyclobutyl ring even in tertiary carbocationic systems.12 Solvent effects on the kinetics of the solvolysis... 

In organic chemistry it is well known that a substituent directly linked to a reaction center has a profound effect on the latter s reactivity. The influence of a remote substituent has also received much attention as more organic molecules are being scrutinized. Thus, neighboring group participations in solvolysis are generally observed. Certainly, the full attribution of a substituent must be the sum of electronic, steric, and stereoelectronic factors. 

The solvolysis of 3-phenyl-2-butyl tosylate is only half as fast as that of 2-butyl tosylate. However, Winstein suggested that the inductive effect of the phenyl group should retard the rate by a factor of ten and that neighboring-group participation therefore has given a fivefold rate enhancement [see note 14(a), p. 273]. See p. 280 for the actual rate enhancement. 

Discrimination can readily be observed between the two possible modes of attack on a carbonium ion (195 196 and 195 197 when the nucleophile is part of the substrate. In such cases, the phenomenon of neighboring group participation is observed (for a review, see ref. 69). For example, solvolysis of the erythro-tosylate isomer 202 in acetic acid gave largely the erythro-acetate isomer 204 vi a the chiral bridged ion 203, whereas the threo isomer 205 yielded a racemic mixture of threo products 207A and 207B via the achiral intermediate 206 (70). 

The exo/endo rate ratio in the unsaturated series compares closely to that observed with the saturated tosylates homoconjugative interaction is therefore not apparent. Also, the products were those expected for solvolysis without participation.373 The dihydrotriquinacenyl tosylates solvolyze more slowly due die inductive contributions of the double bonds. The geometry of the p7T orbitals in the exo isomer are seen to have a very poor alignment for neighboring group participation. 

If R has a / -hydroxyl group, two other modes of decomposition can occur—neighboring group participation by either hydrogen or hydroxyl. Thus, the acid hydrolysis of propylene chlorohydrin gives not only glycol by solvolysis of the carbon-chlorine bonds, but also acetone by a hydrogen shift simultaneous with the departure of X" (14),... 

Evidence for Neighboring Group Participation for Solvolysis of Secondary TTF Salts... 

Another aspect of the mechanism is the reversibility of formation of the bromonium ion. Reversibility has been demonstrated for highly hindered alkenes, and attributed to a relatively slow rate of nucleophilic capture. However, even the bromonium ion from cyclohexene appears to be able to release Br2 on reaction with Br. The bromonium ion can be generated by neighboring-group participation by solvolysis of frfln -2-bromocyclohexyl triflate. If cyclopentene, which is more reactive than cyclohexene, is included in the reaction mixture, bromination products from cyclopentene are formed. This indicates that free Br2 is generated by reversal of bromonium ion formation. Other examples of reversible bromonium ion formation have been found. " ... 

Like the un-ionized hydroxyl group, an alkoxy group is a weak nucleophile, but can be involved in neighboring-group participation when steric and other factors are favorable. For example, solvolysis of the isomericp-bromobenzenesulfonate esters 7 and 8 leads to identical product mixtures, suggesting the involvement of a common intermediate, as would be the case if participation of the methoxy group occurred ... 

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