Neighboring groups, alkenes

Neighboring Group Participation in / -Substituted Chalcogenides. 9.3 Addition to Alkenes. 9.4 Addition to Alkynes... 

The same result can be achieved in one step with m-chloroperoxybenzoic acid and water. Overall anti addition can also be achieved by the method of Prevost (the Prevost reaction). In this method, the alkene is treated with iodine and silver benzoate in a 1 2 molar ratio. The initial addition is anti and results in a (3-halo benzoate (172). These can be isolated, and this represents a method of addition of lOCOPh. However, under the normal reaction conditions, the iodine is replaced by a second PhCOO group. This is a nucleophilic substitution reaction, and it operates by the neighboring-group mechanism (p. 446), so the groups are still anti ... 

Hydrolysis of the ester does not change the configuration. The Woodward modification of the Prevost reaction is similar, but results in overall syn hydroxylation. The alkene is treated with iodine and silver acetate in a 1 1 molar ratio in acetic acid containing water. Here again, the initial product is a (3-halo ester the addition is anti and a nucleophilic replacement of the iodine occurs. However, in the presence of water, neighboring-group participation is prevented or greatly decreased by solvation of the ester function, and the mechanism is the normal Sn2 process, ... 

The cation product 85 can undergo loss of H to give the alkene or add water to give the diol. It has been proposed that 85 is stabilized by neighboring-group... 

When malonate anion [20] was used as nucleophile, surprisingly, the internal alkene of 71 was attacked subsequent elimination afforded 72. When the salt 2a was treated with an oxygen nucleophile, hydroxide anion [20], then elimination occurred to yield the alkyne 73. It is clear from these reactions that Sn2 solvolysis of primary TTF salts is not easy the only successful example of solvolysis (i.e. of 29) is unimolecular and requires neighboring group assistance. 

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. " ... 

Neighboring group effects are important in reactions of peroxyacids, as they were with reactions of hydroperoxides. Reexamination of the epoxidation of 171 using MCPBA rather than TBHP reveals that the peroxyacid gave a 95 5 mixture of 172 and 173. Using MCPBA rather than TBHP led to a slight increase in diastereoselectivity for the reaction, although this depends on the substitution pattern of the alkene. Sharpless... 

Neighboring group effects can compete with steric effects, as illustrated by treatment of oxetane 142 with diethylaluminum-At-methylaniline. This elimination reaction gave a 99% yield of (E)-alkene 143 in a synthesis of humulene. The aluminum coordinated to the oxetane oxygen on the less hindered p face in 144 and 145, the two key rotamers. Rotamer 144 is lower in energy due to the severe nonbonded interaction... 

It is in this sense that cyclic sulfates are superior to epoxides. Further examples of double displacement reactions of cyclic sulfates are cited vide infra). In the absence of neighboring group participation, a -substituted ethylene sulfate may undergo an elimination reaction to furnish olefin. For example, when a cyclic sulfate was treated with sodium naphthalide in TFIF, only alkene 127 was obtained (90SL479) [Eq. (28)]. 

In the examples discnssed so far, the halide and the heterofunction were part of the same coupling component. In contrast, for the reactions in Scheme 43 the heterofunction is located as a neighboring group of the alkene moiety where the carbopalladation takes... 

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