Phenonium ions, also

Scheme 12.15 gives some examples of both acid-catalyzed and nucleophilic ring openings of epoxides. Entries 1 and 2 are cases in which epoxidation and solvolysis are carried out without isolation of the epoxide. Both cases also illustrate the preference for anti stereochemistry. The regioselectivity in Entry 3 is indicative of dominant bond cleavage in the TS. The reaction in Entry 4 was studied in a number of solvents. The product results from net syn addition as a result of phenonium ion participation. The ds-epoxide also gives mainly the syn product, presumably via isomerization to the... 

The study of lactonization via an intermediate phenonium ion has been further pursued for several methyl 4-aryl-5-tosylhexanoates (55) as substrates. The intermediate phenonium ion (56) has two possibilities for ring closure, yielding products (57) or (58). In all the substrates, Ar contained one or two methoxy groups and sometimes also a methyl group. The effects of reaction medium, temperature, and time on the product ratios were examined. It was concluded that substrates (55) give y-lactone (57) selectively under thermodynamic conditions, but 5-lactone (58) under kinetic conditions. Substituents in Ar influence the selectivity through their electronic effects. 

To limit the scope of this review and to reduce overlap with other reviews, some related topics will only be discussed briefly. The addition of salts can affect both the rate and the course of solvolytic reactions and provides very important evidence for ion-pair intermediates. A full discussion of this topic has been published recently (Raber etal., 1974) additional comments are given onpp. 27,32. Also, we have generally excluded solvolyses known to proceed by competitive nucleophilically solvent-assisted and anchimerically assisted pathways. These solvolyses are very common (e.g. even n-propyl tosylate yields 87% of rearranged product during trifluoroacetolysis Reich et al., 1969), but a detailed account has been published recently (Harris, 1974). Recognition that solvolytic reactions could proceed by these two competitive, assisted pathways provided the key to the solution of the controversial phenonium ion problem (Lancelot et al., 1972 Brown et al., 1970), as well as inspiring the reinvestigation of the mechanisms of solvolyses of simple secondary substrates discussed in Section 2. 

Spiro[2.5]octadienyl radical (146), the radical analog of the phenonium ion (105), is a very short-lived species but was detected by its visible absorption and fluorescence on generation by radical abstraction from 147 prior to facile ring-opening to 148 (equation 29). There appeared to be a modest (two-fold) acceleration compared to 1,3- or 1,4-cyclohexadiene in the rate of abstraction from 147, and this was attributed to stabilization by a favorable interaction between cyclopropyl and the adjacent semioccupied orbital in 146. Kinetic acceleration was also proposed to occur in hydrogen atom abstraction from spiro[2.n]alkanes (149). ... 

Ando and coworkers conducted isotope effect studies (entry 11) on the direct displacement reaction of benzyl arenesulfonates with dimethylaniline (DMA)38. They found that an electron-withdrawing substituent in the substrate (Y = 3-Br) caused the TS to shift to a later position along the reaction coordinate, which is consistent with that predicted by the Thornton rule (or anti-Hammond effect). The anilinolysis of phenylethyl arenesulfonates (entries 12 and 13) proceeds also by an SN2 mechanism. The reaction was found to proceed by a dissociative SN2 mechanism with a relatively small degree of aryl participation. The fraction of the phenonium ion intermediate captured by the aniline nucleophile in the aryl-assisted pathway has been shown to increase with a stronger nucleophile, and a four-center TS in an intermolecular SNi mechanism is suggested for the aryl-assisted pathway39,40. Under the same reaction conditions, benzylamine nucleophiles react at a rate ca two times faster than that of anilines. 

Another important development in permitting structural conclusions from NMR studies on carbocations resulted from the use of theoretical computations of and chemical shifts. Known as the MP2-G1AO method,it has also been applied successfully to allylic, cyclopropylmethyl, and phenonium ions. ° ... 

This is also in agreement with the fact that for ions (11) At = P-CH3C6H4 and P-CH3OC6H4, X = At = QH5, X = CP3 (3.6), Br (2.7) , as well as for 9-methyl-9,10-diphenylphenanthrenonium ion the rate of the aryl ring rotation around the C —bond is far lower than that of the 1,2-aryl shift (cf. Ref. This shows that a nonclassical phenonium ion (B) is not involved in the... 

Olah and co-workers used NMR spectroscopy to study the relative stability of phenonium ions, benzylic ions, and phenethyl cations as a function of substituents on the benzene ring. For the imsubstituted benzene ring, the phenonium ion was more stable than the other two jjossible structures. With a p-methoxy substituent, however, the benzylic ion was most stable, while the phenethyl cation was most stable for a p-trifluoromethyl substituent. Olah, G. A. Comisarow, M. B. Kim, C.].J.Am. Chem. Soc. 1969,91,1458. Also see Olah, G. A. Head, N. J. Rasul, G. Prakash, G. K. S. /. Am. Chem. Soc. 1995,117, 875. 

Other Small Ring Intermediates. Arguments and evidence for phenonium ions in a variety of cation-forming reactions of 2-phenylethyl derivatives have been presented. For example, acetolysis of [l- C]-2-phenylethyl triflate proceeds with ca. 32 % rearrangement of the label to the 2-position, and recovered starting material is also isotopically scrambled. ... 

Hi) Loss of proton from the carbocation Although the carbocation is relatively stabler, it is quite reactive as one of the carbon atoms has only six electrons Also, it is not aromatic and tries to stabilise itself by regaining its aromatic structure. This can be achieved by losing a proton to the basic ion, HSO. Although HSO is an extremely weak base, the tendency of the phenonium ion to lose a is so high that even this slight basicity is quite sufficient for capture of a proton. 

Note carefully the geometry of the phenonium ion. The two rings are perpendicular, not coplanar, and the ion is achiral, a meso compound. All chance of optical activity is lost at this point. Note also the close correspondence between this intermediate and the benzenonium ion formed in an electrophilic aromatic substitution reaction (Fig. 21.27). 

Intramolecular oxygenation reactions were also used in the asymmetric synthesis of different substituted lactone derivatives. Using chiral X -iodane 5c, lactonization of 4-aryl-4-pentenoic acids 31 gave rearranged lactones through phenonium ion participation in 56% yield, albeit in only 4% ee (Scheme 14) [57]. Another example of lactonization involving enantioselective... 

The solvolysis of trityl derivatives in strongly acidic aqueous media has also been examined. Rates of hydrolysis were correlated to Hr acidity function and theX excess acidity scale. The solvolysis and substitution reactions of 2-(4-methylphenyl)ethyl tosylate have also been studied. " In trifluoroethanobwater (50 50) solution, the corresponding phenonium ion is generated. The kinetics of azide trapping has been studied and compared with results from an analogous system 2-(4-methoxyphenyl)ethyl tosylate. Owing to electronic effects, the methyl-substituted phenonium ion is a more reactive electrophilic species. 

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