Phenonium ion intermediate

Support for the suggestion that Fig. 13.6 involves a change in actual reaction pathway is provided by acetolysis of the threo diastereoisomer (31) of the brosylate. Acetolysis leads to two different distinguishable, diastereoisomers whose relative proportion will depend on how much of the total reaction proceeds by external nucleophilic attack via the SN2 pathway (erythro product, 32), and how much by internal nucleophilic attack via a cyclic phenonium ion intermediate (threo product, 33) ... 

The first step involves hydride abstraction and a phenonium ion intermediate.198 The low yields are accounted for by the ready dealkylation of sec- and tert-pentyl-benzenes under the reaction conditions (see discussion below). 

Cram provided further evidence for the existence of a phenonium ion intermediate by isolating starting tosylate after reaction had proceeded for 1.5 half-lives he found that the L-t/wo-tosylate was 94 percent racemized but the l-erythro-tosylate was still optically pure.23 These results can be easily understood il it is assumed that the starting material first forms a phenonium-tosylate intimate ion pair, which can either revert to starting materials or go on to products. Th< achiral ion pair from the tAreo-tosylate will return to racemic starting material whereas the chiral intermediate from the erythro isomer will return to optically active starting material. 

A similar aryl participation in saturated compounds to give a phenonium ion intermediate appears in Chapters , p. 000. 

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. 

Defunctionalization. Aryl participation in the ionization of a homobenzylic tosyloxy group is not unusual. Such a phenonium ion intermediate is readily converted to a lactone when a strategically located ester is present in the same molecule. Removal of A/-Boc groups by silica gel with assistance of microwaves is an expedient method. 

The tosylates (374) and (376) are both possible precursors of die spirocyclic phenonium ion (375). The degree of strain required to accomodate the phenonium ion is clearly dependent on ring size. Rate and product studies — including the stereo chemistry of (377)from optically active precursors — suggest intervention of (375) with n 331 With n=2, all evidence is against a phenonium ion intermediate. 

Stable carbonium ions. Olah et a I have reported direct observation (NMR) of a phenonium ion intermediate, first postulated by Cram.6 1 -p-Anisyl-2-chloroethane l.1) on being ionized with FS020H-SbCl5 in SOa at —70° gave the bridged ion of ructure (2) determined by NMR. 

Draw a reaction coordinate diagram for the solvolysis of 2,2,2-triphenylethyl chloride in acetic acid. Pay special attention to the phenonium-ion intermediate. What would be the difference in this diagram if the phenonium ion were a transition state instead of an intermediate ... 

A/ -Tosylindole 87 was prepared by the metal-free C—H amination of Af-tosylaniline 86 and DDQ.The reaction is believed to proceed through a radical cation generated from a SET process and a phenonium ion intermediate. Most of the substituted indoles were isolated in very good yields and a variety of aryl, heteroaryl, halogen, and trifluoromethyl substituents were tolerated (14OL3720). 

Scheme 7.49. A rearrangement during solvolysis in which a phenyl group (CeHs-) migrates from one carbon to another. The rearrangement appears to occur along with the normal solvolysis (in this case). The scrambling of label is accounted for by involving a phenonium ion intermediate (but see Brown, H. C. Kim, C. J. /. Am. Chem. Soc., 1968, 90, 2082 and references therein).

FIGURE 21.26 If the benzene ring acts as a neighboring group, a cyclic phenonium ion intermediate results. Notice that the intermediate has a plane of symmetry and is therefore achiral. 

A class of reactions in which the variation of with ApK has been extensively investigated is electrophilic aromatic substitution, in which the rate-determining step is proton loss from a phenonium-ion intermediate. This includes diazo-coupling [56], nitrosation [63] and aromatic hydrogen exchange [47,51,64,65]. Aromatic hydrogen... 

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