Phenonium intermediate

Contrary to the case of free a-amino acids, deamination-substitution of esters of a-amino acids generally proceeds with racemization with excess inversion about the a-carbon atom. However, reaction of the ethyl ester of phenylalanine and its derivatives with sodium nitrite in trifluoroacetic acid affords substitution products with retention of configuration and migration products with inversion of configuration. This result may be explained by assuming initial formation of the phenonium intermediate (2 Scheme 6). ... 

Equation 35). On the other hand, the acyclic ary 1-2,3-epoxy acylate (57) undergoes rearrangement through C3-cleavage of the oxirane ring via a phenonium intermediate to give ester (58) (Equation 36) [36]. 

Such species with a bridging phenyl group are known as phenonium ions. The neighbouring group effect is even more pronounced with an OH rather than an OMe substituent in the p-position. Solvolysis is found to occur % 106 times more rapidly under comparable conditions, and matters can be so arranged as to make possible the isolation of a bridged intermediate (5), albeit not now a carbocation ... 

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 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. 

The thermodynamic stabilities of phenonium ions have been determined based on bromide-transfer equilibria in the gas phase and, depending on the substituents, the bridged species (1) has been proposed as an intermediate or transition state on the potential-energy surface for the 1,2-aryl rearrangement of triarylvinyl cations (see Scheme 1). Phenonium ion (3) has been presented as an intermediate to account for the fact that lactonization of methyl 4-aryl-5-tosyloxy hexanoate (2) produces y-lactone (4) selectively under thermodynamic conditions, but affords 5-lactone (5) preferentially under kinetic conditions. It has been shown that anodic oxidation of frany-stilbene in alcohols in the presence of KF or BU4NBF4 is accompanied by its electro-oxidative rearrangement into diphenylacetaldehyde acetals. The mechanism outlined in Scheme 2 has been proposed" for the transformation. 

Phenonium ions as discrete intermediates in certain Wagner-Meer-wein rearrangements. J. Amer. chem. Soc. 86, 3767 (1964). 

When the electrophile attacks the ring C bonded to COOH, the intermediate phenonium ion first loses an H from COOH and then loses CO, which is a very good leaving group ... 

These experimental observations were explained by invoking the formation of the most stable benzylic carbocation that then undergoes rearrangement via an intermediate phenonium ion72 (Scheme 4.5). This mechanism accounts for cc-p isotope scrambling, and the low amount of isopropylbenzene is in harmony with the involvement of the primary cation. Other observations, in turn, suggest the involvement of the n complex of the arene and the side-chain carbocation.73-75... 

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). 

Hence rearrangement to carbon 4 to afford 3,4-diphenylphenol might seem an equally reasonable possibility. However, if one inspects the phenonium type intermediates for migration to carbon 2 and for migra-... 

The phenonium ion from the L-erythro-tosylate is chiral (Equation 6.15) and can give chiral products. Examination of the two possible paths of attack of acetic acid (it must come from the opposite side from the bulky phenyl ring) in each of the intermediates confirms that the products expected from them are those that are observed experimentally. 

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. 

---