Cations phenonium ions

Phenylethyl cations, phenonium ions and benzyl cations 262 Cyclopropylcarbinyl cation, bicyclobutonium ion, cyclobutyl cation and related systems 265... 

PHENYLETHYL CATIONS, PHENONIUM IONS AND BENZYL CATIONS... 

The thermodynamic stabilities of phenonium ions relative to the parent have been determined in the gas phase by measuring the position of the equilibrium between (46) and (47)7 The results followed a Yukawa-Tsuno relationship with a p value of -12.6 and an r+ value of 0.62, the general behaviour being more like benzenium ions than benzyl cations, with tt-delocalization less effective than in benzyl cations. A theoretical study of the elimination of molecular H2 from the benzenium ion C6H7+ shows that the barrier to this process appears to be very small.The gas-phase Friedel-Crafts alkylation reaction of CF3C6L6+ (L = H or D) with C2L4 is accompanied by isotopic scrambling, which has been used to elucidate the mechanism of this process. A theoretical calculation shows that the lifetime of triplet phenyl cation must be very short. ... 

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. 

Mass spectra of hydroxy- and alkoxy-coumarins have been very intensively studied. The decomposition sequence of 3-hydroxycoumarin is initiated by carbon monoxide loss from the molecular ion giving a 2-hydroxybenzofuran ion. Subsequent fragmentation occurs by two major pathways, involving a further loss of CO and expulsion of a formyl radical. The former leads to the base peak, and thence by another loss of CO to give the abundant benzene radical cation at m/e 78. The other main pathway gives a benzoyl cation which leads to the phenonium ion at m/e 77 (77IJC(B)816). 

The unsubstituted phenonium ion, as well as other phenonium ions substituted with electron-donating groups, have been recently observed as stable ions in superacid medium.34 That the structure is actually 18 and not an unsym-metrically bridged ion (19) nor a nonclassical ion (20) (see Section 6.2) in which there are three-center bonds was shown by the nmr evidence. The ring carbon that is bonded to the aliphatic carbons was established by 13C shifts to be tetrahedral in nature and 13C and proton chemical shifts in the ring were similar to those of cations shown to have Structure 21. 

Phenonium ion The bridged carbonium cation that results from the departure of a leaving group that is P to the aromatic ring. 

Thus, the treatment of hexamethyl Dewar benzene with acid at ambient temperature causes rearrangement to a phenonium ion (55) 55) while at lower temperature a cation is formed which is converted into an exo-methylene compound on treatment with a base56>. Addition of tetraeyanoethylene to the latter yields an 1,4-addition product (54). 

Again the unimolecular solvolysis which involves formation of a phenonium ion is retarded by incorporation of 195) in cationic micelles whereas the bimolecular reactions are catalyzed. 

Olah et al. (1992) studied the reaction of triphenylcarbenium tetrafluoroborate (10 mmol) with diphenyldiazomethane in dry dichloromethane hoping to detect the 1,1,2,2,2-pentaphenylethyl cation (9.25). The reaction yielded, however, tetraphenyl-ethene (79%) and a small amount (< 0.2%) pentaphenylethane. Using perdeuterated triphenylcarbenium salt or C-enriched diphenyldiazomethane, the authors demonstrate, by analysis of the labeled products, that the results are consistent with the mechanism (9-17), i.e., with the 1,1,2,2,2-pentaphenylethyl cation as steady-state intermediate, which is expected to undergo 1,2-phenyl migration via a phenonium ion and subsequent phenyl group scrambling. 

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). 

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. 

Jenekhe and coworkers found that DDQ converts anthrazo-line 7 to heptacyclic bis(indolo[l,2-a]quinolone) 8 (equation 3) [19]. Youn and Jang used DDQ to convert Af-Ts-2-alkenylanilines into indoles via, it is proposed, a radical cation generated by SET (equation 4) [20]. In the case of p,p-disubstituted 2-alkenylanilines, the 2,3-disubstituted indole product is derived from a C-2 to C-3 aryl migration via a phenonium ion (equation 5). 

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. ... 

With support from labelling, the solvolysis of 2-4-methoxyphenylethyl tosylate in 50 50 CF3CH2OH water is shown to proceed via a symmetrical 4-methoxyphenonium ion (25). °° Azide trapping experiments show a selectivity k lk of 83 mol resulting in an upper limit on the rate constant for solvent trapping of 6 X 10 s . Phenonium ions were obtained as intermediates by the addition of 4-methoxy- and 4-dimethylaminophenyl cations to mono- and tetra-substituted alkenes. These phenonium ions react by several pathways, depending on the aryl substituent, alkene, and solvent. With support from computation, a detailed mechanistic analysis was presented to explain the experimental observations. 

Using group increments and HIA data, provide a best estimate of the relative stabilities of 3-phenyl-2-butyl cation and the corresponding phenonium ion. 

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