Attempts at Solvolytic Generation of Phenyl Cations

Abstract

SummaryNew substrates and reaction conditions which may be expected to yield phenyl cation intermediates have been investigated. The approaches used were: ( a ) solvolysis of PhX in fluorinated alcohols, where X = -N (O)= NOTs (tosyloxyazoxy), -N (O)= NONf (Nf= C4F9S0T) and -OSO$ (CH3)30Tf (Tf= CF3S0,); (b) solvolysis of ArBr, PhOTf and Ph0SO2fi(CH3),OTf (phenyl 'betylate triflate') in superacid solvents (FS03H . SbF5, SbF5, AgSbF6). Analysis of the Rroduct mixtures provided no evidence for the intermediacy of phenyl cations as a major pathway in any of the reactions. This result is remarkable, since the 'betylate', for example, is a better leaving group by a factor of at least lo5 than the 'super' leaving group triflate in the solvolysis of alkyl sulfonates. These results are a further indication of the extremely low stability of phenyl cations, as well as of the very special properties of the nitrogen leaving group in arenediazonium ions.Introduction. -The solvolytic formation of aryl cations as intermediates in the heterolytic dediazoniation of arenediazonium ions is well documented [ 1-61. On the other hand, all attempts at producing phenyl cations by solvolyzing arenesulfonates with 'super' leaving groups, for example trifluoromethanesulfonates (triflates) and nonafluorobutanesulfonates (nonaflates) in polar non-nucleophilic solvents have failed up to now [7] [S].

DFT Study of Substituted and Benzannelated Aryl Cations: Substituent Dependency of Singlet/Triplet Ratio1a

“…Generation of aryl cations from polyaromatic hydrocarbons (PAHs) is also attractive because precursors to such intermediates are widely available, for example, amino-PAHs for diazotization and aryl triflates, which can be made from phenols. However, the search for aryl cations in solution has mostly focused on the parent phenyl cation. − In a search for other potential systems that could produce aryl cations, we have calculated the singlet and triplet 1-naphthyl ( 10 + ), 2-naphthyl ( 10a + ), and 9-anthracenyl ( 11 + ) monocations, as well as 4-OH ( 12 + ), 4-NMe 2 ( 13 + ), and the onium−aryl dications resulting from protonation of 12 + and 13 + , i.e., 12H 2+ and 13H 2+ (the relevant energies are listed in Table ). Whereas in all cases both singlet and triplet states are minima, the preference for the triplet ground state increases as the system becomes more electron rich.…”

“…Although aryl cations are produced in abundance in the gas phase, proof for their existence in solution has been difficult to obtain and quite challenging. The studies of Zollinger and associates on the mechanism of heterolytic dediazoniation (including demonstration of the reversibility of dediazoniation and exchange with external nitrogen) established Ph + as the implied intermediate. , Attempts to generate aryl cations, in low nucleophilicity, highly ionizing solvents including superacids, failed to give evidence for Ph + . , Hyperconjugative stabilization of Ph + was subsequently achieved via ortho substitution with Me 3 Si− or tBu− groups leading to solvolytic generation and trapping of such ions by Sonoda and associates . Ab initio calculations by Apeloig and Arad predicted that the introduction of ortho −SiH 3 groups or ring fusion to a cyclopropane ring should bring about significant stabilization …”

Mechanism of hydrogenolysis. Part 1. Catalytic hydrogenation of vinyl and aryltrifluoromethanesulphonates

“…Phenyl trifluoromethanesulphonate (7) was prepared according to a literature procedure,' 2,3,4,4a,5,6,7,8-octahydronaphthalen-1-yl trifluoromethanesulphonate (9) and trans-3,4,4a,5,6,7,8,8a-octahydronaphthalen-l-y1 trifluoromethanesulphonate (12) were prepared by treating trans-decahydronaphthalen-1-one with triflic anhydride. ', The isomers ( 9) and (12) formed in a ratio of 66 : 34 were separated by preparative gas chromatography (10% Carbowax 20M, Chromosorb P-AW-DMCS, 60-80 mesh, 2 m x 1 / 4 ; 130 "C).…”

Isolation of the Phenyl Cation in a Solid Argon Matrix

“…[8] Owing to its extreme electrophilicity and reactivity, 1 is very short-lived in the condensed phase, and, in contrast to the phenyl radical (3), [9] has never been isolated or characterized spectroscopically. Neither NMR investigations in magic acid [10] nor time-resolved spectroscopy [11] made a direct observation of the phenyl cation possible. Although mass spectrometry and ICR spectroscopy provided some insight into the gas-phase reactivity of 1, [12] no detailed conclusions concerning structural features and electronic properties can be drawn from these experiments.…”