Cations diarylmethyl

The diarylmethyl cations listed in Table 5.1 are 6-7pATr+ units less stable than the corresponding triarylmethyl cations. This indicates that the additional aryl group has a cumulative, although not necessarily additive, effect on stability of the carbocation. Primary benzylic cations (monparylmethyl cations) are generally not sufficiently stable for determination of pATr+ values. A particularly stable benzylic ion, the 2,4,6-trimethyl-phenylmefliyl cation, has a pATr+ of — 17.4. 

Dicationic systems in which two diarylmethyl cations are linked by an 1,2-diphenylethyl,414 biphenyl,415 binaphthyl,415 or naphthalene-1,8-diyl416 417 backbone have attracted great interest recently because of their unique electrochromic properties. The dications 200 were prepared from the corresponding diols416,417 [Eq. (3.50)]... 

The reduction of (C6H5)3C+ SbCl6 occurs at +0.46V (versus Ag/AgCl) in a reversible 1 e-process, whilst the subsequent reduction of the trityl radical at -1,07 V is irreversible 599 Reduction potentials for diarylmethyl cations, which were generated by protonation of the corresponding alcohols or olefins by 97% sulfuric acid or by anhydrous HC104 in methylene chloride have been reported 600... 

Alkenes, with a n orbital as the HOMO and no charge, are inherently soft nucleophiles, and their nucleophilicity ought to bear some relationship to the energy of their HOMOs. The relative rates of attack by different alkenes on several electrophiles have been measured, but most systematically, Mayr has measured the nucleophilicity of a wide range of alkenes 4.18 being attacked by a family of diarylmethyl cations 4.19.15... 

The pXr values of diarylmethyl cations [22C ] were basically those determined according to the same method as for the triarylmethyl cations in aqueous sulphuric acid solutions by Deno and coworkers (Deno et ai, 1955 Deno and Schriesheim, 1955 Deno and Evans, 1957). For reasons of solubility, a wide set of pXr+ values in aqueous acetic acid solutions were determined by Mindl and coworkers (Mindl and Vecera, 1971, 1972 Mindl, 1972). Unfortunately, however, the pXr+ values, determined by means of the protonation-dehydration method for diarylmethanols, could only be obtained for more stable derivatives and not for those less stable than the unsubstituted... 

The correlations, and especially the r values, must be intimately related to the dihedral angle between the substituted phenyl ring and the plane of the carbocation. In fact, the symmetrically substituted series exhibits a linear Y-T correlation with p and r values intermediate between those for symmetrically substituted triarylmethyl and diarylmethyl cations. In the unsymmetrical (X Y) series, the p-MeO group appears to deviate from the correlation, as indicated by the SD values, and a lower r value should be assigned for these systems. The non-linear Y-T correlations with rather lower r values for protonation of diarylethylenes probably result from the conformation of the carbocation intermediates and nearly similar results are also observed in bromination. Both reactions will be considered together in a later section (p. 334). 

Chloride transfer equilibria of triarylmethyl- [34] and diarylmethyl cations [35] have been determined by H NMR spectroscopy they have been combined to give a chloride affinity scale (Scheme 5). 

The relative chloride affinities of the benzhydryl cations from Scheme 5 are now graphically displayed in Scheme 7 (bottom, right). The correlation between the chloride affinities of diarylcarbenium ions and the ethan-olysis rate constants of the corresponding diarylmethyl chlorides [39] allows the chloride affinity scale of the diarylmethyl cations to extend to the less stabilized systems (Scheme 7, bottom left). In the AG° scale shown in Scheme 7, 10 kJ mol-1 corresponds to K = 374. 

Light induced formation of a diarylmethyl cation from the dithienylethanol (418) has been proposed as the initiation step in a chain reaction leading to the dimer (419). ... 

Electrophilic attack on allyl silanes represents one of their most commonly used modes of reaction. The relative reactivity of allyl silanes and alkenes towards diarylmethyl cations has been used to calculate the value of the 0- effect of a trimethylsilyl group. 

Conclusive evidence for the cation intermediate was obtained by detection of the absorption spectra of several diarylmethyl cations following nanosecond or picosecond laser flash photolysis of their respective diazo, diphenylazi-ridinylimine, or 3H-indazole precursors in acidic media [107-112], Photochemically generated vinyl carbenes were recently shown to similarly protonate by deuterium labeling experiments to give allylic cations that were detected by transient absorption spectroscopy [113],... 

The diarylmethyl cations listed in Table 5.2 are 6-7 pJ R- units less stable with respect to the corresponding carbinols than the triarylmethyl cations, illustrating the cumulative, although not necessarily linear, effect of increasing stabilization by aryl groups. 

Cations (12), related dications, and cations (13) were obtained as stable salts and characterized by various spectral properties and X-ray structure determination. The reactivity of diarylmethyl cations in non-protic zeolites was studied. Comparison with solution reactivities shows a significant decrease in sensitivity to substiment in the zeolite. This is explained through a low isokinetic temperature for the zeolite reaction. 

CIL 90 in combination with phthalic acid efficiently catalyzed asymmetric reactions of cyclic ketones 93 with diarylhydroxymethane derivatives 94, affording alkylation products 95 with yields from moderate to high and a broad range of enantiomeric excess (8-87% ee) [104] (Scheme 22.29). Presumably, ketone 93-derived enamines and alcohol 94-sourced diarylmethyl cations were intermediates in these reactions. Desymmetrization products 95 [R, = CH2CH(R)CH2]... 

Examples are known of both alcohols and acids losing OH in 96% H2SO4. The ionization of triarylmethanols to triarylmethyl cations (ArjC+) and diarylmethanols to diarylmethyl cations (Ar2CH+) has been studied (Deno et al., 1955, 1959b, 1960). Triphenylmethyl cation is half-formed from the alcohol in 50% H2SO4 and the diphenylmethyl cation is half-formed from its alcohol in 78% acid. The behavior of monoaryl alcohols is as much in doubt as with monoaryl olefins. 

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