Diphenylmethyl cation

CF3COOH, anisole, CH2Cl2. Anisole is present to scavenge the diphenylmethyl cation liberated during the cleavage reaction. 

Figure 1. Time-dependent absorption spectra obtained after laser excitation (248 nm, 20 ns) of benzophenone (frans-2,3-diphenylaziridin-l -yljimine (134d) in oxygen-saturated TFE.64 Inset influence of the laser dose on the yields of stilbene (monophotonic) and diphenylmethyl cation (biphotonic).

Thus a number of canonical structures is possible. Like triphenylmethyl cation, diphenylmethyl cation is also very stable. In some cases the carbocations are so stable that their salts have been... 

We have reported the synthesis of a series of azulene-substituted methyl cations, i.e., tri(l-azulenyl)methyl, di(l-azulenyl)phenylmethyl, and (1-azul-enyl)diphenylmethyl cations (2a+, 3a+, and 4a+) (Figure 6) (6). In order to examine substituent effects on the azulene rings and to enhance their stabilities, a series of the cations (2b-d+, 3b-d, and 4b-d+) bearing tert-buty groups on each azulene ring were also synthesized (7). 

Is the behavior of fluorenylidene dications, such as 3 or 5 unusual Comparison of the average chemical shifts of fluorenyl cations 7 with the analogous diphenylmethyl cations shows minimal antiaromaticity in the monocations, with paratropic shifts of 0.5 ppm or less (29). The dramatic para-... 

It is especially interesting to examine the thermodynamics of reaction (i) for the trityl and dityl (diphenylmethyl) cations as initiating salts for isobutylene because we can thus provide a theoretical explanation of the experimental fact that trityl salts do not initiate isobutylene polymerisation, but dityl salts do Table 2 shows the relevant data. The solvation energy terms have been omitted since on the basis of a... 

Table 1.3 provides rate constants for the decay of selected carbocations and oxocar-bocations in H2O, TFE, and HFIP. As a general comment, water, methanol, and ethanol are highly reactive solvents where many carbocations that are written as free cations in standard textbooks have very short lifetimes. The diphenylmethyl cation, with two conjugating phenyl groups, has a lifetime in water of only 1 ns. Cations such as the benzyl cation, simple tertiary alkyl cations such as tert-butyl, and oxocarbocations derived from aldehydes and simple glycosides, if they exist at all, have aqueous lifetimes in the picosecond range, and do not form and react in water as free ions. This topic is discussed in more detail in Chapter 2 in this volume. 

A subsequent picosecond electronic absorption spectroscopic study of TPE excited with 266- or 355-nm, 30-ps laser pulses in cyclohexane found what was reported previously. However, in addition to the nonpolar solvent cyclohexane, more polar solvents such as THF, methylene chloride, acetonitrile, and methanol were employed. Importantly, the lifetime of S lp becomes shorter as the polarity is increased this was taken to be evidence of the zwitterionic, polar nature of TPE S lp and the stabilization of S lp relative to what is considered to be a nonpolar Sop, namely, the transition state structure for the thermal cis-trans isomerization. Although perhaps counterinmitive to the role of a solvent in the stabilization of a polar species, the decrease in the S lp lifetime with an increase in solvent polarity is understood in terms of internal conversion from to So, which should increase in rate as the S -So energy gap decreases with increasing solvent polarity. Along with the solvent-dependent hfetime of S lp, it was noted that the TPE 5ip absorption band near 425 nm is located where the two subchromophores— the diphenylmethyl cation and the diphenylmethyl anion—of a zwitterionic 5ip should be expected to absorb hght. A picosecond transient absorption study on TPE in supercritical fluids with cosolvents provided additional evidence for charge separation in 5ip. 

Phenylmethyl halides are similar in SN1 reactivity to 2-propenyl halides. The SN1 reactivity of phenylmethyl derivatives can be ascribed mainly to stabilization of the cation by electron delocalization. Diphenylmethyl halides, (C6HS)2 CHX, are still more reactive and this is reasonable because the diphenylmethyl cation has two phenyl groups over which the positive charge can be delocalized and therefore should be more stable relative to the starting halide than is the phenylmethyl cation ... 

It has been demonstrated9 that photo-protonation of 2-diphenylmethyl-l,3-dimethoxy-benzene (4) to the cyclohexadienyl cation (5) not only results in dissociative cleavage of the diphenylmethyl cation (the reverse of the step in a Friedel-Crafts alkylation that produces the cyclohexadienyl cation), but is accompanied by a surprising rearrangement to the isomeric 2,4-dimethoxybenzenium ion (6). This (see Scheme 3) represents the first example of a system where rearrangement involving two isomeric... 

Product formation was interpreted in terms of transalkylation of substituted triphenylmethanes. Protonation at the ipso position of the substituted phenyl ring to form arenium ion 64 followed by the C—C bond breaking yields the diphenylmethyl cation, which alkylates benzene or is stabilized by hydride transfer (Scheme 5.30). The protonated intermediate 64 is highly unstable when the ring has an electron-withdrawing substituent. Consequently, its transformation is extremely slow and the primary product triphenylmethane can be isolated. 

Charge transfer reactions of 132, cyano-substituted diphenylmethyl radicals (148,152), and 2-phenanthrylmethyl radical (159) with CC14 and CBr4 have also been reported [97,99,101]. A decreased rate constant for CBr4 quenching of the a-cyano-substituted diphenylmethyl radical (152) relative to the parent radical provided indirect evidence of charge transfer [97,145]. More recently, the diphenylmethyl cation was observed directly in acetonitrile solvent with added CC14 [145,149] (Scheme 25). 

The following represent the resonance forms of the diphenylmethyl cation ... 

Swain et al. (1953b) noted that a qualitative relationship exists between the stability of a carbocation and its selectivity. For example, the selectivity of a number of carbocations in aqueous solution and in the presence of azide ion was enhanced with increasing carbocation stability the ratio An /Aw, where An and Aw are the specific rate constants with azide ion and water respectively, was found to increase from 3-9 for the t-butyl cation to 170 for the diphenylmethyl cation, to 240 for the 4,4 -dimethyldiphenylmethyl cation, and to 280,000 for the highly stabilized triphenylmethyl cation. Sneen et al. (1966a) observed that this relationship could be quantified. It was found that a plot of log (An /Aw ) against log A (where A is the solvolytic rate constant) for a number of alkyl chlorides gave a linear correlation. Sneen made the first attempt to utilize such a relationship as a mechanistic tool. The selectivity of... 

Fig. 21 The structures of a-trifluoromethyl-a,a-diphenylmethyl cations, optimized at the RHF/6-31G level. From top, unsubstituted, p,p -dimethoxy, mono-p-methoxy and p-methoxy-3,5-dichloro cations. Reproduced with permission from Fujio et al. (1999). Copyright 1998 John Wiley Sons.

The rate constants for reactions of highly stable triphenylmethyl and diphenylmethyl cations with various ionic and neutral nucleophiles have been measured (Gandler, 1985 McClelland etal., 1986) in aqueous acetonitrile and discussed from the view point of a reactivity-selectivity relationship. 

Greater stabilization can be achieved through the attachment of a second phenyl ring, producittg the diphenylmethyl cation. Ph CH . A third ring gives the triphenylmethyl cation, Ph C whidi is even more stable. The triphenylmethane dyes, e.g. 

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