Triphenylmethyl, stability

Additional phenyl substituents stabilize carbocations even more Triphenylmethyl The triphenylmethyl group is 1, , t 11 1. . 1. .. 

The l,2,3-tri-/-butylcyclopropenium cation is so stable that the perchlorate salt can be recrystallized from water. An X-ray study of triphenylcyclopropenium perchlorate has verified the existence of the carbocation as a discrete species. Quantitative estimation of the stability of the unsubstituted ion can be made in terms of its pXn+ value of —7.4, which is intermediate between those of such highly stabilized ions as triphenylmethyl... 

Additional phenyl substituents stabilize carbocations even more. Triphenylmethyl... 

The N+ relationship, as discussed above, is a systematization of experimental facts. The equation of Scheme 7-4 has been applied to nearly 800 rate constants of over 30 electrophiles with about 80 anionic, neutral, and even cationic nucleophiles covering a range of measured rate constants between 10-8 and 109s 1 (Ritchie, 1978). Only about a dozen rate constants deviated from the predicted values by more than a factor of 10, and about fifty by factors in the range 5-10. It is therefore, very likely that this correlation is not purely accidental. Other workers have shown it to be valid for other systems, e.g., for ferrocenyl-stabilized cations (Bunton et al., 1980), for coordinated cyclic 7r-hydrocarbons (Alovosus and Sweigart, 1985), and for selectivities of diarylcarbenes towards alkenes (Mayr, 1990 Mayr et al., 1990). On the other hand, McClelland et al. (1986) found that the N+ relationship is not applicable to additions of less stable triphenylmethyl cations. 

The first conclusive qualitative evidence for the relative stability of aryldiazenyl radicals was the isolation of phenylazotriphenylmethane (8.50) in the thermolysis of a-(phenylazo)cumene in the presence of triphenylmethyl radicals by Porter et al. (1978), as shown in Scheme 8-32. 

Steric effects have been discussed in free radical chemistry ever since the discovery of the first free radical, triphenylmethyl 1 by M. Gomberg in 19001. To what extent is the dissociation of its dimer, which was believed to be hexaphenylethane 23 till 19682 determined by electronic stabilization of triphenylmethyl 1 or by steric strain in its dimer ... 

Because of the high stability of the triphenylmethyl carbocation, the reductive ether cleavage of trityl ethers with EtySiH/trimethylsilyl triflate (TMSOTf) is highly successful. This reaction even occurs in the presence of highly reactive sugar ketals, leaving the ketals intact (Eq. 126).269... 

The situation with polyarylmethanes is very similar. Due to the stabilization of free valence in arylmethyl radicals, the bond dissociation energy (BDE) of the bond C—02 for example, in triphenylmethyl radical is sufficiently lower than in alkylperoxyl radicals. This radical is decomposed under oxidation conditions (room temperature), and the reaction of Ph3C with dioxygen is reversible ... 

A structure that prevents coplanarity prevents resonance stabilization to a corresponding degree. Thus the triptycyl radical has none of the stability of triphenylmethyl and the benzene rings are tied back in a position with the very maximum deviation from coplanarity.16... 

Since triphenylmethane is not stabilized by any resonance not already present in hexaphenylethane, the difference between the two heats of hydrogenation, or 22 kcal., might be a measure of the steric effect alone. The difference in the heats of dissociation into radicals when ethane and hexaphenylethane are compared is 62 kcal. This leaves about 40 kcal. to be accounted for as resonance stabilization of the radical.16 This degree of resonance stabilization for the triphenylmethyl radical does not violate quantum mechanical expectations. 

Tetra-/>-anisy lhy drazine is green in benzene at room temperature.185 On the other hand, 1,2-diphenyl-1,2-di- >-nitrophenylhydrazine appears not to be dissociated at all. If this difference in degree of dissociation is real, and not a specious one due merely to a difference in color or reactivity of the radicals, it presents a puzzling contrast to the case of diphenylpicrylhydrazyl and to the triphenylmethyl series in which both kinds of substituent stabilize the radical. 

The ortho-ring junction that converts the triphenylmethyl structure into that of the ion LX increases the stability of the carbanion but decreases that of the carbonium ion. It will be recalled that this structural modification of the triphenylcarbonium ion had about the same effect as the introduction of one to two nitro groups. 

The triphenylmethyl carbocation shows its stability because the positive charge on the carbon is distributed uniformly over a number of structures. 

The main limitation of these CSPs is their limited pressure stability, which makes them not very suitable for HPLC application. However, they have proved to be an excellent tool for the preparative separation of drugs by low-pressure HPLC. To make these CSPs accessible to HPLC, silica gel-based phases were developed. " This type of phase is available from Merck (Darmstadt, Germany) under the name Chiraspher. Polymer phases of different types have been developed by Okamoto s group. > They are prepared by the asymmetric polymerization of triphenylmethyl-methacrylate monomers. The original character of these polymers is that they do not possess any chiral centre and therefore their chirality is only due to their helicity. However, clear mechanisms have not been proposed... 

One of the most stable carbocation structures is the employing all three rings. Trityl chloride ionizes read-triphenylmethyl cation (trityl cation). In this struc- ily, and can capture an available nucleophile, ture, the positive charge is stabilized by resonance... 

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