Triarylmethyl derivatives

In the older literature there are reports of colorless but conducting solutions of triarylmethyl derivatives. It seems well-established that triarylmethyl cations are colored, but an oxonium ion formed by the covalent attachment of a carbonium ion to an ether oxygen atom, for example, would be colorless. 

Just as interaction of the negative ion with solvent or with a catalyst can promote the ionization, the same is true of the interactions of the carbonium ion. The triaryl carbonium ions discussed so far should all be colored. Reports of colorless but conducting solutions of triarylmethyl derivatives are fairly common and may represent covalently solvated carbonium ions which are not expected to be colorless. For example, a colored solution of the ion XVI in acetic acid slowly fades when it is diluted with methanol containing enough acid to prevent the formation of any "color base. The fading is attributed to the formation of XVII.m... 

There were several attempts to gain better control on the free radical polymerization process [18, 19], One of these methods was named the iniferter method. The compounds used in this technique can serve as m/tiator, trans/er agent and terminating agent [20-22], Another technique is based on the use of bulky organic compounds such as diaryl or triarylmethyl derivatives [23-25], The main disadvantages of these systems comprise slow initiation, slow exchange, direct reaction of counter radicals with monomers, and their thermal decomposition. Therefore, these techniques did not offer the desired level of control over the polymerization. 

Most carbocations are too reactive to be directly observable in ordinary solvents, and until relatively recently evidence has been obtained indirectly, primarily through the study of reaction kinetics and trapping processes, experiments discussed in Sections 5.1, 5.2, and 5.4. Nevertheless, a few types of compounds have long been known to produce observable concentrations of positive ions relatively easily. The triarylmethyl derivatives were the first of this type to be investigated the halides ionize readily in non-nucleophilic solvents such as sulfur dioxide,70 and the alcohols yield solutions of the ions in concentrated sulfuric acid. Early observations by the freezing-point depression technique (see Section 3.2, p. 130) established that each mole of triphenyl carbinol yields 4 moles of ions in sulfuric acid, the reaction presumably being by way of Equation 5.14.71 Results in methane-sulfonic acid are similar.72... 

A study33 has been made of the rearrangement of /V-triarylmethylanilines to their p-triarylmethyl derivatives. A-Methyl-A-nitrosoaniline has been observed to undergo a Fischer-Hepp rearrangement when treated with montmorillonite clay,34 and /V-aryl-A-nitrosohydroxylamine ammonium salts (30) have been transformed35 into azoxy... 

The other trends in values for the various Rs in Table V are consistent with the trends in pairwise interaction contributions to heats of formation evaluated by Allen (19) and discussed by Hine (20). For example, pyronin has a C-C-X pair interaction in place of an O-C-X interaction for pyridine-4-carboxaldehyde for X = OH this difference favors the aldehyde adduct by approximately 7 kcal, although for other Xs, the difference is somewhat smaller. Thus, the values given for the aldehyde are smaller than those for pyronin. We have already pointed out that steric effects are expected in the triarylmethyl derivatives and that the comparison of these with pyronin derivatives are consistent with that expectation (1). 

Poly(butylene sebacate)-rofaxa-(30-crown-10) Poly(butylene sebacate)-rofaxa-(60-crown-20) Poly(decamethylene sebacate)-rofaxa-(30-crown-10) With and without a triarylmethyl derivative as blocking group efficiency of threading depends on size of the macrocycle blocking group is not always needed for stability. [237] [238]... 

This group was developed for protection of the 5 -OH group in nucleosides. The derivative is prepared from the corresponding triarylmethyl chloride and is cleaved by reductive cleavage (Zn/AcOH) of the phenacyl ether to the / -hydro-xyphenyldiphenylmethyl ether, followed by acidic hydrolysis with formic acid." ... 

The preparation of triarylmethyl azides via the hydrazine derivative and nitrous acid has been reported [52-54]. A more convenient method is via the carbinol and sodium azide [55]. 

Like most photochemical reactions, benzylic fragmentation has been known from the beginning of the twentieth century. The photoinduced cleavage of triphenylmethylcarbinol has been noted by Gomberg in Ann Arbor in 1913 [1] and the generation of triarylmethyl cationic dyes from their leuco form by Lifschitz in Zurich in 1919 [2]. A solution of the colorless derivative obtained from p-rosaniline and cyanide became colored in a few seconds when exposed to an iron arc lamp (Sch. 1, old and new notation). 

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 rate constants for the solvent-recombination process of the carbocations [3C (X,Y,Z)] were determined by the use of the azide clock method (Richard etal., 1984 Richard and Jencks, 1984a,b,c McClelland et al., 1991) and the rate constant of the forward reaction was derived using (38b) as /Ch = /CwXr+ (McClelland et al., 1989,1991). While ordinary Hammett-type relationships were found to be inapplicable to the substituent effects on solvent recombination, there is a rate-equilibrium correlation for all available data on triarylmethyl cations, shown as the linear log/c , vs. p/Cr<- plot, in Fig. 34 with a slope of 0.64. Such a relationship was earlier suggested by Arnett and Hofelich (1983) and Ritchie (1986). The correlation of ky, with the cr scale was... 

Both the results and the reasoning, however, were rigorously confirmed shortly thereafter. Eventually it became clear that Gomberg s synthetic failure led to the discovery of stable radicals, a new class of organic species, derivatives of trivalent carbon. Further studies led to the understanding that the triarylmethyl system is extremely versatile and gives rise to the preparation of other types of derivatives of trivalent carbon, e.g. salts of the triphenylmethyl cation 66b and the triphenylmethyl anion 66c. 

The W -Trt groups are somewhat more stable than the A/ -Trt derivatives. In fact, cleavage of Trt-Lys-OH or Trt-Om-OH with 1% trifluoroacetic acid in dichloromethane occurs within 3 minutes at 0 °C, while for the corresponding A/ -derivatives 2 hours at room temperature are required for deprotection with 5% trifluoroacetic acid in dichloromethane. This differentiated acid lability is exploited for the synthesis of A/ -triarylmethyl-protected lysine and ornithine derivatives that are readily obtained by selective N -deprotection of a,co-bis-pro-tected diamino acids via treatment with mild acids.The subsequent N -protection, e.g. with the Fmoc group, is then carried out by standard procedures under Schotten-Baumann conditions. ... 

The Trt group, as well as the related Mtt and Mmt derivatives, are sometimes preferable over Boc protection, since the low electrophUicity of the extremely bulky triarylmethyl cations that are formed during cleavage does not lead to any side reaction at the level of sensitive amino acid residues such as Trp, Met, and Tyr.b i l As an example, the peptide H-Thr-Thr-Trp-Ser-Met-Ser-Trp-Tyr-OH was obtained with 94% homogeneity by using the Fmoc/Trt strategy as compared to 44% of the peptide prepared by using the standard Fmoc/tBu protection. ... 

In a landmark paper, Breslow and coworkers described the determination of pA), values of weak hydrocarbon acids by use of thermochemical cycles involving electrochemical reduction data for triarylmethyl, cycloheptatrienyl, and triphenyl- and trialkylcyclopropenyl cations and radicals [9aj. Later, they derived pATa data from standard oxidation potentials and bond-dissociation energies [9b, c]. The methodology was further developed by Nicholas and Arnold [10a] for the determination of cation radical acidities, and later modified and extensively used by Bordwell and coworkers [10b, c] so that homolytic bond-dissociation energies and cation radical... 

Anyone who has worked with triarylmethyl or pyronin systems has probably been struck by the distinctions between the solid states of various derivatives. Malachite Green chloride, for example, is a highly colored crystalline ionic material, and the carbinol is a colorless, reasonably low nelting [mp 163 °C (14)] solid. This distinction between ionic and covalent iolids can be considered in another thermodynamic cycle ... 

For triarylmethyl and pyronin derivatives, the solubilities of covalent compounds, such as the carbinols, are on the order of 10-5 M in water. Thus, must be approximately — 7 kcal/mol. 

In water, N3 is much less reactive in aromatic nucleophilic substitution than expected from its reactivity toward carbocations, that is, its N+value. Ritchie (43) initially developed his N+ scale from nucleophilicities toward preformed carbocations and the scale fits the data for nucleophilicities toward many electrophiles, regardless of their charge. However, in water, and similar hydroxy lie solvents, the nucleophilicity of azide ion, relative to that of other anions, seems to be related to the carbocation-like character of the electrophile. An acyl derivative with its sp2 carbonyl group is somewhat akin to a carbocation stabilized by an alkoxide group, >C=0 <-— >C+-0 , just as a triarylmethyl carbocation is stabilized by electron delocalization into the aryl groups and azide ion is a good nucleophile toward these electrophiles. As compared with anions such as OH- or CN , azide ion, in water, is very reactive toward carbocations and in deacylation but is relatively unreactive toward dinitrohaloarenes (44). 

The stereochemistry of nucleophilic substitution reactions has been examined for substrates ranging in complexity from primary alkyl to triarylmethyl. A summary of representative examples is presented in Table 5.12. Chiral 1-butanol-l-rf and its derivatives have small, but measurable, optical rotations and provide useful substrates for the important case of substitution in primary systems. Entry 1 in Table 5.12 illustrates the stereospecific inversion observed in 1-butyl-1-rf... 

It has also been suggested that such conformation must be very rigid (high enantiomerization barrier for the reversal of propeller helici-ty) due to the intramolecular congestion of the three pairs of voluminous ortho-chlorine atoms. Those clathratogenic features encouraged us to start a systematic study of the inclusion properties of PTM radical, functionalized derivatives and other polychlorinated triarylmethyl radicals. ... 

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