Carbonium ions trityl

In contrast to the findings of van Tamelen et al., whose experiments employed carbonium ion concentrations of 10 M, irradiation of the trityl ion in somewhat lower concentrations ( 10 m) in 99% sulfuric acid in the absence of hydrogen donors with oxygen present yielded only two products, ions 109 and 110 (Allen and Owen, 1971). An excited triplet carbonium ion is again invoked, which is thought to give rise to the intermediate 111. This species can then lose an allylic hydride ion... 

The addition of chloride ion to the medium in the case of the trityl chloride reaction reduces the net rate by virtue of the back reaction to regenerate trityl chloride. The product composition in the presence of various added reagents is a quantitative measure of the relative reactivity of the added reagents towards the carbonium ion. The results are summarized in Table VI. 

The addition of a cation to an olefin to produce a carbonium ion or ion pair need not end there but may go through many cycles of olefin addition before the chain is eventually terminated by neutralization of the end carbonium ion. Simple addition to the double bond is essentially the same reaction stopped at the end of the first cycle. The addition of mineral acids to produce alkyl halides or sulfates, for example, may be prolonged into a polymerization reaction. However, simple addition or dimerization is the usual result with olefins and hydrogen acids. The polymerization which occurs with a-methyl-styrene and sulfuric acid or styrene and hydrochloric acid at low temperatures in polar solvents is exceptional.291 Polymerization may also be initiated by a carbonium ion formed by the dissociation of an alkyl halide as in the reaction of octyl vinyl ether with trityl chloride in ionizing solvents.292... 

Hirschler and Hudson (36/6), however, favor the opinion that Bronsted sites are exclusively responsible for the activity of silica-alumina. In studying the adsorption of perylene and of triphenylmethane, they concluded that carbonium ions are not formed by a hydride abstraction mechanism as claimed by Leftin (362). Instead, triphenylmethane is oxidized by chemisorbed oxygen to triphenylcarbinol in a photo-catalyzed reaction, followed by reaction with a Bronsted acid giving water and a triphenylmethyl carbonium ion. After treatment with anhydrous ammonia, the organic compound was recovered by extraction as triphenylcarbinol. About thirteen molecules of ammonia per assumed Lewis site were required to poison the chemisorption of trityl ions. The authors explain the selective inhibition of certain catalyzed reactions by alkali by assuming that only certain of the acidic protons will ion-exchange with alkali ions. 

Richards and Hill have recently obtained quantitative evidence of the stabilization of a -metallocenyl carbonium ions (38, 95). They have shown that sol-volyses of methylmetallocenylcarbinyl acetates proceed via a carbonium ion mechanism, and that these acetates solvolyze with rates greater than even tri-phenylmethyl (trityl) acetate. Further, the relative rates of solvolysis and therefore the order of carbonium ion stabilities increas.e, proceeding from the iron to the osmium acetate. A portion of these data is summarized in Table II. 

Hydride Abstraction from Organic Ligands The removal of a hydride ion from an organic radical is an important method of generating carbonium ions stabilized by metal carbonyl systems. Dauben and Honnen (61) in 1958 were the first to exploit this method by use of the powerful hydride abstractor, triphenyl methyl (or trityl) carbonium ion, which is converted thereby into triphenylmethane. 

When this hydride abstraction was tried with the iron-cycloheptatriene analog, a carbonium ion complex was formed, but by trityl alkylation. 

Lewis acids readily isomerize both 1,3-dioxolanes and 1,3-oxathiolanes in ether solution. The reaction proceeds by coordination with the oxygen atom in the latter case since 1,3-dithiolanes do not isomerize under the same conditions. With trityl carbonium ion, an oxidative cleavage reaction takes place as shown in Scheme 6. Hydride extraction from the 4-position of 2,2-disubstituted 1,3-dioxolanes leads to an a-ketol in a preparatively useful reaction. 1,3-Oxathiolanes are reported to undergo similar cleavage but no mention of products other than regeneration of the ketone has been made (71CC861). Cationic polymerization of 1,3-dioxolane has been initiated by a wide variety of proton acids, Lewis acids and complex catalytic systems. The exact mechanism of the polymerization is still the subject of controversy, as is the structure of the polymer itself. It is unclear if polymerization... 

Kuntz (13) and Dreyfuss et al. (8, 9) have used NMR spectroscopy to study the THF polymerizations initiated by trityl salts. The results seem to indicate that the initial reaction that occurs between THF and the carbonium ion or diazonium ion salts is analogous to the one shown in Reaction 3. 

Copolymerization between an oxonium ion type monomer and a carbonium ion type monomer has never been carried out successfully. Styrene (St) does not form a copolymer with THF (1), BCMO (1), or /3-PL (2, 16). The formation of a homopolymer mixture was confirmed for the St-/ -PL system (18,19, 26). The reason for the absence of cross propagation was discussed elsewhere (6), but the reaction of the trityl cation with fi-PL and the reaction of the triethyloxonium ion with 1,1-diphenylethylene did show the absence of the bonding reaction (6). 

The failure of difluoramine to appear among the final products is not particularly surprising. In the presence of nitric acid and/or nitrogen oxides, it might easily be oxidized and may well constitute the source of the silicon tetrafluoride. The formation of a carbonium ion from trityl-difluoramine would be favored by resonance stabilization. In the tert-butyl case, on the other hand, this driving force is not present and formation of the ion would be expected to occur less readily. In addition, both the tert-butyl carbonium ion and the difluorammonium ion from which it is derived would be more subject to a variety of side reactions than the corresponding trityl species. 

Oxidmion of ketone acetals and ethers. Ketones can be regenerated from the ethylene acetal derivatives by treatment with trityl fluoroborate in dry dichloro-methanc (Nj) at room temperature. Thus the reaction of trityl fluoroborate with cyclohexanone ethylene acetal results in cyclohexanone (80%) and triphenylmethane. The reaction thus involves hydride transfer to the triphenyl carbonium ions. Triethyl-oxonium fluoroborate can also be used but is somewhat less effective than trityl fluoroborate. 

Starting from the hypothesis that azo compounds with an ability to form active carbonium ions in vivo are biologically active, Biichel et al. (1972,1975) synthesised several derivatives of 1-trityl azoles. They established in extensive research work that the derivatives particularly active are those which contain only one substituent in one of the phenol rings, and in which the azole ring remains unsubstituted. This led to the very active antimycotic clotrimazole (see Section 5.7.1), which nowadays plays an important role in the local therapy of human mycoses, and to fluo-trimazole, of specific activity against powdery mildews. 

In the case of triphenylsilane and trityl chloride, this reaction is first order in both reactants in benzene (233). The reaction rate for modified substrates indicates that with respect to R3CX, rate is a function of proclivity to ionize, trityl halides being more rapid than benzhydryl or allyl or tcr/-butyl halides. Indeed the latter required the presence of BBr3 as a catalyst. Within a given series. Cl > Br > I, phenylsilanes react more rapidly than alkyl silanes. A mechanism involving simultaneous nucleophilic attack of halide on silicon, and electrophilic attack on hydrogen by the carbonium ion is reasonable,... 

An essential characteristic of ferrocene chemistry is the stabilization of ferrocenyl carbonium ions. These carbocations are mesomers of the corresponding hexahapto fulvene complexes [FeCp(r -fulvene)]+. They are even more stable than the trityl cation PhsC". The stabilization of the a-ferrocenyl carbonium ions explains the acetolysis of vinylferrocene, the hydrolysis of the acetate formed, the ease of nucleophilic substitution in a position, and the OH" abstraction from the a-ferrocenyl alcohols. This stabilization is still enhanced by going down in the iron column of the periodic table, because the size of the d orbital increases, which facilitates their insertion with the carbocation and accelerates the solvolysis of acetates (Os > Ru > Fe). 

The trityl group is stable to base, may be removed by hydrogenolysis, but is very susceptible to acid. It can be removed under conditions (i.e., 80% acetic acid, hour, 30°C) that will not affect the tert-BOC and CBz functions. A variety of acids may be used which include acetic acid, HCl in water, acetic acid, methanol and chloroform, and trifluoroacetic acid. The case of acidolysis is the result of the stabilized carbonium ion ... 

As with the amino acids, the trityl function may be removed from the nucleoside by treatment with acid (i.e., 80% acetic acid, pyridine-acetic acid buffer). Again, this proceeds by way of a stabilized carbonium ion intermediate. Further stabilization is achieved by the introduction of p-methoxy functions. Thus, the acid lability of the tritylated nucleoside is increased approximately by a factor of ten for each p-methoxy group present. In the case of nucleosides and nucleotides, removal of the p-methoxytrityl function... 

As predicted from the pKr+ values (Table 7.4) the S-di-p-methoxybenzhydryl group [18] is cleaved much more rapidly than other S-benzhydryl thioethers. Konig et al. [55] report complete removal in 2 hr with trifluoroacetic acid (70 ) or in 10 min with added anisole. Photaki et al. [58] report 80% removal in 2 hr (20-25°) with trifluoroacetic acid and 15% phenol. The pKr value of this cation predicts it to be more stable than the S-trityl cation which in turn su ests a more rapid conversion of the conjugate acid of the thioether to the carbonium ion and thiol and a selectivity comparable with trityl in the reaction with nucleophiles. This prediction is not entirely borne out by facts and indeed the S-di-p-methoxybenzhydryl group falls between the benzhydryl and trityl in reactivity. This lowered reactivity is probably due to the presence of significant concentrations of the less reactive dication. 

The trityl carbonium ion is well documented as a hydride-ion acceptor indeed it can be reduced by aldehyde acetals in an intramolecular hydride-transfer process. This is pivotal in a new method for the deacetaliza-tion of ketone acetals by oxidative hydride transfer trityl fluoroborate is used as reagent, and the glycol moiety is oxidized to an a-ketol, as is shown in Scheme 79. This novel procedure is successful also with hemithioacetals, but fails with thioacetals. 

Steroidal alcohols, masked as their tetrahydropyranyl ethers, benzyl ethers or bismethylenedioxy-derivatives, can be deprotected by the trityl carbonium ion in a hydride-transfer process. This method of regeneration should be equally applicable to other protected groups such as amines and carboxylic acids. 

Direct evidence in support of the analysis of eq. (2) is found in the work of Pocker (73), who investigated the effect of added ionic chloride on the ultraviolet absorption spectra of solutions of trityl chloride in liquid SO2 and found that only part of the characteristic carbonium ion absorption can be suppressed by addition of excess chloride. This... 

In order to examine the behaviour of Eq. (6) we need to define likely values of c/K and fi. For reasons which will be explained later, values of c greater than about 10 5 M need not be considered for carbonium polymerisations, and a useful lower limit is probably about 10 10 M. As far as K is concerned, in solvents such as ethylene and methylene dichloride, whose D.C. lies between ca. 9 and 16, according to the temperature, the evidence (6, 7, 8, 9) shows that under these conditions the dissociation constants of trityl salts are in the range 10 4 to 10 5 M. This is the only information which we have on the dissociation constants of carbonium salts, and it is of little direct relevance to the likely dissociation constants of ion-pairs involving... 

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