Trityl salt

X 10 M at 25°C for the trityl salts, and = 0.5 x 10 M at 0°C for the tropylium salts. Note that those Tables contain some misprints and the ordinal references should be consulted for the correct values. A recent study by Gogolczyk et al. is particularly interesting in this context because while it confirms the above value of for trityl salts in dichloromethane, it also reports for the first time values of the dissociation constant in nitromethane and in mixtures of the two solvents. The value in pure nitromethane is more than a hundred times higher than that in dichloromethane, as expected from the considerable increase in polarity. Obviously then, the proportion of free ions in CH3NO2 is very high and at salt concentrations below 10 M they will be the predominant species. It must be emphasised that these ionic salts are only sparingly soluble in solvents of low dielectric constant such as carbon tetrachloride and therefore polymerisations caimot be carried out in these media. [Pg.190]

Having established some general criteria concerning the state of these salts in the solvents suitable for cationic polymerisation, it is advisable briefly to survey the most relevant unwanted reactions of these q ecies which could arise in the course of their use as initiators. [Pg.190]

Brauman and Archie used NMR spectroscopy to study the reaction of trityl fluo-roborate with D2O in acetonitrile with fairly high salt concentrations. They established the occurrence of a series of equilibria leading to the formation of the carbinol [Pg.190]

They could not obtain a value of K, but concluded that the second order rate constant for the hydration reaction must be greater than 10 sec . Slomkowskietal. stud- [Pg.191]

Reactions of trityl salts with other strong nucleophEes such as alcohols, amines etc., are also very fast and efficient as recently shown by the excellent work of Dorfman s poup Recent work has also shown the high reactivity of this salt towards ethers and acetals [Pg.191]

Alkoxy ketones. These ketones can be prepared by an aldol-type reaction of enol ethers with acetals catalyzed by a trityl salt. Methoxymethyl (MOM) enol ethers are more reactive than methyl enol ethers. [Pg.345]

Representative Procedure for Trityl Salt Promoted Clycosylation with Glycosyl Phenyl Carbonate Donors [360]... [Pg.151]

The initiation of the cationic polymerisation of alkenes is examined in detail by means of simple thermodynamic concepts. From a consideration of the kinetic requirements it is shown that the ideal initiator will yield a stable, singly charged anion and a cation with a high reactivity towards the monomer by simple, well defined reactions. It must also be adequately soluble in the solvent of choice and for the experimental method to be used. The calculations are applied to carbocation salts as initiators and a method of predicting their relative solubilities is described. From established and predicted data for a variety of carbocation salts the position of their ion molecule equilibria and their reactivity towards alkenes are examined by means of Born-Haber cycles. This treatment established the relative stabilities of a number of anions and the reason for dityl, but not trityl salts initiating the polymerisation of isobutene. [Pg.189]

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... [Pg.202]

Despite these limitations the results in Table 2 predict correctly that dityl salts will initiate the polymerisation of isobutylene but that trityl salts will not, provided that under the reaction conditions AH10 has the same sign as AH % and that the AH 0soi(R+) and the TAS terms are such that the AG10 have the same sign as the corresponding AH 01O, all of which seems to be probable. [Pg.203]

On the basis of this argument it is probable that the tris-p-halogen substituted trityl salts will be useful initiators for the polymerisation of isobutylene since their ionisation potentials probably exceed 690 kj mol1 [38], and the p-substitution would inhibit the back-biting reactions which make both trityl and dityl salts unstable [39]. [Pg.204]

We consider first the problem arising from the nature and shape of the cations. Many workers have attempted to estimate the KD of the ion-pair P+nA" for various growing cations with various A", basing their estimates on the KD of the corresponding trityl salts in the same solvent. These attempts were misguided because of the geometrical and electrostatic differences between the propeller-shaped trityl cation and the most common... [Pg.513]

Acid-catalyzed dealkoxylation is particularly suitable for the preparation of highly reactive, cationic iron(IV) carbene complexes, which can be used for the cyclopropanation of alkenes [438] (Figure 3.11). Several reagents can be used to catalyze alkoxide abstraction these include tetrafluoroboric acid [457-459], trifluoroacetic acid [443,460], gaseous hydrogen chloride [452,461], trityl salts [434], or trimethylsilyl triflate [24,104,434,441,442,460], In the case of oxidizing acids (e.g. trityl salts) hydride abstraction can compete efficiently with alkoxide abstraction and lead to the formation of alkoxycarbene complexes [178,462] (see Section 2.1.7). [Pg.85]

This method exploits the relative weakness of the E-H bond and involves the transfer of the hydride from the element to a strong Lewis acid, in most cases to trityl cation. The easy access of trityl salts with a wide variety of weakly coordinating counteranions is a clear advantage of this method. The reaction can be applied in polar solvents such as sulfolane, ethers and nitriles but also in chlorinated... [Pg.157]

The reactions proceeded efficiently under mild conditions in short time. The silyl enol ethers reacted with the activated acetals or aldehydes at -78 °C to give predominant erythro- or threo-products [136, 137] respectively. In the same manner, the aldol reaction of thioacetals, catalyzed by an equimolar amount of catalyst, resulted in <-ketosulfides [139] with high diastereoselectivity. In the course of this investigation, the interaction of silyl enol ethers with a,]3-unsaturated ketones, promoted by the trityl perchlorate, was shown to proceed regioselec-tively through 1,2- [141] or 1,4-addition [138]. The application of the trityl salt as a Lewis acid catalyst was spread to the synthesis of ]3-aminoesters [142] from the ketene silyl acetals and imines resulting in high stereoselective outcome. [Pg.373]

The next investigation condncted by the gronp of Chen [147], involved the chiral trityl salt 53 and thus brought clearness to a certain extent in the nnderstanding of the mechanism of the catalysis (Scheme 55). Since enantiomeric excess was achieved, the carbenium-mediated catalysis should not be disregarded. The correlation of the results manifested the concurrence of the catalytic species and dependence of their participation in the catalysis on the silyl substituents and counter ions in the trityl salts. [Pg.374]

Finally, one example of trityl salt analogues in the phase-transfer catalysis is presented. The highly stable triazatriangulenium cations 62 [161, 162] were jnst recently introduced to the phase-transfer chemistry [163], Persistent to strongly basic and nncleophilic conditions, these salts revealed efficient catalytic activity in addition reactions (Scheme 64). Modification of the alkyl side chains on nitrogen allowed matching the fair hydro/lipophilicity with the optimised conditions in the alkylation, epoxidation, aziridination and cyclopropanation reactions. The results are comparable to those of tetrabutylammonium salts and in some cases showed even a better outcome. [Pg.378]

Cationic polymerization of thietanes initiated by trityl salts mechanism of the initiation reaction. ... [Pg.279]

Various agents used for aromatization of 4//-thiopyrans are listed in Table IX. Perchloric acid and trityl salts are the most frequently applied reagents. [Pg.235]

Alkoxy substituted thiopyrylium compounds are obtained by hydride abstraction from the thiin precursor with trityl salts, or by elimination of halogen from a thiin. By contrast, alkylthio substituents may be generated in creating the cationic species as shown in equation (115), though of course only 2- and 4-substituents are available in this way (76CC899). Amino substituted compounds may be prepared by substitution of the methylthio groups, but the nature of the product is strongly dependent on the nature of the amine (see Section 2.25.3.4). [Pg.938]

Aldol reactions using a carbocation as an organocatalyst An organocatalytic aldol reaction based on a different concept was developed by the Chen group. The chiral triarylcarbenium ion 34 was used as a novel non-metallic Lewis acid catalyst in a Mukaiyama-type aldol reaction which led to enantiomerically enriched aldol products (Scheme 6.17) [67]. Although non-chiral trityl salt-mediated catalytic aldol reactions had previously been reported by Mukaiyama and co-workers [68], the construction of a suitable chiral carbenium ion remained a challenge. Optically active salts of type 34 were synthesized as Lewis acids based on a reactive carbe-... [Pg.146]

The effectiveness of these anions depends to a large degree on the integrity of the jl-X linkage. The p,-NH2 compound is air and moisture stable and exhibits a particularly strong bridge, possibly by virtue of four N-H- F hydrogen bonds between ortho fluorine atoms and the amido NH protons. Activation of common metallocenes with the trityl salt... [Pg.53]

Bochmann has also produced a novel class of large dianionic borates XV based on the non-labile fefra-cyano Ni and Pd(II) metallates [K12 M(CN) i 2. 215a The metals in these WCAs are strictly square planar and as such the anion assumes a flat rather than spherical shape, with a torus of fluorination on the periphery of the plane. The bis-trityl salts of these dianions afford catalysts that are not as productive as those formed from the XIV class (X = CN), although they are still much more active than borane activated catalysts. The lower activity was attributed to the inherently greater cation/anion attraction in a monocation/dianion pair. [Pg.54]

Michael reactions. This system promotes a Michael addition of silyl enol ethers to a,(3-unsaturated thiolesters, which are excellent Michael acceptors. Trityl salts are less effective, as are Lewis acids in combination with SnCl2. [Pg.15]

Reaction of otfi-unsaturated acetals with silyl nucleophiles. This trityl salt is an efficient catalyst for reaction of the dimethyl acetal (2) of cinnamaldehyde with successive silyl nucleophiles to form y,5-unsaturated p-azido ketones. The azide group can be reduced to an amino group by the Staudinger reaction.1... [Pg.360]

Several 1,3-dithiolylium salts have been prepared by hydride removal from 2H-1,3-dithioles. Naturally, this elimination is favored by electron-releasing substituents in the 2-position, whereas 4,5-dicyano-l,3-dithiole remains stable toward oxidizing agents. The reaction of the 1,3-dithioles (30) with trityl salts to produce the 1,3-dithiolylium ions (31) offers one such example <80AHC(27)15l). [Pg.820]

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