Trityl cations acidity

Furthermore, several functionalities remained unaffected, namely the acid-labile TBDMS or PMB groups.118 Deprotection yields were in the range of 85-95% when methanol was used at room temperature as the solvent, whereas acetonitrile or dichloromethane led to very sluggish or nonexistent reactions, respectively. Cleavage of primary trityl ethers was also accomplished using the same conditions in a very rapid and effective fashion. The trityl pyranosides and furanosides assayed were selectively deprotected in 2-3 h and yields higher than 85% were achieved. This reaction was also more efficient when conducted in methanol, which acts as a nucleophile to trap the generated trityl cation. 

For protonation-dehydration processes, such as trityl cation formation from triphenylcarbinols, equation (24), the water activity has to be included if the formulation of the activity coefficient ratio term is to be the same as that in equation (7), which it should be if linearity in X is to be expected see equation (25). The excess acidity expression in this case becomes equation (26) this is a slightly different formulation from that used previously for these processes,36 the one given here being more rigorous. Molarity-based water activities must be used, or else the standard states for all the species in equation (26) will not be the same, see above. For consistency this means that all values of p/fR listed in the literature will have to have 1.743 added to them, since at present the custom... 

In view of the useful interpretations which can be given to the differences in E1/2 values, an obvious step is to select the E1/2 of one convenient ion as a zero or reference point. We follow Taft [14] and Breslow [15] in selecting E1/2 for the triphenylmethylium (trityl) cation, as this ion is easy to procure and is stable under acidic conditions. It seems useful to define a quantity Ef/2 which is 1/2 of the species in question minus EV2 of the triphenylmethylium ion measured under the same conditions of concentration, temperature, solvent, and nature and concentration of base electrolyte, and against the same reference electrode ... 

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... 

The allyl cation (9) is the simplest member of the class of resonance-stabilized cations that includes the alkyl-substituted cyclopentenyl cations. But one could also say that the carbenium ion (CH3) is the simplest member of a class of cations that includes the trityl cation. In each case, 10 or so orders of magnitude of acidity separate the primitive member from its more elaborate derivatives. 

Maciel (117) described the formation of the trityl cation 16 on silica-alumina in a 1984 symposium in retrospect it is surprising that no one followed up on this work until much later. 16 easily forms from triphenylcar-binol and other precursors in solutions of modest acid strength. The early observation of such an easy cation had the unintended effect of suggesting that real carbenium ions would not so easily be detected in NMR studies of solid acids. Over a decade elapsed before we characterized the rert-butyl cation on A1C13 powder (43). 

The principal components of the trityl cation in zeolite HY are <5 = 282 ppm and <5j = 55 ppm. It is instructive to tabulate all of the 13C principal component data measured for free carbenium ions in zeolites as well as for a few carbenium ions characterized in other solid acid media (Table III). The zeolitic species, in addition to the trityl cation (119), are the substituted cyclopentenyl cation 8 (102), the phenylindanyl cation 13, and the methylindanyl cation 12 (113). Values for the rert-butyl cation 2 and methylcyclopentyl cation 17 (prepared on metal halides) (43, 45) are included for comparison. Note that the ordering of isotropic chemical shifts is reasonably consistent with one s intuition from resonance structures i.e., the more delocalized the positive charge, the smaller the isotropic shift. This effect is even more apparent in the magnitudes of the CSA. Since... 

As mentioned earlier, steric effects can be important in determining the outcome of the hydride abstraction reaction. This is particularly vexing in cases where an alkyl substituent is present at the sp carbon of the cyclohexadiene complex. For example, complexes such as (47 equation 19) are untouched by trityl cation, provided traces of acid are not present (these are formed by hydrolysis of the trityl tetra-fluoroborate due to atmospheric moisture, and will cause rearrangement of the diene complex). This is due to the fact that only the hydride trans to the Fe(CO)3 group can be removed, and the methyl substituent prevents close approach to this hydrogen. 

Trityl Cation Assay.22 DMT-functionalized resin (10-20 mg) was weighed into a 10-ml volumetric flask. Trichloroacetic acid (3% solution = 1.50 g TCA in 50 ml dichloromethane) in CH2C12 was added and a 200- to 300-pl aliquot was transferred to another 10-ml volumetric flask and diluted with 3% TCA/ CH2C12. Analysis was done by UV-VIS absorption at 504 nm (A504). Loading calculation ... 

When nitronium tetrafluoroborate was attempted to react with the trityl cation in CH2CI2 or sulfolane, no nitration occurred due to the deactivating effects of the carbenium ion center in 215. Nitration of deactivated substrates is also readily accomplished by reaction with NO2CI with three mole excess AICI3 suggesting Lewis acidic electrophilic solvation of the nitronium cation (217, eq 62).105... 

Benzyl 2,2,2-trichloroacetimidate (bp 106-114 °C/0.07 kPa) alkylates alcohols in the presence of trifluoromethanesulfomc acid.311 Esters, imides, isopropylidene and benzylidene acetals are unaffected. This method allows the formation of benzyl ethers in molecules that are base-sensitive. In the example [Scheme 4,167],312 benzylation of the P-hydroxy ester 167.1.1 under the usual basic conditions would lead to retro-aldol reactions and/or elimination. A synthesis of the cellular messenger L-a-phosphatidyl-D-myo-inositol 3,4-bisphosphate exemplifies the use of trityl cation-promoted benzylation of two adjacent hydroxyls [Scheme 4.168].313314... 

Triphenylmethyl (trityl) groups can be cleaved under very mild acidic conditions and the resulting trityl cations display a lower electrophilicity than ferf-butyl cations therefore, trityl side-chain protection is especially useful in SPPS of sensitive peptides containing Met and Trp. In a recent comparative study, crude products of higher purity were obtained using side-chain-trityl-protected amino acids compared to standard Fmoc/tBu amino acid derivatives,... 

The O-trityl protection can be removed under the same conditions as used for cleavage of peptides from 2-chlorotrityl resin, i.e. with ACOH/TFE/CH2Q2 (2 2 6, rt, 1 h) as the cleavage nnixture or with 1% TFA without affecting tert-butyl side-chain protection. Silanes are added as hydride donors to scavenge the trityl cations and to avoid retritylation. Most commonly, the trityl derivatives of tyrosine, serine, and threonine are used for the synthesis of phosphopeptides by postsynthetic methods on a solid support. In contrast to the use of protected phosphoamino acid derivatives, such as Fmoc-Tyr(POOBzl)-OH,f l both the phosphorylated and nonphosphorylated peptides can be prepared from one syn-... 

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