Rates of cleavage

The relative rates of cleavage for a variety of uridine-protected acetals are given in the table below. 

FeCl3, AC2O, 55-75% yield.The relative rates of cleavage for the 6-, 3-, and 2-O-benzyl groups of a glucose derivative are 125 24 1. Sulfuric acid has also been used as a catalyst. ... 

Several methoxy-substituted benzyl ethers have been prepared and used as protective groups. Their utility lies in the fact that they are more readily cleaved oxidatively than the unsubStituted benzyl ethers. The table below gives the relative rates of cleavage with dichlorodicyanoquinone (DDQ). ... 

PhS02CH2CH20C0Cl, Pyr, 20°, 74-99% yield. 4-Substituted phenylsul-fonyl analogs of this protective group have also been prepared and their relative rates of cleavage studied /,2 (min) (TEA, Pyr, 20°) 4-H, 180 4-Me, 1140 4-Cl, 60 4-NO2, 10. ... 

The following derivatives represent protective groups that contain an auxiliary functionality, which when chemically modified, results in intramolecular, assisted cleavage, thus increasing the rate of cleavage over simple basic hydrolysis. 

Tri.-nethylsilyl triflate (TMSOTf), PhSCH, CF3COOH. These conditions also cleave the following protective groups used in peptide synthesis (MeO)Z-, Bn-, Ts-, CI2C6H3CH2-, BOM (benzyloxymethyl)-, Mts-, MBS-, r-Bu-SR, Ad-SR, but not a BnSR, Acm, or Arg(N02) group. The rate of cleavage is reported to be faster than with TfOH/TFA. 

The rate of cleavage is 4-5 times faster if dimethyl sulfide is included in the TFA/PhSMe mixture. ... 

Mg, MeOH or Mg(OMe)2 iri MeOH. The acetate is cleaved in the presence of the benzoate and pivaloate (76-96% yield). The relative rates of cleavage are as follows p-nitrobenzoate > acetate > benzoate > pivaloate acetamide. Tertiary acetates are not cleaved. ... 

CH3SO3H, m-cresol, CH2CI2. The addition of m-cresol greatly accelerates the rate of cleavage. ... 

Nitrophenyl)ethyl Carbamate. The photolytic removal of this group occurs twice as fast as does the 2-nitrobenzyl carbamate. Additionally, substitution at the alpha carbon increases the rate of cleavage even more. 

This group is stable to TEA and morpholine in pyridine at 20°. It is cleaved by oxidation with MCPBA followed by elimination with TEA in Pyr, 10 min, 20°." The rate of cleavage is proportional to the strength of the electron-withdrawing group on the phenyl ring. ... 

Protodemercuration was first studied kinetically by Kharasch et al.604, who measured, qualitatively, the rates of cleavage of diarylmercury compounds by acids, viz. 

A further kinetic investigation of the rates of cleavage of diphenylmercury (and some dialkylmercurials) showed similar kinetic features608. The first-order rate for the reaction of diphenylmercury with acetic acid at 25 °C was 4.98 x 10", which agreed quite well with the value from the above determination (2 x 10-4 at 42 °C with dioxan). In the presence of perchloric acid, second-order kinetics were found to be obeyed (for dineophyl mercury and presumably for diarylmercurials as well) for a twofold concentration change in both mercurial and perchloric acid. Two separate mechanisms were proposed for the reactions in the absence, and presence, of perchloric acid. Under the former conditions an SEi process (244) was... 

Most of the kinetic studies have been carried out by Eaborn and by Benkeser and their coworkers, and the first study was that of Eaborn656, who measured, spectrophotometrically, the rates of cleavage of 4-methoxyphenyltrimethylsilane by hydrochloric and perchloric acids in methanol, and by hydrochloric acid in aqueous dioxan, both at 47.9 °C (Table 226). The reaction was first-order in silane... 

Further mechanistic evidence was provided by Benkeser and Krysiak658, who determined the effects of added salts and water on the rates of cleavage of xylyltrimethylsilanes by p-toluenesulphonic acid in acetic acid at 25 °C, the progress of the reaction being followed by dilatometry the first-order rate coefficients are given in Table 227. Clearly the addition of water retards the reaction, as... 

Contrary to the results obtained with carbon tetrachloride solvent and entirely in accord with the postulate that the effect arises from the steric hindrance to solvation, the rates of cleavage of ArSnR3 compounds in methanol decrease on increasing the size of the group R. This is shown by the rate coefficients in Table 265, though it is difficult to draw any conclusion from the Arrhenius parameters... 

Here, a change in the aromatic —S02 conjugation at the anion radical stage should destabilise differently the C—S bonds. Unfortunately, no kinetic data on the rate of cleavage of the different sulphonic (hindered or not) anion radicals are available for the moment. 

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