Rates cleavage

From the table it is clear that there are considerable differences in the cleavage rates of the various ethers that should prove quite useful. 

Comparison of Cleavage Rates for Various Carbamate Proi ctive Groups... 

Alkyldithio carbamates are prepared from the acid chloride (Et N, EtOAc, 0°) and amino acid, either free or as the O-silyl derivatives (70-88% yield). The N- i-propyldithio) carbamate has been used in the protection of proline during peptide synthesis. Alkyldithio carbamates can be cleaved with thiols, NaOH, Ph P/TsOH. They are stable to acid. Cleavage rates are a function of the size of the alkyl group as illustrated in the table below. 

Cleavage rates for 1,3-dioxanes are greater than for 1,3-dioxolanes/ but hydrolysis of a trans-fused dioxolane is faster than that of the dioxane. In substrates having more than one acetonide, the least hindered and more electron-rich acetonide can be hydrolyzed selectively." In a classic example, 1,2-5,6-diace-toneglucofuranose is hydrolyzed selectively at the 5,6-acetonide. 

Hydrochloric acid, 80% dioxane/water. Cleavage rates for various ring sizes were examined. 

The mechanisms for hydrolysis of 0,5-acetals have been reviewed. The following acid-catalyzed cleavage rates show that the 0,5-acetals have a stability that lies between thioacetals and acetals ... 

The trityl group was introduced on a primary amide, RCONH2, in the presence of a secondary amide with TrOH, AC2O, H2SO4, AcOH, 60°, 75% yield. It is stable to BOC removal with 1 N HCl in 50% isopropyl alcohol, 30 min, 50°, but can be cleaved with TFA. The following table gives the cleavage rates with TFA for a number of protected primary amides. 

Mercaptobenzothiazole, A-methylpyrrolidone, DIPEA. The reagent has the advantage that it is odorless and does not lead to intemucleotide cleavage, but the cleavage rate is 10 times slower than when thiophenol is used. ... 

The following table compares the cleavage rates for a variety of benzyl phosphates using thiols or pyridine for the reaction" " ... 

Benzylic, allylic and propargylic positions enhance the cathodic cleavage rate of C— heteroatom bonds as, for example, in the reduction of benzylic and allylic halides or alcohols56. Similar activated sulphones, due to their acidity, are in a class apart. Figure 8 shows the similitude between the cathodic behaviour of an allylic sulphone and its isomer, i.e., the corresponding vinylic sulphone when the electrolyses are run in an aprotic solvent. However, in the presence of an excess of proton donor, discrepancies appear. 

Photolabile linkers play an important role in solid-phase organic synthesis (SPOS) due to their stability under both acidic and basic conditions. The ONb photolabile linker was modified to improve cleavage rates and yields Fmoc-Tos-OFI was released in 87% yield after 23 h (Scheme 4) [24]. Specifically, the primary alcohol was changed to a secondary benzylic alcohol and the attachment to the resin was through an alkyl chain as opposed to an amide function. Linker 20 was used for the production of carboxylic acids or carbohydrates. A second example... 

The results of a recent investigation of the dependence of the cleavage rate constant upon the solvent of two similar anion radicals, those of 3-nitrobenzyl chloride and 3-chloroacetophenone,85 may likewise be interpreted as the outcome of a competition between the Lewis acid solvation of the developing halide ion and of the negatively charged oxygen atoms in the initial state. 

In total, the situation is less favorable in terms of quantum yields than in the preceding case. Indeed, even if the cleavage rate constant is large enough to overcome back electron transfer from the ion pair (kc k act), the maximal value of the quantum yield is 1/(1 + P), which would reach unity only in the unlikely case where the ground-state electron transfer was entirely non-adiabatic (H = 0). 

Break up of Sn—Fe or Sn—Mn bonds is observed, possibly as a result of one more ET. The alternative fragments are found in nearly equimolecular amounts. It is uncertain whether this is due to similar cleavage rates or the results of follow-up equilibration. The results of this study can be considered as corresponding to the electrosynthesis of complexes described earlier in this section. 

Effect of Water Addition on the Cleavage Rate of Anion Radicals in Acetonitrile Another manifestation of the importance of solvation as a factor controlling the rate of cleavage of anion radicals can be found in experiments carried out in acetonitrile, where the effect of water addition has been investigated38 for the anion radicals of the three compounds shown... 

Values in parentheses cleavage rate constant in s 1 in the stepwise case. Cyclic voltammetry. 

In the stepwise case, the intermediate ion radical cleaves in a second step. Adaptation of the Morse curve model to the dynamics of ion radical cleavages, viewed as intramolecular dissociative electron transfers. Besides the prediction of the cleavage rate constants, this adaptation opens the possibility of predicting the rate constants for the reverse reaction (i.e., the reaction of radicals with nucleophiles). The latter is the key step of SrnI chemistry, in which electrons (e.g., electrons from an electrode) may be used as catalysts of a chemical reaction. A final section of the chapter deals... 

---