Solvolysis base catalyzed

This group is more labile to hydrolysis than the TBDMS group and has been used to protect in alcohol where the TBDMS group was too resistant to cleavage. The DEIPS group is approximately 90 times more stable than the TMS group to acid hydrolysis and 600 times more stable than the TMS group to base-catalyzed solvolysis. 

The concept of preassembly as a requirement for substitution may throw light upon the vexed question of the mechanism of the base hydrolysis reaction. It has long been known that complexes of the type, [Co en2 A X]+n can react rapidly with hydroxide in aqueous solution. The kinetic form is cleanly second-order even at high hydroxide concentrations, provided that the ionic strength is held constant. Hydroxide is unique in this respect for these complexes. Two mechanisms have been suggested. The first is a bimolecular process the second is a base-catalyzed dissociative solvolysis in which the base removes a proton from the nitrogen in preequilibrium to form a dissociatively labile amido species (5, 19, 30). 

BASE CATALYZED SOLVOLYSIS AND SUBSTITUTION 7.1.5.1 General Remarks... 

The mechanism provides (in equation 1) a pathway for proton exchange in amine complexes. (In fact, it was the observation that proton exchange in [Co(NH3)6]3+313 and the base hydrolysis of (Co(NH3)5Br]2+298 had the same kinetic form that led Garrick to propose this mechanism in 1937.314) The pathway for base catalyzed solvolysis is made up of (1) + (2) + (3) and the pathway for base catalyzed substitution is (1) + (2) + (4). It was the observation of base catalyzed ligand substitution that provided the first strong evidence for the dissociative nature of the process.315... 

Other systems are ambiguous and require a careful consideration of the magnitudes of the derived rate constants before a conclusion can be drawn. An extreme case can be found in the pH dependence of the solvolysis of m-[Co(en)2(H20)Cl]2+.330 The rate is independent of pH in the range 7—9, where the complex is almost entirely in the form of ris-[Co(en)2(OH)Cl]+ and it is usually, and probably correctly, assumed that the pH independent rate constant is that for the uncatalyzed aquation of this species.180 However, consideration ought to be given to the possibility that the observed process is the base catalyzed hydrolysis of the aquo complex in which a primary amine proton is removed. Problems of this sort are discussed in ref. 301, p.84. 

The relatively large value of 1.67 for s seems to fit an SN2 -Si or SN2 -Si mechanism better rather than an SN2-Si type mechanism. In addition, SN2-type mechanisms generally give a small negative p value. For example, solvolysis of primary alkyl p-toluenesulfonaies in ethanol has a p value of —0.746 [53]. The large positive p for base catalyzed hydrolysis is inconsistent with a simple SN2-type mechanism. 

For the removal of this protecting group, tetrabutylammonium fluoride in oxolane is the most frequently used [388, 389, 409-411]. A much simpler reagent to prepare, potassium fluoride — crown ether, has been introduced for the same purpose [427]. Silyl group at 0-2 of nucleosides is cleaved more rapidly [411] than at 0-5. Acyl migrations occurred under the tetrabutylammonium fluoride-catalyzed desilyla-tion [432, 434, 443], Differencies between the primary and secondary position were also observed for acid- or base-catalyzed solvolysis [391, 409-412], 5 -0-(7ert-butyl-dimethylsilyl)nucleosides are much more labile towards acid than either 2 - or 3 -silyl ethers [391, 410-412], whereas the situation is reversed for base hydrolysis [411], /V-Bromosuccinimide in aqueous DMSO is another alternative for the removal of this type of silyl group [444]. 

The hydrolysis and solvolysis of esters can occur through acid-catalyzed, pH-independent, and base-catalyzed mechanisms. These reactions can be classified further according to the type of bond fission and the molecularity of the rate determining step. It is important to realize, however, that there are borderline cases in which classification of the molecularity of the reaction is merely a matter of its academic... 

Relative stabilities of trialkylsilyl ethers towards base-catalyzed solvolysis... 

Burrows, H. D., Topping, R. M. Base-catalyzed intramolecular nucleophilic keto-group participation in the solvolysis of the hindered ester, 2-acetylphenyl mesitoate acetal formation under basic conditions as a mechanistic consequence of such participation. J. Chem. Soc. B 1970, 1323-1329. 

Meloscandine, which accompanies the above alkaloids, may also be converted to meloscine (I) either by heating or base-catalyzed solvolysis. The empirical formula, C20H20N2O2, and an extra carbonyl absorption at 1748 cm i suggest a formyl substituent at C-3 (structure CXL) but the absence of a formyl proton in the NMR-spectrum casts doubt on the monomeric structure. 

The base catalyzed solvolysis and substitution reactions of cobalt(III) complexes has provided some of the most fertile ground for the study of dissociatively activated processes and the attempts to distinguish between and D mechanisms in terms of the lifetime of the intermediate species. The dissociative intimate mechanism has been demonstrated by a variety of trapping, competition and stereochemical studies, The base catalyzed substitutions of [Co(NH3)5X]"+, in which addition of small amounts of OH in the presence of excess leads to a mixture of [Co(NH3)5Y] and [Co(NH3)50H], are all consistent with a dissociatively activated process in which there is competition between Y and H2O for the place vacated by At the first approximation, the ratio... 

The characteristics of the hydrolysis process are illustrated by observations with the insecticide fenitrothion. ° From Figure 8.11 it is seen that there is no significant acid-catalyzed reaction and that the base-catalyzed reaction is more active than the direct solvolysis reaction. At 23°C, a of 5.55 x 10 min and a fej, of 0.15 min was reported. At pH values of 9 and above, only the phenol... 

In base-catalyzed solvolysis studies, the stage was set by Price s work on KOH/EtOH, HOH cleavage of trialkylsilanes. The reaction is first order in silane, and first order in base. As water concentration is varied, the plots of log obs vs. [HOH] suggest that a molecule of water can be tied up in the transition state, although later work suggests that this observation is related to changes in [OH ] by a medium effect. Eaborn has also pointed out that the cation of the base affects the rate and that added LiC104 impedes reaction. 

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