Deprotonation Processes

Hydrogen exchange, in thiazole, especially deuteration, has been quantitatively investigated (379,380), but the mechanism of the reaction carried out at acidic or neutral pH corresponds to a protonation-deprotonation process (380), different from electrophilic substitution and is discussed in section I.3.E. 

Vanadium(iv) is even more strongly polarizing. The first deprotonation process is not observable in aqueous solution. The pentaaquahydroxy ion may be present in... 

Adequate basicity to ensure the selective deprotonation process for enolate generation... 

The proton transfer processes described above induce interesting effects on the geometry of these metal complexes upon protonation (see also Section II). If it is assumed that the equatorial cyano ligands form a reference plane and are stationary for any of these distorted octahedral cyano oxo complexes, the protonation/deprotonation process as illustrated in Scheme 3 is responsible for the oxygen exchange at the oxo sites. This process effectively induces a dynamic oscillation of the metal center along the O-M-O axis at a rate defined by kmv, illustrated in Fig. 15. This rate of inversion is determined by the rate at which the proton is transferred via the bulk water from the one... 

The reaction of 2,3-allenamide 547 with I2 and aqueous formic acid in aqueous THF afforded /f-iodobutenolides 548 and butenolides 549, respectively. With anhydrous formic acid, 2,4-alkadienamide 550 was formed via a protonation and deprotonation process [256]. 

Deprotonation processes are exclusively governed by intrinsic structural effects, without any external stabilization. Therefore, the thermodynamic data are affected by the nature of the atom bound to the proton, its hybridization, the adjacent functionality and the transmitted substituent effects. 

It is obvious that the isobutene unit provides a good starting point for the generation of trimethylenemethane radical anions. However, even isobutane units can be used to produce these more highly unsaturated species. In a preliminary work aimed at two-and three-fold deprotonation processes in solution, Kuck, de Meijere and coworkers165 have subjected triquinacene (41) and the tribenzotriquinacenes 44 to NCI conditions with... 

This indicates the occurrence of the following deprotonation process ... 

It is interesting to note that no reaction is observed with oxirane 72a if the hydroxyl group is protected. Moreover, whereas deuterium labeling experiments indicate a clean /3-deprotonation process for both oxiranes 69 and 72a, the same enantiomer of base 71 furnishes the corresponding allylic alcohols 70 and 73a with the opposite absolute configurations (Scheme 30 vs. 31). The same studies on vicinal disubstituted analogues 72b,c showed that both the sense and the level of enantioselectivity are unchanged, which... 

Attempts at deprotonation of epoxyketone 190 with EDA or n-BuLi at —95°C result only in reduction or addition to the carbonyl moiety. By contrast, deprotonation of the epoxy imine 191 leads to the aziridine 192 through a stereoselective dimerization/aza-Darzens//3-deprotonation process (Scheme 84). No example of reaction of this lithiooxirane with other electrophiles is reported. 

Cationic allenylidene complexes containing a hydrogen atom in the 6 position, that is, [M]" =C=C=C(R )CHR R, are known to undergo deprotonation processes upon treatment with bases affording neutral 0-enynyl derivatives [M]-C=CC(R )= CR R. Representative examples include deprotonation of [Ru(t] -C9H7)(=C=... 

Falvey examined the reactions of A-methyl-A-phenylnitrenium ion 65 (Fig. 13.37) in the presence and absence of chloride. It was found the yield of aniline (resulting from hydrolysis of the product iminium ion) was unaffected by added base. This finding ruled out a deprotonation process and led to the conclusion that a 1,2-hydride shift had occurred. Cramer et al. modeled this process using ab initio methods. 

We emphasize that the above linear relationships are endowed with an important predictional power and can be used to estimate thermodynamic data. These relationships are also valuable for assessing the sites involved in protonation and deprotonation processes. 

Magnesium amides have also found good utility in enantioselective deprotonation processes. A range of chiral amines has been prepared by Henderson and coworkers and it was found after conversion to their Mg-bisamide derivatives that it react with 4- and 2,6-substituted cyclohexanones with good to excellent selectivities (see Section m). Structures of some chiral magnesium amides are given in Chart 1. 

One function of amine and other basic promoters may be to facilitate cluster transformation by allowing facile protonation/deprotonation processes to occur. The [Rhl3(CO)24H3]2 cluster is a sufficiently strong acid to be deprotonated by NMM, as seen in (44). The HRh(CO)4 species has been found to be fully deprotonated by NMM and A.A-dimethylaniline... 

Some other interesting observations regarding free radicals in these systems are noteworthy. In many instances, multiple conformations of radicals are found at lower but not higher temperatures. This indicates that the radicals exist in shallow energy wells at low temperature this phenomenon was observed very early, in the 4 K ENDOR investigation of radical formation in amino acids.23 Unlike the process in DNA. In which it is well understood that the thymine anion radical protonates at C6 to form T(C6)H-, in the crystalline state there is a not clear link between pyrimidine electron adducts and H-addition radicals. We finally note that a deuterium isotope effect of protonation/deprotonation processes was found in cytosine.HCl and 2 -deoxycytidine.HCl, as evidenced by a lower propensity for these processes to occur in partially deuterated systems than in predated ones. 

This question of equilibration of the protonation and deprotonation processes leads to another fundamental problem in the case of excited state reactions between which states can a protolytic equilibrium be at all established A molecule has only one ground state, so there can be no ambiguity about the thermal protolytic equilibrium which connects of course the ground states of the acid and base forms. However, there are many excited states of both these forms, excited states which can differ greatly in electron distribution (e.g. mr and 7T7T states) or even in multiplicity (e.g. singlet and triplet states). 

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