Reverse deprotonation reactions

In so far as values of pATn2o for the hydration of alkenes are known or can be estimated,47 values of pATR can be derived by combining rate constants for protonation of alkenes with the reverse deprotonation reactions of the carbocations. The protonation reactions seem much less likely to be concerted with attack of water on the alkene than the corresponding substitutions. Indeed arguments have been presented that even protonation of ethylene in strongly acidic media involves the intermediacy of the ethyl carbocation.97,98... 

Once formed the tetrahedral intermediate can revert to starting materials by merely reversing the reactions that formed it or it can continue onward to products In the sec ond stage of ester hydrolysis the tetrahedral intermediate dissociates to an alcohol and a carboxylic acid In step 4 of Figure 20 4 protonation of the tetrahedral intermediate at Its alkoxy oxygen gives a new oxonium ion which loses a molecule of alcohol m step 5 Along with the alcohol the protonated form of the carboxylic acid arises by dissocia tion of the tetrahedral intermediate Its deprotonation m step 6 completes the process... 

Although there is a tendency to associate coordinated water with Werner-type complexes, where it is extensively established, organometallic aqua ions are known.945 The simple [(Cp )Co(OH2)3]2+ has been established, and is prepared via Equation (8). The lower pATa is 5.9, similar to values in aminecobalt(III) compounds, and reversible deprotonation and dimerization has been identified as part of the reactions of the aqua ion.946... 

Thus, the role of zinc in the dehydrogenation reaction is to promote deprotonation of the alcohol, thereby enhancing hydride transfer from the zinc alkoxide intermediate. Conversely, in the reverse hydrogenation reaction, its role is to enhance the electrophilicity of the carbonyl carbon atom. Alcohol dehydrogenases are exquisitely stereo specific and by binding their substrate via a three-point attachment site (Figure 12.7), they can distinguish between the two-methylene protons of the prochiral ethanol molecule. 

There is little or no well-defined chemistry of 2-amino-pyrones, -pyrylium salts, etc., because of the ease with which these systems undergo ANRORC reactions to give substituted pyridones (equation 88). 4-Aminopyrylium salts are reversibly deprotonated to the imines (equation 89). Amino groups in the 3- and 5-positions of pyrones can be diazotized normally. 

Carbyne complexes are formed by reaction of DzO with 26, in a reversal of the deprotonation reaction used to form it (35) ... 

It has been mentioned above that the pyrimidine radical cations are reasonably strong acids and rapidly deprotonate at a heteroatom. As all protonation/ deprotonation reactions at heteroatoms are reversible [e.g., equilibrium (22)], the radical cations are regenerated upon reprotonation. Deprotonation at carbon or reaction with water yield the final free-radical products. For the l,3Me2Thy system, where the deprotonation/reprotonation equilibria such as reaction (22) fall away, reactions (25)-(28) have been postulated to account for the fact that in the presence of 02 l,3Me25HOMeUra and l,3Me25(CHO)Ura [reaction (29)] are formed in a combined yield of 80% of primary S04 radicals (Rashid et al. 1991). The formation of these products has been taken as evidence that a free radical cation must be an intermediate. It is, however, also possible that the allylic radical is formed in a concerted reaction HS04 elimination. For such a process, a six-membered transition state can be written. 

When covalently attached to electron transfer active subunits, the DHA-VHF couple can facilitate chemical and physical switching of electronic properties, as a result of photochemically induced rearrangement accompanied by a change in the redox potential. An interesting example of such a switching system is the compound containing a dihydroazulene component and a covalently attached anthraquinone moiety.1311 This system is able to act as a multimode switch, assisted by various processes such as photochromism, reversible electron transfer, and protonation-deprotonation reactions (Scheme 8). 

If a Michael addition of an enolate forms a ketone enolate as the primary reaction product, this enolate will be almost completely protonated to give the respective ketone. The reaction medium is of course still basic, since it still contains OH or RO ions. The Michael adduct, a ketone, is therefore reversibly deprotonated to a small extent. 

You can t make esters from carboxylic acids and alcohols under basic conditions because the base deprotonates the carboxylic acid (see p. 288). However, you can reverse that reaction and hydrolyse an ester to a carboxylic acid (more accurately, a carboxylate salt) and an alcohol. 

There are many kinetic evidences for the fact that the nitronium cation and the aromatic substrate are involved in a reversible bimolecular reaction to form a o-complex, which, being a strong acid, undergoes fast deprotonation (Scheme 2). 

The reaction is strongly affected by base which may reversibly deprotonate HRh(CO)2L2 to an anionic species which then reacts with formaldehyde and [H base]+ to give the hydroxymethyl intermediate. Using iridium analogues as models, several postulated reaction intermediates could be isolated.66... 

The study was intended to prove the intermediacy of nitroalkene 8 in the unexpected deacetona-tion-methoxylation of 1,2 3,5-diisopropylidene-6-deoxy-6-nitro-a-n-glucofuranose(ll), which occurred upon treating the latter carbohydrate with dilute methanolic sodium hydroxide60. In this reaction, deacetylated 9 and 10 were formed in a 4 1 ratio, too. Interestingly, under identical conditions, the cpimeric L-idose 12 was reversibly deprotonated and did not lose acetone. 

Complexation/decomplexation of metal ions or of neutral organic molecules, protonation/deprotonation reactions, and oxidation/reduction processes can all be exploited to alter reversibly the stereoelectronic properties of one of the two recognition sites, thus affecting its ability to sustain noncovalent bonds [30-34, 41]. These kinds of switchable [2 catenanes can be prepared following the template-directed synthetic strategy illustrated in Figure 5, wherein one of the two macrocyclic components is preformed and then the other one is clipped around it with the help of noncovalent bonding interactions. 

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