Deprotonation reversible

It should be noted that the sequence deprotonation/reverse Brook rearrangement between the triisopropylsilyloxy allene and isopropylmagnesium chloride in THE does not provide the magnesium enolate. 

The amino acid ester with the free NH2 group can also react as a base. Hence, the amino acid chloride is deprotonated—reversibly—to the enolate.This is so readily pos-... 

Accordingly, trimethylsilyl enol ethers are enolate precursors (Figure 10.16). Fortunately, they can be prepared in many ways. For instance, silyl enol ethers are produced in the silylation of ammonium enolates. Such ammonium enolates can be generated at higher temperature by partial deprotonation of ketones with triethylamine (Figure 10.18). The incompleteness of this reaction makes this deprotonation reversible. Therefore, the regioselectivity of such deprotonations is subject to thermodynamic control and assures the preferential formation of the more stable enolate. Consequently, upon... 

Another rationalization for a lack of buildup of iminium ion is that both reactions which destroy it, namely hydration (equation 16) and deprotonation (reverse of equations... 

Successful reactions of complexes 15 and 33, some occurring in very high yield, with various organic halides (Equations 14-19) will be presented in approximately chronological order. Acyl and aroyl chlorides react to form hydrazido and diazenido complexes, respectively. The latter complexes were treated with one mole of hydrogen chloride to form the hydrazido complexes.The hydrazido complexes can be deprotonated reversibly to give the corresponding diazenido complexes (Equation 14) ... 

Hydrazido complexes with two bidentate phosphine ligands can be deprotonated reversibly to give the corresponding neutral diazenido complex (Equation 26) ... 

Reverse reaction Loss of the weak nucleophile, followed by deprotonation. Reverse reaction ... 

Thermodynamic Enolate- Reversible deprotonation to give the most stable enolate more highly substituted C=C of the enol form... 

Retrosynthetic path b in Scheme 3.1 corresponds to reversal of the electrophilic and nucleophilic components with respect to the Madelung synthesis and identifies o-acyl-iV-alkylanilines as potential indole precursors. The known examples require an aryl or EW group on the iV-alkyl substituent and these substituents are presumably required to facilitate deprotonation in the condensation. The preparation of these starting materials usually involves iV-alkyla-tion of an o-acylaniline. Table 3.3 gives some examples of this synthesis. 

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... 

Chemical off—on switching of the chemiluminescence of a 1,2-dioxetane (9-benzyhdene-10-methylacridan-l,2-dioxetane [66762-83-2] (9)) was first described in 1980 (33). No chemiluminescence was observed when excess acetic acid was added to (9) but chemiluminescence was recovered when triethylamine was added. The off—on switching was attributed to reversible protonation of the nitrogen lone pair and modulation of chemically induced electron-exchange luminescence (CIEEL). Base-induced decomposition of a 1,2-dioxetane of 2-phen5l-3-(4 -hydroxyphenyl)-l,4-dioxetane (10) by deprotonation of the phenoHc hydroxy group has also been described (34). 

The relative stability of the anions derived from cyclopropene and cyclopentadiene by deprotonation is just the reverse of the situation for the cations. Cyclopentadiene is one of the most acidic hydrocarbons known, with a of 16.0. The plCs of triphenylcyclo-propene and trimethylcyclopropene have been estimated as 50 and 62, respectively, from electrochemical cycles. The unsubstituted compound would be expected to fall somewhere in between and thus must be about 40 powers of 10 less acidic than cyclopentadiene. MP2/6-31(d,p) and B3LYP calculations indicate a small destabilization, relative to the cyclopropyl anion. Thus, the six-7c-electron cyclopentadienide ion is enormously stabilized relative to the four-7c-electron cyclopropenide ion, in agreement with the Hixckel rule. 

Tire deprotonation of thiazolium salts (see Section II) under argon at room temperature allowed the characterization of nonfused DTDAF of types 52 and 53 by cyclic voltammetry. Their very good donor properties were confirmed by two quasi-reversible peaks of equal intensity (93CC601). It is noteworthy that upon a second scan the first oxidation peak was shifted from -0.03 to -0.04 V. Upon further scans the voltam-mogram remains unchanged. Tliis interesting feature has been observed previously with TTF analogs. It was demonstrated that the neutral form... 

Reaction of the cyclopentadienyl rhodium and iridium tris(acetone) complexes with indole leads to the species 118 (M = Rh, Ir) [77JCS(D)1654 79JCS(D)1531]. None of these compounds deprotonates easily in acetone, but the iridium complex loses a proton in reaction with bases (Na2C03 in water, r-BuOK in acetone) to form the ri -indolyl complex 119. This reaction is easily reversed in the presence of small amounts of trifluoroacetic acid. 

Hydroxysanguinarinebetaine (367) is formed on Oppenauer oxidation of Chelidonine (366) as a red compound (65MI2) (Scheme 109). Deprotonation of the benzo[c]phenanthridine alkaloid Fagaronine (93T10305), which is known to inhibit various reverse transcriptases (77MI1), resulted in the... 

Although the previous protocol suggests it is not necessary to deprotonate the sulfonamide prior to exposure to the zinc carbenoid, a experimentally simpler procedure can be envisioned wherein the alcohol and promoter are deprotonated in a single flask (Fig. 3.15). In protocol IV, the alcohol and promoter are combined in flask A and are treated with diethylzinc, thus forming the zinc alkoxide and zinc sulfonamide. In sub-protocol IVa, this solution is transferred to flask C which contains the zinc carbenoid. Sub-protocol IVb represents the reversed addition order. Sub-protocol IVa is not only found to be the superior protocol in this sub-set, it is found to out-perform all of the previous protocols Despite the persistence of the induction period, a large rate enhancement over the uncatalyzed process is observed. This considerable rate enhancement also translates to a reduction in the overall reaction time when compared to sub-protocols la and Ilia. Selectivity rises... 

Basic hydrolysis occurs by nucleophilic addition of OH- to the amide carbonyl group, followed by elimination of amide ion (-NH2) and subsequent deprotonation of the initially formed carboxylic acid by amide ion. The steps are reversible, with the equilibrium shifted toward product by the final deprotonation of the carboxylic acid. Basic hydrolysis is substantially more difficult than the analogous acid-catalyzed reaction because amide ion is a very poor leaving group, making the elimination step difficult. 

Although lithium aldolates generally display a rather moderate preference for the u/f/z-isomer4, considerable degrees of diastereoselectivity have been observed in the reversible addition of doubly deprotonated carboxylic acids to aldehydes20. For example, the syn- and uw/z-alkox-ides, which form in a ratio of 1.9 1 in the kinctically controlled aldol addition, equilibrate in tetrahydrofuran at 25 C after several hours to a 1 49 mixture in favor of the anti-product20. 

A possible side reaction in A-acyliminium chemistry is caused by deprotonation, giving rise to the formation of an enamide. Though this tautomerization is in principle reversible in acid media, this is not always the case. The enamide may react as a nucleophile with the /V-acyliminium ion still present, to produce dimers14. 

The reversibility of aromatic diazotization in methanol may indicate that the intermediate corresponding to the diazohydroxide (3.9 in Scheme 3-36), i. e., the (Z)-or (is)-diazomethyl ether (Ar — N2 — OCH3), may be the cause of the reversibility. In contrast to the diazohydroxide this compound cannot be stabilized by deprotonation. It can be protonated and then dissociates into a diazonium ion and a methanol molecule. This reaction is relatively slow (Masoud and Ishak, 1988) and therefore the reverse reaction of the diazomethyl ether to the amine may be competitive. Similarly the reversibility of heteroaromatic amine diazotizations with a ring nitrogen in the a-position may be due to the stabilization of the intermediate (Z)-diazohydroxide, hydrogen-bonded to that ring nitrogen (Butler, 1975). However, this explanation is not yet supported by experimental data. 

The main problem of interest, however, is that of finding a way to determine Kx and K2 separately for cases where Kx < K2. Such a separation of Kx and K2 is possible by taking advantage of the fact that the addition of hydroxide ion to the diazonium ion (rate constant kx in Scheme 5-1) is slower than the deprotonation of the diazohydroxide (rate constant k2). An analogous relationship holds for the two reverse reactions (k 2>k i). From the values of kx and k x one can, of course, calculate Kx and, if KXK2 is known, K2. Such measurements of Kx and K x were, however, difficult in the 1950s. 

Challis and Long497 have used the fast flow technique described above (p. 217) to measure the equilibrium protonation of azulene in a range of aqueous perchloric acid media at 7.5 °C and hence the rates of the forward protonation and reverse deprotonation, the overall exchange rate being the sum of these. Some representat i ve values are given in Table 141. Coupled with data obtained at other temperatures... 

RATE COEFFICIENTS FOR PROTONATION (ki FORWARD) AND DEPROTONATION (Aij REVERSE) OF AZULENE IN AQUEOUS HCIO4. AT 7.5 °C197... 

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