Deprotonation with acid

The dianions derived from furan- and thiophene-carboxylic acids by deprotonation with LDA have been reacted with various electrophiles (Scheme 64). The oxygen dianions reacted efficiently with aldehydes and ketones but not so efficiently with alkyl halides or epoxides. The sulfur dianions reacted with allyl bromide, a reaction which failed in the case of the dianions derived from furancarboxylic acids, and are therefore judged to be the softer nucleophiles (81JCS(Pl)1125,80TL505l). 

In rtido- and arac/ino-carboranes is more acidic than H and is the one removed on deprotonation with NaH. 

Methylation of nicotine to the pyridinium iodide with methyl iodide, followed by its conversion to the hydroxide with silver oxide in water, oxidation with potassium permanganate to the A -methyl nicotinic acid hydroxide and subsequent deprotonation with silver oxide yielded Trigollenine as colorless needles (1897CB2117). In a later publication, the formation of nicotinic acid from nicotine was described. Esterification followed by aminolysis and methylation yielded the A -methylnicotinamide... 

Using oxathiane 11, ( + )-(i )-2-methoxy-2-phenylpropanoic acid was obtained in 97% ee, however, the synthesis contains some inconvenient reaction steps. Thus, reduction of ( + )-10-camphorsulfonic acid (8) leads in low yield to a mixture of 10-mercaptoisoborneol (9 A) and 10-mercaptoborneol (9B) which must be separated by chromatography. The oxathiane 10 resists deprotonation with butyllithium and, therefore,, y -butyllithium had to be employed. Furthermore, after addition of methylmagnesium iodide, cleavage of the oxathiane moiety 12, with iodomethane did not proceed as well as with the simpler oxathianes 3. 

A completely different dipolar cycloaddition model has been proposed39 in order to rationalize the stereochemical outcome of the addition of doubly deprotonated carboxylic acids to aldehydes, which is known as the Ivanov reaction. In the irreversible reaction of phenylacetic acid with 2,2-dimethylpropanal, metal chelation is completely unfavorable. Thus simple diastereoselectivity in favor of u f/-adducts is extremely low when chelating cations, e.g., Zn2 + or Mg- +, are used. Amazingly, the most naked dianions provide the highest anti/syn ratios as indicated by the results obtained with the potassium salt in the presence of a crown ether. 

Citric acid, which is extracted from citrus fruits and pineapples, undergoes three successive deprotonations with pK, values of 3.14, 5.95, and 6.39. Estimate the pH of (a) a 0.15 m aqueous solution of the monosodium salt (b) a 0.075 M aqueous solution of the disodium salt. 

Complexes with deprotonated hydroxamic acids, catechols, and related ligands 319... 

Proton dissociation in the excited states commonly occurs much easier than in the ground states, and the great difference in proton dissociation constants by several orders of magnitude is characteristic for photoacids [47]. These dyes exist as neutral molecules and their excited-state deprotonation with the rate faster than the emission results in new red-shifted bands in emission spectra [48]. Such properties can be explored in the same manner as the ground-state deprotonation with the shift of observed spectral effect to more acidic pH values. 

In addition the structure of the 1,2-azathiabenzene 78 was also confirmed by chemical evidence as shown in Scheme 10. Protonation of 78a (R1 = R2 = Me) with 70% perchloric acid yielded the corresponding cyclic amino sulfonium salt 82a in 87% yield, but not the starting sulfonium compound 76a, suggesting predominance of sulfilimine structure 78a rather than cyclic sulfonium ylide stmcture 80a. Thus, compound 78 could be recognized as the first example of a 1,2-azathiabenzene having sulfur at a bridgehead position. A proposed mechanism for the formation of 78 and 79 is shown in Scheme 9. The most acidic proton adjacent to sulfur in 76 is deprotonated with... 

Equation 44).57 The amide hydrogen is acidic in these compounds and can be deprotonated with standard organometallic bases such as raBuLi to give the imido complex 28 (Equation 44). 

The probable pathway resulting in the stereoselective formation of silylated ene nitrile (586) from enoxime (584) is presented on the right of Scheme 3.282. At higher temperature, the latter eliminates trimethylsilanol to give ene-nitrile (586) under the action of silyl Lewis acid (TfOSiMe3). Evidently, the reaction of compound (585) with TfOSiMe3 at room temperature involves initial silylation of the nitrogen atom to form the cationic intermediate B, which is deprotonated with triethylamine, followed by the thermodynamically favorable l,3-N,C-shift... 

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