Second deprotonation

The large sulfur atom is a preferred reaction site in synthetic intermediates to introduce chirality into a carbon compound. Thermal equilibrations of chiral sulfoxides are slow, and parbanions with lithium or sodium as counterions on a chiral carbon atom adjacent to a sulfoxide group maintain their chirality. The benzylic proton of chiral sulfoxides is removed stereoselectively by strong bases. The largest groups prefer the anti conformation, e.g. phenyl and oxygen in the first example, phenyl and rert-butyl in the second. Deprotonation occurs at the methylene group on the least hindered site adjacent to the unshared electron pair of the sulfur atom (R.R. Fraser, 1972 F. Montanari, 1975). 

The first proton to be removed from iV-methylpyrrole by w-butyllithium is from an a-position a second deprotonation occurs to give a mixture of 2,4- and 2,5-dilithiated derivatives. The formation of a 2,4-dilithio derivative is noteworthy since in the case of both furan and thiophene initial abstraction of a proton at C-2 is followed by proton abstraction from C-5 (77JCS(P1)887). iV-Methylindole, benzo[6]furan and benzo[6]thiophene are also deprotonated at C-2. Selenophene and benzo[6]selenophene and tellurophene and benzo[6]tellurophene similarly yield 2-lithio derivatives (77AHC(21)119). 

Sulfuric acid is the only common polyprotic acid for which the first deprotonation is complete. The second deprotonation adds to the H,0+ concentration slightly, and so the overall pH will be slightly less than that due to the first deprotonation alone. For example, in 0.010 M H2S04(aq) the first deprotonation is complete ... 

Step 2 Find the concentration of HA2 from the second deprotonation equilibrium (that of H2A ) by substituting the concentrations of H fO and H2A" from step 1 into the expression for K a. 

Step 2 Now we use Kal = 6.2 X 10 8 to find the concentration of HP042. Because Ka2 K,, we can safely assume that the H30+ concentration calculated in step 1 is unchanged by the second deprotonation. The proton transfer equilibrium is... 

We can conclude that the second deprotonation proceeds only to a very small extent. Step 3 The loss of the final proton from HP042 produces the phosphate ion,... 

Calculate the pH of each of the following acid solutions at 25°C ignore second deprotonations only when that approximation is justified, (a) 1.0 X 10 4 M H BO (aq), boric acid acts as a monoprotic acid (b) 0.015 m H,P04(aq) ... 

Like sulfuric acid, a certain diprotic acid, H,A, is a strong acid in its first deprotonation and a weak acid in its second deprotonation. A solution that is 0.020 M H,A(aq) has a pH of 1.66. What is the value of fCa2 for this acid ... 

The contribution from a second-order [OH-] dependence in the base hydrolysis of [Co(NH3)5-(OCOCF3)]2+ is associated with C—O bond fission and comes from a pathway in which the first hydroxide adds to the carbonyl carbon and the second deprotonates it.334... 

Calculate the pH of the following diprotic acid solutions at 25°C, ignoring second deprotonations only when the approximation is justified (a) 0.10 M H2S(aq) (b) 0.15 m H2C4H4Oe(aq) (tartaric acid) ... 

The reversible formation of a monoanionic trigonal bipyrimidal mtennediate with hypervalent sulfur (325) has been supported in the alkaline hydrolysis of the /J-sultam (326).297 A second deprotonation by hydroxide takes place to give (327) before the... 

After preparation of sulphonate salt, a second deprotonation step effects the ring closure ... 

In flavylium compounds that bear OH substituents in their 4 - and/or 7-positions, deprotonation of the OH group can result in other forms being obtained, not seen in the case of the 4 -methoxyflavylium compound discussed above. Figure 8 illustrates this for the 4 -hydroxyflavylium ion.1171 The new species are the quinoidal base A, obtained by simple deprotonation of the AH+ flavylium cation, and the dianionic Cc2- and Ct2- forms, obtained by second deprotonations of Cc and Ct. The roles played by these forms depend on the specific compound and the pH conditions. For... 

A similar study on the deprotonation/silylation process of iV-benzyl acetophenone imine led to similar results. Again n-butyllithium was shown to be regioselective. A second deprotonation-trimethylsilylation sequence leads to the expected BSMA derivative.150... 

The introduction of functional groups at C-4 causes special problems, as the introduction of a heteroatom results in an unstable, aminal-like structure, which is readily hydrolyzed. Dimeric (aminoethyl)sulfonyl /3-sultams 129 were isolated in nearly quantitative yield by addition of a solution of butyllithium at — 78 °C to a N-substituted /3-sultam followed by hydrolytic workup. When the reaction was performed on iV-benzoyl /3-sultam, a second deprotonation occurred yielding the trimeric compound 130 as a mixture of two diastereoisomers (Scheme 39) <2004HCA1574>. 

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