Acetone, deprotonation

TABLE 10. Calculated activation energies (in kcalmol" ) for acetone deprotonation by the cyclic and open mixed dimer mechanisms (equations 20 and 21)... 

Figure S. Free energy relationship (activation barrier versus ApKa) comprising experimental data on acetone deprotonation by various bases ([19], squares) and on base catalyzed phosphate elimination reactions [10] of DHAP (circles) and LGAP (triangles), using the estimated pKa values of DHAP and LGAP. The latter pA, was related to the former by the observed equilibrium constant of 1/22 between LGAP and DHAP in solution [10]. All values have been corrected for the number of equivalent protons.

Deprotonation at carbon adjacent to a carbonyl group leads to an enolate anion, e.g., deprotonation of acetone. 

Is the most delocalized enolate also the most easily formed enolate Calculate relative deprotonation energies from the enolate precursors using the deprotonation energy of acetone as a standard. 

In general, alkyl hydrogens are not very aeidie. However, alkyl hydrogens adjacent to carbonyl groups can be deprotonated by strong bases to give enolate anions, e.g., for acetone. 

The 5-substituted 1,3-dioxolan-4-one 23 is readily deprotonated at the 5 position and can be alkylated with a variety of alkyl halides. The resulting products 24 decompose upon flash vacuum pyrolysis (FVP) at 600°C with loss of acetone... 

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. 

For the classical Williamson synthesis an alcohol is initially reacted with sodium or potassium to give an alkoxide, e.g. 1. Alternatively an alkali hydroxide or amide may be used to deprotonate the alcohol. Phenols are more acidic, and can be converted to phenoxides by treatment with an alkali hydroxide or with potassium carbonate in acetone. ... 

Problem 8.9 ] The pKa of acetone, CH3COCH3, is 19.3. Which of the following bases is strong enough to deprotonate acetone ... 

Because carbonyl compounds are only weakly acidic, a strong base is needed for enolate ion formation. If an alkoxide such as sodium ethoxide is used as base, deprotonation takes place only to the extent of about 0. l% because acetone is a weaker acid than ethanol (pKa - 16). If, however, a more powerful base such as sodium hydride (NaH) or lithium diisopropylamide ILiNO -CjHy ] is used, a carbonyl compound can be completely converted into its enolate ion. Lithium diisopropylamide (LDA), which is easily prepared by reaction of the strong base butyllithium with diisopropylamine, is widely used in the laboratory as a base for preparing enolate ions from carbonyl compounds. 

The a-alkoxy iron-acyl complex 5 may be deprotonated to generate the lithium enolate 6, which undergoes a highly diastereoselective aldol reaction with acetone to generate the adduct 7 as the major product. Deprotonation of acetone by 6 is believed to be a competing reaction 30% of the starting complex 5 is found in the product mixture48 40. 

With OH and SH, the nucleophilic substitution of Cl has been reported. Thus, with NaOH, there is a report of successful nucleophilic substitution in 50% aq. acetone at room temperature to give the phenol complex in 36% yield. The latter is then spontaneously deprotonated to give the cyclohexadienyl complex (Eq. (24)). An identical reaction was carried out using NaSH in MeCN (50% yield) to give the thiophenol complex which was deprotonated [72] Eq. (25). These reactions would be especially valuable because direct synthesis of the phenol or thiophenol complexes from ferrocene is not possible due to the strong interaction between the heteroatom and A1C13 [11, 19]. Recent improvement and use of this reaction were achieved [88],... 

In principle, numerous reports have detailed the possibility to modify an enzyme to carry out a different type of reaction than that of its attributed function, and the possibility to modify the cofactor of the enzyme has been well explored [8,10]. Recently, the possibility to directly observe reactions, normally not catalyzed by an enzyme when choosing a modified substrate, has been reported under the concept of catalytic promiscuity [9], a phenomenon that is believed to be involved in the appearance of new enzyme functions during the course of evolution [23]. A recent example of catalytic promiscuity of possible interest for novel biotransformations concerns the discovery that mutation of the nucleophilic serine residue in the active site of Candida antarctica lipase B produces a mutant (SerlOSAla) capable of efficiently catalyzing the Michael addition of acetyl acetone to methyl vinyl ketone [24]. The oxyanion hole is believed to be complex and activate the carbonyl group of the electrophile, while the histidine nucleophile takes care of generating the acetyl acetonate anion by deprotonation of the carbon (Figure 3.5). 

The condensation of acetone can also occur over acidic sites as shown by a number of authors [1,9], Generally, when this occurs other products are formed such as isobutene and acetic acid, by the cracking of DAA. Additionally mesitylene can be formed by the internal 2,7-aldol condensation of 4,6-dimethylhepta-3,5-dien-2-one which is in turn obtained by the aldol condensation of MO with a deprotonated acetone molecule [7, 8], As these species are not observed we can concluded that any acidic sites on the silica support are playing no significant role in the condensation of acetone. 

The cationic tantalum dihydride Cp2(CO)Ta(H)2]+ reacts at room temperature with acetone to generate the alcohol complex [Cp2(C0)Ta(H01Pr)]+, which was isolated and characterized [45]. The mechanism appears to involve protonation of the ketone by the dihydride, followed by hydride transfer from the neutral hydride. The OH of the coordinated alcohol in the cationic tantalum alcohol complex can be deprotonated to produce the tantalum alkoxide complex [Cp2(C0)Ta(01Pr)]. Attempts to make the reaction catalytic by carrying out the reaction under H2 at 60 °C were unsuccessful. The strong bond between oxygen and an early transition metal such as Ta appears to preclude catalytic reactivity in this example. 

Griesbeck et al. successfully transformed w-phthalimidoalkanoates via PET with concomitant decarboxylation and C,C combination leading to medium- and large-ring compounds with yields in the range 60-80%. Thereby, the solvent system acetone/water and K2CO3 employed for the deprotonation of the carboxylic acids were crucial (Scheme 45) [66]. 

When different types of reactive sites are compared, however, there can be dramatic inversions of acidity order. Fluoride is a relatively weak base in solution, with pAjfHF) = 3.2 in aqueous solution. In contrast, in the gas phase, fluoride is strong enough to exothermically deprotonate acetone, which has a p of about 20... 

Hydrazones can also be deprotonated to give lithium salts which are reactive toward alkylation at the j> carbon. Hydrazones are more stable than alkylimines and therefore have some advantages in synthesis.79 The / A Alimcthy I hydrazones of methyl ketones are kinetically deprotonated at the methyl group. This regioselectivity is independent of the stereochemistry of the hydrazone.80 Two successive alkylations of the A A -dimethylby-drazone of acetone can provide unsymmetrical ketones. 

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