Acetaldehyde, protonated

The catalytic cycle proposed for ethylene to acetaldehyde is shown in Fig. 8.2. The tetrachloro palladium anion 8.1 is used as the precatalyst. Conversion of 8.1 to 8.3 involves substitution of two chloride ligands by ethylene and water. Nucleophilic attack on coordinated ethylene leads to the formation of 8.4. The latter then undergoes substitution of another Cl- ligand. Conversion of 8.5 to 8.6 involves /3-hydride abstraction and coordination by vinyl alcohol. Intramolecular hydride attack to the coordinated vinyl group leads to the formation of 8.7. The latter eliminates acetaldehyde, proton, and CF and in the process is reduced to a palladium complex of zero oxidation state. 

Based on the flavonoid reactivity pattern (figure 2), the dichotomy between electrophilic and nucleophilic characters is strongly related to pH. The more acidic the pH is, the more cationic charges prevail. Thus, the balance between flavylium cations (AH+) and hydrated forms (AOH) is pH-dependent as well as the interflavanic bond cleavage yielding the flavanol cations (F+). In addition, acetaldehyde protonation is also controlled by pH. Therefore, it can be expected that reactions involving flavylium, flavanol or protonated acetaldehyde cations are favored at lower pH values. Based on carbocation reactivity, these cationic species can be classified as follows ... 

Conversion of 83 to 8.6 involves /3-hydride abstraction and coordination by vinyl alcohol. Intramolecular hydride attack to the coordinated vinyl group leads to the formation of 8.7. The latter eliminates acetaldehyde, proton, and chloride and in the process is reduced to 8.8 where palladium is in the zero oxidation state. 

The enol can be observed by NMR spectroscopy and at —20°C has a half-life of several hours. At -1-20°C the half-life is only 10 minutes. The presence of bases causes very r id isomerization to acetaldehyde via the enolate. Solvents have a significant effect on the lifetime of such unstable enols. Solvents such as DMF and DMSO, which are known to slow rates of proton exchange by hydrogen bonding, increase the lifetime of unstable enols. ... 

The hydration reaction has been extensively studied because it is the mechanistic prototype for many reactions at carbonyl centers that involve more complex molecules. For acetaldehyde, the half-life of the exchange reaction is on the order of one minute under neutral conditions but is considerably faster in acidic or basic media. The second-order rate constant for acid-catalyzed hydration of acetaldehyde is on the order of 500 M s . Acid catalysis involves either protonation or hydrogen bonding at the carbonyl oxygen. 

It is also possible to carry out the aldol condensation under acidic conditions. The reactive nucleophile is then the enol. The mechanism, as established in detail for acetaldehyde, involves nucleophilic attack of the enol on the protonated aldehyde. 

Experimental evidence, obtained in protonation (3,6), acylation (1,4), and alkylation (1,4,7-9) reactions, always indicates a concurrence between electrophilic attack on the nitrogen atom and the -carbon atom in the enamine. Concerning the nucleophilic reactivity of the j3-carbon atom in enamines, Opitz and Griesinger (10) observed, in a study of salt formation, the following series of reactivities of the amine and carbonyl components pyrrolidine and hexamethylene imine s> piperidine > morpholine > cthyl-butylamine cyclopentanone s> cycloheptanone cyclooctanone > cyclohexanone monosubstituted acetaldehyde > disubstituted acetaldehyde. 

Figure 19.18 1H NMR spectrum of acetaldehyde. The absorption of the aldehyde proton appears at 9.8 8 and is split into a quartet.

Hydrogens on the carbon next to a carbonyl group are slightly deshielded and normally absorb near 2.0 to 2.3 8. The acetaldehyde methyl group in Figure 19.18, for instance, absorbs at 2.20 8. Methyl ketones are particularly distinctive because they always show a sharp three-proton singlet near 2.1 8. 

There is evidence (in the self-condensation of acetaldehyde) that a water molecule acts as a base (even in concentrated H2SO4) in assisting the addition of the enol to the protonated aldehyde Baigrie, L.M. Cox, R.A. Slebocka-Tilk, H. Tencer, M. Tidwell, T.T. J. Am. Chem. Soc., 1985, 107, 3640. 

Aerobic living features metabolize sugars and fatty acids to carbon dioxide. Accordingly, there are some kinds of decarboxylation reactions. TPP-mediated decarboxylation of pyruvic acid to acetaldehyde is one of the most important steps of the metabolism of sugar compounds (Fig. 1). When the intermediate reacts with lipoic acid instead of a proton, pyruvic acid is converted to acetylcoenzyme A, which is introduced to TCA cycle (Fig. 2). 

Infrared spectroscopy has also been employed to follow the formation of acetaldehyde and acetic acid on Pt during ethanol electro-oxidation. On the basal planes, acetaldehyde could be observed starting at about 0.4 V (vs. RHE), well before the onset of CO oxidation, while the onset of acetic acid formation closely follows CO2 formation [Chang et al., 1990 Xia et al., 1997]. This is readily explained by the fact that both CO oxidation and acetic acid formation require a common adsorbed co-reactant, OHads, whereas the formation of acetaldehyde from ethanol merely involves a relatively simple proton-electron transfer. 

Detailed structural calculations have been carried out for this system. This is because the neutral isomer, C2HsO, which is implicated in the thermochemistry of ethanol, is of interest in pollution control, atmospheric chemistry, and combustion. Also, there is new information available from photoionization experiments with which to compare theoretical calculations. For details of these comparisons, see Curtiss et al.73 In the earlier theoretical studies of Nobes et al.,74 calculations were performed at the MP2 and MP3 levels with basis sets of double plus polarization (6-13G ) with electron correlation. These studies revealed four stable minima for the system protonated acetaldehyde, CHj-C H-OH <-> CH3-CH=0+H the methoxymethyl cation, CH3OCH2 protonated oxirane, (CH2)2OH+ and vinylox-... 

Protonated acetaldehyde has been accessed in studies reported in the literature24,25 by rapid proton transfer reactions of acetaldehyde with HCO+, C2H30+, and H30+, and by ... 

The ethoxide anion resulting from hydride transfer to acetaldehyde is then protonated by the enzyme to form ethanol. 

The formylmethyl complex 21 also serves as a source of free acetaldehyde, and one equivalent of trifluoromethanesulfonic acid in CH2CI2 releases it from 21 within one hour at room temperature. Acetaldehyde was identified by its 2,4-dinitrophenylhydrazone (isolated in 42% yield), and was determined directly (48%) by quantitative analysis of its IR v 1716 cm"l absorption. The protonation of 21 presumably generates a n2-vinyl alcohol compound 19 (R=H) [IR observable v 1983 cm" ], which then dissociates acetaldehyde. We have overall converted selectively two carbonyls on CpFe(CO)3+ (1) to acetaldehyde. 

Finally, Nagaoka et al have made a very interesting study applying MC-FEP techniques to the vinyl alcohol - acetaldehyde tautomerism.32 Using a cluster of the solute with three water molecules as a solute , the free energy for the tautomerism was calculated along different reaction pathways, which had been previously found by ab initio calculations including an SCRF solvation term. They were able to deduce that a two-step mechanism is favoured over a concerted one for the transfer of the proton. 

Since many of the transformations undergone by metabolites involve changes in oxidation state, it is understandable that cofactors have been developed to act as electron acceptors/ donors. One of the most important is that based on NAD/NADP. NAD+ can accept what is essentially two electrons and a proton (a hydride ion) from a substrate such as ethanol in a reaction catalysed by alcohol dehydrogenase, to give the oxidized product, acetaldehyde and the reduced cofactor NADH plus a proton (Figure 5.2). Whereas redox reactions on metal centres usually involve only electron transfers, many oxidation/reduction reactions in intermediary metabolism, as in the case above, involve not only electron transfer but... 

Carbamate substituents have also been found to permit the direct removal of allylic, propargylic and allenic protons by organolithium reagents [32, 33]. In the latter case, the resulting lithioallenes can be converted to the allenyltitanium reagents with ClTi(OiPr)3 (Eq. 9.28) [8]. As illustrated, subsequent addition to acetaldehyde proceeds with only modest diastereoselectivity. 

Such stability is only relative, however, given the possibility of the acid-catalyzed 1,2-shift of a proton observed in some olefin epoxides of general structure 10.10 (Fig. 10.3) [12], Such a reaction occurs in the in vivo metabolism of styrene to phenylacetic acid the first metabolite formed is styrene oxide (10.10, R = Ph, Fig. 10.3, also 10.6), whose isomerization to phenyl-acetaldehyde (10.11, R = Ph, Fig. 10.3) and further dehydrogenation to phenylacetic acid has been demonstrated by deuterium-labeling studies. A com-... 

The carbon dioxide anion-radical was used for one-electron reductions of nitrobenzene diazo-nium cations, nitrobenzene itself, quinones, aliphatic nitro compounds, acetaldehyde, acetone and other carbonyl compounds, maleimide, riboflavin, and certain dyes (Morkovnik and Okhlobystin 1979). The double bonds in maleate and fumarate are reduced by CO2. The reduced products, on being protonated, give rise to succinate (Schutz and Meyerstein 2006). The carbon dioxide anion-radical reduces organic complexes of Co and Ru into appropriate complexes of the metals(II) (Morkovnik and Okhlobystin 1979). In particular, after the electron transfer from this anion radical to the pentammino-p-nitrobenzoato-cobalt(III) complex, the Co(III) complex with thep-nitrophenyl anion-radical fragment is initially formed. The intermediate complex transforms into the final Co(II) complex with the p-nitrobenzoate ligand. 

The homolytic acylation of protonated heteroaromatic bases is, as with alkylation, characterized by high selectivity. Only the positions a and y to the heterocyclic nitrogen are attacked. Attack in the position or in the benzene ring of polynuclear heteroaromatics has never been observed, even after careful GLC analysis of the reaction products. Quinoline is attacked only in positions 2 and 4 the ratio 4-acyl- to 2-acylquinoline was 1.3 with the acetyl radical from acetaldehyde, 1.7 with the acetyl radical from pyruvic acid, and 2.8 with the benzoyl radical from benzaldehyde. 

An alternative approach to mixed aldol reactions, and the one usually preferred, is to carry out a two-stage process, forming the enolate anion first using a strong base like EDA (see Section 10.2). The first step is essentially irreversible, and the electrophile is then added in the second step. An aldol reaction between butan-2-one and acetaldehyde exemplifies this approach. Note also that the large base EDA selectively removes a proton from the least-hindered position, again restricting possible combinations (see Section 10.2). 

---