Paal mechanism

The mechanism for the redistribution in oxidation states begins similarly to that of the Paal thiophene synthesis. However, upon formation of dithione 38, nucleophilic addition of one thiocarbonyl into the other produces the intermediate zwitterion 39. A 1,3-tautomerization of hydrogen then gives... 

Probably, with ethyl substituent, a 7t-allylic adsorption [such as suggested by Paal and Dobrovolszky (97)] competes with the dual site mechanism. 

There are two principal routes to pyrroles. One is called the Paal-Knorr synthesis, in which pyrroles are formed by the interaction of 1,4-dicarbonyl compounds and ammonia. No intermediates have ever been isolated, so the mechanism shown in Scheme 6.13 is speculative. 

Even though the Paal-Knorr pyrrole synthesis has been around for 120 years, its precise mechanism was the subject of debate. In 1991, V. Amarnath et al. investigated the intermediates of the reaction and determined the most likely mechanistic pathway. The formation of pyrroles was studied on various racemic and meso-3,4-diethyl-2,5-hexanediones. The authors found that the rate of cyclization was different for the racemic and meso compounds and the racemic isomers reacted considerably faster than the meso isomers. There were two crucial observations 1) the stereoisomers did not interconvert under the reaction conditions and 2) there was no primary kinetic isotope effect for the hydrogen atoms at the C3 and C4 positions. These observations led to the conclusion that the cyclization of the hemiaminal intermediate is the rate-determining (slow) step. 

Give all steps and the mechanism for the synthesis of 3,4-dimethylpyrrole by the Paal-Knorr reaction. 

Chapter 4 discussed two of the traditional methods of pyrrole synthesis, the Paal-Knorr synthesis and the Knorr synthesis. The basic reactions are repeated here as Scheme 9.3. Details of the mechanisms were given in sections 4.2.1 and 4.2.3, respectively. 

The mechanism of the Paal-Knorr fiiran synthesis was established by Amath and co-workers using the d,l- and /we o-diastereomers of 2,3-disubstituted 1,4-diketones. Protonation of one of the carbonyls of the P-diketone, followed by deprotonation of the proton adjacent to the unprotonated ketone, results in the formation of an enolate that attacks the protonated ketone to form the corresponding dihydrofuran. Elimination of water then generates the furan. 

This is an example of the so-called Paal-Knorr pyrrole synthesis. Additionally, there is formation of an unsaturated ester, which suffers a conjugated addition. A plausible mechanism is the following one ... 

Wootsch, A., Paal, Z. (2002). Reactions of -hexane on Pt catalysts reaction mechanism as revealed by hydrogen pressure and compensation effect. Journal of Catalysis, 205, 86. 

Furans are prepared by modification of the chemistry used to make pyrrole derivatives. Because furans have an oxygen atom rather than a nitrogen atom, a modification of the reactive partners is required. When 2,5-hexanedione (115) is treated with acid, the product is a furan, 124. This is called the Paal-Knorr furan synthesis, and it begins with protonation of one carbonyl and attack of the oxygen atom of the second carbonyl to close the ring. Elimination of water leads to the furan because it generates an aromatic system. The reaction of protonated ketones and aldehydes with oxygen nucleophiles was discussed in Chapter 18 (Section 18.6). The mechanism for formation of 124 is therefore related to the chemistry presented in Chapter 18. 

The last equation is related to the first gas phase studies of the solubility of H2 in iron (and other metals) generally attributed to Sieverts (Sieverts etal, 1911), who found experimentally that amounts of H2 dissolved in metal is directly proportional to the square root of the hydrogen pressure (Equation [2.4]). Various aspects related to the degradation of mechanical properties due to the entry of hydrogen into metals or alloys can also be found in Lewis (1967) and in Paal and Menon (1988). 

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