Aldolization kinetic study

A full kinetic study of the proline-mediated aldol reaction based on a detailed catalytic reaction mechanism will be published separately. 

The aldol reaction can be applied to dicarbonyl compounds in which the two groups are favorably disposed for intramolecular reaction. Kinetic studies on cyclization of 5-oxohexanal, 2,5-hexanedione, and 2,6-heptanedione indicate that formation of five-membered rings is thermodynamically somewhat more favorable than formation of six-membered rings, but that the latter is several thousand times faster.170 A catalytic amount of acid or base is frequently satisfactory for formation of five- and six-membered rings, but with more complex structures, the techniques required for directed aldol condensations are used. 

In 1998, Hasanayn and Streitwieser reported the kinetics and isotope effects of the Aldol-Tishchenko reaction . They studied the reaction between lithium enolates of isobu-tyrophenone and two molecule of beuzaldehyde, which results iu the formation of a 1,3-diol monoester after protonation (Figure 28). They analyzed several aspects of this mechanism experimentally. Ab initio molecular orbital calculatious ou models are used to study the equilibrium and transition state structures. The spectroscopic properties of the lithium enolate of p-(phenylsulfonyl) isobutyrophenone (LiSIBP) have allowed kinetic study of the reaction. The computed equilibrium and transition state structures for the compounds in the sequence of reactions in Figure 28 are given along with the computed reaction barriers and energy in Figure 29 and Table 6. 

A kinetic study of the Ph2BOH-catalysed reactions of several aldehydes with 2 revealed that the rate of the disappearance of 2 followed first-order kinetics and was independent from the reactivity of the aldehydes used. Taking into account this result, we have proposed the reaction mechanism in which a silyl enol ether is transformed to the corresponding diphenylboryl enolate before the aldol addition step takes place (Scheme 13.1). The high diastereoselectivity is consistent with the mechanism, in which the aldol step proceeds via a chair-like six-membered transition state. The opposite diastereoselectivity in the reaction with the geometrical isomers of the thioketene silyl acetal shown in Table 13.3 also supports the mechanism via the boron enolate, because this trend was also observed in the classical boron enolate-mediated reactions in dry organic solvents. Although we have not yet observed the boron enolates directly under the reaction conditions, this mechanism can explain all of the experimental data obtained and is considered as the most reasonable one. As far as we know, this is the first example of... 

Other reports of kinetic studies deal with mechanisms of thermal oxidation of a variety of simple ketones monitored via gas evolution (CO, CO2, H2, etc.), alkaline oxidation of aldehydes with copper and silver tellurates, [M (H2Te06)2] , and oxidation of acetals of simple aldehydes in aqueous acetic acid with (i) N-chlorobenzamide (H2OCI+ is the oxidant inferred) and (ri) W-chlorosaccharin. Accounts of the reductive coupling of two molecules of ketone via the McMurry alkene synthesis have been described earlier under Miscellaneous Aldols. 

The formation of 64 using catalyst (S,S)-62 exhibits a positive nonlinear effect, fitting well with Kagan s two ligand model [78] whereas the more hindered catalyst (S,S)-63 led to a perfect linear asymmetric induction suggesting that the product arose from a transition structure involving only one chiral phosphoramide. The kinetic study of this aldol reaction is in accordance with these re-... 

The bulkier 1,2-epoxyoctane can be rearranged over zeolites HY (Si/Al = 15) and H-OFF (Si/Al 3.6) at 160 °C in an autoclave, with toluene as solvent this results in 45 % and 46 %, respectively, seleetivity to octanal 17 %/25 % to allylic alcohols, and 18%/14% to aldolization products, e. g. 2-/z-hexyldec-2-en-l-al [21], A kinetic study of this reaction over different acidic heterogeneous catalysts, e. g. phosphoric acid, ZnCU immobilized on various supports, sulfated zirconia, zeolites, and dodecatungstenophosphoric acid, was reported by Yadav and Satoskar [22],... 

Several weakly basic anion-exchange resin catalysts were screened in laboratory-scale and the results showed that a very good product distribution with respect to aldol can be obtained. Extensive kinetic studies demonstrated that weakly basic anion-exchange resin catalysts promoted both the aldolization and elimination processes, but the product distribution can be steered by selection of the solvent and the ratio of the reactants. A kinetic model based on the aldolization and elimination reaction mechanisms was developed and sin5>lified. The model predictions were in good agreement with the... 

Finally, kinetic studies for the aldol reaction showed that this process was characterized by a well-behaved positive order kinetics, conversely to that observed in other related organocatalytic processes [315]. 

This type of catalyst was named Lewis acid-surfactant combined catalyst (LASC), and was expected to possess the characters of both a Lewis acid and a surfactant. Sc(DS)3 showed quite high activity in aldol reactions in water without using any organic solvents (Scheme 12.63). A kinetic study on the initial rate of this reaction revealed that the reaction in water is about 130 times faster than that in dichloromethane. It was assumed that hydrophobic reaction environments were created by combining Sc(DS)3 and substrates under the conditions, and that they were concentrated in water to realize efficient catalysis. Interestingly, under neat conditions, the reaction proceeded much slower to give the desired adduct in a much lower yield. 

The lithium enolate of t-amyl acetate exists as a doubly chelated dimer in the presence of TMEDA (A,A,A, At -tetramethylethylenediamine). Reaction with a simple aldimine such as pflra-F-C6H4-CH=N-Ph gives an iV-lithiated -amino ester as a monomer, observed by Li- and i N-NMR. Kinetic studies by i F-NMR give a reaction order consistent with a TS of stoichiometry [(ROLi)2(TMEDA)2(imine)], supported by DPT calculations. That such aza-aldol condensations involve dimeric mechanistic routes runs counter to many claims that monomers are more reactive. 

Based on mechanistic and kinetic studies of the higher alcohol synthesis from synthesis gas, it has been shown that the ethanol in the mixed-oxygenate product is produced from intermediates derived from methanol, not CO [103,109]. Kinetic models of the synthesis have been developed that are able to explain the observed product distribution [110,111]. These models are based on a detailed understanding of the reaction mechanism in which two types of reactions dominate aldol condensation, which yields primarily 2-methyl branched alcohols, and Cl coupling reactions, which yield linear alcohols [106,111]. Estimates of the parameters of the kinetic models that quantitatively describe the oxygenate product distributions suggest that the rate of ethanol formation is about an order of magnitude lower than the rate of production of branched alcohols [111,112]. On the Cs/Cu/Zn catalysts, this results in a minimum in yield of ethanol compared with the yields of methanol, 1-propanol, and 2-methyl-1 propanol. Althou methanol conversion to ethanol has been confirmed as part of the hi er alcohol synthesis from synthesis gas, this synthesis does not offer a plausible route for the conversion of methanol to ethanol. Under the reaction conditions methanol rapidly decomposes, even at a pressme of 0.1 MPa [113], to yield an equilibrium mix of methanol, CO, and H2. Furthermore, as shown by the data in T able 7, the yield of ethanol remains low even with methanol in the feed. 

Kinetic studies revealed that whereas the addition of the copper enolate to the carbonyl compound is a rapid reaction, the trapping of the copper aldolate is the rate-determining step. In order to enhance its rate, the additives (EtO)jSiF and PhBFgK are used that are assumed to facilitate the sUyl transfer to the copper aldolate [134]. 

It turned out that the dodecylsulfate surfactants Co(DS)i Ni(DS)2, Cu(DS)2 and Zn(DS)2 containing catalytically active counterions are extremely potent catalysts for the Diels-Alder reaction between 5.1 and 5.2 (see Scheme 5.1). The physical properties of these micelles have been described in the literature and a small number of catalytic studies have been reported. The influence of Cu(DS)2 micelles on the kinetics of quenching of a photoexcited species has been investigated. Interestingly, Kobayashi recently employed surfactants in scandium triflate catalysed aldol reactions". Robinson et al. have demonshuted that the interaction between metal ions and ligand at the surface of dodecylsulfate micelles can be extremely efficient. ... 

The ratio of products 15 and 16 is dependent on the structures, base, and the solvent. The kinetics of the reaction is likewise dependant on the structures and conditions of the reaction. Thus addition or cyclization can be the rate-determining step. In a particularly noteworthy study by Zimmerman and Ahramjian, it was reported that when both diastereomers of 20 were treated individually with potassium r-butoxide only as-epoxy propionate 21 was isolated. It is postulated that the cyclization is the rate-limiting step. Thus, for these substrates, the retro-aldolization/aldolization step reversible. ... 

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