Unsaturated aldehydes mechanism

The simplest a 3 unsaturated aldehyde acrolein is prepared by heating glycerol with an acid catalyst Suggest a mechanism for this reaction... 

The simple hydroperoxide mechanism so far discussed is incomplete for representing reactions with significant products other than hydroperoxides. It can be adequate for oxidations of certain unsaturates, aldehydes, and alkylaromatics where the yield of the corresponding hydroperoxide can exceed 90%. 

Skraup proposed a simple mechanism involving imine formation followed by an acid-mediated cyclization. Unfortunately the observed regioselectivity is not consistent with the proposed mechanism when, for example, electron-rich aniline 4 reacts with a, 3-unsaturated aldehyde 5 to give quinoline 6. ... 

In all the reactions described so far a chiral Lewis acid has been employed to promote the Diels-Alder reaction, but recently a completely different methodology for the asymmetric Diels-Alder reaction has been published. MacMillan and coworkers reported that the chiral secondary amine 40 catalyzes the Diels-Alder reaction between a,/ -unsaturated aldehydes and a variety of dienes [59]. The reaction mechanism is shown in Scheme 1.73. An a,/ -unsaturated aldehyde reacts with the chiral amine 40 to give an iminium ion that is sufficiently activated to engage a diene reaction partner. Diels-Alder reaction leads to a new iminium ion, which upon hydrolysis af-... 

Fermenting baker s yeast also catalyzes the 1,4-addition of a formal trifluoroethanol-d1-synthon to a,/i-unsaturated aldehydes, to give optically active l,l,l-trifluoro-2-hydroxy-5-alka-nones52. Presumably, the mechanism involves oxidation of the alcohol to the corresponding aldehyde followed by an umpolung step with thiamine pyrophosphate and Michael addition to the a,/i-unsaturated aldehyde. For example, l,l,l-trifluoro-2-hydroxy-5-hexanone (yield 26%, ee 93%) is thus obtained from trifluoroethanol and l-bnten-3-one. 

The product is a P-hydroxy aldehyde (called an aldol) or ketone, which in some cases is dehydrated during the course of the reaction. Even if the dehydration is not spontaneous, it can usually be done easily, since the new double bond is in conjugation with the C=0 bond so that this is a method of preparing a,P-unsaturated aldehydes and ketones as well as P-hydroxy aldehydes and ketones. The entire reaction is an equilibrium (including the dehydration step), and a,P-unsaturated and P-hydroxy aldehydes and ketones can be cleaved by treatment with OH (the retrograde aldol reaction). There is evidence that an SET mechanism can intervene when the substrate is an aromatic ketone. ... 

The general mechanistic features of the aldol addition and condensation reactions of aldehydes and ketones were discussed in Section 7.7 of Part A, where these general mechanisms can be reviewed. That mechanistic discussion pertains to reactions occurring in hydroxylic solvents and under thermodynamic control. These conditions are useful for the preparation of aldehyde dimers (aldols) and certain a,(3-unsaturated aldehydes and ketones. For example, the mixed condensation of aromatic aldehydes with aliphatic aldehydes and ketones is often done under these conditions. The conjugation in the (3-aryl enones provides a driving force for the elimination step. 

The Claisen rearrangement is an electrocyclic reaction which converts an allyl vinyl ether into a y,8-unsaturated aldehyde or ketone, via a (3.3) sigmatropic shift. The rate of this reaction can be largely increased in polar solvents. Several works have addressed the study of the reaction mechanism and the electronic structure of the transition state (TS) by examining substituent and solvent effects on the rate of this reaction. 

Choppin GR (2003) Actinide speciation in the environment. Radiochim Acta 91 645-649 Claus P (1998) Selective hydrogenation of a,P-unsaturated aldehydes and other C = O and C = C bonds containing compounds. Top Catal 5 51-62 Colon D, Weber EJ, Anderson JL, Winget P, Suarez LA (2006) Reduction of nitrobenzenes and N-hydroxylanilines by Fe(II) species Elucidation of the reaction mechanism. Environ Sci Technol 40 4449-4454... 

Hong and co-workers have described a formal [3-t-3] cycloaddition of a,P-unsaturated aldehydes using L-proline as the catalyst (Scheme 72) [225], Although the precise mechanism of this reaction is unclear a plausible explanation involves both iminium ion and enamine activation of the substrates and was exploited in the asymmetric synthesis of (-)-isopulegol hydrate 180 and (-)-cubebaol 181. This strategy has also been extended to the trimerisation of acrolein in the synthesis of montiporyne F [226],... 

The aldol formed by the aldol reaction, especially if heated, can react further. The heating causes dehydration (loss of H2O), and the overall reaction involving an aldol reaction followed by dehydration is the aldol condensation. The product of an aldol condensation, favored by the presence of extended conjugation, is an a,(3-unsaturated aldehyde (an enal) or ketone. The mechanism for dehydration (Figure 11-13) begins where the mechanism of the aldol reaction (Figure 11-12) ends. This process works better if extended conjugation results. The aldol reaction and condensation are reversible. 

Entries 1 and 2 in Scheme 2.1 illustrate the preparation of aldol reaction products by the base-catalyzed mechanism. In entry 1, the product is a /< -hydroxya I dehyde, whereas in entry 2 dehydration has occurred and the product is an a,/f-unsaturated aldehyde. 

The 8-hydroxyl group has an inevitable role in this reaction as simple a,(5-unsaturated aldehydes without a 8-hydroxyl group did not yield a bicyclic adduct. The probable mechanism is shown in Fig. 32 and seems to involve the... 

Problem 20.37 Ouinoline is prepared by the Skraup reaction of aniline, glycerol and nitrobenzene. Suggest a mechanism involving Michael addition of aniline to an a,)3-unsaturated aldehyde, ring closure, and then dehydration and oxidation. 

Evidence for this mechanism is (1) two equivalents of RLi are required (2) the hydrogen in the product comes from the water and not from the adjacent carbon, as shown by deuterium labeling 209 and (3) the intermediates 31-33 have been trapped.210 This reaction, when performed in tetramethylenediamine, can be a synthetically useful method211 of generating vinylic lithium compounds (33), which can be trapped by various electrophiles such as D20 (to give deuterated alkenes), C02 (to give a, 3-unsaturated carboxylic acids—6-34), or DMF (to give a, 3-unsaturated aldehydes—0-105). 

Certain unsaturated aldehydes may be converted to cyclic ketones by a related mechanism. The formyl group reacts with Rh(I) complexes to form an acyl-Rh hydride species, which undergoes intramolecular reaction with the olefinic linkage present in the same molecule (117a). Asymmetric induction is observed with a chiral diphosphine ligand (Scheme 53) (117b-d). Enantioselective cyclization of 4-substituted 4-pentanals into 3-substituted cyclopentanones in greater than 99% ee is achieved with a cationic BINAP-Rh complex. 

The hydroperoxides formed in the autoxidation of unsaturated fatty acids are unstable and readily decompose. The main products of hydroperoxide decomposition are saturated and unsaturated aldehydes. The mechanism suggested for the formation of aldehydes involves cleavage of the isomeric hydroperoxide (I) to the alkoxyl radical (II), which undergoes carbon-to-carbon fission to form the aldehyde (III) (Frankel et al. 1961). 

Unimolecular (Section 4.8) Describing a step in a reaction mechanism in which only one particle undergoes a chemical change at the transition state. ol, (i-Unsaturated aldehyde or ketone (Section 18.11) Aldehyde or ketone that bears a double bond between its a and (3 carbons as in... 

The activity and selectivity of a Cs unsaturated aldehyde (prenal) has been investigated in gaseous phase on a platinum polycrystalline sample. The aim of this work was the study of the respective effects of the pretreatment of the sample by oxygen and of the presence of a small partial pressure of H2S in the gas phase. The results gave us a better understanding of the surface mechanism and proved the importance of the chemisorption mode of the molecule. 

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