Aldehydes reaction mechanism

This reaction, applicable only to the preparation of hydroxy-aldehydee, is alternative to the Gattermann aldehyde reaction (or the Adams modification of it) given under 4. The yields are usually smaller, but a large quantity of the phenol may be recovered. The following mechanism is consistent with the known facts ... 

Methods of the first type have been used for both qualitative and quantitative investigation. An important limitation is that the rates of interconversion of the tautomeric forms must be small as compared with those of the test reaction (s). The method is further complicated since the test reactions are sometimes complex and it is difficult to be certain that only one tautomer is reacting. An even more fundamental objection is that much chemical evidence is based on incorrect reaction mechanisms. Thus, the formation of condensation products (30) with aldehydes has repeatedly been quoted as evidence for structures of type 31 and against type 32,. whereas if 31 does react with an aldehyde it must either first tautomerize to 32 or ionize to 33. 

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-... 

Chiral salen chromium and cobalt complexes have been shown by Jacobsen et al. to catalyze an enantioselective cycloaddition reaction of carbonyl compounds with dienes [22]. The cycloaddition reaction of different aldehydes 1 containing aromatic, aliphatic, and conjugated substituents with Danishefsky s diene 2a catalyzed by the chiral salen-chromium(III) complexes 14a,b proceeds in up to 98% yield and with moderate to high ee (Scheme 4.14). It was found that the presence of oven-dried powdered 4 A molecular sieves led to increased yield and enantioselectivity. The lowest ee (62% ee, catalyst 14b) was obtained for hexanal and the highest (93% ee, catalyst 14a) was obtained for cyclohexyl aldehyde. The mechanism of the cycloaddition reaction was investigated in terms of a traditional cycloaddition, or formation of the cycloaddition product via a Mukaiyama aldol-reaction path. In the presence of the chiral salen-chromium(III) catalyst system NMR spectroscopy of the crude reaction mixture of the reaction of benzaldehyde with Danishefsky s diene revealed the exclusive presence of the cycloaddition-pathway product. The Mukaiyama aldol condensation product was prepared independently and subjected to the conditions of the chiral salen-chromium(III)-catalyzed reactions. No detectable cycloaddition product could be observed. These results point towards a [2-i-4]-cydoaddition mechanism. 

The reaction mechanism involves deprotonation of the carboxylic anhydride 2 to give anion 4, which then adds to aldehyde 1. If the anhydride used bears two a-hydrogens, a dehydration takes place already during workup a /3-hydroxy carboxylic acid will then not be isolated as product ... 

From this initiation mechanism, the important role of the aldehyde group in the reaction mechanism of 1,2-diol and l-amino-2-hydroxy compound and Ce(IV) ion initiation systems can again be seen. 

In general, the reaction mechanism of elastomeric polymers with vulcanisation reagents is slow. Therefore, it is natural to add special accelerators to rubber compounds to speed the reaction. Accelerators are usually organic compounds such as amines, aldehyde-amines, thiazoles, thiurams or dithio-carbamates, either on their own or in various combinations. 

We said in Section 17.4 that carboxylic acids are reduced by L1AIH4 to give primary alcohols, but we deferred a discussion of the reaction mechanism at that time. In fact, the reduction is a nucleophilic acyl substitution reaction in which —H replaces -OH to give an aldehyde, which is further reduced to a primary alcohol by nucleophilic addition. The aldehyde intermediate is much more reactive than the starting acid, so it reacts immediately and is not isolated. 

Classical examples of this type of reaction are the various dimethylaminobenz-aldehyde reagents (q.v.) and vanillin-acid reagents, of which one, the vanillin-phosphoric acid reagent, is already included in Volume 1 a. The aldol condensation of estrogens is an example for the reaction mechanism (cf. Chapter 2, Table 6). According to Maiowan indole derivatives react in a similar manner [1]. Longo has postulated that catechins yield intensely colored triphenylmethane dyes [2]. 

This is an example of the Doebner synthesis of qulnollne-4-carboxyUc acids (cinchoninic acids) the reaction consists in the condensation of an aromatic amine with pyruvic acid and an aldehyde. The mechanism is probably similar to that given for the Doebner-Miller sj-nthesis of quinaldiiie (Section V,2), involving the intermediate formation of a dihydroquinoline derivative, which is subsequently dehydrogenated by the Schiff s base derived from the aromatic amine and aldehyde. 

The catalysts used in hydroformylation are typically organometallic complexes. Cobalt-based catalysts dominated hydroformylation until 1970s thereafter rhodium-based catalysts were commerciahzed. Synthesized aldehydes are typical intermediates for chemical industry [5]. A typical hydroformylation catalyst is modified with a ligand, e.g., tiiphenylphoshine. In recent years, a lot of effort has been put on the ligand chemistry in order to find new ligands for tailored processes [7-9]. In the present study, phosphine-based rhodium catalysts were used for hydroformylation of 1-butene. Despite intensive research on hydroformylation in the last 50 years, both the reaction mechanisms and kinetics are not in the most cases clear. Both associative and dissociative mechanisms have been proposed [5-6]. The discrepancies in mechanistic speculations have also led to a variety of rate equations for hydroformylation processes. 

Venturini, A., Joglar, J., Fustero, S., Gonzalez, J., 1997, Diels-Alder Reactions of 2-Azabutadienes With Aldehydes Ab Initio and Density Functional Theoretical Study of the Reaction Mechanism, Regioselectivity, Acid Catalysis, and Stereoselectivity , J. Org. Chem., 62, 3919. 

Scheme 8 Plausible reaction mechanism for the Ni-catalyzed mono- and bis-allylation of aldehyde with butadiene, promoted by In(I)I...

Together with the fast oxidation (at low temperatures) of NO to N02, the plasma causes the partial HC oxidation (using propylene, the formation of CO, C02, acetaldehyde and formaldehyde was observed). Both the effects cause a large promotion in activity of the downstream catalyst [86]. For example, a "/-alumina catalyst which is essentially inactive in the SCR of NO with propene at temperatures 200°C allows the conversion of NO of about 80% (in the presence of NTP). Formation of aldehydes follows the trend of NO concentration suggesting their role in the reaction mechanism. Metal oxides such as alumina, zirconia or metal-containing zeolites (Ba/Y, for example) have been used [84-87], but a systematic screening of the catalysts to be used together with NTP was not carried out. Therefore, considerable improvements may still be expected. 

Havel Sb. Ved. Praci, Vysoka Chem. Tech-nol, Pardubice 1 (83), 1965] has suggested the following chain reaction mechanism for the Co+ + + catalyzed oxidation of aldehyde to peracetic acid. 

Modern MCRs that involve isocyanides as starting materials are by far the most versatile reactions in terms of available scaffolds and numbers of accessible compounds. The oldest among these, the three-component Passerini MCR (P-3CR), involves the reaction between an aldehyde 9-1, an acid 9-2, and an isocyanide 9-3 to yield a-acyloxycarboxamides 9-6 in one step [8], The reaction mechanism has long been a point of debate, but a present-day generally accepted rational assumption for the observed products and byproducts is presented in Scheme 9.1. The reaction starts with the formation of adduct 9-4 by interaction of the carbonyl compound 9-1 and the acid 9-2. This is immediately followed by an addition of the oxygen of the carboxylic acid moiety to the carbon of the isocyanide 9-3 and addition of this carbon to the aldehyde group, as depicted in TS 9-5 to give 9-5. The final product 9-6 is... 

A theoretical study of the reaction mechanism for addition of organozincate complexes to aldehydes was recently performed using density functional theory.298 It has been suggested that the addition takes place through formation of a four-centered transition state and, therefore, it can be considered a typical nucleophilic reaction. 

The methylation of secondary amines works better than for primary amines because there is no competition between the formation of mono- or dimethylated products. The best results for the microwave-enhanced conditions were obtained when the molar ratios of substrate/formaldehyde/formic acid were 1 1 1, so that the amount of radioactive waste produced is minimal. The reaction can be carried out in neat form if the substrate is reasonably miscible with formic acid/aldehyde or in DM SO solution if not. Again the reaction is rapid - it is complete within 2 min at 120 W microwave irradiation compared to longer than 4 h under reflux. The reaction mechanism and source of label is ascertained by alternatively labeling the formaldehyde and formic acid with deuterium. The results indicate that formaldehyde contri-... 

The readsorption and incorporation of reaction products such as 1-alkenes, alcohols, and aldehydes followed by subsequent chain growth is a remarkable property of Fischer-Tropsch (FT) synthesis. Therefore, a large number of co-feeding experiments are discussed in detail in order to contribute to the elucidation of the reaction mechanism. Great interest was focused on co-feeding CH2N2, which on the catalyst surface dissociates to CH2 and dinitrogen. Furthermore, interest was focused on the selectivity of branched hydrocarbons and on the promoter effect of alkali on product distribution. All these effects are discussed in detail on the basis... 

In Fischer-Tropsch synthesis the readsorption and incorporation of 1-alkenes, alcohols, and aldehydes and their subsequent chain growth play an important role on product distribution. Therefore, it is very useful to study these reactions in the presence of co-fed 13C- or 14 C-labeled compounds in an effort to obtain data helpful to elucidate the reaction mechanism. It has been shown that co-feeding of CF12N2, which dissociates toward CF12 and N2 on the catalyst surface, has led to the sound interpretation that the bimodal carbon number distribution is caused by superposition of two incompatible mechanisms. The distribution characterized by the lower growth probability is assigned to the CH2 insertion mechanism. 

Amino acid formation in the Urey-Miller experiment and almost certainly in the prebiotic environment is via the Stecker synthesis shown in Figure 8.3. This reaction mechanism shows that the amino acids were not formed in the discharge itself but by reactions in the condensed water reservoir. Both HCN and HCO are formed from the bond-breaking reactions of N2 and H2O in a plasma, which then react with NH3 in solution. The C=0 group in formaldehyde or other aldehydes is replaced by to form NH and this undergoes a reaction with HCN to form the cyano amino compound that hydrates to the acid. The Strecker synthesis does not provide stereo-control over the carbon centre and must result in racemic mixtures of amino acids. There is no room for homochirality in this pathway. 

The decarbonylations, which do not appear to be affected by light, are reasonably selective with aromatic aldehydes, yielding the expected product however, significant amounts of other products are obtained with non-aromatic substrates (e.g. cyclohexane-aldehyde gives methylcyclopentane and small amounts of n-hexane, as well as the expected cyclohexane and cyclohexen-4-al gives both cyclohexene and cyclohexane). Indeed, the unexpected products perhaps provided a major clue to an understanding of the reaction mechanism(s) involved. 

Wacker (1) A general process for oxidizing aliphatic hydrocarbons to aldehydes or ketones by the use of oxygen, catalyzed by an aqueous solution of mixed palladium and copper chlorides. Ethylene is thus oxidized to acetaldehyde. If the reaction is conducted in acetic acid, the product is vinyl acetate. The process can be operated with the catalyst in solution, or with the catalyst deposited on a support such as activated caibon. There has been a considerable amount of fundamental research on the reaction mechanism, which is believed to proceed by alternate oxidation and reduction of the palladium ... 

Catalytic coupling reaction of aldehydes, alkynes, and secondary amines promoted by less than 3 mol.% of Ag(l) salt was reported by Li et al,517 In this reaction, pure water was used as solvent and Agl was found to be the best catalyst without need of any additives or co-catalysts (Table 9). The reaction mechanism has been proposed as shown in Scheme 110. 

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. 

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