Catalysts aldol condensations

The effect of the basicity of aldol condensation catalysts on their activity was thoroughly investigated by Malinowski et al. [372—379]. The observed linear dependence of the rate coefficients of several condensation reactions on the amount of sodium hydroxide contained in silica gel (Figs. 12 and 13) supported the view that the basic properties of this type of catalyst were actually the cause of its catalytic activity, though the alkali-free catalyst was not completely inactive. The amphoteric nature of the catalysis by silica gel, which can act also as an acid catalyst, was demonstrated [380]. By a stepwise addition of sodium acetate to a HN03-pretreated silica gel catalyst the original activity for acetaldehyde self-condensation was decreased to a minimum (when an equivalent amount of the base was added) by further addition of sodium acetate, the activity increased again because of the transition to a base... 

ALDOL CONDENSATION, CATALYSTS Bis(p-methoxypheny])telluroxide. Phase-transfer catalysts. Zinc chloride. 

Tsuji and coworkers have developed diisobutylaluminum phenoxide-pyridine as an effective aldol condensation catalyst and applied it to the macrocyclization of 2,15-hexadecanedione (equation 132). Addition of the diketone at high dilution to a solution of the catalyst in hexane provides a mixture of cis and trans isomers of the and A enones. Catalytic hydrogenation of the mixture affords ( )-muscone. The authors explain the regioselectivity of the process by assuming that the aluminum phenoxide functions as a Lewis acid, coordinating to the carbonyl group. Pyridine functions as a base to remove a proton from the less hindered methyl group. 

The reaction of MP and methanol was conducted using a conbined catalyst system consisting of a methanol conversion catalyst and an aldol-condensation catalyst. As the methanol conversion catalyst, a 1.0 g portion of Ag-MOH/Si02 (M = alkali metal, Ag/M/Si atomic ratio = 4/20/1000) was used. As the catalyst for the aldol-condensation, a 20 g portion of CsOH/Zr-SiOa (Cs/Zr/Si atomic ratio = 22/10/100) was used. A mixed gas of MP, methanol, oxygen, and nitrogen was passed at 360 °C. The feed rate of MP/methanol/oxygen/nitrogen was 50/75/10/250 mmol/h. The results areshown in Table 6. They are summarized as follows. 

Various benzylic alcohols, as well as cinnamyl alcohol and cyclohexanol, were dehydrogenated using large amounts of GiO under sonication [71] or GeO [72]. Despite the high GeO loadings (200 wt%), the material acted as an aerobic oxidation catalyst, since hardly any reaction occurred under nitrogen and the material could be recycled many times. Interestingly, GiO acted as a multifunctional tandem alcohol oxidation-alkyne hydration-aldol condensation catalyst for chal-cone formation directly from phenylacetylenes and benzyl alcohols [73]. 

Scheme 5.131 Domino hydroformylation-cross aldol addition with a chiral rhodium catalyst and a chiral aldol condensation catalyst.

Clalsen aldol condensation. This consists in the condensation of an aromatic aldehyde and an ester R—CHjCOOCjHj in the presence of finely divided sodium and a trace of alcohol at a low temperature. The catalyst is the alkoxide ion aqueous alkalis caimot be employed since they will hydrolyse the resulting ester. The product is an ap-unsaturated ester, for example ... 

In an intramolecular aldol condensation of a diketone many products are conceivable, since four different ends can be made. Five- and six-membered rings, however, wUl be formed preferentially. Kinetic or thermodynamic control or different acid-base catalysts may also induce selectivity. In the Lewis acid-catalyzed aldol condensation given below, the more substituted enol is formed preferentially (E.J. Corey, 1963 B, 1965B). 

The base-catalyzed reaction of acetaldehyde with excess formaldehyde [50-00-0] is the commercial route to pentaerythritol [115-77-5]. The aldol condensation of three moles of formaldehyde with one mole of acetaldehyde is foUowed by a crossed Cannizzaro reaction between pentaerythrose, the intermediate product, and formaldehyde to give pentaerythritol (57). The process proceeds to completion without isolation of the intermediate. Pentaerythrose [3818-32-4] has also been made by condensing acetaldehyde and formaldehyde at 45°C using magnesium oxide as a catalyst (58). The vapor-phase reaction of acetaldehyde and formaldehyde at 475°C over a catalyst composed of lanthanum oxide on siHca gel gives acrolein [107-02-8] (59). 

Reaction of one mole of acetaldehyde and excess phenol in the presence of a mineral acid catalyst gives l,l-bis(p-hydroxyphenyl)ethane [2081-08-5], acid catalysts, acetaldehyde, and three moles or less of phenol yield soluble resins. Hardenable resins are difficult to produce by alkaline condensation of acetaldehyde and phenol because the acetaldehyde tends to undergo aldol condensation and self-resinification (see Phenolic resins). 

Lithium Iodide. Lithium iodide [10377-51 -2/, Lil, is the most difficult lithium halide to prepare and has few appHcations. Aqueous solutions of the salt can be prepared by carehil neutralization of hydroiodic acid with lithium carbonate or lithium hydroxide. Concentration of the aqueous solution leads successively to the trihydrate [7790-22-9] dihydrate [17023-25-5] and monohydrate [17023-24 ] which melt congmendy at 75, 79, and 130°C, respectively. The anhydrous salt can be obtained by carehil removal of water under vacuum, but because of the strong tendency to oxidize and eliminate iodine which occurs on heating the salt ia air, it is often prepared from reactions of lithium metal or lithium hydride with iodine ia organic solvents. The salt is extremely soluble ia water (62.6 wt % at 25°C) (59) and the solutions have extremely low vapor pressures (60). Lithium iodide is used as an electrolyte ia selected lithium battery appHcations, where it is formed in situ from reaction of lithium metal with iodine. It can also be a component of low melting molten salts and as a catalyst ia aldol condensations. 

Propane, 1-propanol, and heavy ends (the last are made by aldol condensation) are minor by-products of the hydroformylation step. A number of transition-metal carbonyls (qv), eg, Co, Fe, Ni, Rh, and Ir, have been used to cataly2e the oxo reaction, but cobalt and rhodium are the only economically practical choices. In the United States, Texas Eastman, Union Carbide, and Hoechst Celanese make 1-propanol by oxo technology (11). Texas Eastman, which had used conventional cobalt oxo technology with an HCo(CO)4 catalyst, switched to a phosphine-modified Rh catalyst ia 1989 (11) (see Oxo process). In Europe, 1-propanol is made by Hoechst AG and BASE AG (12). 

Citral reacts in an aldol condensation using excess acetone and a basic catalyst, usually sodium hydroxide. The excess acetone can be recovered for recycle. The resulting intermediate pseudoionone [141-10-6] (83) after cyclization with phosphoric acid gives predominantly a-ionone [127-41 -3] (84), which is the isomer commercially important in flavors and fragrances. A hydrocarbon solvent is generally necessary in order to get high yields. P-Ionone [14901-07-6] (85) is the predominant isomer if sulfuric acid is used as the catalyst but lower temperature than that for cyclization to a-ionone is required. y-Ionone [79-6-5] (86) is also produced. 

Neo acids are prepared from selected olefins using carbon monoxide and acid catalyst (4) (see Carboxylic Acids, trialkylacetic acids). 2-EthyIhexanoic acid is manufactured by an aldol condensation of butyraldehyde followed by an oxidation of the resulting aldehyde (5). Isopalmitic acid [4669-02-7] is probably made by an aldol reaction of octanal. 

Kelkar and McCarthy (1995) proposed another method to use the feedforward experiments to develop a kinetic model in a CSTR. An initial experimental design is augmented in a stepwise manner with additional experiments until a satisfactory model is developed. For augmenting data, experiments are selected in a way to increase the determinant of the correlation matrix. The method is demonstrated on kinetic model development for the aldol condensation of acetone over a mixed oxide catalyst. 

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. 

Cope rearrangement 17, 213 f., 643 Corey catalysts 74 f., 80 -, aldol condensations with 74 -, carbonyl allylations with 74 -, carbonyl reductions with 74 f. 

Hydroformylation is an important industrial process carried out using rhodium phosphine or cobalt carbonyl catalysts. The major industrial process using the rhodium catalyst is hydroformylation of propene with synthesis gas (potentially obtainable from a renewable resource, see Chapter 6). The product, butyraldehyde, is formed as a mixture of n- and iso- isomers the n-isomer is the most desired product, being used for conversion to butanol via hydrogenation) and 2-ethylhexanol via aldol condensation and hydrogenation). Butanol is a valuable solvent in many surface coating formulations whilst 2-ethylhexanol is widely used in the production of phthalate plasticizers. 

Antibody-catalyzed aldol condensation was demonstrated in a [bmim][PFg] solvent system by Kitazume and co-workers (Fig. 17). They tested recyclable use of antibody catalyst in the solvent system and, very interestingly, found that the chemical yield was increased for the second cycle (89%) over the initial run (21%). 

Examples of commercially applied solid base catalysts are much fewer than for solid acids. Nevertheless, much attention is currently focused on the development of novel solid base catalysts for classical organic reactions such as aldol condensations, Michael additions, and Knoevenagel condensations, to name but a few. 

As in the case of homogeneous acids as catalyst, we would also benefit from using solid ba.ses instead of dissolved bases as catalyst. A number of industrially important reactions are carried out with bases as catalyst. A well know example is the aldol condensation of acetone to diacetone alcohol, where dissolved NaOH in ethyl alcohol is u.sed as a catalyst at about 200 to 300 ppm level. However, heterogeneous pelleted sodamide can be used as a catalyst for this reaction and it obviates the problem of alkali removal from the product, which would otherwise lead to reversion of diacetone alcohol to acetone during distillation of the product mixture. 

The n-butyraldehyde is treated with a 2 per cent w/w aqueous sodium hydroxide and undergoes an aldol condensation at a conversion efficiency of 90 per cent. The product of this reaction, 2-ethylhexanal, is separated and then reduced to 2-ethylhexanol by hydrogen in the presence of a Raney nickel catalyst with a 99 per cent conversion rate. In subsequent stages of the process (details of which are not required), 99.8 per cent of the 2-ethylhexanol is recovered at a purity of 99 per cent by weight. 

The Aldol Condensation of Acetone Over a CsOH/Si02 Solid Base Catalyst... 

The solid base catalysed aldol condensation of acetone was performed over a CsOH/Si02 catalyst using a H2 carrier gas. The products observed were diacetone alcohol, mesityl oxide, phorone, iso-phorone and the hydrogenated product, methyl isobutyl ketone. Deuterium tracer experiments were performed to gain an insight into the reaction mechanism. A mechanism is proposed. 

The aldol condensation of acetone to diacetone alcohol is the first step in a three-step process in the traditional method for the production of methyl isobutyl ketone (MIBK). This reaction is catalysed by aqueous NaOH in the liquid phase. (3) The second step involves the acid catalysed dehydration of diacetone alcohol (DAA) to mesityl oxide (MO) by H2S04 at 373 K. Finally the MO is hydrogenated to MIBK using Cu or Ni catalysts at 288 - 473 K and 3- 10 bar (3). 

Annual Volume 71 contains 30 checked and edited experimental procedures that illustrate important new synthetic methods or describe the preparation of particularly useful chemicals. This compilation begins with procedures exemplifying three important methods for preparing enantiomerically pure substances by asymmetric catalysis. The preparation of (R)-(-)-METHYL 3-HYDROXYBUTANOATE details the convenient preparation of a BINAP-ruthenium catalyst that is broadly useful for the asymmetric reduction of p-ketoesters. Catalysis of the carbonyl ene reaction by a chiral Lewis acid, in this case a binapthol-derived titanium catalyst, is illustrated in the preparation of METHYL (2R)-2-HYDROXY-4-PHENYL-4-PENTENOATE. The enantiomerically pure diamines, (1 R,2R)-(+)- AND (1S,2S)-(-)-1,2-DIPHENYL-1,2-ETHYLENEDIAMINE, are useful for a variety of asymmetric transformations hydrogenations, Michael additions, osmylations, epoxidations, allylations, aldol condensations and Diels-Alder reactions. Promotion of the Diels-Alder reaction with a diaminoalane derived from the (S,S)-diamine is demonstrated in the synthesis of (1S,endo)-3-(BICYCLO[2.2.1]HEPT-5-EN-2-YLCARBONYL)-2-OXAZOLIDINONE. 

This possible mechanism should be evaluated in relation to the catalysts. If the catalytic action is to be ascribed to the acid character of the catalysts, the condensation under consideration may differ from the ordinary aldol condensation, which is catalyzed preferentially by basic agents. Nevertheless, many condensations of the aldol type are effected with the aid of acidic reagents. Moreover, the condensation of sugars with dicarbonyl compounds has been carried out in aqueous alcoholic media which are non-acidic hence, there also exists the possibility of a mechanism catalyzed simultaneously by acid and by base, somewhat like that suggested by Lowry46 in another connection. A transition state with an amphiprotic structure has been postulated. Its formation can be catalyzed by either acids or bases. 

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