In the Tishchenko reaction

Yields in the Tishchenko Reaction of Furfural on Alkaline Earth Metal Oxides in a Batch Reactor (178)... 

Aluminum methoxide Al(OMe)3 is a solid which sublimes at 240 °C in vacuum. Aluminum isopropoxide melts in the range 120-140 °C to a viscous liquid which readily supercools. When first prepared, spectroscopic and X-ray evidence indicates a trimeric structure which slowly transforms to a tetramer in which the central Al is octahedrally coordinated and the three peripheral units are tetrahedral.162,153 Intramolecular exchange of terminal and bridging groups, which is rapid in the trimeric form, becomes very slow in the tetramer. There is MS and other evidence that the tetramer maintains its identity in the vapour phase.164 Al[OCH(CF3)2]3 is more volatile than Al[OCH(Me)2]3 and the vapour consists of monomers.165 Aluminum alkoxides, particularly Al(OPr )3, have useful catalytic applications in the synthetic chemistry of aldehydes, ketones and acetals, e.g. in the Tishchenko reaction of aldehydes, in Meerwein-Pondorf-Verley reduction and in Oppenauer oxidation. The mechanism is believed to involve hydride transfer between RjHCO ligands and coordinated R2C=0— A1 groups on the same Al atom.1... 

Ogata, Y. and Kawasaki, A. Alkoxide transfer from aluminum alkoxide to aldehyde in the Tishchenko reaction. Tetrahedron, 1969, 25, 929. 

Potassium fluoride on alumina is a solid base.69 (It forms potassium hydroxide by exchange of fluoride for hydroxyl groups on the surface of the alumina.) An example of its use is given by Reaction 6.21.70 It has also been used without solvent in the Tishchenko reaction of benzaldehyde to give benzyl benzoate if 94% yield.71 Potassium fluoride on an aluminum phosphate molecular sieve was a weaker base in the isomerization of 1 -butene.72... 

Scheme 18.25 Maruoka s complex in the Tishchenko reaction a mechanism proposal.

Magnesium ethylate is used as a drying agent for organic solvents, as an intermediate for the manufacture of organomagnesium and other organic products, as catalyst in the Tishchenko (85) and other reactions (86—91), in the condensation of esters (92), in alkylation reactions, and in polymeri2ations (84,91). 

When aldehydes are reduced, the Tishchenko reaction may be a side-reaction. It is the result of an attack of the oxygen atom of the alkoxide on the carbonyl function of the aldehyde. In particular, aldehydes lacking an a-hydrogen atom such as benzaldehyde are prone to form esters (Scheme 20.24) [108]. It has been reported that many aldehydes can be converted into Tishchenko esters at room temperature, almost quantitatively and with high turnovers, using Sml2 catalysts [109] or a bi-aluminum catalyst [8],... 

In mixed Tishchenko reactions using benzaldehyde combined with fural or 2,3-dimethyoxybenzaldehyde in a 1 1 ratio (Scheme 12.24/b), the Sm-supported material 17 gave a better selectivity in cross products P (53-67%) than the molecular Sm[N(SiMe3)2]3 (36-51%). Again, the changed selectivity was attributed to phenomena like spatial restriction and diffusion controlled surface confine-... 

The Tishchenko reaction is a dimerization of aldehydes to the corresponding esters, which is classically carried out in homogeneous media using aluminum... 

The results obtained with the Tishchenko reaction of furfural using alkaline earth metal oxides as base catalysts are presented in Table III. Because the strength of basic sites increases in the order MgOacid strengths is the reverse 73), it was concluded that CaO and SrO— which have moderate acid and base sites in comparison with MgO and BaO—are appropriate... 

Solid base catalysts were tested for this reaction, which was carried out in benzene as a solvent at 313 K. Among them, MgO, CaO, and SrO exhibited good catalytic properties, affording phthalide exclusively with yields between 86% and 100% after a short time (15min) in a batch reactor, whereas with y-alumina, phthalide was obtained with excellent yields and selectivity after 4h. The Tishchenko reaction of 2,3-naphthalenedicarbaldehyde was also carried out with CaO and with y-alumina at 333 K yields of the corresponding five-membered lactone were 94% and 100% after 2 and 20 h, respectively. 

Unlike the reactions described in the previous two sections, competition between insertion and j5-hydrogen transfer is usually not an issue here. Ketone polymerization is nearly thermoneutral and disfavoured by entropy. However, aldehyde insertion is thermodynamically more favourable, and the Tishchenko reaction mentioned in the previous section can plausibly be written as a sequence of insertions and j -hydrogen transfer reactions (Scheme 4). 

When aldehydes, with or without a hydrogen, are treated with aluminum ethoxide, one molecule is oxidized and another reduced, as in 9-69, but here they are found as the ester. The process is called the Tishchenko reaction. Crossed Tishchenko reactions are also possible. With more strongly basic alkoxides, such as magnesium or sodium alkoxides, aldehydes with an a hydrogen give the aldol reaction. Like 9-69, this reaction has a mechanism that involves hydride transfer.751 The Tishchenko reaction can also be catalyzed752 by ruthenium complexes.753 by boric acid,754 and, for aromatic aldehydes, by disodium tetracarbonylferrate Na2Fe(CO)4,755 OS I, 104. 

Formic acid, methyl formate, and CO were detected when photoreduction was performed in Ti silicalite molecular sieve using methanol as electron donor.173 Mechanistic studies with labeled compounds indicated, however, that CO originates from secondary photolysis of formic acid, whereas methyl formate emerges mainly from the Tishchenko reaction of formaldehyde, the initial oxidation product of methanol. 

Esters. The monoisobutyrate ester of 2,2,4-trimethyl- 1,3-pentanediol is prepared from isobutyraldehyde in a Tishchenko reaction (58,59). Diesters, such as trimethylpentane dipelargonate (2,2,4-trimethylpentane 1,3-dinonanoate), are prepared by the reaction of 2 mol of the monocarboxylic acid with 1 mol of the glycol at 150—200°C (60,61). The lower aliphatic carboxylic acid diesters of trimethylpentanediol undergo pyrolysis to the corresponding ester of 2,2,4-trimethyl-3-penten- l-ol (62). These unsaturated esters reportedly can be epoxidized by peroxyacetic acid (63). 

Aldehydes and Ketones. Esters are obtained readily by condensation of aldehydes in the presence of alcoholate catalysis such as aluminum ethylate, AI(OC H5) , by the Tishchenko reaction. 

For the reduction of aldehydes, anhydrous isopropyl alcohol is recommended as the solvent in order to avoid side reactions such as the Tishchenko reaction.13 In those reductions for which aluminum ethox-ide is satisfactory either ethyl or isopropyl alcohol may be employed as a solvent. For most aldehydes, the temperature of boiling isopropyl alcohol is satisfactory for rapid and complete reduction. Certain sensitive aldehydes or ketones may be reduced at room temperature by letting the mixture stand for several days. When aldehydes are reduced, a slow stream of dry nitrogen or hydrogen is frequently employed for reactions requiring long periods of time (including most of the aluminum ethoxide reductions), or when the reduction products are sensitive to air. 

Ruthenacyclobutenone complex 141 is observed as a minor product during the course of the Tishchenko reaction of benzaldehyde catalyzed by (MeaP RuF this complex is considered the likely resting state in the catalytic cycle (Equation 55) <1994JOM265>. 

The Tishchenko reaction of furfural has been found to be difficult when carried out by traditional homogeneous catalysis, but excellent results for the Tishchenko reaction of furfural and 3-furaldehyde[90,91] using CaO and SrO as catalysts have been obtained. The use of other solid base catalysts such as La203, Zr02, ZnO, 7-alum in a, hydrotalcite and KOH/alumina, was unsuccessful. An investigation of the influence of the pretreatment temperature of the MgO and CaO catalysts showed that the active basic sites for this transformation are not OH groups, but rather O2 ions on the MgO surface. 

Tormakangas, O. P., Koskinen, A. M. P. The Tishchenko reaction and its modifications in organic synthesis. Recent Research Developments in Organic Chemistry 2001, 5, 225-255. 

The reaction of benzaldehyde with triethylsilane in the presence of KF/AI203 in DMF gave only benzyloxytriethylsilane [19], However, when hexane or THF were used instead of DMF, the Tishchenko reaction occurred to give, mainly, benzyl benzoate (Scheme 5.7). 

The Tishchenko reaction (see page 334) may take place as a side reaction to reduction of aldehydes, but for aliphatic aldehydes it is suppressed by using an excess of isopropoxide. Also the aldehyde may undergo an aldol condensation (see page 868) with itself or with the acetone produced. Recently the formation of 2-methyl-2,4-pentanediol as by-product was observed.361 These side reactions, however, occur only occasionally and in no way detract from the value of this method of reduction. 

Hydrosilylation and the Tishchenko reaction. Aromatic aldehydes give both ArCH20SiEt3 and ArCOOCH2Ar on treatment with KO -lS-crown-b in benzene or THFat40°. ... 

Redox coupling and addition. The Tishchenko reaction products of aldehydes, which are formed by the action of the organosamarium species, are trapped by vinyl esters in situ. For the Tishchenko reaction, (Cp >2LaCH(TMS)2 is also effective. ... 

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