Tishchenko condensation

Metal alkoxides cataly2e the Tishchenko condensation of aldehydes (62), the transesterification of carboxyhc esters, the Meerwein-Poimdorf reaction (63), and other enolization and condensation reactions. 

The iodoalkoxide Sml2(Ol-Bu) promotes the intramolecular Tishchenko reaction via a metal-chelate intermediate to form perfectly stereocontrolled lactones (eq. (24)) [189]. Ethyllanthanide iodide complexes, EtLnl (Ln = Pr, Nd, Sm) etc. [25 b, 186], also serve as catalyst precursors for the Tishchenko condensation reaction [190]. 

Yokoo, K., Mine, N., Taniguchi, H., Fujiwara, Y. Chemistry of organolanthanoids lanthanoid-catalyzed Tishchenko condensation of aldehydes to esters. J. Organomet. Chem. 1985, 279, C19-C21. 

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). 

The direct conversion of ethyl alcohol to ethyl acetate is beheved to take place via acetaldehyde and its condensation to ethyl acetate (Tishchenko reaction) (28-34). 

Condensation of butanol has been carried out on alkaline earth metal oxides at 273 K (13,121). This condensation reaction yields 2-ethyl-3-hydroxy-hexanal as a main product other products, such as 2-ethyl-2-hexenal (arising from the dehydration of 2-ethyl-3-hydroxy-hexanal), n-butyl-K-butyrate (arising from the Tishchenko reaction of butyraldehyde), and 2-ethyl-3-hydroxy- -hexyl butyrate (arising from the Tishchenko reaction of 2-ethyl-3-hydroxy-hexanal), are also formed (Scheme 12). 

The order of activity per unit surface area was equal to that in the case of selfcondensation of acetone and in agreement with the order of basicity of the solids, namely, SrO > CaO > MgO. However, the authors found that the rate-determining step for aldol condensation of n-butyraldehyde is the a-hydrogen abstraction by the active sites, which are the surface ions. The differences in rate-determining step and active sites in the condensation of butyraldehyde and aldol condensation of the acetone were attributed to differences in acidity of the a-hydrogen in the two molecules. CaO was slightly more active than MgO at 273 K after a reaction time of 1 h, maximum conversions of 41% were observed with selectivities to 2-ethyl-3-hydroxy-hexanal and to the corresponding Tishchenko reaction product (2-ethyl-3-hydroxy- -hexyl butyrate) of 39.8 and 56.9%, respectively. 

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. 

An outstanding input into the chemistry of metal alkoxides was made by the dissertation of V. E. Tishchenko, which was devoted to aluminium alkoxides. It was published in 1899 (unfortunately only in Russian) [1585] and—in contrast to his works on organic chemistry — is practically unknown by foreign chemists. A number of general synthetic approaches to metal alkoxides have been developed. The main regularities for the variation of the properties in the homologous sales and ramification of the radical were found to determine the main directions of the thermal decomposition. He made also the proposal to use the Al(OR)3 in ester condensation of aldehydes (Tishchenko reaction) (see also Chapter 12, Section 12.5). 

Tishchenko, W. The effect of aluminum alcoholates on aldehydes. The ester condensation as a new condesationform of aldehydes. Chem. Zentr. 1906, II, 1309-1311. 

Rates for competing processes such as self-condensation of cyclohexanone and Cannizzaro or Tishchenko reactions of benzaldehyde are negligible compared with the Claisen-Schmidt condensation of aldehyde with ketone. 

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. 

Different conditions may lead to methanol and formic acid via Cannizarro-Tishchenko reaction (a), to methanol by hydrogenation of formaldehyde (b), or to Cg - Cj g polyoxy aldehydes via condensation of glycol aldehyde with formaldehyde (c). Interestingly, the formation of cyclic acetals (d) prevents these side reactions (Scheme 6.121) [22]. 

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