Selective Dimerization of Aldehydes to Esters Catalyzed by Hydridoruthenium Complexes

Abstract

RuH2(PPh3)4 and other hydridoruthenium complexes catalyze selective conversion of aldehydes into esters in high yields. The method is applicable to most aliphatic aldehydes as well as to aromatic aldehydes. The purity of aldehydes is critical for achieving high conversions, since the presence of carboxylic acid completely inhibits the reaction and alcohol and triphenylphosphine reduce the yields of esters. RuH2(PPh3)4 is converted into Ru(CO)3(PPh3)2 through the reaction indicating the occurrence of decarbonylation of aldehyde. A mechanism involving the acyl-H cleavage of aldehyde is proposed to account for the catalysis and formation of compounds accompanying the reaction. The mechanism is compared with an alternative one which comprises of consecutive insertions of two aldehyde molecules into Ru–H bond followed by β-hydrogen abstraction from an alkoxo intermediate formed. Addition of water changes the reaction course to give carboxylato carbonyl complexes Ru(OCOR)2(CO)m(PPh3)2 (m=1 and 2). Cross esterification studies showed the reactivity order of RCHO as R=Et>Me>n-Pr>i-Pr>>Ph.

Lanthanide Formamidinates as Improved Catalysts for the Tishchenko Reaction

“…The use of the standard aluminum catalyst Al(OiPr) 3 [21,22] under the reaction conditions described above led to a low yield of product ( [33] or other aluminum-based catalysts (Table 1, Entry 13) [34] did not reach the level of activity of the lanthanum compounds. All classical transition-metal catalysts, [23][24][25] as well as boric acid [22] ( Table 1, Entries 14-18), failed to catalyze the reaction to complete conversion. Some more recent examples from transition-metal chemistry show high yields, but are still slower than the lanthanide systems (Table 1, Entry 19).…”

“…More recently, other catalysts such as boric acid [23] and a few transition-metal complexes have been used. [24][25][26] However, these alternative catalysts are either only reactive under extreme reaction conditions (e.g. boric acid) or they are slow (e.g.…”

“…[(C 5 H 5 ) 2 ZrH 2 ]), [24] expensive (e.g. [H 2 Ru(PPh 3 ) 2 ]), [25] or give low yields (e.g. K 2 [Fe(CO) 4 ]).…”

Monoalcoholates of 1,3-diols as effective catalysts in the Tishchenko esterification of 1,3-dioxan-4-ols

“…3 was found to be inactive as a catalyst. It is possible that catalytic aluminium coordinates to the primary hydroxyl group located at the end of the side chain of 1 instead of forming the transition state related to 6.…”

“…2 The reaction has been carried out with a number of other catalysts such as ruthenium complexes, 3 alkali earth metal oxides, 4 boric acid, 5 sodium alkoxides 6 (with aryl aldehydes), alkali metals, 7 zirconocenes, 8 and lanthanide (amide) complexes. 9 The mechanism of the Tishchenko reaction involves a hydride shift.…”