Hydrogen reaction with ketones

A -Heterocyclic carbene complexes of Ir(I) and Ir(III) have also demonstrated high reactivity in transfer hydrogenation reactions of ketones (Scheme 2) [4]. Complex 4 catalyzed the reduction of a range of ketones into the corresponding alcohols, including the reduction of pinacolone 7 into alcohol 8 with a low catalyst loading and short reaction time [5]. The chelating bis(Af-heterocyclic carbene) complex 5 was shown to catalyze the reduction of ketones, and in the case of the reduction of benzophenone 9 to alcohol 10, the reaction was complete within 4 min [6]. 

In the first route, methylbutenol is made from acetone and acetylene followed by hydrogenation. Reaction with methyl isopro-penyl ether yields methylheptenone (6). The second route involves the reaction of isobutylene, formaldehyde and acetone (7 ). Methyl vinyl ketone is an intermediate. Finally, methylheptenone is made by alkylation of acetone with prenyl chloride which is derived from isoprene (8). The initial product is the terminal olefin which is isomerized to the desired isopropylidene compound. 

In contrast to unfunctionalized ketones, Wilkinson-type catalysts are quite effective in the hydrogenation of 2-oxo esters. With in situ catalysts consisting of [Rh(cod)Cl]2 2 and a proline derived chelate phosphane BPPM 3l4, quantitative hydrogenation of 2-oxo esters to (7 )-2-hydroxy esters was achieved. Dry benzene or dry tetrahydrofuran as solvent were superior to alcohols usually used in hydrogenation reactions with Wilkinson-type catalysts. While methyl 2-oxopropanoate was reduced to methyl (R)-2-hydroxypropanoate in only 66% eel5, propyl and 2-methylpropyl 2-oxopropanoate gave the (R)-alcohols with 76% and 71 % ee, respectively (Table 2)15,10. 

The investigated supported complexes 22 and 23, outlined in Fig. 8, were used for hydrogenations of alkenes, nitriles and a,jS-unsaturated ketones. Furthermore, 23 was used in the reduction of different heterocycles like benzoth-iophene, quinoline, indole, dibenzothiophene and acridine. The supported chiral Rh complexes, depicted in Fig. 9, were used for hydrogenation reactions with prochiral olefins. 

The phosphonate XXIV was converted to the respective ylid with sodium hydride and yielded on reaction with ketones or aldehydes the corresponding Schiff bases XXV. These products, in contrast to the aldehyde or ketimine adducts with their intramolecular hydrogen bonds, can easily be converted to the unsaturated carbonyl compound in good yield. 

The Ci3-ketone /-ionone (2) is built up in five stages (5 + 4—from the starting materials acetone (3) and acetylene (4), with sigmatropic rearrangements playing a key role (Scheme 1). The process comprises four basic reactions, ethynylation, partial hydrogenation, reaction with wo-propenyl methyl ether (9) and rearrangement, which can be carried out inexpensively [11]. 

Chitosan is a multi-nucleophilic polymer due to the presence of the NH2 and OH functional groups. The initial sites where substitution occurs are the more nucleophilic amino groups. However, the experimental conditions and protection of the NH2 groups reduces the intermolecular hydrogen bonding and creates space for water molecules to fill in and solvate the hydrophilic groups of the polymer backbone (Sashiwa and Shigemasa 1999). A -alkylated derivatives can be obtained by the treatment of chitosan with aldehydes or ketones via formation of Schiff base intermediates, aldimines (from reactions with aldehydes), or ketimines (from reactions with ketones) followed by reduction of the imine with sodium borohydride. 

The applicability of the various reactions leading to the formation of centers of asymmetry in total synthesis is characterized by the data of Table 5. The formation of each center of asymmetry is carried out by means of very diverse reactions from which, however, we can single out some of the most characteristic. Thus, the C3 and C9 centers are formed predominantly in reductions with alkali metals, the and 0 3 centers in electrophilic reactions with ketones, and the 0 4 center in catalytic hydrogenation. 

The reaction with sodium is by no means an infallible practical test for alcohols since, strictly speaking, it is applicable only to pure anhydrous liquids. Traces of water, present as impurities, would give an initial evolution of hydrogen, but reaction would stop after a time if an alcohol is absent furthermore, certain esters and ketones also evolve hydrogen when treated with sodium (compare Section XI,7,6). It may, however, be assumed that if no hydrogen is evolved in the test, the substance is not an alcohol. 

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