The hydrogenation of a,3-unsaturated

The effect on selectivity and activity for the hydrogenation of 3-methyl crotonaldehyde using Pt/SiOj modified with cations from successive groups in the periodic table was studied by Ponec et The most effective promoter was found to be Fe and Sn both of which raised the selectivity to the unsaturated alcohol from 21% to about 80%. Interestingly, this increase in selectivity was also accompanied by a substantial rate enhancement from 2.0 to 12.7 pmol s (g cat) in the case of Pt Sn prepared with a 4 1 ratio. Similar selectivity and activity enhancement were reported by Vannice and Sen for crotonaldehyde hydrogenation over Pt/TiOj in the vapour phase using a Hg aldehyde ratio of 22.7 Under both low tem- 

The fact that Pt has a high alkene hydrogenation capability has lead to a considerable amount of research being undertaken on a less active metal, Cu. This, combined with the ability of sulphur to promote the formation of unsaturated alcohol by the formation of 6 sites on the active surface as described earlier, lead to an in-depth study by Hutchings and Rochester involving Cu and Pd and later extended to Au. Some important points are illustrated in Table 5.2. 

Catalyst Promoter Temp/K TOSVmin Conv / % Butyraldehyde Crotyl alcohol Butanol Others  

All these reactions occurring over supported metal catalysts utilise the full range of metal crystal faces normally present on microcrystalline metals. A theoretical study by Delbecq and SauteT showed that the adsorption geometries of acrolein, crotonaldehyde and 3-methyl crotonaldehyde differed depending onto which face they were adsorbing. Pt(lll) yielded di-o adsorption, whilst Pt(lOO) 

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In any of the schemes presented, hydrogen dissociative adsorption on Pt is possible after Sn addition, as was checked by hydrogen chemisorption. From these results, it is possible to think of a scheme to represent the main reaction pathway during the hydrogenation of a,(3-unsaturated aldehydes. Depending on the catalyst used, such a scheme is shown in Figure 6.12, which summarizes the results from the characterizations and catalytic tests performed in this work [47]. 

Most of the predictions concerned with the stereochemical outcome of the hydrogenation of a,/3-unsaturated ketones have been based on empirical conclusions derived from studies on model compounds. This is particularly true in the steroid field [e.g., Slomp et al. (7)]. One result of this type of empirical approach is the development of the classic method for preparing 5/3 steroids by hydrogenating the corresponding A4-3-keto species in the presence of hydroxide ions (7, 8). A more complete discussion of other generalizations which have been made concerning the hydrogenation of steroids is beyond the scope of this review but such information is available elsewhere (9,10). 

None of these mechanistic proposals is sufficiently general to use to rationalize all of the stereochemical data observed on the hydrogenation of a,[3-unsaturated ketones. By a judicious combination of segments of each of these proposals along with the Horiuti-Polanyi mechanism (2), it is possible, however, to develop a uniform mechanistic rationale that can be useful in determining the effect of solvent on product stereochemistry. In addition, the influence of hydrogen availability, the type and quantity of catalyst, and the nature of other substituents on the reacting molecule on the product isomer distribution can also be more readily understood. 

Simple a-substituted styrenes are reduced in the presence of RuCl2(DuPhos)(DMF) . The reactivity of the ruthenium catalyst is enhanced by the addition of potassium te/t-butoxide, which may facilitate generation of a ruthenium hydride. The products are obtained under low hydrogen pressures and selectivities obtained are up to 89% ee (eq 8). Neutral Rh-DuPhos complexes catalyze the hydrogenation of a,3-unsaturated acids such as tiglic acid (eq 9). The product is obtained in quantitative yield and good enantioselectivity. ... 

Cationic Rh complexes [RhL(C0)(PPh3)2]C104 (L = unsaturated nitrile) show catalytic activities for the hydrogenation of a,3-unsaturated nitriles under mild conditions (30 °C). Selective reduction of the carbon-carbon double bonds of a,3-unsaturated carbonyls also occurs in high yield with [ Rh( 1,5-hexa-diene)Cl 2] and a phase transfer catalyst in aqueous media. ... 

The complexes derived from chloroplatinic acid and tin(II) chloride can catalyze the hydrogenation of a,3-unsaturated ketones (equation... 

The binuclear palladium complex [(tBu2PH)PdPtBu2]2 with oxygen gave a very efficient catalyst for the hydrogenation of a,/3-unsaturated carbonyls. 0 Glueck and co-workers have undertaken studies on aspects of platinum-catalyzed hydrophosphinations of activated olefins.71,72... 

The hydrogenation of a,/3-unsaturated aldehydes over modified metal catalysts... 

At lower rhodium concentrations, the activity of the catalyst decreases, the decrease is probably caused by the low pH of the aqueous solution in the presence of carbon dioxide and the limited stability of the active species of the hydroformy-lation cycle under acidic conditions. More recently, it has been shown that similar catalytic systems show excellent activity and very promising recycling characteristics in the hydrogenation of a,(3-unsaturated carboxylic acids such as itaconic acid [Eq. (14)] [42],... 

Virtanen P., Karhu H., Kordas K Mikkola J.P. (2007). The effect of ionic liquid in supported ionic liquid catalysts (sdca) in the hydrogenation of a, 3-unsaturated aldehydes. Chem. Eng. Set, vol.62, n°14, pp.3660-3671, (July 2007), ISSN 0009-2509 Washiro S., Yoshizawa M., Nakajima H. and Ohno H. (2004). Highly ion conductive flexible films composed of network polymers based on polymerizable ionic liquids. Polymer vol.45, n°5, pp. 1577-1582 (March 2004), ISSN 0032-3861 Wasserscheid P. Keim W.A. (2000). Ionic Liquids—New "Solutions" for Transition Metal Catalysis, Ang. Chem. Int. Ed., vol.39, pp.3772-3789, (November 2000), Online ISSN 1521-3773... 

This method ensures the deposition of very reactive metal nanoparticles that require no activation steps before use. We shall review here the following examples of catalytic reactions that are of interest in line chemical synthesis (a) the hydrogenation of substituted arenes, (b) the selective hydrogenation of a, 3-unsaturated carbonyl compounds, (c) the arylation of alkenes with aryl halides (Heck reaction). The efficiency and selectivity of commercial catalysts and of differently prepared nanosized metal systems will be compared. 

Catalytic systems at very low metal loading 0.1% (w/w) obtained in this way can be conveniently used in the hydrogenation of a,P-unsaturated ketones to the corresponding saturated carbonyl compounds with very high efficiencies and selectivities. In Table 4 we report the results obtained in the selective hydrogenation of 4-(6-methoxy-2-naphthyl)-3-buten-2-one, 1, and 2-acetyl-5,8-dimethoxy-3,4-dihydronaphthalene, 2, to the corresponding saturated carbonyl products (I), which are important intermediates... 

Thus, [HRh(C0)(TPPTS)3]/H20/silica (TPPTS = sodium salt of tri(m-sulfophenyl)phopshine) catalyzes the hydroformylation of heavy and functionalized olefins,118-122 the selective hydrogenation of a,/3-unsaturated aldehydes,84 and the asymmetric hydrogenation of 2-(6 -methoxy-2 -naphthyl)acrylic add (a precursor of naproxen).123,124 More recently, this methodology was tested for the palladium-catalyzed Trost Tsuji (allylic substitution) and Heck (olefin arylation) reactions.125-127... 

The [RhCl(CO)2]2 dimer immobilized on a cross-linked polystyrene containing pyrrolidine effects the same novel selectivity as the homogeneous analog in hydrogenation of a,/3-unsaturated aldehydes to the unsaturated alcohols, Eq. (30) (162). 

Bentley et al.m recently improved upon Julia s epoxidation reaction. By using urea-hydrogen peroxide complex as the oxidant, l,8-diazabicyclo[5,4,0]undec-7-ene (DBU) as the base and the Itsuno s immobilized poly-D-leucine (Figure 4.2) as the catalyst, the epoxidation of a, (3-unsaturated ketones was carried out in tetrahydrofuran solution. This process greatly reduces the time required when compared to the original reaction using the triphasic conditions. 

Analogously, the SAPC catalyzed hydroformylation reaction was carried out using other water-soluble metal complexes of Pt and Co. Pt complexes in the presence of an Sn co-catalyst underwent hydrolysis of the Pt-Sn bond, which led to lower reaction selectivity. With the corresponding Co catalyst, good hydroformylation selectivities and conversions could be achieved, provided excess phosphine was used. Other authors performed hydrogenation of a,(3-unsaturated aldehydes using SAPC, and Ru and Ir water-soluble complexes. 

The selective hydrogenation of a,/3-unsaturated aldehydes to give the corresponding unsaturated alcohols [Eq. (9)] was investigated with the ruthenium complex catalysts, initially present as [Ru(H)(Cl)(tppts)3] or [Ru(H)2(tppts)4] (91). 

The Stereochemistry of Hydrogenation of a,/3-Unsaturated Ketones Robert L. Augustine Asymmetric Homogeneous Hydrogenation J. D. Morrison, W. F. Masler, and M. K. Neuberg... 

Prochiral organic acids were hydrogenated on clay-supported Rh-chiral phosphine complexes.205,206 Hectorite-supported chiral Rh(I)-phosphine complexes were used for the asymmetric hydrogenation of a, 3-unsaturated carboxylic acids.207 It was found that the interaction between the a-ester group of itaconates and phenyl groups of phosphine can play an important role in the determination of the configuration of products. 

As already mentioned, the stereochemistry of simple olefin hydrogenation can usually be understood by utilizing the classic Horiuti-Polanyi mechanism (1,2). A number of different mechanistic rationales have been put forth, however, to account for the stereochemical data obtained on hydrogenation of a, /3-unsaturated ketones in different media. Actually, no single explanation can be used to account for all of the stereochemical observations, but it is possible to blend the various proposals to give a mechanistic framework from which it is possible by extrapolation to obtain the desired stereochemical information. 

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