Metals, activated with aldehydes, formation

Ir catalysts supported on binary oxides of Ti/Si and Nb/Si were prepared and essayed for the hydrogenation of a,P-unsaturated aldehydes reactions. The results of characterization revealed that monolayers of Ti/Si and Nb/Si allow a high metal distribution with a small size crystallite of Ir. The activity test indicates that the catalytic activity of these solids is dependent on the dispersion obtained and acidity of the solids. For molecules with a ring plane such as furfural and ciimamaldehyde, the adsorption mode can iirfluence the obtained products. SMSI effect (evidenced for H2 chemisorption) favors the formation of unsaturated alcohol. 

In real systems (hydrocarbon-02-catalyst), various oxidation products, such as alcohols, aldehydes, ketones, bifunctional compounds, are formed in the course of oxidation. Many of them readily react with ion-oxidants in oxidative reactions. Therefore, radicals are generated via several routes in the developed oxidative process, and the ratio of rates of these processes changes with the development of the process [5], The products of hydrocarbon oxidation interact with the catalyst and change the ligand sphere around the transition metal ion. This phenomenon was studied for the decomposition of sec-decyl hydroperoxide to free radicals catalyzed by cupric stearate in the presence of alcohol, ketone, and carbon acid [70-74], The addition of all these compounds was found to lower the effective rate constant of catalytic hydroperoxide decomposition. The experimental data are in agreement with the following scheme of the parallel equilibrium reactions with the formation of Cu-hydroperoxide complexes with a lower activity. 

The effectiveness of various substituted BINOL ligands 12-16 in the Zr(IV)-or Ti(IV)-catalyzed enantioselective addition of allyltributyltin to aldehydes was also investigated by Spada and Umani-Ronchi [21], The number of noteworthy examples of asymmetric allylation of carbonyl compounds utilizing optically active catalysts of late transition metal complexes has increased since 1999. Chiral bis(oxazolinyl)phenyl rhodium(III) complex 17, developed by Mo-toyama and Nishiyama, is an air-stable and water-tolerant asymmetric Lewis acid catalyst [23,24]. Condensation of allylic stannanes with aldehydes under the influence of this catalyst results in formation of nonracemic allylated adducts with up to 80% ee (Scheme 3). In the case of the 2-butenyl addition reac-... 

The common route to bis(indoyl)methanes is via condensation of indoles with aldehydes or ketones in the presence of either protic or Lewis acids. The reaction has been evaluated in tetrafluoroborate and hexafluorophosphate ionic liquids and of the metal salts tested best results were obtained with In(OTf)3 and FeCl3-6H20. Although In(OTf)3 is somewhat more active, its higher price makes the use of iron(III)chloride more attractive. Furthermore, whereas the activity of In(OTf)3 decreases quickly upon recycling, ionic liquid solutions of FeCl3 remain reasonably active for at least four runs.[62] It was found that in hydrophilic ionic liquids the reaction did not proceed at all, whereas fast conversion was observed with the [PF6] -anion, see Scheme 9.17. As water is produced in the course of the reaction it is possible that elimination of the water from the reaction medium helps to protect the catalyst, however, it cannot be excluded that at least some of the catalytic activity is due to the formation of HF. While the... 

The a-heterofunctionaUsation of aldehydes and ketones can be achieved via the reaction of chiral noirracemic metal-enolates with suitable electrophiles or by activation of the electrophilic heteroatom source with an enantiomerically pure Lewis add catalyst. Alternatively, metal-free a-functionalisation can be achieved via the reversible formation of enamines and this approach has proved a successful method for the synthesis of a range of enantioenriched a-heteroaldehydes and ketones. 

These. supramolecular catalysts showed high substrate selectivity in competition hydrogenation experiments and exceptional activity in the hydroformylation reactions. In contrast to the simple methylated P-cyclodextrin previously mentioned, even internal and cyclic olefins were converted into aldehydes. Such improvements were explained with the formation of an inclusion complex at the phase boundary, with the cylodextrin host fixing the substrate in the proximity of the catalytically active metal center (Fig. 

Alternatively, the iminium-activation strategy has also been apphed to the Mukaiyama-Michael reaction, which involves the use of silyl enol ethers as nucleophiles. In this context, imidazolidinone 50a was identified as an excellent chiral catalyst for the enantioselective conjugate addition of silyloxyfuran to a,p-unsaturated aldehydes, providing a direct and efficient route to the y-butenolide architecture (Scheme 3.15). This is a clear example of the chemical complementarity between organocatalysis and transition-metal catalysis, with the latter usually furnishing the 1,2-addition product (Mukaiyama aldol) while the former proceeds via 1,4-addition when ambident electrophiles such as a,p-unsaturated aldehydes are employed. This reaction needed the incorporation of 2,4-dinitrobenzoic acid (DNBA) as a Bronsted acid co-catalyst assisting the formation of the intermediate iminium ion, and also two equivalents of water had to be included as additive for the reaction to proceed to completion, which... 

The formation of an active complex 81 consisting of three components, tin(II) trifiate, chiral diamine 80, and dibutyltin acetate is assumed in these aldol reactions. The three-component complex would activate both aldehyde and silyl enolate (double activation), i.e. the chiral diamine-coordinated tin(II) trifiate activates aldehyde while oxygen atoms of the acetoxy groups in dibutyltin acetate interact with the silicon atom of the silicon enolate. Because it has been found that the reaction does not proceed via tin(II) or tin(IV) enolates formed by silicon-metal exchange, silicon enolate is considered to attack the aldehydes directly [65]. The problem of this aldol reaction is that (Z) enolates [63] react with aldehydes more slowly, consequently affording the aldols in lower yield and with lower diastereo- and enantio-selectivity. 

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