Aldehydes bimetallic systems

Addition of CX4 to CHO.1 This bimetallic system effects addition of CX4 to aldehydes in DMF at 25° in >75% yield. PbBr2 can be replaced by PbCI2, Pb, or SnCl2 with only slightly lower yields. The system also effects a 1,2-elimination from... 

In contrast to oxidation in water, it has been found that 1-alkenes are directly oxidized with molecular oxygen in anhydrous, aprotic solvents, when a catalyst system of PdCl2(MeCN)2 and CuCl is used together with HMPA. In the absence of HMPA, no reaction takes place(100]. In the oxidation of 1-decene, the Oj uptake correlates with the amount of 2-decanone formed, and up to 0.5 mol of O2 is consumed for the production of 1 mol of the ketone. This result shows that both O atoms of molecular oxygen are incorporated into the product, and a bimetallic Pd(II) hydroperoxide coupled with a Cu salt is involved in oxidation of this type, and that the well known redox catalysis of PdXi and CuX is not always operalive[10 ]. The oxidation under anhydrous conditions is unique in terms of the regioselective formation of aldehyde 59 from X-allyl-A -methylbenzamide (58), whereas the use of aqueous DME results in the predominant formation of the methyl ketone 60. Similar results are obtained with allylic acetates and allylic carbonates[102]. The complete reversal of the regioselectivity in PdCli-catalyzed oxidation of alkenes is remarkable. 

In 1996, Liu et al. reported the selective hydrogenation of cinnamaldehyde, an a,/ -unsaturated aldehyde, to cinnamyl alcohol, an a,/ -unsaturated alcohol, by means of PVP-protected Pt/Co bimetallic colloids prepared by the polyol process [111]. The colloids were obtained as a dark-brown homogeneous dispersion in a mixture of ethylene glycol and diethylene glycol, and characterized by TEM and XRD. These authors prepared different samples of nanoparticles with Pt Co ratios of 3 1 and 1 1, the mean diameters of which measured 1.7 and 2.2 nm, respectively. These colloidal systems were also compared with the single metal-... 

Organomagnesium compounds react with imines, prepared from 3-methoxy-2-naphth-aldehydes by a 1.4-addition mechanism. This reaction can be performed with high diastere-oselectivity. The method was applied for the synthesis of optically pure S-tetralones . Vinyhnagnesium bromide reacts as an acceptor with a ketone dimethyl hydrazone zincate 207, yielding a 1,1-bimetallic species, which can be reacted sequentially with two different electrophiles (equations 131 and 132) . The reaction proceeds via a metalla-aza-Claisen rearrangement, where the dimethylhydrazone anion behaves as an aza-allylic system . 

This hypothesis has been supported by experiments with a modified bimetallic, two-center system. Initial treatment of bis(dimethylaluminum) derivative 188 with 1 equiv. MeLi generates a new amphiphilic alkylation system 199 which has both electrophilic and nucleophilic centers in one reagent (Sch. 150). This system is found to be much more effective than symmetric 188, and the carbonyl alkylation of aldehydes proceeds even at -78 to -40 °C. A similar process with monoalmninum derivative 190 and its complex with MeLi, however, gave a trace of methylation product 200 (R = Ph) (< 3 % yield), indicating that appropriate internal arrangement of the two metal centers is essential to achieve this remarkable rate enhancement in the new amphiphilic alkylation. 

These results compare well with those reported by Mallat (7) in his studies on the oxidation of cinnamyl alcohol to cinnamaldehyde using a 5% Pt, 3% BPC catalyst in an aqueous solvent. Careful control of the system pH was critical to achieve high selectivities to the aldehyde. In our system using organic solvents rather than the water/base solvent no such control of the pH is necessary and a simpler monometallic catalyst can be used rather than the Bi promoted bimetallic catalyst. 

The hydrolormylation of alkynes is usually accompanied by extensive hydrogenation, producing saturated aldehydes or alkenes. However, the use of bimetallic catalytic systems, such as Pd-Co, Pd-W or Pd-Fe. can achieve excellent yields in the hydroformylation of symmetric internal alkynes, affording conjugated un-salurated aldehydes (Eq. 14). ... 

The C=C bond in furfural is made much less reactive (compared with that in the above mentioned aldehydes) by the structure of the molecule. The reactions to he suppressed here are the hydrogenolyses leading to either 2 or 6. The routes 1 to 2 and 1 to 6 are the most important side-reactions which were observed in the hydrogenation of furfural over platinum catalysts. The reaction of 1 to 6 is a CO elimination and reaction 1 to 2 is most likely a common hydrogenolytic fission of a C-0 bond. To study the role of promoters we have chosen platinum as active metal in the bimetallic catalytic system. Platinum belongs to the most active metals for hydrogenation, while it shows a low activity for various hydrogenolytic fissions. 

The other significant difference between the two bimetallic catalysts is that the monocationic monohydride dirhodium catalyst needs to oxidatively add Hj in order to gain the hydride(s) to allow the reductive elimination of aldehyde. The dicationic dihydride system has the second hydride already present and ready to go for the acyl reductive elimination step. H2 then oxidatively adds to the dicationic catalyst to regenerate the dihydride llr/llr. But since the monocationic catalyst system has a low activation barrier for H2 oxidative addition (8.7 kcal), this is not a bottleneck in the catalysis cycle. 

Toshima and coworkers have prepared various nanoparticle systems stabilized by organic polymers such as poly(vinylpyrrolidone) (PVP), poly(vinyl alcohol) (PVA), and poly(methylvinyl ether) by alcohol reduction. Eturing the reduction of metal ions, alcohols having a-hydrogen oxidize to the corresponding aldehyde. A similar approach has been applied for the synthesis of bimetallic nanoparticles, as well.P ... 

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