Cocatalysts carbonylation

Tertiary stibines have been widely employed as ligands in a variety of transition metal complexes (99), and they appear to have numerous uses in synthetic organic chemistry (66), eg, for the olefination of carbonyl compounds (100). They have also been used for the formation of semiconductors by the metal—organic chemical vapor deposition process (101), as catalysts or cocatalysts for a number of polymerization reactions (102), as ingredients of light-sensitive substances (103), and for many other industrial purposes. 

The three-component synthesis of benzo and naphthofuran-2(3H)-ones from the corresponding aromatic alcohol (phenols or naphthols) with aldehydes and CO (5 bar) can be performed under palladium catalysis (Scheme 16) [59,60]. The mechanism involves consecutive Friedel-Crafts-type aromatic alkylation and carbonylation of an intermediate benzylpalla-dium species. The presence of acidic cocatalysts such as TFA and electron-donating substituents in ortho-position (no reaction with benzyl alcohol ) proved beneficial for both reaction steps. 

Figure 8.6 Main step showing the role of the cocatalyst in the iridium-catalyzed methanol carbonylation reaction.

Reactions of the same carbonyl ylide intermediate with aldehydes are even more fruitful. The Rh2(OAc)2 catalyzed reaction proceeds at room temperature in the presence of 2 mol% of the catalyst, but the diastereoselectivity is disappointingly low (endo/exo = 49 51, Scheme 11.56). However, when 10 mol% of the cocatalyst Yb(OTf)3 is added, the reaction becomes highly exo-selective (endo/ exo = 3 97) (198). Suga has extended this Lewis acid catalyzed carbonyl ylide cycloaddition reaction to catalyzed asymmetric versions. The chiral cocatalyst employed is ytterbium(III) tris(5)-1,1 -binaphthyl-2,2 -diyl phosphonate, Yb[(S) BNP]3 (10 mol%). In the reaction of methyl o-(diazoacetyl)benzoate with benzyloxyacetaldehyde in the presence of Rh2(OAc)2 (2 mol%) at room temperature with the chiral Yb catalyst, the diastereoselectivity is low (endo/exo = 57 43) and the enantiopurity of the endo-cycloadduct is 52% ee. 

Carbonylation of Alcohols - Pd(tppts)3 catalyses the carbonylation of benzylic alcohols to the corresponding phenylacetic acids, in the presence of a Bronsted acid cocatalyst such as H2S04 or p-CH3C6H4S03H in biphasic aqueous/organic media (no organic solvent).305,451 For example, benzyl alcohol was converted to phenylacetic acid (Equation 6) and l-(4-isobutylphenyl)ethanol (IBPE) to ibuprofen (Figure 9). 

Nicotinamide coenzymes act as intracellular electron carriers to transport reducing equivalents between metabolic intermediates. They are cosubstrates in most of the biological redox reactions of alcohols and carbonyl compounds and also act as cocatalysts with some enzymes. 

Good yields of carbonyl compounds have also been obtained from the vapor-phase oxidation of alkenes by steam and air over palladium catalysts supported on charcoal.413 In this case, no copper cocatalyst is needed, presumably because palladium(II) is not reduced to palladium(O), but remains in the form of a stabilized palladium(Il) hydride which can react with 02 to give the hydroperoxidic species. 

Palladium-Catalyzed Conjugate Reduction of a,s-Unsaturated Carbonyl Compounds with Diphenylsilane and Zinc Chloride Cocatalyst a,B-Dihydro-B-Ionone... 

Carbonylation of allylic acetates. This reaction is effected by catalysis with this and a few other Pd(0) complexes, but requires bromide ion as a cocatalyst. It provides a route to (E)-p,y-unsaturated esters in generally high yield from primary allylic acetates. 

A frequent theme in alcohol carbonylations by transition metals is the use of a halide or pseudo-halide promoters or cocatalysts. Despite major problems of corrosion associated with its use, iodide is almost always found to be most effective in this capacity. This is because the halide serves several purposes, for each of which iodide is ideally suited. One of the most important roles of these anion promoters can be that of facilitating the formation of metal-carbon bonds via the formation of intermediate alkyl halides. Under typical catalytic conditions for a variety of systems, at least some portion of the added halide is converted to the corresponding strong halo-acid. In fact, conditions are generally set so that this event is maximized. 

The majority of catalytic carbonylations employ palladium catalysts and the water soluble complex, Pd(tppts)3, is easily prepared by reduction of PdCl2/tppts with CO in water at room temperature [39]. Hence, this complex is readily generated in situ, under carbonylation conditions. It was shown to catalyze the car-bonylation of alcohols [40, 41] and olefins [42-44], in the presence of a Bronsted acid cocatalyst (Fig. 7.10). 

Although electron-rich aromatics typically undergo 1,2-carbonyl addition, the iminium ions derived from 4/f-imidazol-4-one 456a are inert to the 1,2-pathway due to steric constraints imposed by the catalyst framework. The heteroaromatic nucleophiles react via the less sterically demanding 1,4-addition pathway (Equation 109). With TEA as the cocatalyst, ee values of 89-97% were obtained with a range of substituted pyrroles (R = Me, Bn, allyl). In addition, ee values of 87-93% were obtained with alkyl, aromatic, or electron-withdrawing substituents on the ot, 3-unsaturated aldehydes (R = Me, Pr , Bn, Ph, MeC02). 

The essence of the Wacker process is the invention of the reoxidation process for Pd° by using CuCh as a cocatalyst. Cu" salts are good reoxidants, but chlorination of carbonyl compounds takes place with CuCh. For example, chloroacetaldehyde is a by-product of the Wacker process. Chlorohydrin is another by-product from the reaction of ethylene with PdCh and CuCb. - Thus, a number of other reoxidants were introduced. When CuCl, pretreated with oxygen, is used, no chlorination of ketones takes place and the rate of the reaction is higher. - Also Cu(N03)2 and Cu(OAc)2 have been used. Oxidation of cy-clopentene with PdCl2/Fe(C104)3 combined with electrochemical oxidation was carried out. Benzoqui-none was used at first by Moiseev et al and later by many other researchers as a good reoxidant, but a stoichiometric amount is necessary. The oxidation of alkenes can be carried out smoothly with catalytic... 

These methods suggested in the present form by Caunt83) rely on inhibition (retardation) effects of strong catalyst poisons on polymerization. Typical poisons potentially usable for this purpose are carbon oxides, carbonyl sulfide, carbon disulfide, acetylenes and dienes. All these substances exhibit a strong unsaturation they have either two double bonds or one triple bond. Most of the works devoted to application of the poisons to determination of active centers 10,63 83 102 1O7) confirm a complicated nature of their interaction with the catalytic systems. To determine the active centers correctly, it is necessary to recognize and — as much as practicable — suppress side processes, such as physical adsorption and chemisorption on non-propagative species, interaction with a cocatalyst, oligomerization and homopolymerization of the poison and its copolymerization with the main chain monomer. 

A useful synthesis of ( )-j -ethoxycarbonylvinylsilanes by palladium-catalyzed regio- and stereospecific hydroesterification (EtOH -I- CO) (or carboethoxylation) of trimethylsilylacetylenes has been reported recently [210]. Alkoxycarbonyl or carbonyl functionalization of vinylsilanes are useful synthetic intermediates [211, 212]. The use of PdCl2(dppf) as a catalyst (with SnCl2 2 H2O as cocatalyst) is found to be superior and gives excellent yields. A key step in the reaction is thought to involve hydropalladation to give 60 or 61. The preference for 60 to 61 is understood... 

The carbonyl [Ru3(CO),2] is a good cocatalyst for the low pressure hydroformylation of internal alkenes using the classic rhodium phosphine [HRh(CO)(PPh3),] system in the presence of an excess of triphenylphosphine (P/Rh = 200) (22). Starting from a mixture of hex-2- and hex-3-ene, the addition of [Ru3(CO),2l (Rh/Ru = 1/1) increased both the reaction rate and the n/iso ratio of heptanals. More recently, Poilblanc and coworkers (23) have prepared a mixed ruthenium-rhodium complex formulated as [CIRh(/i-CO)(//-dppm)2Ru(CO)2] (dppm is Ph2PCH2PPh2). This species shows catalytic activity in the hydroformylation of pent-l-ene at 40 bar (H2/C0= 1/1) and 75°C. Conversion to hexanals was 90% in 24 hours and the linearity reached 70%. No further report has appeared to determine the role of the two metals in this catalysis. 

A particularly broad potential for application in syngas reactions is shown by ruthenium carbonyl clusters. Iodide promoters seem to favor ethylene glycol (155,156) the formation of [HRu3(CO),]- and [Ru(CO)3I3]- was observed under the catalytic conditions. These species possibly have a synergistic effect on the catalytic process. Imidazole promoters have been found to increase the catalytic activity for both methanol and ethylene glycol formation (158-160). Quaternary phosphonium salt melts have been used as solvents in these cases the anion [HRu3(CO)u] was detected in the mixture (169). Cobalt iodide as cocatalyst in molten [PBu4]Br directs the catalytic synthesis toward acetic add (163). With... 

Satoh T, Kokubo K, Miura M et al (1994) Effect of copper and iron cocatalysts on the palladium-catalyzed carbonylation reaction of iodobenzene. Organometallics 13 4431 1436... 

Enol triflates have emerged as attraetive alternatives to vinyl halides in the Stille coupling partner due to their ease of preparation from readily available carbonyl compounds. The addition of LiCI has been found to be beneficial to Stille enol triflate coupling reactions. Thus, it was not surprising that coupling 5-tributylstannylpyrimidine to enol triflate 134 proceeded in good yield in the presence of LiCI. In this case, the addition of Cul as cocatalysts was found to also be beneficial to the reaction outcome [57]. 

No dimers were formed in the controlled thermal decomposition a binuclear ir complex Co2(CO)4(C7Hg)2(57). However, when the decomposition of this complex was performed in the presence of AlBrs, Binor-S was obtained in almost quantitative yield (52). Interaction of the complex with AlBrs may cause breaking of the bridging carbonyl groups and thus cause free rotation in the Co-Co axis, which may be necessary to obtain Binor-S selectivity. Similarly, CosfCOls becomes an effective Binor-S catalyst in the presence of as little as 0.5-1 mole of AlBi s or BFs O(C2H5)2 per mole of the carbonyl. These catalysts are very active and produce Binor-S essentially with quantitative conversion and selectivity (52). Other metal carbonyls and their mixtures with Lewis acids were evaluated as well. Low conversion to dimers other than Binor-S took place in some instances in the majority of cases, however, polymerization of norbornadiene occurred. The catalyst systems studied included Ni(CO)4, Fe(CO)5, Mn2(CO)io, Cr(CO)g, Mo(C0)fl, and W(CO)g in combination with BPg O(C2H5)2 cocatalyst (52). 

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