Acetylene catalysts, palladium complexes

A new type of asymmetric hydrosilylation which produces axially chiral allenylsilanes has been reported by use of a palladium catalyst coordinated with the bisPPFOMe ligand 51b.64 The hydrosilylation of l-buten-3-ynes substituted with bulky groups such as tert-butyl at the acetylene terminus took place in a 1,4-fashion to give allenyl(trichloro)-silanes with high selectivity. The highest enantioselectivity (90% ee) was observed in the reaction of 5,5-dimethyl-T hexen-3-yne with trichlorosilane catalyzed by the bisPPFOMe-palladium complex (Scheme 13). 

Primary propargylic formates decarboxylate in the presence of Pd(acac)2 and Bu3P at room temperature to give mainly allenic products (Eq. 9.115) [91]. Initial formation of a propargylic palladium complex, which rearranges to the more stable allenylpalladium species, accounts for this transformation. Under similar conditions, a terminal allenyl formate afforded a 99 1 mixture of allene and acetylene product (Eq. 9.116) [91]. However, a mixture of enyne elimination products was formed when a secondary propargylic carbonate was treated with a palladium catalyst (Eq. 9.117). 

Sonogashira has proposed a catalytic cycle (Figure 4) which shows 1) the reduction of the palladium complex, 2) coordination of the aryl halide and acetylene with the palladium (0) complex and 3) the reductive elimination of the substituted aryl acetylene and regeneration of the active catalyst.(10)... 

The use of a basic solvent (in this case diethylamine) is important to stabilize acetylenic anions.(9) The third catalyst system component, triphenyl phosphine, is presumably added to help replace lost triphenyl phosphine ligands on the palladium complex and thus prevent metal agglomeration. 

In support of the concept that acetylenes form tt complexes with a single surface atom of the catalyst, McQuillin el al. (39) have cited the parellelism between the effect of substances such as amines and phosphines as inhibitors for the hydrogenation of butynediols to butenediols on a palladium catalyst with the ability of these same substances to form complexes with metals of the class to which palladium belongs (40). 

Highly selective transformation of terminal acetylenes to either linear or branched carboxylic acids or esters may be achieved by appropriately selected catalyst systems. Branched esters are formed with high selectivity when the acetylenes are reacted with 1-butanol by the catalyst system Pd(dba)2/PPh3/TsOH (dba = dibenzylideneacetone) or palladium complexes containing PPh3. Pd(acac)2 in combination with various N- and O-containing phosphines and methanesulfonic acid is also an efficient catalyst for the alkoxycarbonylation of 1-alkynes to yield the branched product with almost complete selectivity.307,308... 

Since the substitution reaction succeeded so well with olefins, the obvious extension to acetylenes was tried. Of course, only terminal acetylenes could be used if an acetylenic product was to be formed. This reaction has been found to occur but probably not by a mechanism analogous to the reaction of olefins (43,44). It was found that the more acidic acetylene phenylacetylene reacted with bromobenzene in the presence of triethylamine and a bisphos-phine-palladium complex to form diphenylacetylene, while the less acidic acetylene, 1-hexyne did not react appreciably under the same conditions. The reaction did occur when the more basic amine piperidine was used instead of triethylamine, however (43). Both reactions occur with sodium methoxide as the base (44). It therefore appears that the acetylide anion is reacting with the catalyst and that a reductive elimination of the disubstituted acetylene is... 

Acetylene hydrogenation. Selective hydrogenation of acetylene to ethylene is performed at 200°C over sulfided nickel catalysts or carbon-monoxide-poisoned palladium on alumina catalyst. Without the correct amount of poisoning, ethane would be the product. Continuous feed of sulfur or carbon monoxide must occur or too much hydrogen is chemisorbed on the catalyst surface. Complex control systems analyze the amount of acetylene in an ethylene cracker effluent and automatically adjust the poisoning level to prepare the catalyst surface for removing various quantities of acetylene with maximum selectivity. 

The Heck-type reaction. The Heck reaction135 (or some modified procedure of it) is certainly one of the most powerful tools used in the preparation of precursors with acetylenic and vinylic subunits. For instance, in the case of precursors 46-49 the synthesis is conveniently achieved by a cross-coupling reaction in the presence of palladium complexes as catalyst. Two pathways are possible, as represented by equations 9 and 10108. 

The polymer of methyl methacrylate (MMA) is known as Perspex. It is a clear transparent glasslike material with high hardness, resistance to fracture, and chemical stability. The conventional route, as shown by reaction 4.10, involves the reaction between acetone and hydrocyanic acid, followed by sequential hydrolysis, dehydration, and esterification. This process generates large quantities of solid wastes. An alternative route based on a homogeneous palladium catalyst has recently been developed by Shell. In this process a palladium complex catalyzes the reaction between propyne (methyl acetylene), methanol, and carbon monoxide. This is shown by reaction 4.11. The desired product is formed with a regioselectivity that could be as high as 99.95%. 

Palladium complexes provide a catalytic means for introducing a carbonyl functionality to many organic substrates [185]. The unique value of PdCl2(dppf) as a carbonylation catalyst was realized over a decade ago in the synthesis of acetylenic ketones from terminal acetylenes and organic halides (Scheme 1-25) [186]. Catalytic carbonylation of arylacetylenes in the presence of HI in methanol gives methyl... 

As in the case of quinolines, isoquinolines can be prepared by metal-mediated annotation processes. Reaction of t-butylimines of iodo-benzaldehydes with acetylenes in the presence of palladium catalyst affords 3,4-disubstituted isoquinolines. The power of this reaction relies on its ability to introduce different type of substituents on 4-position of the heterocycle by using the formed intermediate palladium complex in the cross-coupling reaction with aryl and alkyl halides or alkenes. This methodology was successfully applied to the total synthesis of decumbenine... 

Cobalt, nickel, iron, ruthenium, and rhodium carbonyls as well as palladium complexes are catalysts for hydrocarboxylation reactions and therefore reactions of olefins and acetylenes with CO and water, and also other carbonylation reactions. Analogously to hydroformylation reactions, better catalytic properties are shown by metal hydrido carbonyls having strong acidic properties. As in hydroformylation reactions, phosphine-carbonyl complexes of these metals are particularly active. Solvents for such reactions are alcohols, ketones, esters, pyridine, and acidic aqueous solutions. Stoichiometric carbonylation reaction by means of [Ni(CO)4] proceeds at atmospheric pressure at 308-353 K. In the presence of catalytic amounts of nickel carbonyl, this reaction is carried out at 390-490 K and 3 MPa. In the case of carbonylation which utilizes catalytic amounts of cobalt carbonyl, higher temperatures (up to 530 K) and higher pressures (3-90 MPa) are applied. Alkoxylcarbonylation reactions generally proceed under more drastic conditions than corresponding hydrocarboxylation reactions. 

Silylboration and borylstannation of acetylenes have also been achieved easily in the presence of a catalyst of palladium complexes. In contrast, palladium complexes such as Pd(PPh3)4 or Pd(OAc)2-isocyanide were ineffective for diboration of alkynes. However, the development of another new ligand will solve this difficulty in the future. 

The reductive carbonylation of acetylenes proceeds via a different mechanism compared to the carbonylation of olefins, but through the addition of palladium hydride species to the triple bond. The most probable source of PdH is the WGS reaction, so water is required at two key steps of this catalytic cycle. Depending on conditions, the nature of the catalyst, and promoter additives, the carbonylation of acetylenes can lead to different products. An important role of cationic palladium complexes that readily form in the presence of water has been disclosed. "... 

Alkyl- and aryl-acetylenes, e.g. (70), but not acetylene itself, co-dimerize with allylic halides (71) in the presence of catalytic amounts of palladium complexes to give halogenated 1,4-dienes (72) in excellent yields. " The most active catalyst appears to be the [PdX2(PhCN)2] complex. The procedure involves very careful addition of the acetylenic compound to the allylic halide solution at 20 °C to prevent polymerization in this exothermic reaction. The co-dimerization of isobutene with trichloroethylene can produce useful quantities of l,l-dichloro-4-methylpenta-1,4-diene in the presence of t-butyl peroxide at 500 °C in a gas-phase reactor. The reaction probably occurs by a radical transfer mechanism. 

The substituted 1,4-dienes (60) are synthesized in good yield (70—90%) by the selective co-dimerization of terminal acetylenes with allyl chloride in the presence of a palladium catalyst. Several palladium complexes are catalytically active, but the best is [PdCl2(PhCN)2]. The reaction also works for substituted allyl chlorides, but yields are lower. Carboxy, ester, halogeno, or hydroxy groups can be tolerated in the acetylenic compound. 

An alternative route based on a homogeneous palladium catalyst was developed by Shell. In this process, a palladium complex catalyzes the reaction between propyne (methyl acetylene), methanol, and carbon monoxide. This is shown by reaction 4.5.2, and the product is formed with a regioselectivity that could be as high as 99.95%. 

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