Decarbonylation, with palladium

Wilkinson s catalyst has also been reported to decarbonylate aromatic acyl halides at 180°C (ArCOX ArX). This reaction has been carried out with acyl iodides, bromides, and chlorides. Aliphatic acyl halides that lack an a hydrogen also give this reaction, but if an a hydrogen is present, elimination takes place instead (17-16). Aromatic acyl cyanides give aryl cyanides (ArCOCN—> ArCN). Aromatic acyl chlorides and cyanides can also be decarbonylated with palladium catalysts. °... 

The reaction of carboxylic acid chlorides with low-valent metal complexes leads to the formation of acyl complexes, which can undergo decarbonylation to give a-bonded metal alkyls, alkenyls, or aryl complexes. This decarbonylation has been used to synthesize aiylsilanes from aryl acid chlorides in the presence of nickel(0) or palladium(0) complexes as catalysts. Similarly, acyl cyanides undergo decarbonylation with palladium(O) complexes to afford nitriles or alkenes, depending on the substrate. ... 

Aromatic acid chlorides are decarbonylated to aryl chlorides when they are heated to 300-360 C with palladium on carbon. The reaction proceeds by way of an aroylpalladium chloride, then to an arylpalla-dium chloride and finally through a reductive elimination to the aryl chloride. If the reaction is conducted in the presence of a reactive alkene under mild conditions the aroylpalladium chloride intermediate will sometimes acylate the alkene, as noted in Section 4.3.5.3.I. More usually, however, decarboxylation is more rapid than acylation, especially at higher temperatures (>100 C), and decarbonylation occurs. The... 

The most characteristic catalytic activity of the rhodium complex was observed with the reaction of aroyl halides. The decarbonylation of aroyl halides was not satisfactory with palladium catalyst whereas they decarbonylated smoothly on heating to 200°C. with the rhodium complex. For example, when benzoyl chloride was heated with the complex at 200°C., chlorobenzene distilled oflF rapidly ith the evolution of carbon monoxide. Benzoyl bromide reacts similarly to give bromo-benzene. Phenylacetyl chloride was coi verted into benzyl chloride. Additional results are in Table II. 

On the other hand, the oxidative addition of aliphatic acid chlorides occurs in the absence of alkyne, but the oxidative addition complex could not be isolated due to fast decarbonylation followed by facile /1-hydrogen elimination. The decarbonylation of carboxylic acid was reported with palladium catalysts as well [47-56], In general, the reactions to acid anhydride as the intermediate need relatively high temperatures. 

Aldehydes, both aliphatic and aromatic, can be decarbonylated by heating with chlorotris(triphenylphosphine)rhodium or other catalysts such as palladium. The compound RhCl(Ph3P)3 is often called Wilkinson s catalyst.In an older reaction, aliphatic (but not aromatic) aldehydes are decarbonylated by heating with di-tert-peroxide or other peroxides, usually in a solution containing a hydrogen donor, such as a thiol. The reaction has also been initiated with light, and thermally (without an initiator) by heating at 500°C. 

The next task was removal of the C3,C3 -esters. Although the palladium-catalyzed decarboxylation protocol performed well in previous systems, a competing C-H insertion reaction was discovered with the methylidene bridge needed for cercosporin (see below). Since reexamination of alternate decarboxylation methods [48] led to no success, a decarbonylation strategy was explored [49]. Formation of the requisite dialdehyde was best accomplished by overreduction using DIB AL and... 

A 5% palladium on charcoal when heated with some aromatic aldehydes brings about its decarbonylation. 

In an alternative strategy functionalized phenols, such as iodophenol, were involved in palladium-catalyzed carbonylation of alkynes or allenes, producing coumarin or chromone derivatives (Scheme 23) [130-133]. After oxidative addition of the iodoarene to the Pd(0) catalyst the order of insertion of either CO or the unsaturated substrate mainly depends on the nature of the substrate. In fact, Alper et al. reported that CO insertion occurs prior to allene insertion leading to methylene- or vinyl-benzopyranone derivatives [130]. On the contrary, insertion of alkynes precedes insertion of CO, affording couma-rine derivatives, as reported by Larock et al. According to the authors, this unusual selectivity can be explained by the inability of the acyl palladium species to further react with the alkyne, hence the decarbonylation step occurs preferentially [131-133]. 

Pure decarbonylation typically employs noble metal catalysts. Carbon supported palladium, in particular, is highly elfective for furan and CO formation.Typically, alkali carbonates are added as promoters for the palladium catalyst.The decarbonylation reaction can be carried out at reflux conditions in pure furfural (165 °C), which achieves continuous removal of CO and furan from the reactor. However, a continuous flow system at 159-162 °C gave the highest activity of 36 kg furan per gram of palladium with potassium carbonate added as promoter. In oxidative decarbonylation, gaseous furfural and steam is passed over a catalyst at high temperatures (300 00 °C). Typical catalysts are zinc-iron chromite or zinc-manganese chromite catalyst and furfural can be obtained in yields of... 

The synthetic potential of palladium-mediated cross-coupling reactions (Heck, Suzuki, Stille, Sonogashira, Buchwald-Hartwig) led to the search for a practical synthesis of p-[ F]fluoroiodo- and p-[ F]fluorobromobenzene. p-[ F]Fluoroio-dobenzene (G, X = iodine) can be obtained in poor yield from p F]fluoride and a trimethylammonium precursor (P7). p-p F]Fluorobromobenzene can be prepared in a more reproducible way from 5-bromo-2-nitrobenzaldehyde (radiochemical yields > 70%). The synthesis involves a two-step procedure radiofluorination (F for NO2 substitution), then a catalysed decarbonylation [190,191]. Also very efficient is the one-step reaction of p F]fluoride with a suitable diaryliodonium salt (P6) giving >70% radiochemical yield [192-194]. 

Perhaps the most industrially feasible approach has been developed by Rich and co-workers at General Electric, a palladium-catalyzed silylative decarbonylation reaction of aromatic acid chlorides with disilanes [Eq. (35)].97 One of the silicon centers from the disilane is transferred to the arene whereas the other acts as a chloride acceptor to produce the chlorosi-... 

Scheme 2 shows a similar mechanistic pathway for a Heck reaction taking place on a Pd octahedral comer. This mechanism is based on that established for soluble Pd catalysts (ref. 5). Adsorption of the aryl halide (or aryl acid chloride after decarbonylation) gives the aryl Pd halide, 15, by way of the adsorbed intermediate, 14. Vinyl ether adsorption, as in 16, takes place as described in Scheme 1. Aryl insertion gives the halometalalkyl, 17, which on f) elimination to the available 4dxy orbital gives the aryl enol ether, 2 (or 1 depending on which hydrogen is eliminated in 17). The resulting halo palladium hydride, 18, then reacts with the tertiary amine to give the amine hydrochloride and regenerates the octahedral comer for further reaction.

Normally, the most practical vinyl substitutions are achieved by use of the oxidative additions of organic bromides, iodides, diazonium salts or triflates to palladium(0)-phosphine complexes in situ. The organic halide, diazonium salt or triflate, an alkene, a base to neutralize the acid formed and a catalytic amount of a palladium(II) salt, usually in conjunction with a triarylphosphine, are the usual reactants at about 25-100 C. This method is useful for reactions of aryl, heterocyclic and vinyl derviatives. Acid chlorides also react, usually yielding decarbonylated products, although there are a few exceptions. Likewise, arylsulfonyl chlorides lose sulfur dioxide and form arylated alkenes. Aryl chlorides have been reacted successfully in a few instances but only with the most reactive alkenes and usually under more vigorous conditions. Benzyl iodide, bromide and chloride will benzylate alkenes but other alkyl halides generally do not alkylate alkenes by this procedure. 

In the proposed mechanism, the RPdCl generated by decarbonylation reacts with the diene to give a (7r-allyl)palladium intermediate. Reaction of this intermediate with the disilane replaces the chloride by trimethylsilyl. Subsequent reductive elimination gives the product. In mechanistic studies, the chlorodimer 31 corresponding to the (5r-allyl)palladium chloride intermediate in Scheme 8-5 was prepared and allowed to react with Me3SiSiMe3. This led to Me3SiCl (characterized by Si NMR) and l-phenyl-4-(trimethylsilyl)but-2-ene [Eq.(31)]. 

It is known that insertion of carbon monoxide to form an acyl complex is reversible, in which results depend on the pressure of carbon monoxide and temperature. If the above-mentioned mechanisms are correct, then acyl halides and aldehydes should be decarbonylated to form olefins provided that an acyl-palladium bond is formed by the oxidative addition of acyl halides or aldehydes to metallic palladium. This proved to be the case. When acyl halide was heated with a catalytic amount of metallic palladium or palladium chloride at 200°C. in a distilling flask, carbon monoxide and hydrogen halide were evolved rapidly, and olefin was collected in a good yield. This reaction is a new and useful preparative method of olefins. In the same way, aldehydes can be decarbonylated smoothly, but in this case, both olefin and the corresponding paraffin Were obtained. The latter probably arises by the hydrogenation of the olefin. Decarbonylation of certain aldehydes has been reported by several workers (3, 6), but no reasonable mechanism has been known. The mechanism of the palladium-catalyzed aldehyde formation discussed above gives clear explanation for the palladium catalyzed decarbonylation of aldehydes. 

It is well known that there is a close analogy between a transition metal complex and a transition metal surface with respect to their reactions with hydrogen, hydrogen halides, carbon monoxide and some other reagents. From this consideration, the carbonylation and decarbonylation reactions by metallic palladium and by rhodium complexes discussed in this paper have great significance. 

Tsuji, J., Ono, K., Kajimoto, T. Organic syntheses with noble metal compounds. XX. Decarbonylation of acyl chloride and aldehyde catalyzed by palladium and its relation with the Rosenmund reduction. Tetrahedron Lett. 1965, 4565-4568. 

Terminal arylalkynes can be prepared by oxidation-decarbonylation of 3-arylpropargyl alcohols using manganese dioxide in the presence of alkali. The corresponding arylpiopargyl dcohols are available by palladium-catalysed cross-coupling of aryl halides with commercial propargyl alcohol. The yield of the second step can be improved by the addition of a phase-transfer catalyst (18-crown-6 Scheme 18). ... 

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