Decarbonylation of Acyl Halides and Aldehydes

Formation of the aroylpalladium 495 by insertion of CO to the arylpalladium 494 (M = Pd) is reversible. Aroylpalladium complexes 495, prepared directly by the 

The decarbonylation of acyl halides and aldehydes proceeds under mild conditions to give aryl halides ArX and arenes ArH with a stoichiometric amount of RhCl(Ph3P)3. At the same time, RhCl(CO)(Ph3P)2 is formed which is inactive at moderate temperatures, and the reaction is stoichiometric [245-248], 

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Rhodium(I) complexes are effective reagents and/or catalysts for the decarbonylation of acyl halides and aldehydes 9 11,34,195,230,231,236). The compound Rh(PPh3)3Cl, especially, has received considerable attention. The first step in such reactions involves oxidative addition to Rh(I) of the organic molecule, exemplified by the following ... 

XXXI. Carbonylation of Olefins and Decarbonylation of Acyl Halides and Aldehydes... 

Tsuji, J., Ohno, K. Organic syntheses by means of noble metal compounds. XXXI. Carbonylation of olefins and decarbonylation of acyl halides and aldehydes. Advances in Chemistry Series 1968, No. 70, 155-167. 

Tsuji, J. Other reactions of acylpalladium derivatives palladium-catalyzed decarbonylation of acyl halides and aldehydes. Handbook of Organopaiiadium Chemistry for Organic Synthesis 2002, 2, 2643-2653. 

In addition to Pd-catalyzed decarbonylation, rhodium complexes catalyze the decarbonylation efficiently. In this section, decarbonylation of acyl halides and aldehydes using palladium catalysts is surveyed. ... 

Pd-catalyzed decarbonylation of acyl halides and aldehydes is summarized. The decarbonylation proceeds mainly using Pd on carbon as a catalyst at high tanperature. 

Insertion processes are reversible in certain cases, subject to thermodynamic factors in Eq. 7.1. Particularly important among the deinsertion processes is the decarbonylation by which a compound with one less carbon unit is produced. De-carbonylation of acyl halides and aldehydes are utilized for removing a carbonyl... 

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. 

Thus, this reaction is a most facile and selective method of decarbonylation of aldehydes. The decarbonylation of acyl halides by using the complex (XI) was then tried, and again decarbonylation proceeded smoothly though it was necessary to warm the solution for complete decarbonylation. 

It would be more interesting and useful if the reaction could be made catalytic. Actually, catalytic decarbonylation reaction was found to be possible by using chlorocarbonylbis(triphenylphosphine) rhodium (XII) (26). This complex is reasonably stable, and more importantly it is four-coordinated and coordinatedly unsaturated, so that it may expand to a six-coordinated complex by the oxidative addition of acyl halides or aldehydes. The oxidative addition of methyl iodide to similar complexes was reported by Heck (5). 

The radical reduction of acyl halides and related compounds, such as acyl chal-cogenides, to aldehydes may find significance for primary, vinyl, and aromatic acyl radicals whose decarbonylation rates are significantly slower than those of the corresponding secondary and tertiary radicals [6], In practice, however, this method is restricted to substrates which have serious incompatibilities with more traditional methods. 

The reduction of acyl halides with hydrogen to form aldehydes using Pd catalyst is well known as the Rosenmund reduction[756]. Some acyl chlorides give decarbonyiation products rather than aldehydes under Rosenmund conditions. The diene 890 was obtained by decarbonyiation in an attempted Rosenmund reduction of acetyloleanolic acid chloride (889)[757], Rosenmund reduction of sterically hindered acyl chlorides such as diphenyl- and tnpheny-lacetyl chloride (891) gives the decarbonylated products 892[758],... 

Many carboxylic acids lose carbon dioxide on either direct or sensitized irradiation, and in some cases (4.10 the evidence points to the operation of an initial electron-transfer mechanism rather than primary a-deavage. Cleavage occurs readily with acyl halides, and this can [ead to overall decarbonylation (4.11). Aldehydes also cleave readily, since the (0=)C—H bond is more prone to homolysis than the (0= C-C bond. This offers a convenient method for replacing the aldehydic hydrogen by deuterium in aromatic aldehydes (4.12. and a similar initial reaction step accounts for the production of chain-Iengtheped amides when formamide is irradiated in the presence of a terminal alkene (4.13). 

Palladium chloride and metallic palladium are useful for carbonylating olefinic and acetylenic compounds. Further, palladium is active for decarbonylation of aldehydes and acyl halides. Homogeneous decarbonylation of aldehydes and acyl halides and carbonylation of alkyl halides were carried out smoothly using rhodium complexes. An acyl-rhodium complex, thought to be an intermediate in decarbonylation, was isolated by the oxidative addition of acyl halide to chlorotris(triphenylphosphine)rhodium. The mechanisms of these carbonylation and decarbonylation reactions are discussed. 

The rhodium complex [RhCl(PPh3)3] readily brings about stoichiometric decarbonylation of aldehydes, acyl halides and diketones. A typical aldehyde decarbonylation is illustrated by equation (69). a,3-Unsaturated aldehydes are decarbonylated stereospecifically (equation 70), while with chiral aldehydes the stereochemistry is largely retained (equation 71). ° ... 

One of the standard methods for the preparation of aldehydes involves the reduction of acid halides. A variety of stoichiometric reducing systems are available for this transfomiation, which include NaAlH(OBu-r)3, LiAlHfOBu-O.i, NaBHfOMe). Catalytic hydrogenation with H2 and Pd on carbon is also a popular method. In contrast, methods based on the radical reduction of acyl halides are synthetically less important. Radical reduction methods involve generation and subsequent hydrogen abstraction as key steps, which is complicated by decarbonylation of the intermediate acyl radicals. The first example in Scheme 4-1 shows that this competitive reaction is temperature dependent, where an acyl radical is generated from an acyl phenyl selenide via the abstraction of a phenylseleno group by tributyltin radical [5]. 

D. MECHANISM OF PALLADIUM-CATALYZED DECARBONYLATION OF ALDEHYDES AND ACYL HALIDES, AND ITS RELATION WITH ROSENMUND REDUCTION... 

Decarbonylation of Aldehydes and Acyl Halides Carbonyl-extrusion... 

The insertion of CO into M—C bonds to give acyls is reversible (Section 21-5). However, CO can be irreversibly removed from organic molecules these reactions involve acyl intermediates. Thus aldehydes, acyl, and aroyl halides can be decarbo-nylated either stoichiometrically or catalytically by complexes such as RhCl(PPh3)3 or RhCl(CO)(PPh3)2. An example of a stoichiometric decarbonylation is... 

Tsuji J, Ohno K (1968) Organic synthesis by means of noble metal compounds. XXXV. Novel decarbonylation reactions of aldehydes and acyl halides using rhodium complexes. J Am Chem Soc 90 99-107... 

It was noted in Section 3 that unsaturated acyl halides often formed anomalous products. This tendency is even more pronounced in the case of aldehydes. The decarbonylation of 2-allylbenzaldehyde gives a high yield of indan, and very... 

The complex- imns-RhCI(CO)(PPh3)2 has recently been show-n to act as a catalyst for the hydroformylation of olefins and acetylenes under mild conditions, and for decarbonylation of aldehydes and of acyl and aroyl halides. ... 

In the foregoing, the formation of organic molecules on transition metal complexes is explained by stepwise processes of oxidative addition, insertion, and reductive elimination. One typical example, which can be clearly explained in this way, are the carbonylation and decarbonylation reactions catalyzed by rhodium complexes 10-137). Tsuji and Ohno found that RhCl(PPh3)3 decarbonylates aldehydes and acyl halides under mild conditions stoichiometrically. Also this complex and RhCl(CO) (PPh3)2 are active for the catalytic decarbonylation at high temperature. 

The decarbonylation of aldehydes, acyl halides, aroyl halides, alcohols, or ketones is a useful and important reaction in organic synthesis/ Although several methods not utilizing transition metals are known " (including various deformylation reactions and thermal and photochemical decarbonylatlons), they are not general and not usually applicable under mild conditions where undesirable side reactions are minimized/ ... 

Fifth, although the relative inertness of carbonyl componnds excluding acyl halides was emphasized above, most everything in chemistry is relative, and organopalladinm chemistry is no exception. Thus, in the absence of faster reaction paths, Pd and its complexes may react with aldehydes via C—H activation to give acylpalladinm derivatives and snbsequent decarbonylation (Sect. VL5.1), while ketones may be rednced to alcohols and even to hydrocarbons, as discussed in Sect. VII.2.3.1, althongh the presence of proximal 7T- or n-donors may be critical in snch reactions. 

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