Acetic acid decarbonylation

The proposed mechanism involves the formation of ruthenium vinylidene 97 from an active ruthenium complex and alkyne, which upon nucleophilic attack of acetic acid at the ruthenium vinylidene carbon affords the vinylruthenium species 98. A subsequent intramolecular aldol condensation gives acylruthenium hydride 99, which is expected to give the observed cyclopentene products through a sequential decarbonylation and reductive elimination reactions. 

An interesting reaction of methyl formate is its isomerization to give acetic acid. Based on patent literature, a number of companies have recently reinvestigated this isomerization which has been known for over 30 years ( ). It is unlikely that it can compete with the Monsanto process however, since it doesn t need pure CO and may be operable at milder reaction conditions, some potential may be seen. Combining isomerization to acetic acid and decarbonylation to methanol and CO, could provide a direct synthesis for acetic anhydride starting directly from methyl formate (Equation 13). 

The four-coordinate alkyl complex, LNiI(C0)CH3, may coordinate with carbon monoxide to regenerate the five coordinate alkyl species, and this leads to insertion to form Ni-acyl complex. This complex, LNil (CO)(COCH3), can be cleaved either by water yielding acetic acid or by methanol to give methyl acetate. However, in the presence of high iodide concentration formation of acetyl iodide may predominate (29). This step is reversible and can lead to decarbonylation under low carbon monoxide partial pressure. Similar decarbonylations of acyl halides by nickel complexes are known (34). 

While most of the above carbonylations are carried out at pressures greater than 40 atm (isocyanate and acetic acid formations are exceptions), decarbonylations are low pressure reactions. Decarbonylation of acyl halides catalyzed by (3P)2RhCOCl leads either to halides (65) (Reaction 19)... 

The cadmium secocorrinoid carboxylic acid (102 M = Cd) also undergoes photocyclization to the acid (103 M = Cd), which on transmetallation to the nickel(II) complex (103 M = Ni) and treatment with triethylamine and acetic acid yields the parent corrin complex (100 M = Ni).268 The decarboxylation process is extremely facile. A related base-catalyzed cyclization of the secocorrinoid aldehyde (104) gives the corrin complex (105), which can be decarbonylated to the parent complex (100 M = Ni) by treatment with potassium hydroxide (Scheme 66).268... 

Benzoic acid is an important chemical intermediate which can also be used as a phenol precursor by decarbonylation in the presence of copper catalysts (Lummus process). It is produced industrially by oxidation of toluene by air in the presence of cobalt catalysts (Dow and Amoco processes equation 240). The reaction can be carried out without solvent, or in an acetic acid solvent. The oxidation of toluene without solvent uses a cobalt octoate catalyst and operates at higher temperature (180-200 CC). Yields of benzoic acid are about 80% for ca. 50% toluene conversion.361 In an acetic acid solution and in the presence of cobalt acetate, the reaction occurs at lower temperature conditions (110-120 °C) and gives higher yields in benzoic acid (90%).83,84... 

By increasing the temperature over > 300°C, the phenyl glycidic acid esters are converted to phenyl acid esters. That occurs in a consecutive decarbonylation of a-ketone carboxylic acid esters as intermediate. In the case of p-tert-butyl glycidic acid ester in the presence of a boron-pentasil zeolite at 350 °C the ratio between a-ketone carboxylic acid ester and the phenyl acetic acid ester is 65 35. 

Decarbonylations of furfuraldehyde to furan continue to be of commercial interest and various new catalysts have been recommended.253 Decarboxylations are still occasionally useful,254 and the selective decarboxylation of furan-3,4-dicarboxylic acid to furan-3-carboxylic acid is said to be much improved by omitting any solvent.255 The easy decarboxylation of furan acetic acid derivatives is formulated in structure 139, although the acidic conditions need not preclude ring opening.2553... 

Cyclic exponents of the same elimination type are of particular interest. Thus, numerous cyclopentanones photolytically decarbonylate to give 1,4-dienes (p. 876 ff. in Ref. 108)). With thujone (194) this [l,2,(3)4]-elimination of carbon monoxide proceeds quantitatively to give 195 109). With silver nitrate the norcaradiene 196 yields 197 (95%)110) apparently regioselectively, and the [l,2,(3)4]-elimination of methanethiol from 198 to give 199 was realized thermally (16%), acid-catalyzed (59%), and photochemiciiily (ca. 5%)105). For the acid-catalyzed reaction (acetic acid, 100 °C) a non-stereospecific process has been proved 105). 

One of the byproducts observed in some cases was tetramethylindanone (24) in acetic acid it constituted 6% and in benzene 14% of the total product. The most reasonable mechanism for formation of 24 is via cleavage and decarbonylation to give the tertiary-benzilic diradical 25 which can cyclize to 24. The process may be facilitated by the considerable stabilization present in 25. 

Increasing the temperature to 350 °C results in decarbonylation of the phenylpyruvic acid methyl ester derivatives and the phenyl acetic ester is formed with a ratio of 65 % a-ketoester to 35 % acetic acid ester. Until now the industrial process for the synthesis of phenylacetic acid ester has started from benzyl chloride, which is converted to benzyl cyanide by KCN, followed by hydrolysis. Every step of this reaction must be performed in a separate reactor and special measures must be taken for handling large amounts of toxic KCN. The new route is certainly an environmentally benign alternative [8,27]. 

Reactions in which a substance decomposes by losing CO are called decarbonylation reactions. The decarbonylation of acetic acid proceeds as follows ... 

A number of highly stable azulene-substituted cations, (17) and several others, have been prepared and studied some of them have pA R+ values of 14 and above." The amazingly stable species (18) is also reported with = morpholino the p.STr+ value is 21.5, and with R = Me it is 24.3, which means that these species remain as cations even in superbase media." While en route to these species, it was found that 1-azulenecarbaldehydes can be decarbonylated under fairly mild conditions by treating them with pyrrole in acetic acid." ... 

A new strategy for the protection of the 1- and/or 3-position of azulene rings is based on the ease of decarbonylation of 1-azulene carbaldehydes and 1,3-dicarbaldehydes promoted by pyrrole in acetic acid the decarbonylation is a consequence of the ease of protonation of the azulene ring in acidic conditions and the electron-donating properties of the pyrrole ring. 

With the above methodology in hand, a similar strategy was attenq)ted for the synthesis of the aryl acetic acid 7 (Scheme 7). The aniline 5 was treated with 2,5-dimethoxytetrahy ofuran in toluene and acetic acid to get the W-aryl pyrrole 28. The Vilsmeier/Triedel-Crafts acylation of 28 followed by decarbonylation afforded the keto ester 30. However, the reduction of the keto ester proved to be difficult. Most of the general methods employed for the reduction of ketones gave a mixture of products. However, a two step process involving the formation of the thioketal 31 followed by desulfurization with Ni proved to be successful. Although, the standard Wolff-Kishner conditions could not be employed in the system due to the susceptibility of the nitrile to hydrolysis, a modified Wolff-Kishner reduction (8) proved to be fruitful. 

Although the above route was successful in providing an easy access to the aryl acetic acid 7, it was still plagued by the use of p-(p-cyanophenyl)aniline (5), which is a suspected carcinogen. Thus it was decided to use a different p-substituted aniline and later extend it via a Pd mediated process. Commercially available and inexpensive p-bromo aniline was the compound of first choice. Condensation with dimethoxy tetrahydrofuran followed by Vilsmeier/Friedel-Crafls reaction sequence resulted in die formation of the required aldehyde 35. However, the decarbonylation of this aldehyde proved to be extremely difficult. With fewer options available, we decided to convert the bromide to the boronate ester 37 by reacting with bis(pinacolato)diboron 36. To our surprise, the boronate ester 37 underwent decarbonylation readily with Pd/alumina to give the keto ester 38. The keto group was reduced by treatment with either Ra-Ni or Pd-... 

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