Carbon monoxide sources

Table 7. Typical Analyses of Carbon Monoxide Sources ...

A somewhat related process, the cobalt-mediated synthesis of symmetrical benzo-phenones from aryl iodides and dicobalt octacarbonyl, is shown in Scheme 6.49 [100]. Here, dicobalt octacarbonyl is used as a combined Ar-I bond activator and carbon monoxide source. Employing acetonitrile as solvent, a variety of aryl iodides with different steric and electronic properties underwent the carbonylative coupling in excellent yields. Remarkably, in several cases, microwave irradiation for just 6 s was sufficient to achieve full conversion An inert atmosphere, a base or other additives were all unnecessary. No conversion occurred in the absence of heating, regardless of the reaction time. However, equally high yields could be achieved by heating the reaction mixture in an oil bath for 2 min. 

After extensive screening of various aldehydes to optimize the reaction conditions, it was found that aromatic aldehydes were able to serve as a carbon monoxide source, in which the electronic nature of the aldehydes is responsible for their ability to transfer CO efficiently [24]. Consequently, aldehydes bearing electron-withdrawing substituents are more effective than those bearing electron-donating substituents, with pentafluoro-benzaldehyde providing optimal reactivity. Interestingly, for all substrates tested the reaction is void of any complications from hydroacylation of either the alkene or alkyne of the enyne. Iridium and ruthenium complexes, which are known to decarboxylate aldehydes and catalyze the PK reaction, demonstrated inferior efficiency as compared to... 

Palladium-catalyzed aminocarbonylations, using solid Mo(GO)6 as the carbon monoxide source, have been performed under microwave-assisted conditions, with both 5-bromopyrimidine and 2-substituted-5-bromopyrimidines 273 <2007TL2339>. 

Kakiuchi and co-workers found that aromatic aldehydes bearing electron-withdrawing substituents were able to serve as a carbon monoxide source.A stoichiometric amount of pentafluorobenzaldehyde in refluxing xylene is generally enough to perform this transformation (Table 4). 

Bromobenzyl alcohol and its derivatives were converted to phthalides by the palladium catalysed insertion of carbon monoxide and intramolecular quenching of the formed acylpalladium complex. 2-Hydroxymethyl-1-bromonaphthaline, for example, gave the tricyclic product in excellent yield (3.34.). An interesting feature of the process is the use of molybdenum hexacarbonyl as carbon monoxide source. The reaction was also extended to isoindolones, phthalimides and dihydro-benzopyranones 42... 

Scheme 2.29 In situ aminocarbonylations with Mo(CO)6 as the carbon monoxide source (palladacycle = frans-di(acetate)bis[o-(di-o-tolylphosphino)benzyl]dipalladium(II)).

Scheme 2.31 In situ hydroxycarbonylation with Mof 00)6 as the carbon monoxide source.

A series of rapid microwave-mediated ester syntheses using Mo(CO)6 as the carbon monoxide source were published in 200374. In this paper, a range of valuable ester-protected acids (butyl-, benzyl- and trimethylsilylethyl esters) were smoothly produced both in solution (Scheme 2.32) and on solid phase (TentaGel S RAM-resin, Scheme 2.33) after 15-20 min of single-mode microwave irradiation. The use of DMAP as a nucleophilic additive increased the product yields slightly. Unfortunately, the sterically hindered ferf-butanol furnished little or no product formation at all. 

The concept of in situ liberation of carbon monoxide would be even more attractive if a metal-free material could serve as the carbon monoxide source. In the ideal carbonylation method, the organic solvent itself could be exploited for controlled generation of carbon monoxide. In 2002, Wan et al. addressed this issue and developed a microwave-promoted carbamoylation process based on the commonly used solvent dimethylformamide (DMF) as the carbon monoxide precursor75. Firstly, it was discovered that aryl dimethyl amides were accessible from the corresponding bromides in the presence of a nucleophilic catalyst, imidazole (Scheme 2.34). Secondly, tertiary benzamides other than dimethylamides were synthesised by addition of 3 equiv of an external amine (Scheme 2.34). 

Georgsson, J., Hallberg, A. and Larhed, M., Rapid palladium-catalysed synthesis of esters from aryl halides utilizing Mo(CO)6 as a solid carbon monoxide source, /. Comb. Chem., 2003, 5, 350-352. 

Wan, Y.Q., Alterman, M., Larhed, M. and Hallberg, A., Dimethylformamide as a carbon monoxide source in fast palladium-catalysed aminocarbonylations of aryl bromides, /. Org. Chem., 2002, 67, 6232-6235. 

The [2+2+1] cycloaddition of an alkene, an alkyne and carbon monoxide is commonly known as the Pauson-Khand reaction. This transformation has been adopted many times in the synthesis of complex natural products and related compounds, which contain a cyclopentenone moiety, for example, prostaglandins. Two independent reports of this reaction appeared almost simultaneously in late 2002 by Iqbal and co-workers25 and Fisher and co-workers26, respectively. They not only used very similar substrate systems in their studies, but they also reached very similar conclusions Toluene was found to be the preferred solvent in this reaction, even though it is a very poor microwave absorber. A reaction time between 5 and 10 min, using dicob alto ctacar-bonyl or dicobalthexacarbonyl as the carbon monoxide source, and a temperature of 100-120°C resulted in high yields of the products. Fisher and co-workers used 20 mol% Co2(CO)8 and cyclohexylamine as an additive (Scheme 5.12), since this system had been used previously in order to allow a catalytic reaction. Iqbal and co-workers did not use cyclohexylamine, but instead used 1 equiv. of the carbon monoxide (Co2(CO)6) source. In both reports, the products were formed in 40-70% yield. 

The recent dramatic increase in the price of petroleum feedstocks has made the search for high selectivities more urgent. Several new processes based on carbon monoxide sources are currently competing with older oxidation processes.103,104 The more straightforward synthesis of acetic acid from methanol carbonylation (Monsanto process) has made the Wacker process obsolete for the manufacture of acetaldehyde, which used to be one of the main acetic acid precursors. Several new methods for the synthesis of ethylene glycol have also recently emerged and will compete with the epoxidation of ethylene, which is not sufficiently selective. The direct synthesis of ethylene... 

Table 9.3. Global carbon monoxide sources and sinks fluxes in units of 1012 moles C y1. (After Seiler, 1974.)...

Scheme 1 Microwave-promoted aminocarbonylations of aryl chlorides using Mo(CO)g as a solid carbon monoxide source...

At the beginning of this decade we developed an aminocarbonylation [62, 63] method using Mo(CO)6 as the carbon monoxide source [64,65] with the intention of applying this reagent in laboratory-scale medicinal chemistry. A more precise goal was to enable fast generation of new amido-decorated HIV-1 protease inhibitors. The idea was that the microwave-promoted bis-... 

Herrmann F, List H (1995) Transdermal therapeutic system comprosong active substances representing carbon monoxide sources. Patent WO 95/35105... 

Alberto RA (2001) Carbon monoxide source for preparation of transition metal carbonyl complexes. Patent WO 2001/025243... 

Carbonylation reactions. The formation of derivatives 966 is an example of palladium-catalyzed ami-nocarbonylation This reaction, which uses solid Mo(CO)6 as the carbon monoxide source, proceeds well with S-bromopyrimidines <2007TL2339>. 

Their retro synthetic study was based around the Pauson-Khand cyclization (6), which couples an alkene, an alkyne, and a carbon monoxide source (typically dicobalt octacarbonyl) to give a cyclopentenone ring (Fig. 3.5, top). This reaction has been widely used for synthetic purposes, and some excellent reviews (7,8) have covered its principal features and the recent improvements to its experimental conditions. This reaction, in its intramolecular version, is ideal for the assembly of the l//-[2]pyrindi-none scaffold in two distinct versions, differing in the stereochemistry of the ring junction (Fig. 3.5, bottom). Hence, the readily available unsaturated amino acid derivatives 3.1a,b undergo intramolecular Pauson-Khand reaction to produce the two unsamrated scaffolds 3.2a,b. 

O.C. Zafiriou, S.A. Andrews, W. Wang. Concordant estimates of oceanic carbon monoxide source and sink processes in the Pacific yield a balanced global blue-water CO budget, Global Biogeochem. Cycles, in press. 

SCHEME 25.2(C) Microwave assisted carbonylation using Mo(CO)g as an in situ carbon monoxide source (Reference 37). 

The Pauson-Khand reaction has also been carried out under different conditions, such as the use of chiral ligands, including PuPHOS, CamPHOS, and camphorsultam the use of aldehydes as the carbon monoxide source > the use of solid-supported cobalt catalyst to enhance purification, such as the dry-state adsorption the use of colloidal cobalt nanoparticle and the use of metallic cobalt supported on mesoporous silica prepared by decomposing Co2(CO)8 on mesoporous silica supports (SBA-15 and MCM-41) in the refluxing toluene solution.Other modifications include different promoting methods, such as photo-irradiation, microwave irradiation, molecular sieves, TEMPO,A-oxides, and supercritical fluids. Furthermore, the cycloaddition between allene and carbon monoxide under similar conditions is known as the allenic Pauson-Khand reaction,27,41 jjjg reaction among alkyne, carbodiimide, and CO is referred to as the aza-Pauson-Khand reaction. ... 

Palladium-catalyzed aminocarbonylations of alkenyl phosphates have been investigated using Mo(CO)6 as a solid carbon monoxide source. The reactions afforded a wide variety of acrylamides (78) after 20 min of microwave irradiation in moderate to good yields (Scheme 16). ... 

Garbon monoxide exposures are quite common. In fact, low exposures are almost impossible to avoid. This means that some of the hemoglobin molecules in your blood are always bound to carbon monoxide. Any organic material that imdei oes incomplete combustion will liberate carbon monoxide. Sources include auto exhausts, smoldering leaves, lighted cigars or cigarettes, and charcoal burners. In the United States alone, combustion sources of all types dump about 200 million tons of carbon monoxide per year into the atmosphere, where it mixes with the other molecules in the air. 

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