2-cyclopentenone dicobalt octacarbonyl

The [2+2+1] cycloaddition of an alkene, an alkyne, and carbon monoxide is known as the Pauson-Khand reaction and is often the method of choice for the preparation of complex cyclopentenones [155]. Groth and coworkers have demonstrated that Pauson-Khand reactions can be carried out very efficiently under microwave heating conditions (Scheme 6.75 a) [156]. Taking advantage of sealed-vessel technology, 20 mol% of dicobalt octacarbonyl was found to be sufficient to drive all of the studied Pauson-Khand reactions to completion, without the need for additional carbon monoxide. The carefully optimized reaction conditions utilized 1.2 equivalents of... 

The alkyne-cobalt carbonyl complex 3 formed from the alkyne 1 and dicobalt octacarbonyl 2 should lose at least one of the GOs on the metal to provide the vacancy for the incoming olefins. Subsequently, an olefin-bound complex 5 rearranged oxidatively to yield a metallacyclic intermediate 6. Migratory insertion of GO of 6 would provide the homologated ring intermediate 7, and the following two successive reductive eliminations afford the cyclopentenone... 

Ethynylcyclopropanes, like normal acetylenes, react with dicobalt octacarbonyl in ether to form stable dinuclear cluster-like hexacarbonyl complexes (equation 170)236. The complex with l-chIoro-2,2,3,3-tetramethylethynylcyclopropane reacts stereo- and regioselec-tively with norbomene in a typical Pauson-Khand reaction to give the exn-2-cyclopropyl substituted cyclopentenone (equation 171). Similarly, the reaction of 2-ethoxycyclo-propylacetylene with cyclopentene in the presence of Co2(CO)8 under CO gave 3-(2-ethoxycyclopropyl)-cw-bicyclo[3.3.0]oct-3-en-2-one (equation 172)242. 

A further addition-cyclization process that leads to complex fused-ring systems is the dicobalt octacarbonyl-mediated Pauson-Khand reaction which, applied to enynes 209 and 211, gives respectively and in modest yields the tricyclic cyclopentenones 210215 and 212.216... 

The initial catalytic version of the PK reaction actually appeared in 1973, in the first paper detailing the preparation of cyclopentenones by Pauson and co-workers.3 By stirring a solution of dicobalt octacarbonyl (10 mol % with respect to the alkene) and norbornene (or norbornadiene), first under a pressure of acetylene and then under a pressure of mixed acetylene/CO (1 1), it was possible to obtain a good yield of the desired cyclopentenone products (42 and 8 - Scheme 16). It was possible to lower the loading of dicobalt octacarbonyl to 2.5 mol % and still obtain a 62% yield of the cyclopentenone. When norbornadiene was employed as the alkene, a lower yield of the desired cyclopentenone was obtained but this was due to the product reacting further under the conditions employed. 

In 1990, Rautenstrauch and co-workers reported the catalytic reaction of 1-heptyne (43) with ethene (Scheme 17).51 They were able to use just 0.22 mol % of dicobalt octacarbonyl (with respect to the alkyne) in the reaction and obtained a moderate yield of the desired cyclopentenone (44). 

It was thought that the formation of inactive cobalt clusters such as Co4(CO)i2, formed by dimerisation of the remaining cobalt carbonyl species after release of the cyclopentenone product, were responsible for the shutdown of the catalytic cycle when dicobalt octacarbonyl was employed.51 Krafft and co-workers were able to show that Co4(CO)12 can actually be exploited as a catalytic species in the PK reaction and were able to obtain excellent yields if cyclohexylamine was introduced as an additive alongside the metal cluster.57,58 The use of metal clusters as catalysts for the reaction has been extended to involve mixed metal clusters.59... 

The expected hexacarbonyl dicobalt complex was formed when the al-kyne and dicobalt octacarbonyl were stirred in CH2C12 (73 —> 74). If the same reaction was performed in the presence of ten equivalents of NMO, the novel pentacarbonyl complex could be isolated (73->75). On dissociation of the CO ligand, accelerated by the addition of NMO, the sulfur atom is able to coordinate to the vacant site creating a stable complex. Treatment of the complex with CO resulted in regeneration of the hexacarbonyl complex and, unlike the isolated alkene-pentacarbonyldicobalt-alkyne species 68, heating of the pentacarbonyl complex 75 led to the formation of the desired cyclopentenone. A similar sulfur stabilised intermediate was isolated by Pericas and co-workers while investigating tethered chiral auxiliaries in the PK reaction.87,88... 

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. 

Cyclopropene can also be used as the aUcene component and affords bicyclo[3.1.0]hexen-2-ones upon reaction with alkyne dicobalt octacarbonyl complexes in the presence of NMO (Scheme 250). Vinyl ethers and vinyl esters serve as ethene equivalents in Pauson-Khand reactions. For example, reaction of vinyl benzoate with complex (169) furnished cyclopentenone (170) (Scheme 251). This reaction was used in a synthesis of (-l-)-taylorine and nortaylorine. Allenes participate in intermolecular Pauson-Khand reactions affording alkylidene-substituted cyclopentenones (Scheme 252). ... 

An interesting cyclization reaction was reported that involved the reaction of dienes, diynes, or ene-ynes with transition metals to form cyclopentenone derivatives in the presence of carbon monoxide.363 in a simple example, ene-yne 444 was heated with dicobalt octacarbonyl and CO to give a 68% yield of 445.364 jjjj transformation has become an important synthetic tool known as the Pauson-Khand reaction.365 jhe mechanism probably involves insertion of the alkene (or alkyne) into the transition metal bond, which is why it is presented in this section. Formally, it is a [2+2+l]-cycloaddition, but the accepted mechanism is the one proposed by Magnus,364 and shown in Figure 13.8.366 n has been stated that further study is required to... 

The Pauson-Khand reaction is the formation of cyclopentenones 486 by the action of dicobalt octacarbonyl on an alkyne, followed by an alkene. The process involves the intermediacy of cobalt complexes 485 (equation 53). (For reviews, see References 302 and 303.) Terminal alkynes, including acetylene and arylacetylenes, give better yields than internal acetylenes. An example of an intramolecular Pauson-Khand reaction is the conversion of the enyne 487 into the cyclopentanocyclopentenone 488 ". ... 

A significant advance in the Pauson-Khand reaction was made by the discovery that various additives, such as tertiary amine A-oxides, promote the cycloaddition reaction. For example, treatment of the dicobalt complexed alkyne 187 with trimethylamine A-oxide at only 0 °C provides the cyclopentenone 188 in good yield (1.192). More recent advances have been made in catalytic Pauson-Khand reactions. " Only 3 mol% of dicobalt octacarbonyl [Co2(CO)8] under one atmosphere of CO effects the formation of the cyclopentenone 188 from the alkyne 189 in benzene at 70 °C (an improvement in the yield to 90% was achieved in the presence of the additive Bu3P=S) (1.193). " ... 

Apart from cobalt carbonyl catylyzed hydroformylation, Pauson-Khand (PK) reaction is another type of reaction catalyzed with bimetallic carbonyl complex. Formally Pauson-Khand (PK) is a [2 -i- 2 -i- 1] cycloaddition of an alkyne, an alkene, and a CO group into cyclopentenone [128-130]. This process was initially discovered in 1973 [131], and early studies focused on using dicobalt octacarbonyl as both reaction mediator and the source of the carbonyl functional group. Since several variants of the original thermal protocol were introduced, PK reaction has received more and more fundamental and organic synthesis interests [132, 133]. 

The reaction was first reported by Khand and Pauson et al. in 1973d It is the dicobalt octacarbonyl [Co2(CO)8l mediated or promoted one-step synthesis of a,p-unsaturated cyclopentenone from the [2+2+1] cycloaddition of alkyne, alkene and carbon monoxide, through an intermediate of alkynedicobalt hexacarbonyl complex. Therefore, this reaction is generally known as the Pauson-Khand reaction, Pauson-Khand cyclization, or Pauson-Khand cycloaddition. Occasionally, this reaction is also referred to as the Pauson-Khand annulation, Pauson-Khand multicomponent cycloaddition, Pauson-Khand carbonylative cocyclization, Pauson-Khand bicyclization, Khand annulation, Khand cycloaddition, Khand cyclization (cyclisation ), or Khand reaction.Among these names, the Pauson-Khand reaction is the one used most often. 

The Pauson-Khand reaction is a recent addition to the armamentarium of synthetic organic chemistry. In this process, a carbonyl group (provided initially as a ligand attached to dicobalt octacarbonyl [Co2(CO)g]) bridges an alkyne to an alkene to produce, finally, a cyclopentenone. While many of the details of the process remain sketchy, a general outline of the process is provided in Scheme 9.92. 

The dicobalt octacarbonyl catalysed the three-component reaction of alkene, alkyne and carbon monoxide to give cyclopentenone, which was an important breakthrough in organic synthesis [8]. A three-component direct condensation to yield rings or fragments was very difficult earlier to the estabhshment of the Pauson-Khand reaction. 

Formal [2 + 2+1] cycloaddition of an alkene, alkyne, and carbon monoxide mediated by octacarbonyl dicobalt to form cyclopentenones. 

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