Ruthenium Pauson-Khand reaction

Scheme 12 Ruthenium-catalyzed hetero Pauson-Khand reactions with alkynes and al-lenes...

Ruthenium carbonyl complexes have been shown to catalyze a number of car-bonylation processes. The ruthenium-catalyzed intramolecular Pauson-Khand reaction was found to proceed in the presence of Ru3(CO)12 (Eq. 105) [165,166]. The reaction is a valuable tool for selective organic synthesis. 

Although a whole series of carbonyl complexes of other transition metals (Fe, Mo, W, Ni) could only be used in stoichiometric Pauson-Khand reactions [11], two Japanese laboratories have since independently reported efficient ruthenium-catalyzed (intramolecular) reactions. The desired cy-clopentenones are formed in good to excellent yields in dimethylacetamide [12] or dioxane [13] in the presence of 2 mol% of [Ru3(CO),2] at 140-160 °C and 10-13 atm CO pressure. 

The Pauson-Khand reaction is a well-known method for preparing cydopente-nones by the [2 + 2 + 1] cycloaddition reaction of alkyne, alkene and CO. While reactions using stoichiometric amounts of Co2(CO)g were initially examined, catalytic versions with cobalt, titanium, rhodium, iridium, and ruthenium complexes have recently been developed. Whilst the intramolecular version is rather easy, the inter-molecular version is a very difficult problem that has not yet been solved [76]. 

The intramolecular versions catalyzed by ruthenium complexes, and developed independently by the groups of Murai [77] and Mitsudo [78] in 1997, opened the door to the chemistry of noble metal-catalyzed Pauson-Khand reactions (Eq. 11.35). 

Considerable effort has been devoted to achieving the intermolecular catalytic Pauson-Khand reaction. The mthenium complex-catalyzed reaction of an alkyne with an alkene such as ethylene or 2-norbornene under CO gave hydroquinone derivatives [79], with CO (2 mol) being introduced into the products (Eq. 11.36). This reaction is the first example of the preparation of hydroquinone derivatives by the reaction of alkynes and alkenes with CO, while hydroquinone is synthesized by the ruthenium-catalyzed reaction of 2 mol acetylene with 2 mol CO (Eq. 11.37) [80]. 

The catalytic [2 + 2 + 1]-cycloaddition reaction of two carbon—carbon multiple bonds with carbon monoxide has become a general synthetic method for five-membered cyclic carbonyl compounds. In particular, the Pauson-Khand reaction has been widely investigated and established as a powerful tool to synthesize cyclopentenone derivatives.110 Various kinds of transition metals, such as cobalt, titanium, ruthenium, rhodium, and iridium, are used as a catalyst for the Pauson-Khand reaction. The intramolecular Pauson-Khand reaction of the allyl propargyl ether and amine 91 produces the bicyclic ketones 93, which bear a heterocyclic ring as shown in Scheme 31. The reaction proceeds through formation of the bicyclic metallacyclopentene intermediate 92, which subsequently undergoes insertion of CO to give 93. 

Kang et al. reported that the ruthenium-catalyzed intramolecular hetero-Pauson—Khand reaction of the allenyl aldehydes and ketones 387 gave the a-meth-ylene-y-butyrolactones 388 (Scheme 123).186... 

As follow-ups of the original reaction procedure, numerous improvements have been made, including the lowering of the reaction temperature, use of other transition metals, and the catalytic use of various metal species. In the 1990s, cobalt, titanium, and ruthenium complexes were found to serve as catalysts for the Pauson-Khand reaction, but these catalytic systems often need to employ medium to high pressures of... 

The group of Murai [14] could demonstrate that ruthenium-catalyzed cyclocar-bonylation of yne-imines resulted in formation of lactams (Scheme 1.4a). Catalytic amounts of Ru3(CO)j2 promote this cyclocarbonylation of 1,6- and 1,7-yne-imines, giving bicychc a,fi-unsaturated lactams. Similar to the Pauson-Khand reaction, the... 

Complexes of other transition metals have been reported to catalyze Pauson-Khand reactions. Buchwald reported intramolecular PKRs with 1.2 atm of CO at 90 °C in the presence of CpjTi(CO)2. " However, most other catalytic Pauson-Khand reactions have been conducted with late transition metal catalysts. Murai and Mitsudo simultaneously reported intramolecular PKRs catalyzed by ruthenium carbonyl clusters in dioxane or DMAc at 140-160 °C under 10-15 atm of CO. The first Rli-catalyzed PKR was reported by Narasaka. ° In this case, the reaction occurred with acceptable rates, even with CO pressures less than 1 atm. Shibata reported PKRs in refluxing xylenes under 1 atm of CO in the presence of catalytic amounts of PPli and [Ir(COD)Cl]2. Adrio and Carretero showed that the solvated molybdenum carbonyl complex Mo(DMF)3(CO)3 catalyzed intramolecular PKRs with monosubstituted olefins, as well as with disubstituted electron-poor olefins, and Hoye showed that W(CO)5(THF) catalyzes intramolecular PKRs. Iron and palladium complexes have also been reported to catalyze the PKR. 

A problem is that the Pauson-Khand reaction uses two equivalents of cobalt. More efficient versions, many of them catalytic, using other metals have been developed. These include carbonyl complexes of titanium, molybdenum, tungsten (Scheme 7.15), rhodium and ruthenium (Scheme 7.16). Rhodium, iridium and iron (Scheme 7.17) have also been used with two alkynes to give cyclopentadienones, often as complexes 7.59. A version of the Pauson-Khand reaction employing a nickel catalyst and an isonitrile in place of CO has been developed. The product is an imine, which can be hydrolysed to a cyclopentenone. 

The cobalt-mediated cycloaddition of an alkyne, an alkene, and CO leading to a cyclopentenone has been known as the Pauson-Khand (PK) reaction [78], Due to its synthetic importance, numerous variants - especially catalytic reactions - have been developed to date [79]. The first ruthenium-catalyzed PK reaction of enynes has been achieved using Ru3(CO)i2 by two research groups independently (Scheme... 

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