Cyclopentene, 3- Pauson-Khand reaction

Co2(CO)6 fragments at the C=C triple bonds further from the Pt center. The Pauson-Khand reaction between tra 5-Pt C=CC2H[Co2(CO)6] (PR3)2 (R = Et, Bu) (197) and norbomene or cyclopentene gives the corresponding cyclopen-tenones 198 (Scheme 44). ... 

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. 

In a representative example of this type, complexed l-ethoxy-2-ethynylcyclopropane 7 was used in the Pauson-Khand reaction with cyclopentene to give the cyclopropyl-substituted bi-cyclo[3.3.0]octenone 8, which on thermal rearrangement of the vinylcyclopropane unit gave the triquinane ring system 9 ... 

Numerous synthetic applications of the inteimolecular Pauson-Khand reaction have been reported. Pauson has reported a number of very direct tqrplications of cycloadditions of ethylene in the synthesis of prostanoids and jasmone analogs (e.g. equations 15 and 16). - This is a reliable entry to 2-sub-stituted cyclopentenones. The suitability of cyclopentene and dihydrofuran as substrates has permitted the extension of this work to the preparation of still further varieties of prostaglandin analogs (e.g. equations 27 and 51). Simple 4,5-disubstituted 2-cyclopentenones are not as directly accessible, but may be prepared from the cycloaddition products of norbomadiene (equation 45). A sequence of conjugate addition followed by retro-Diels-Alder reaction affords the product (Scheme 5). Dihydrofuran cycloadditions have been used by Billington in the syntheses of the antibiotic methylenomycin B (Scheme 6), as well as cyclomethylenomycin A (synthetic precursor to the antibiotic methylenomycin A), cyclosarko-mycin (precursor to the antitumor agent sarkomycin) ° and the iridoid Jq>anese hop ether. ... 

The typical Nazarov strategy 51 followed by 53 disconnects the rest of the ring from the double bond, but you cannot be quite sure where the double bond will end up in the ring. Another special method, the Pauson-Khand reaction,24,25 differs in both respects. It adds the enone portion of the ring to the rest of the molecule and you can be quite certain where the new double bond will be. The reaction is between an alkene, say cyclopentene 97, an alkyne, and Co2(CO)8 to form a cyclopentenone26 98 in one step. A complex 99 is first formed between the alkyne and the cobalt atoms with the two n-bonds replacing two CO molecules (both are two-electron donors). You may see this complex drawn as 99a but it really has a tricyclic tetrahedrane structure 99b composed of three-membered rings and with a Co-Co bond. 

It is accepted that this reaction involves the formation of the alkynedicobalt hexacarbonyl complex from an alkyne and Co2(CO)s by the evolving of two CO ligands, followed by the alkene coordination at one of the two enantiotopic Co atoms with concomitant CO insertion, and final reductive elimination of the metal to an a,(3-unsaturated cyclopentenone. In the traditional protocol, the reaction mixture is heated in toluene at 110°C, or tertiary amine A-oxides are added to promote the reaction at ambient temperature. For the purpose of stereochemical control, many Pauson-Khand reactions are designed as intramolecular reactions P " or using cyclic alkenes, such as norbornene. It has been found that the reactivity of cyclic alkenes is in the order of cyclohexene < cyclopentene < norbornene. For the intermolecular Pauson-Khand reaction, alkene is positioned adjacent to the less bulky acetylenic substituent during coordination because of steric hindrance, and a subsequent C-C bond forms between an alkenic... 

The scope of the intramolecular Pauson-Khand has been rapidly expanded as both more complex and heteroatom-containing substrates have been employed. Cycloadditions involving a cycloalkene reaction partner afford the direct construction of tricyclic systems in a single step. Triquinacene derivatives are efficiently obtained from 3-(3-butynyl)cyclopentenes [Eq. (55)]. An unusual characteristic of this system is the epimerization that occurs at the pro-pargylic position subsequent to cobalt complexation but prior to cyclization. The steric demands on the reaction are evidently so large that one stereoisomer is unable to cyclize and, instead, isomerizes through a cobalt-stabilized propargylic cation [123]. 

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