Pauson-Khand reactions catalyzed by entrapped rhodium complexes

An entrapped Rh complex prepared by a sol-gel process has been used as a catalyst in the Pauson-Khand reaction under mild reaction conditions; the catalyst is easily recovered and reused at least 10 times without losing catalyst activity.     

Cobalt nanoparticles on charcoal: a versatile catalyst in the Pauson-Khand reaction,hydrogenation, and the reductive Pauson-Khand reaction

[formula: see text] Dispersions of nanometer-sized cobalt particles with very high stability were prepared in charcoal and analyzed by electron microscopy and X-ray analysis. The resulting cobalt nanoparticles on charcoal (CNC) were successfully used as a catalyst for the carbonylative cycloaddition of alkyne, alkene, and carbon monoxide (Pauson-Khand reaction), hydrogenation, and the reductive Pauson-Khand reaction.     

Sequential actions of palladium and cobalt nanoparticles immobilized on silica: one-pot synthesis of bicyclic enones by catalytic allylic alkylation and Pauson-Khand reaction

[reaction: see text] Palladium and cobalt nanoparticles immobilized on silica have been used as catalyst in a sequential allylic alkylation and Pauson-Khand reaction to form bicyclic enones.     

Chiral and achiral phosphine derivatives of alkylidyne tricobalt carbonyl clusters as catalyst precursors for (asymmetric) inter- and intramolecular Pauson-Khand reactions

Phosphine derivatives of alkylidyne tricobalt carbonyl clusters have been tested as catalysts/catalyst precursors in intermolecular and (asymmetric) intramolecular Pauson-Khand reactions. A number of new phosphine derivatives of the tricobalt alkylidyne clusters [Co3(micro3-CR)(CO)9] (R = H, CO2Et) were prepared and characterised. The clusters [Co3(micro3-CR)(CO)9-x(PR'3)x] (PR'3 = achiral or chiral monodentate phosphine, x = 1-3) and [Co3(micro3-CR)(CO)7)(P-P)] (P-P = chiral diphosphine; 1,1'- and 1,2-structural isomers) were assayed as catalysts for intermolecular and (asymmetric) intramolecular Pauson-Khand reactions. The phosphine-substituted tricobalt clusters proved to be viable catalysts/catalyst precursors that gave moderate to very good product yields (up to approximately 90%), but the enantiomeric excesses were too low for the clusters to be of practical use in the asymmetric reactions.     

Thioureas as ligands in the pd-catalyzed intramolecular Pauson-Khand reaction

[reaction: see text] The thiourea-Pd complex was established as a novel type of catalyst in the PKR of allylpropargylamine, and the demonstrated chemistry may prove to be valuable for developing thiuorea as a ligand for the Pd-catalyzed Pauson-Khand reaction.     

Catalyst precursors for the catalytic Pauson-Khand reaction

Dicobalt hexacarbonyl complexes of enynes serve as Co₂(CO)₈ surrogates for the intramolecular Pauson-Khand cycloaddition. Enynes with polar functional groups allow for easy separation of the catalyst cyclopentenone by-product (acid or base wash, silica gel plug column) from the desired cyclopentenone.     

The intermolecular Pauson-Khand reaction

Five membered carbocycles are important building blocks for many biologically active molecules. Moreover, substituted cyclopentenones (e.g. cyclopentenone prostaglandins) exhibit characteristic biological activity. The efficiency and atom economy of the Pauson-Khand reaction render this process potentially one of the most attractive methods for the synthesis of such compounds. Although it was discovered in its intermolecular form, the scope of the intermolecular Pauson-Khand reaction has always been limited by the poor reactivity and selectivity of the alkene component. The past decade, especially the last three years, has seen concerted efforts to broaden the scope of this reaction. In this overview, we provide a comprehensive and critical coverage of the intermolecular Pauson-Khand reaction based on the reactivity characteristics of different classes of alkenes and a rationalization of successes and misfortunes in this area.     

Co-complexes derived from alkene insertion to alkyne-dicobaltpentacarbonyl complexes: insight into the regioselectivity of pauson-khand reactions of cyclopropenes

Described are the X-ray crystallographic and spectral properties of Co-complexes that were isolated from two Pauson-Khand reactions of chiral cyclopropenes. These are the first examples of isolated Co-complexes derived from the putative alkene-insertion intermediates of Pauson-Khand reactions. The binuclear Co-complexes are coordinated to mu-bonded, five-carbon "flyover" carbene ligands. It is proposed that the complexes result from cyclopropane fragmentation subsequent to alkene insertion. The observation of these metal complexes provides a rationale for the origin of regioselectivity in Pauson-Khand reactions of cyclopropenes.     

Advances in the Pauson-Khand reaction: development of reactive cobalt complexes

Three conceptually new methods employing reactive cobalt carbonyl complexes have been developed for the Pauson-Khand reaction. The first method involves activation of alkyne-dicobalt hexacarbonyls with "hard" Lewis bases. Primary amines dramatically promote the stoichiometric Pauson-Khand reaction. The active cobalt complexes produced in the system exhibit high reducing ability. The second method involves activation of dicobalt octacarbonyl with less electron-donating "hard" Lewis bases such as 1,2-dimethoxyethane and water. These Lewis bases were successfully utilized as a promoter for the catalytic Pauson-Khand reaction. The third method employs methylidynetricobalt nonacarbonyl cluster, which exhibits high reactivity in the catalytic Pauson-Khand reaction in the absence of activator. This may be due to facile coordination of the substrate to the metal center through dissociative and associative mechanisms and/or cleavage of one of the cobalt-cobalt bonds.     

Thiocarbonyl induced heterocumulenic Pauson-Khand type reaction: expedient synthetic method for thieno[2,3-b]indol-2-ones

The first examples of C[double bond, length as m-dash]S induced Pauson-Khand type reactions are described; 2-alkynylphenyl isothiocyanates were converted to 3-substituted-2H-thieno[2,3-b]indol-2-ones in the presence of a stoichiometric amount of Mo(CO)(6) or Co(2)(CO)(8), or a catalytic amount of Rh catalyst under an atmospheric pressure of carbon monoxide.     

Rh(CO)2Cl]2-catalyzed domino reactions involving allylic substitution and subsequent carbocyclization reactions

[reaction: see text] Three novel domino reaction processes have been discovered and developed that employ the regioselective and stereoselective [Rh(CO)(2)Cl](2)-catalyzed alkylations of allylic trifluoroacetates with alpha-substituted sodiomalonates followed by an intramolecular Pauson-Khand annulation, a [5 + 2] cycloaddition, or a cycloisomerization. A unique aspect of the methodology is that a single catalyst is used to effect sequential transformations simply by increasing the temperature for the second reaction.     

Cobalt-phosphite-catalyzed asymmetric Pauson-Khand reaction

A complex formed from dicobalt octacarbonyl and a chiral aryl bisphosphite served as a catalyst for the intramolecular asymmetric Pauson-Khand reaction. Bicyclic cyclopentenones were obtained in up to 75% enantiomeric excess. For a terminal 1,6-enyne, the incremental enantiomeric excess was found to increase from 4 to 26% over the course of the reaction. The scope of this process was examined for a variety of 1,6- and 1,7-enynes, and a moderate degree of enantioselectivity was maintained only in the case of aryl-substituted 1,6-enynes.     

Rhodium catalyzed asymmetric Pauson-Khand reaction using SDP ligands

~~Rhodium catalyzed asymmetric Pauson-Khand reaction using SDP ligands1. Khand, I. U., Knox, G R., Pauson, P. L. et al., Organocobalt complexes, Part Ⅱ. Reaction of acetylenehexacarbonyldicobalt complexes, (R1C2R2)Co2(CO)6, with norbomene and its deri…     

Selective enhancement effects of silver salts on the transition metal-catalyzed synthesis of gem-difluorinated heterocyclics using 2,2-difluorohomopropargyl amides

The addition of silver salts had an effect on the catalyst activity in the Pd(0)-catalyzed cyclization-coupling tandem reaction, as well as in the Rh(I)-catalyzed Pauson-Khand reaction. The cationic palladium complex generated from Pd(PPh₃)₄ (2.5 mol%) with AgSbF₆ (1.5 equiv.) activates the triple bond of 2,2-difluoropropargylic amides to give the 4,5-disubstituted 3,3-difluoro-γ-lactams, through a sequential 5-endo-dig cyclization and cross-coupling reaction. The γ-lactam was transformed into ring-opened monofluorovinylic compounds after silica-gel chromatography. Pauson-Khand reaction of fluorinated 1,7-enyne amides using catalytic amounts of [Rh(COD)₂]₂ (5 mol%) and AgOTf (20 mol%) gave the corresponding gem-difluorinated bicyclic lactam.     

Diversity oriented synthesis of tricyclic compounds from glycals using the Ferrier and the Pauson-Khand reactions

Diversity oriented synthesis of tricyclic compounds was achieved using a combination of the Ferrier reaction and the Pauson-Khand reaction. Ferrier reaction was effected using NbCl5, and the Pauson-Khand reaction was carried out using Co2(CO)8, acetonitrile-dimethoxyethane. Michael additions using various alkyl, aryl, and heterocyclic thiols were also performed successfully. The Ferrier, Pauson-Khand, and Michael addition reactions were found to be highly diastereoselective.     

Stereoselective total syntheses of three Lycopodium alkaloids, (-)-magellanine, (+)-magellaninone, and (+)-paniculatine, based on two Pauson-Khand reactions

The total syntheses of (-)-magellanine, (+)-magellaninone, and (+)-paniculatine were completed from diethyl l-tartrate via the common intermediate in a stereoselective manner. The crucial steps in these syntheses involved two intramolecular Pauson-Khand reactions of enynes: the first Pauson-Khand reaction constructed the bicyclo[4.3.0] carbon framework, the corresponding A and B rings of these alkaloids in a highly stereoselective manner, whereas the second Pauson-Khand reaction stereoselectively produced the bicyclo[3.3.0]skeleton, which could be converted into the C and D rings of the target natural products.     

Carbonylative dimerisation of norbornene by cobalt carbonyl: an overlooked by-product of the Pauson-Khand reaction

Treatment of norbornene with either dicobaltoctacarbonyl or with preformed alkyne-dicobalthexacarbonyl complexes gives enol-lactone dimer 3, derived from carbonylative coupling of two molecules of norbornene. This product which has not been recorded before under these conditions appears likely to be an overlooked by-product of many Pauson-Khand reactions.     

Efficient construction of the bicyclo[5.3.0]decenone skeleton based on the Rh(I)-catalyzed allenic Pauson-Khand reaction

A reliable procedure for constructing a bicyclo[5.3.0]deca-1,7-dien-9-one ring system by rhodium-catalyzed Pauson-Khand reaction (PKR) of allenynes with a sulfonyl group has been developed. Investigation of the rhodium-catalyzed PKR on 19 examples of 1,2-nonadien-8-yne derivatives demonstrated that (i) acceptable yields could be consistently achieved through the proper choice of the rhodium catalyst ([RhCl(CO)(2)](2) or [RhCl(CO)dppp](2)) depending on the starting allenyne and that (ii) an ester functionality as well as hydroxy and siloxy groups could be tolerated in this rhodium-catalyzed PKR.     

Type-II Pauson-Khand reaction of 1,8-enyne in the attempt of building 7/5 ring of (-)-caribenol A and DFT understanding

An attempt to access the fused 7/5 ring of the highly biologically active terpenoid caribenol A by employing intramolecular Pauson-Khand reaction of 1,8-enyne gave bridged 8-5 ring, the type-II Pauson-Khand reaction product. DFT study has been carried out to elucidate this unexpected regioselectivity.