Phosphine–Alkene Ligands as Mechanistic Probes in the Pauson–Khand Reaction

Ferrer, Catalina; Benet‐Buchholz, Jordi; Riéra, Antoni

doi:10.1002/chem.201000357

“…[9] Our approach circumvents the long-standing challenge associated with studying Co 2 (CO) 8 by utilizing aw ell-defined d 9 -d 9 Ni 2 complex as afunctional model. [10] These results complement experiments reported by the groups of Gimbert, [11] McGlinchey, [12] and Verdaguer, [13] which access unstable intermediates along the Pauson-Khand pathway by conducting reactions in the gas phase or by using unreactive,c onformationally constrained enyne substrates (Figure 1b). As an entry into Pauson-Khand reactivity,w ef irst examined the accessibility of stable alkyne adducts using the [ i-Pr NDI]Ni 2 platform (NDI = naphthyridine-diimine).…”

supporting

confidence: 82%

Well‐Defined Models for the Elusive Dinuclear Intermediates of the Pauson–Khand Reaction

Hartline

¹

,

Zeller

²

,

Uyeda

³

2016

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The mechanism of the Pauson-Khand reaction has attracted significant interest due to the unusual dinuclear nature of the Co 2 (CO) x active site.Experimental and computational data have indicated that the intermediates following the initial Co 2 (CO) 6 (alkyne) complex are thermodynamically unstable and do not build up in appreciable concentrations during the course of the reaction. As ac onsequence,t he key steps that control the scope of viable substrates and various aspects of selectivity have remained largely uncharacterized. Herein, ad irect experimental investigation of the dinuclear metallacycle-forming step of the Pauson-Khand reaction is reported. These studies capitalizeo nw ell-defined d 9 -d 9 dinickel complexes supported by an aphthyridine-diimine (NDI) pincer ligand as functional surrogates of Co 2 (CO) 8 .Cycloaddition reactions are key targets for the development of new synthetic methods,p roviding efficient routes to ring systems from comparatively simple linear precursors.T ransition-metal-mediated processes have significantly expanded the scope of viable cycloadditions beyond the subset that are favorable under thermal conditions. [1] TheP auson-Khand reaction is one of the earliest examples and represents ap rototype for three-component couplings. [2, 3] Ther eaction produces cyclopentenones from the selective union of an alkene,a na lkyne,a nd CO.D espite its 40-year history,a nd numerous applications in organic synthesis, [4] little is known about the mechanism of the Pauson-Khand reaction. The initial ligand substitution step between Co 2 (CO) 8 and the alkyne component is well-characterized-over 500 crystal structures of Co 2 (CO) 6 (alkyne) adducts have been reported [5] -however,n os ubsequent intermediates are detectable under standard reaction conditions.G iven the unique role that metal-metal bonds are thought to play in the Pauson-Khand reaction, ad etailed understanding of the elementary steps would aid in the development of multinuclear platforms for abroader range of cycloadditions.

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“…[9] Our approach circumvents the long-standing challenge associated with studying Co 2 (CO) 8 by utilizing aw ell-defined d 9 -d 9 Ni 2 complex as afunctional model. [10] These results complement experiments reported by the groups of Gimbert, [11] McGlinchey, [12] and Verdaguer, [13] which access unstable intermediates along the Pauson-Khand pathway by conducting reactions in the gas phase or by using unreactive,c onformationally constrained enyne substrates (Figure 1b). As an entry into Pauson-Khand reactivity,w ef irst examined the accessibility of stable alkyne adducts using the [ i-Pr NDI]Ni 2 platform (NDI = naphthyridine-diimine).…”

supporting

confidence: 87%

Well‐Defined Models for the Elusive Dinuclear Intermediates of the Pauson–Khand Reaction

Hartline

¹

,

Zeller

²

,

Uyeda

³

2016

View full text Add to dashboard Cite

The mechanism of the Pauson-Khand reaction has attracted significant interest due to the unusual dinuclear nature of the Co2 (CO)x active site. Experimental and computational data have indicated that the intermediates following the initial Co2 (CO)6 (alkyne) complex are thermodynamically unstable and do not build up in appreciable concentrations during the course of the reaction. As a consequence, the key steps that control the scope of viable substrates and various aspects of selectivity have remained largely uncharacterized. Herein, a direct experimental investigation of the dinuclear metallacycle-forming step of the Pauson-Khand reaction is reported. These studies capitalize on well-defined d(9) -d(9) dinickel complexes supported by a naphthyridine-diimine (NDI) pincer ligand as functional surrogates of Co2 (CO)8 .

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“…In that case, however, the phosphine and alkene are both bonded to the same cobalt. Interestingly, even though the alkene is well‐aligned with the alkyne in 14 , and the potential PKR product would not be particularly strained, the process halts at this point . Moreover, we note that the particularly favourable geometry of the 5 H ‐dibenzo[ a,d ]cycloheptatrienyl unit that allows coordination of the C10=C11 double bond to a metal centre, as in 2 , 12 or 13 , has also been elegantly exploited by Grützmacher, especially in a very extensive series of 5‐phosphino derivatives, exemplified by molecules such as 15 or 16 , shown in Figure …”

Section: Resultsmentioning

confidence: 86%

Alkynyldicobalt Derivatives of Dibenzosuberenol and Dibenzocyclooctatrien‐5‐ol: Ring Conformations, Ease of Carbonyl Elimination and Relevance to Pauson–Khand Cyclization

Brusey

¹

,

Shen

²

,

Müller‐Bunz

³

et al. 2017

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Under very mild conditions, (5-alkynyl-5H-dibenzo-[a,d]cycloheptatrien-5-ol)Co 2 (CO) 6 clusters 1 lose a carbon monoxide ligand and form the corresponding η 2 -alkene-μ-alkyne-Co 2 (CO) 5 complexes 2 (the first isolated examples of the initial phase of the Pauson-Khand reaction). This behaviour is not paralleled by the homologue in which an additional methylene group has been inserted within the seven-membered ring, as in the [5-alkynyl-6H-dibenzo[a,e]cyclooctatrienol]Co 2 -(CO) 6 system 8, for which several X-ray crystal structures are reported. The apparent additional flexibility of this molecular framework does not translate into conformations appropriate for the formation of η 2 -alkene-μ-alkyne-Co 2 (CO) 5 complexes. The relative inertness of the ethynyl complex 1a, compared [a] 2051 ing of the seven-membered ring towards the cluster, such that this distance, d, shortens to 2.04 Å. In contrast, in the eightmembered ring complex 8a, not only is the corresponding bond-centroid-to-cobalt distance, d, considerably longer (ca. 4.2 Å), but the C11=C12 double bond is also twisted away from the horizontal, as depicted in Figure 6b.

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Phosphine–Alkene Ligands as Mechanistic Probes in the Pauson–Khand Reaction

Cited by 10 publications

References 60 publications

Well‐Defined Models for the Elusive Dinuclear Intermediates of the Pauson–Khand Reaction

Well‐Defined Models for the Elusive Dinuclear Intermediates of the Pauson–Khand Reaction

Well‐Defined Models for the Elusive Dinuclear Intermediates of the Pauson–Khand Reaction

Alkynyldicobalt Derivatives of Dibenzosuberenol and Dibenzocyclooctatrien‐5‐ol: Ring Conformations, Ease of Carbonyl Elimination and Relevance to Pauson–Khand Cyclization

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