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

Abstract

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.

Catalytic Dinuclear Nickel Spin Crossover Mechanism and Selectivity for Alkyne Cyclotrimerization

“…The different selectivity (cyclotrimerization vs cyclotetramerization) for dinuclear catalyst 1 compared with mononuclear Ni catalysts suggests the possibility of a unique reaction mechanism and catalytic cycle influenced by the Ni–Ni interaction and NDI ligand scaffold. Scheme b shows the previously proposed catalytic cycle . This mechanism involves alkyne coordination and C–C bond coupling at a single Ni metal center with a metallacyclopentadiene intermediate.…”

“…Homodinuclear complex ( i ‑Pr NDI)­Ni 2 (C 6 H 6 ) catalyst 1 involves a d 9 –d 9 Ni–Ni covalent bond (HOMO–5) and coordination of benzene across both Ni metal centers (Figure a) . The HOMO of 1 is a π orbital delocalized across the NDI ligand.…”

“…The formation of 1,2,4-substituted benzene isomer (4′-phenyl-1,1′:2′,1″-terphenyl) is preferable to that of the 1,3,5-substituted isomer (5′-phenyl-1,1′:3′,1″-terphenyl). This dinuclear catalyst contrasts many mononuclear Ni catalysts that generally give cyclooctatetraenes as the major product, especially with N-donor ligands . However, addition of phosphine or NHC ligands to some mononuclear catalysts can result in cyclotrimerization …”

“…We are especially interested in homodinuclear catalysts based on first-row transition metals. Uyeda recently reported the homodinuclear Ni–Ni naphthyridine-diimine (NDI) catalyst 1 [( i ‑Pr NDI)­Ni 2 (C 6 H 6 )] that induces cyclotrimerization of several monosubstituted alkynes (Scheme a). , For example, phenylacetylene undergoes selective cyclotrimerization to yield only terphenyl aromatics. The formation of 1,2,4-substituted benzene isomer (4′-phenyl-1,1′:2′,1″-terphenyl) is preferable to that of the 1,3,5-substituted isomer (5′-phenyl-1,1′:3′,1″-terphenyl).…”

“…While this [4 + 2] cycloaddition mechanism is reasonable, there are several viable alternative mechanisms to evaluate, especially considering that likely intermediates may access multiple electronic spin states. This proposal is based on the isolated ( i ‑Pr NDI)­Ni 2 (Et 2 SiH 2 ) complex that has a singlet–triplet energy gap of <3 kcal/mol measured by variable-temperature nuclear magnetic resonance (NMR) . Here we present DFT calculations that reveal the dinuclear Ni–Ni interaction combined with the NDI ligand in catalyst 1 results in a spin crossover mechanism that involves multiple migratory insertion steps and a nonclassical bridging metallacycloheptatriene intermediate.…”