Alkyne gem‐Hydrogenation: Formation of Pianostool Ruthenium Carbene Complexes and Analysis of Their Chemical Character

Abstract

Parahydrogen (p‐H2) induced polarization (PHIP) NMR spectroscopy showed that [CpXRu] complexes with greatly different electronic properties invariably engage propargyl alcohol derivatives into gem‐hydrogenation with formation of pianostool ruthenium carbenes; in so doing, less electron rich CpX rings lower the barriers, stabilize the resulting complexes and hence provide opportunities for harnessing genuine carbene reactivity. The chemical character of the resulting ruthenium complexes was studied by DFT‐assisted analysis of the chemical shift tensors determined by solid‐state 13C NMR spectroscopy. The combined experimental and computational data draw the portrait of a family of ruthenium carbenes that amalgamate purely electrophilic behavior with characteristics more befitting metathesis‐active Grubbs‐type catalysts.

Mechanistic Divergence in the Hydrogenative Synthesis of Furans and Butenolides: Ruthenium Carbenes Formed by gem‐Hydrogenation or through Carbophilic Activation of Alkynes

“…[9] In other cases such as 5-7,t he furan itself was sufficiently stable for isolation. In line with our previous investigations, [2][3][4] different propargylic substituents (for example,-OR, -OSiR 3 , -OMOM) were found to instigate gem-hydrogenation. Gratifyingly though, even substrates devoid of such directing groups led to the formation of butenolides 10 and 11; [10] in these cases,t he ester itself might serve as ad irecting group, fostering regioselective carbene formation by gem-hydrogenation.…”

“…Surprisingly,small amounts of ketone 18 were also isolated in one run from the crude mixture,w hich seems to indicate incidental oxidation of an intermediate of type G featuring the carbene site proximal to the directing -OMe substituent. Such ar egiochemical course violates the mechanistic rationale outlined above for the ester series and is inconsistent with all other available information on alkyne gem-hydrogenation, [1][2][3][4] not least with the X-ray structure shown in Figure 1. Formation of G might be…”

“…Thes ubtlety of the transformations described above is deemed remarkable:hydrogenation of enynes carrying atethered carbonyl group with the help of [Cp*RuCl] 4 as the catalyst invariably affords highly substituted furan products (Scheme 6). Depending on the nucleophilicity of the carbonyl substituent, however, the reaction takes place along two distinctly different pathways,e ach of which involves pianostool ruthenium carbenes as the key intermediates.T hese reactive species evolve,however, on the opposite ends of the central alkyne subunit of the substrate.Inthe first case,the pcomplex initially formed binds H 2 ,w hich in turn triggers the unorthodox gem-hydrogenation of the triple bond with concomitant formation of ar uthenium carbene; [1][2][3][4] hydrogenation and carbene formation are hence synchronized. Attack of the tethered ester carbonyl onto the electrophilic carbene center delivers the heterocyclic product and, at the same time,regenerates the catalyst.…”

“…Detailed spectroscopic and computational data indicate that the resulting pianostool ruthenium complexes basically exhibit aF ischercarbene character with ac ertain overtone reminiscent of Grubbs-type catalysts. [3] This view is corroborated by the fact that they participate in either intramolecular cyclopropanation or metathesis reactions,d epending on the chosen substrate. [4] Fort heir largely electrophilic nature,s uch complexes should be able to participate in various other catalytic transformations too.I ft his is the case, gem-hydrogenation might eventually evolve into an attractive alternative to diazoalkane decomposition, which is arguably the most common gateway to highly reactive late-transition metal carbenes.…”

Mechanistic Divergence in the Hydrogenative Synthesis of Furans and Butenolides: Ruthenium Carbenes Formed by gem‐Hydrogenation or through Carbophilic Activation of Alkynes

“…At the current stage of development, [Cp*RuCl] 4 is the precatalyst of choice; moreover, heteroatom substituents in vicinity of the triple bond are often necessary to render the reaction efficient. Detailed spectroscopic and computational data indicate that the resulting pianostool ruthenium complexes basically exhibit a Fischer‐carbene character with a certain overtone reminiscent of Grubbs‐type catalysts . This view is corroborated by the fact that they participate in either intramolecular cyclopropanation or metathesis reactions, depending on the chosen substrate …”