Cp*RuCl‐Vinyl Carbenes: Two Faces and the Bifunctional Role in Catalytic Processes

Abstract

Ruthenium vinyl carbenes derived from Cp/Cp*RuCl‐based complexes (Cp=cyclopentadiene, Cp*=1,2,3,4,5‐pentamethylcyclopentadiene) have been routinely invoked as key intermediates in tandem reactions involving a carbene/alkyne metathesis (CAM). A priori, these intermediates resemble the Grubbs‐type family of catalysts, but they exhibit a completely different reactivity pattern that few, if any, other catalytic system can reproduce so far. The reactivity of these species with α‐unsubstituted and α‐substituted alkynals showcases the peculiarities of these intermediates. Although Z‐vinyl dihydrooxazines are preferentially obtained with the former, Z‐vinyl epoxypyrrolidines are obtained with the latter. A combination of spectroscopic and computational data now prove that a η3‐coordination mode of the ruthenium vinyl carbene and the presence of a Lewis basic chloride ligand give rise to two markedly different stereoelectronic faces, which are responsible for the unconventional reactivity of these species.

C–H Insertion via Ruthenium Catalyzedgem-Hydrogenation of 1,3-Enynes

“…78 Once again, the conclusions drawn from our experimental data tally well with the computational results of Saáand coworkers, who suggested that the vinylcarbene species formed by CAM evolve via a "hydride transfer" mechanism. 19,20 In addition, our own computations of the C−H insertion pathway populated by ruthenium α-oxocarbene complexes such as D (Scheme 2) showed a very obtuse angle between the carbene center and the incoming reaction partner. 81 ■ CONCLUSIONS gem-Hydrogenation is a conceptually novel mode of H 2 transfer to an organic substrate, which our group was able to discover after a century of intense research devoted to catalytic hydrogenation in innumerous academic as well as industrial laboratories.…”

“…These experimental and spectroscopic data concur with the conclusions drawn by Saá and co-workers, who studied the fate of ruthenium vinylcarbene intermediates generated by CAM in silico. , These authors suggested that the η 3 -bound isomer accounts for the downstream chemistry; it draws its higher reactivity from an out-of-plane distortion that causes an electronic perturbation and, at the same time, renders the site sterically more exposed.…”

“…Once again, the conclusions drawn from our experimental data tally well with the computational results of Saá and co-workers, who suggested that the vinylcarbene species formed by CAM evolve via a “hydride transfer” mechanism. , In addition, our own computations of the C–H insertion pathway populated by ruthenium α-oxocarbene complexes such as D (Scheme ) showed a very obtuse angle between the carbene center and the incoming reaction partner…”

“…Although these preliminary data spoke for a narrow window of opportunity, they sparked our interest. Additional motivation was drawn from a literature survey, which showed that certain piano-stool ruthenium carbenes generated by an entirely different route, namely, carbene/alkyne metathesis (CAM), − are capable of inserting into secondary or tertiary C–H bonds of suitably disposed acetals or ethers in moderate to good yields (Scheme B); the reaction was carefully studied by computational means. − If one were able to generate the presumed vinylcarbenes of type E by gem -hydrogenation, one could avoid the use of hazardous diazoalkanes altogether . At the same time, it might be possible to enlarge the scope of the reaction to a considerable extent, since, in practice, the CAM-based route had worked well only for the addition of trimethylsilyl diazomethane to terminal alkynes (Scheme B); , a hydrogenative approach should not face such limitations.…”

Intramolecular Interception of the Remote Position of Vinylcarbene Silver Complex Intermediates by C(sp3)−H Bond Insertion

“…May [18] described the isolation and mechanistic relevance of ring-fused cyclopropenes with rhodium catalysts (path I). Some of us [12a] postulated the formation of a rhodium(I) η 3vinylcarbene intermediate through a [ π 2 s + π 2 a ] addition that subsequently evolves to the rhodium(I) η 1 -vinylcarbene, a proposal also shared by Saá [19] in their intermolecular Rucatalyzed CAM cascade processes (path II, Scheme 4). Yu and Xu [20] proposed a 5-exo-dig cyclization leading to a very reactive zwitterionic vinyl cationic species that upon a Rh-1,3-shift converts to the rhodium(II) η 1 -vinylcarbene (path III).…”