Hydrogenative Cyclopropanation and Hydrogenative Metathesis

Abstract

The unusual geminal hydrogenation of ap ropargyl alcohol derivative with [Cp X RuCl] as the catalyst entails formation of pianostool ruthenium carbenes in the first place; these reactive intermediates can be intercepted with tethered alkenes to give either cyclopropanes or cyclic olefins as the result of aformal metathesis event. The course of the reaction is critically dependent on the substitution pattern of the alkene trap.Recent investigations into the trans-hydrogenation of internal alkynes with the aid of [Cp*Ru]-based catalysts (Cp* = pentamethylcyclopentadienyl) showed that the perplexing stereochemical outcome of this reaction can be reached along two competing pathways (Scheme 1). [1][2][3][4] Theroutes bifurcate

Catalytic Asymmetric Fluorination of Copper Carbene Complexes: Preparative Advances and a Mechanistic Rationale

“…In stark contrast, attack from the bottom face shows an activation barrier of no less than 12.3 kcal mol −1 (see the Supporting Information). Qualitatively, this differential is rather intuitive and very well in line with the conclusions previously drawn from the solid‐state structure of electrophilic donor/acceptor dirhodium carbenes: Any incoming nucleophile, especially when charged, will avoid dipole repulsion with the lone‐pairs of the ester O‐atoms and hence preferentially follow a Bürgi–Dunitz trajectory alongside the aryl substituent of the donor/acceptor carbene …”

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.…”

“…Abstract: Enynes with at ethered carbonyl substituent are converted into substituted furan derivatives upon hydrogenation using [Cp*RuCl] 4 as the catalyst. Paradoxically,t his transformation can occur along two distinct pathways,each of which proceeds via discrete pianostool ruthenium carbenes.In the first case,h ydrogenation and carbene formation are synchronized ("gem-hydrogenation"), whereas the second pathway comprises carbene formation by carbophilic activation of the triple bond, followed by hydrogenative catalyst recycling.Representative carbene intermediates of either route were characterized by X-ray crystallography;t he structural data prove that the attacko ft he carbonyl group on the electrophilic carbene center follows aBürgi-Dunitz trajectory.…”

“…[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. [5,6] Thef oray along these lines outlined below was inspired by ar ecent publication describing an innovative entry into highly substituted furan derivatives (Scheme 1).…”

trans‐Hydroboration of Propargyl Alcohol Derivatives and Related Substrates

“…[11,[16][17][18][19][20] Since the originald iscovery of the ruthenium-catalyzed trans-hydroboration of internal alkynes, it becamec leart hat the unconventional stereochemical outcomei sb yn om eansa singularity. [21] Rather,t his remarkable addition mode is only one exemplification of am ore general reactivity manifold that is also observedi ntrans-hydrogenation, [22][23][24] trans-hydrosilylation, [25][26][27][28] trans-hydrogermylation [28,29] and trans-hydrostannation reactions. [30,31] All of these transformations can be effected by cationic complexes such as [Cp*Ru(MeCN) 3 ]PF 6 as well as by neutral, chloride-containingp recatalysts such as [Cp*RuCl] 4 .…”