Light‐Driven gem Hydrogenation: An Orthogonal Entry into “Second‐Generation” Ruthenium Carbene Catalysts for Olefin Metathesis

Abstract

The newly discovered light‐driven gem hydrogenation of alkynes opens an unconventional yet efficient entry into five‐coordinate Grubbs‐type ruthenium carbene complexes with cis‐disposed chloride ligands. Representatives of this class featuring a chelate substructure formed by an iodo‐substituted benzylidene unit react with (substituted) 2‐isopropoxystyrene to give prototypical “second‐generation” Grubbs‐Hoveyda complexes for olefin metathesis. The new approach to this venerable catalyst family is safe and versatile as it uses a triple bond rather than phenyldiazomethane as the ultimate carbene source and does not require any sacrificial phosphines.

An Anionic Dinuclear Ruthenium Dihydrogen Complex of Relevance for Alkynegem‐Hydrogenation

“…Initially, however, the yield of complex 3 a was rather low, likely because of the clash between the ethyl substituent on the incipient carbene ligand and the N‐aryl group of the NHC (Scheme 2A) [11] . Therefore, an indirect route was pursued, in which the actual gem ‐hydrogenation step affords the iodo‐chelate complex 4 in the first place, wherein the ruthenacyclic array is turned by 90 °C to prevent any such interligand repulsion; indeed, this reaction proved much more efficient (Scheme 2B) [13] . Subsequent cross metathesis of 4 with styrenes of type 5 leads to prototype Hoveyda‐Grubbs‐type catalysts 3 b , c in good overall yield [12, 14] …”

“…[11] Therefore, an indirect route was pursued, in which the actual gem-hydrogenation step affords the iodochelate complex 4 in the first place, wherein the ruthenacyclic array is turned by 90 °C to prevent any such interligand repulsion; indeed, this reaction proved much more efficient (Scheme 2B). [13] Subsequent cross metathesis of 4 with styrenes of type 5 leads to prototype Hoveyda-Grubbs-type catalysts 3 b, c in good overall yield. [12,14] Despite this significant advance, it seemed equally relevant to investigate the fate of the ruthenium, because a better understanding for potentially competing processes might also help to improve the reaction.…”

“…A high oxidation state is also clearly helpful, and first row transition metals are more likely to form σcomplexes than their higher homologues. Conversely, anionic σ-complexes were unknown until very recently and continue to be exceedingly rare (Figure 6): except for 10, [41] which has only been observed in an Ar-matrix, the few wellcharacterized examples (11)(12)(13) are distinguished by the presence of Lewis-acidic metallo-ligands that exert an "inverse trans-influence" and hence deprive the anionic…”

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

“… The conversion of the receiving C atom into a methylene group is accompanied by the formation of a discrete metal carbene at the adjacent position. For the time being, such “ gem -hydrogention” reactions have been accomplished with [Cp X RuCl] (or closely related metal fragments) , as well as with [NHC­(η 6 -cymene)­RuCl 2 ]; this latter system is photochemically driven and opens an unconventional entry into second-generation Grubbs-type catalysts. , The use of [Cp X RuCl], in contrast, provides access to piano-stool ruthenium carbene complexes, the electrophilic character of which manifests itself in multifarious reactivity (Scheme ): so far, it could be harnessed in the form of hydrogenative cyclopropanation, , hydrogenative metathesis, , hydrogenative heterocycle syntheses, , hydrogenative ring expansion reactions, and hydrogenative rearrangements, − all of which are conceptually new types of transformations; some of them are even counterintuitive when judged on the basis of conventional chemical logic…”

Hydrogenative Cycloisomerization and Sigmatropic Rearrangement Reactions of Cationic Ruthenium Carbenes Formed by Catalytic Alkyne gem‐Hydrogenation

“…[ 9 , 10 ] A second system employing [(NHC)(η 6 ‐cymene)MCl 2 ] (M=Ru, Os; NHC=N‐heterocyclic carbene) is photochemically driven; it opens an unconventional hydrogenative entry into “second generation” Grubbs carbenes for use in olefin metathesis. [ 11 , 12 ]…”