Hydration of Aliphatic Nitriles Catalyzed by an Osmium Polyhydride: Evidence for an Alternative Mechanism

Hydration of Nitriles Catalyzed by Ruthenium Complexes: Role of Dihydrogen Bonding Interactions in Promoting Base-Free Catalysis

Ru(II) complexes of amide-phosphine-based tridentate ligands additionally containing pyridine, isoquinoline, and quinoline rings have been synthesized, and their catalytic utility for the selective hydration of nitriles to amides is explored under the base-containing as well as base-free conditions. The chloride-ligated complexes 1-3 exhibited significant catalytic activity in the presence of a base, whereas hydride-ligated complexes 4-6 carried out the hydration of nitrile without the requirement of any base. The mechanistic studies revealed the involvement of [Ru-H] species as the active catalyst in the catalytic cycle. The [Ru-H] species assisted in the polarization of an incoming water molecule through [Ru-H···H-OH] dihydrogen bonding interaction and consequently aided in the attack of a positioned water molecule to a nitrile coordinated to a ruthenium center. Substrate binding studies and kinetic experiments further supported the mechanism. A wide variety of aromatic nitriles containing both electron-withdrawing and electron-releasing groups as well as other substrates including aliphatic nitriles, base-sensitive nitriles, and a few biologically relevant nitriles were employed for the selective hydration.

Reactions of an Osmium-Hexahydride Complex with 2-Butyne and 3-Hexyne and Their Performance in the Migratory Hydroboration of Aliphatic Internal Alkynes

Reactions of the hexahydride OsH₆(PⁱPr₃)₂ (1) with 2-butyne and 3-hexyne and the behavior of the resulting species toward pinacolborane (pinBH) have been investigated in the search for new hydroboration processes. Complex 1 reacts with 2-butyne to give 1-butene and the osmacyclopropene OsH₂(η²-C₂Me₂)(PⁱPr₃)₂ (2). In toluene, at 80 °C, the coordinated hydrocarbon isomerizes into a η⁴-butenediyl form to afford OsH₂(η⁴-CH₂CHCHCH₂)(PⁱPr₃)₂ (3). Isotopic labeling experiments indicate that the isomerization involves Me-to-C_Os hydrogen 1,2-shifts, which take place through the metal. The reaction of 1 with 3-hexyne gives 1-hexene and OsH₂(η²-C₂Et₂)(PⁱPr₃)₂ (4). Similarly to 2, complex 4 evolves to η⁴-butenediyl derivatives OsH₂(η⁴-CH₂CHCHCHEt)(PⁱPr₃)₂ (5) and OsH₂(η⁴-MeCHCHCHCHMe)(PⁱPr₃)₂ (6). In the presence of pinBH, complex 2 generates 2-pinacolboryl-1-butene and OsH{κ²-H,H-(H₂Bpin)}(η²-HBpin)(PⁱPr₃)₂ (7). According to the formation of the borylated olefin, complex 2 is a catalyst precursor for the migratory hydroboration of 2-butyne and 3-hexyne to 2-pinacolboryl-1-butene and 4-pinacolboryl-1-hexene. During the hydroboration, complex 7 is the main osmium species. The hexahydride 1 also acts as a catalyst precursor, but it requires an induction period that causes the loss of 2 equiv of alkyne per equiv of osmium.

Metathesis between E-C(spn ) and H-C(sp3 ) σ-Bonds (E=Si, Ge; n=2, 3) on an Osmium-Polyhydride

The silylation of a phosphine of OsH6 (Pi Pr3 )2 is performed via net-metathesis between Si-C(spn ) and H-C(sp3 ) σ-bonds (n=2, 3). Complex OsH6 (Pi Pr3 )2 activates the Si-H bond of Et3 SiH and Ph3 SiH to give OsH5 (SiR3 )(Pi Pr3 )2 , which yield OsH4 {κ1 -P,η2 -SiH-[i Pr2 PCH(Me)CH2 SiR2 H]}(Pi Pr3 ) and R-H (R=Et, Ph), by displacement of a silyl substituent with a methyl group of a phosphine. Such displacement is a first-order process, with activation entropy consistent with a rate determining step occurring via a highly ordered transition state. It displays selectivity, releasing the hydrocarbon resulting from the rupture of the weakest Si-substituent bond, when the silyl ligand bears different substituents. Accordingly, reactions of OsH6 (Pi Pr3 )2 with dimethylphenylsilane, and 1,1,1,3,5,5,5-heptamethyltrisiloxane afford OsH5 (SiR2 R')(Pi Pr3 )2 , which evolve into OsH4 {κ1 -P,η2 -GeH-[i Pr2 PCH(Me)CH2 SiR2 H]}(Pi Pr3 ) (R=Me, OSiMe3 ) and R'-H (R'=Ph, Me). Exchange reaction is extended to Et3 GeH. The latter reacts with OsH6 (Pi Pr3 )2 to give OsH5 (GeEt3 )(Pi Pr3 )2 , which loses ethane to form OsH4 {κ1 -P,η2 -GeH-[i Pr2 PCH(Me)CH2 GeEt2 H]}(Pi Pr3 ).

Homogeneous catalysis with polyhydride complexes

This review analyzes the role of transition metal polyhydrides as homogeneous catalysts for organic reactions. Discussed reactions involve nearly every main organic functional group.

Dissimilarity in the Chemical Behavior of Osmaoxazolium Salts and Osmaoxazoles: Two Different Aromatic Metalladiheterocycles

The preparation of aromatic hydride-osmaoxazolium and hydride-oxazole compounds is reported and their reactivity toward phenylacetylene investigated. Complex [OsH(OH)(≡CPh)(IPr)(PiPr3)]OTf (1; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolylidene, OTf = CF3SO3) reacts with acetonitrile and benzonitrile to give [OsH{κ2-C,O-[C(Ph)NHC(R)O]}(NCR)(IPr)(PiPr3)]OTf (R = Me (2), Ph (3)) via amidate intermediates, which are generated by addition of the hydroxide ligand to the nitrile. In agreement with this, the addition of 2-phenylacetamide to acetonitrile solutions of 1 gives [OsH{κ2-C,O-[C(Ph)NHC(CH2Ph)O]}(NCCH3)(IPr)(PiPr3)]OTf (4). The deprotonation of the osmaoxazolium ring of 2 and 4 leads to the oxazole derivatives OsH{κ2-C,O-[C(Ph)NC(R)O]}(IPr)(PiPr3) (R = Me (5), CH2Ph (6)). Complexes 2 and 4 add their Os-H and Os-C bonds to the C-C triple bond of phenylacetylene to afford [Os{η3-C 3 ,κ1-O-[CH2C(Ph)C(Ph)NHC(R)O]}(NCCH3)2(IPr)]OTf (R = Me (7), CH2Ph (8)), bearing a tridentate amide-N-functionalized allyl ligand, while complexes 5 and 6 undergo a vicarious nucleophilic substitution of the hydride at the metal center with the alkyne, via the compressed dihydride adduct intermediates OsH2(C≡CPh){κ2-C,O-[C(Ph)NC(R)O]}(IPr)(PiPr3) (R = Me (9), CH2Ph (10)), which reductively eliminate H2 to yield the acetylide-osmaoxazoles Os(C≡CPh){κ2-C,O-[C(Ph)NC(R)O]}(IPr)(PiPr3) (R = Me (11), CH2Ph (12)).

Azolium Control of the Osmium-Promoted Aromatic C-H Bond Activation in 1,3-Disubstituted Substrates

The hexahydride complex OsH₆(PⁱPr₃)₂ promotes the C-H bond activation of the 1,3-disubstituted phenyl group of the [BF₄]^- and [BPh₄]^- salts of the cations 1-(3-(isoquinolin-1-yl)phenyl)-3-methylimidazolium and 1-(3-(isoquinolin-1-yl)phenyl)-3-methylbenzimidazolium. The reactions selectively afford neutral and cationic trihydride-osmium(IV) derivatives bearing κ²-C,N- or κ²-C,C-chelating ligands, a cationic dihydride-osmium(IV) complex stabilized by a κ³-C,C,N-pincer group, and a bimetallic hexahydride formed by two trihydride-osmium(IV) fragments. The metal centers of the hexahydride are separated by a bridging ligand, composed of κ²-C,N- and κ²-C,C-chelating moieties, which allows electronic communication between the metal centers. The wide variety of obtained compounds and the high selectivity observed in their formation is a consequence of the main role of the azolium group during the activation and of the existence of significant differences in behavior between the azolium groups. The azolium role is governed by the anion of the salt, whereas the azolium behavior depends upon its imidazolium or benzimidazolium nature. While [BF₄]^- inhibits the azolium reactions, [BPh₄]^- favors the azolium participation in the activation process. In contrast to benzimidazolylidene, the imidazolylidene resulting from the deprotonation of the imidazolium substituent coordinates in an abnormal fashion to direct the phenyl C-H bond activation to the 2-position. The hydride ligands of the cationic dihydride-osmium(IV) pincer complex display intense quantum mechanical exchange coupling. Furthermore, this salt is a red phosphorescent emitter upon photoexcitation and displays a noticeable catalytic activity for the dehydrogenation of 1-phenylethanol to acetophenone and of 1,2-phenylenedimethanol to 1-isobenzofuranone. The bimetallic hexahydride shows catalytic synergism between the metals, in the dehydrogenation of 1,2,3,4-tetrahydroisoquinoline and alcohols.

Alternative Conceptual Approach to the Design of Bifunctional Catalysts: An Osmium Germylene System for the Dehydrogenation of Formic Acid

The reaction of the hexahydride OsH₆(PⁱPr₃)₂ with a P,Ge,P-germylene-diphosphine affords an osmium tetrahydride derivative bearing a Ge,P-chelate, which arises from the hydrogenolysis of a P–C(sp³) bond. This Os(IV)–Ge(II) compound is a pioneering example of a bifunctional catalyst based on the coordination of a σ-donor acid, which is active in the dehydrogenation of formic acid to H₂ and CO₂. The kinetics of the dehydrogenation, the characterization of the resting state of the catalysis, and DFT calculations point out that the hydrogen formation (the fast stage) exclusively occurs on the coordination sphere of the basic metal center, whereas both the metal center and the σ-donor Lewis acid cooperatively participate in the CO₂ release (the rate-determining step). During the process, the formate group pivots around the germanium to approach its hydrogen atom to the osmium center, which allows its transfer to the metal and the CO₂ release.

An alternative class of bifunctional catalysts can be assembled by coordination of σ-donor Lewis acids to platinum-group-metal basic fragments. In contrast to what happens with the previously reported bifunctional catalysts, this design allows enhancing the basicity of the base and the acidity of the acid. According to this, a bifunctional catalyst for the dehydrogenation of formic acid, based on an osmium(IV)-germylene cooperative system, has been prepared and the mechanism of the catalysis established.