Acid-Promoted Homogeneous Hydrogenation of Alkenes Catalyzed by the Ruthenium−Hydride Complex (PCy3)2(CO)(Cl)RuH: Evidence for the Formation of 14-Electron Species from the Selective Entrapment of the Phosphine Ligand

Hexene hydrogenation catalysed by the complex monohydrid complexes: A DFT study of associated vs dissociated pathways

A DFT study was conducted to elucidate the mechanism of hexene hydrogenation catalysed by a series of ruthenium (II) monohydride catalysts: RuH(CO)(Cl)(LL') where L and L' represent C(cyclohexyl), Me (methyl) and IMe (N, N '-bis (mesityl) imidazole-2-ylidene). This investigation explores the feasibility of two different proposed mechanisms: the first describes the dissociated pathway and exploits a single phosphine complex. The second is the associative one and uses a two phosphines complex. The detailed pathways have been explored for the catalyst model with L = L' = Me. Three possibilities have been supported for the dissociative route. Pathway (A) begins with a phosphine release. The initial addition of hexane or a dihydrogen molecule on the ruthenium catalyst generates the pathways (B) and (C), respectively. Pathways (B) and (C) merge with the pathway (A) before and after the first proton transfer, respectively. Activation energies in the first hydrogen migration (the key-step of the mechanism) are close. Therefore, both mechanisms (A) and (B) are possible but the former is more probable. The substitution of the catalyst model RuHCl(CO)(PMe₃)₂ by the real catalysts RuHCl(CO)(PCy₃)₂ or RuHCl(CO)(IMes)(PMe₃) shows no significant influence on the energetic barriers of hexene hydrogenation mechanism. The energy profile of the first hydrogen migration for the catalyst RuHCl(CO)(PCy₃)₂ is characteristic of a concerted asynchronous mechanism while our calculation led to two separated synchronous steps when the model catalyst is used. The associative pathway (D) integrates the two experimentally detected intermediates and generates activation energies close to those of dissociative pathways (A) and (B). The rationale to explain the experimentally detected species is achieved by considering the four proposed mechanisms where they occur simultaneously and with different rates (ie. The dissociative mechanism has the highest rate).

CNN pincer ruthenium complexes for efficient transfer hydrogenation of biomass-derived carbonyl compounds

The ligand HCNNOMe (6-(4-methoxyphenyl)-2-aminomethylpyridine) is easily prepared from the commercially available 6-(4-methoxyphenyl)pyridine-2-carbaldehyde by the reaction of hydroxylamine and hydrogenation (H2, 1 atm) with Pd/C. The pincer complexes cis-[RuCl(CNNOMe)(PPh3)2] (1) and [RuCl(CNNOMe)(PP)] (PP = dppb, 2; and dppf, 3) are synthesized from [RuCl2(PPh3)3], HCNNOMe and PP (for 2 and 3) in 2-propanol with NEt3 at reflux and are isolated in 85-93% yield. Carbonylation of 1 (CO, 1 atm) gives [RuCl(CNNOMe)(CO)(PPh3)] (4) (79% yield) which cleanly reacts with Na[BArf4] and PCy3, affording the cationic trans-[Ru(CNNOMe)(CO)(PCy3)(PPh3)][BArf4] (5) (92% yield). These robust pincer complexes display remarkably high catalytic activity in the transfer hydrogenation (TH) of lignocellulosic biomass carbonyl compounds, using 2-propanol at reflux in a basic medium (NaOiPr or K2CO3). Thus, furfural, 5-(hydroxymethyl)furfural and Cyrene are reduced to the corresponding alcohols with 2 and 3, at S/C in the range of 10 000-100 000, within minutes or hours (TOF up to 1 500 000 h-1). The monocarbonyl complex 5 was found to be extremely active in the TH of cinnamaldehyde, vanillin derivatives and ethyl levulinate at S/C in the range of 10 000-50 000. Vanillyl alcohol is also obtained by the TH of vanillin with 5 (S/C = 500) in 2-propanol in the presence of K2CO3.

Acid promoted Ir–P^N complex catalyzed hydrogenation of heavily hindered 3,4-diphenyl-1,2-dihydronaphthalenes: asymmetric synthesis of lasofoxifene tartrate

An asymmetric hydrogenation of minimally functionalized tetrasubstituted cyclic olefins has been developed using an Ir–P^N catalyst and an acid additive enabling the synthesis of lasofoxifene tartrate.

Dihydrobis(methimazolyl)borato complexes of ruthenium and osmium

The reaction of Na[H₂B(mt)₂] (mt = 2-mercapto, 3-methylimidazol-1-yl, methimazolyl) with [Ru(X)Cl(CO)(PPh₃)_n] (n= 3 X = H;n= 2 BO₂C₆H₄, SiCl₃, SiMe₃) affords the complexes [Ru(X)(CO)(PPh₃){κ²-H,S,S′-H₂B(mt)₂}].

Regioselective Hydrohydroxyalkylation of Styrene with Primary Alcohols or Aldehydes via Ruthenium-Catalyzed C-C Bond Forming Transfer Hydrogenation

Transfer hydrogenative coupling of styrene with primary alcohols using the precatalyst HClRu(CO)(PCy3 )2 modified by AgOTf or HBF4 delivers branched or linear adducts from benzylic or aliphatic alcohols, respectively. Related 2-propanol mediated reductive couplings also are described.

Cyclic bent allene hydrido-carbonyl complexes of ruthenium: highly active catalysts for hydrogenation of olefins

A new family of ruthenium complexes bearing the carbodicarbene-type ligand "cyclic bent allene" (CBA) have been synthesized from the common precursor RuHCl(CO)(PPh3)3. Complexes were evaluated for catalytic activity in the room-temperature hydrogenation of unactivated olefins and were found to be significantly more active than known ruthenium hydrido-carbonyl phosphine or NHC complexes. In particular, RuH(OSO2CF3)(CO)(SIMes)(CBA) was found to be among the most active hydrogenation catalysts, achieving comparable activity to Crabtree's catalyst in the hydrogenation of unactivated trisubstituted olefins and superior activity in the hydrogenation of styrene derivatives in side-by-side catalytic runs. RuH(OSO2CF3)(CO)(SIMes)(CBA) was also found to be highly active in olefin selective hydrogenation in the presence of a variety of unsaturated functional groups, and can achieve exceptional diastereoselectivity in functional-group-directed hydrogenations at very low catalyst loadings.

[Cp*Ru]-catalyzed selective coupling/hydrogenation

Selective coupling and hydrogenation catalyzed by [Cp*Ru] have been achieved affording valuable polyfunctionalized cyclic enamide derivatives.

Regioselective Intermolecular Coupling Reaction of Arylketones and Alkenes Involving C-H Bond Activation Catalyzed by an In-Situ Formed Cationic Ruthenium-Hydride Complex

The cationic ruthenium-hydride complex, formed in-situ from the treatment of the tetranuclear ruthenium-hydride complex {[(PCy(3))(CO)RuH](4)(mu(4)-O)(mu(3)-OH)(mu(2)-OH)} with HBF(4).OEt(2), was found to be a highly effective catalyst for the intermolecular coupling reaction of arylketones and 1-alkenes to give the substituted indene and ortho-C-H insertion products. The formation of the indene products was resulted from the initial alkene isomerization followed by regioselective ortho-C-H insertion of 2-alkene and the dehydrative cyclization. The preliminary mechanistic studies revealed a rapid and reversible ortho-C-H bond activation followed by the rate-limiting C-C bond formation step for the coupling reaction.

Dihydrogen complexes of rhodium: [RhH2(H2)x (PR3)2]+ (R = Cy, iPr; x = 1, 2)

Addition of H2 (4 atm at 298 K) to [Rh(nbd)(PR3)2][BAr(F)4] [R = Cy, iPr] affords Rh(III) dihydride/dihydrogen complexes. For R = Cy, complex 1a results, which has been shown by low-temperature NMR experiments to be the bis-dihydrogen/bis-hydride complex [Rh(H)2(eta2-H2)2(PCy3)2][BAr(F)4]. An X-ray diffraction study on 1a confirmed the {Rh(PCy3)2} core structure, but due to a poor data set, the hydrogen ligands were not located. DFT calculations at the B3LYP/DZVP level support the formulation as a Rh(III) dihydride/dihydrogen complex with cis hydride ligands. For R = iPr, the equivalent species, [Rh(H)2(eta2-H2)2(P iPr3)2][BAr(F)4] 2a, is formed, along with another complex that was spectroscopically identified as the mono-dihydrogen, bis-hydride solvent complex [Rh(H)2(eta2-H2)(CD2Cl2)(P iPr3)2][BAr(F)4] 2b. The analogous complex with PCy3 ligands, [Rh(H)2(eta2-H2)(CD2Cl2)(PCy3)2][BAr(F)4] 1b, can be observed by reducing the H2 pressure to 2 atm (at 298 K). Under vacuum, the dihydrogen ligands are lost in these complexes to form the spectroscopically characterized species, tentatively identified as the bis hydrides [Rh(H)2(L)2(PR3)2][BAr(F)4] (1c R = Cy; 2c R = iPr; L = CD2Cl2 or agostic interaction). Exposure of 1c or 2c to a H2 atmosphere regenerates the dihydrogen/bis-hydride complexes, while adding acetonitrile affords the bis-hydride MeCN adduct complexes [Rh(H)2(NCMe)2(PR3)2][BAr(F)4]. The dihydrogen complexes lose [HPR3][BAr(F)4] at or just above ambient temperature, suggested to be by heterolytic splitting of coordinated H2, to ultimately afford the dicationic cluster compounds of the type [Rh6(PR3)6(mu-H)12][BAr(F)4]2 in moderate yield.