Nickel-Catalyzed Reductive Hydroesterification of Styrenes Using CO₂ and MeOH

Complexes [(dippe)­Ni­(μ-H)]₂ (<b>A</b>) (dippe = 1,2-bis-di-isopropylphosphino)­ethane) and [(dtbpe)­Ni­(μ-H)]₂ (<b>B</b>) (dtbpe = 1,2-bis-di-<i>tert</i>-butylphospino)­ethane) catalyze the reductive hydroesterification of styrenes with the use of CO₂ and MeOH. The latter acts as a hydrogen source and as an esterificating agent, to yield the corresponding branched and linear esters in moderate to good yields. In all of the studied reactions the linear esters were obtained in higher amounts than the branched ones. When the hydroesterification reaction was carried out using a stoichiometric metal/substrate ratio, the complexes [(P–P)­Ni­(CO)₂] and [(P–P)­Ni­(CO₃)] (P–P = dippe or dtbpe) were isolated and characterized by standard spectroscopic methods. Compounds [(dtbpe)­Ni­(CO)₂] and [(dtbpe)­Ni­(CO₃)] were also fully characterized by single-crystal X-ray diffraction

Nickel-Catalyzed Alkylation and Transfer Hydrogenation of α,β-Unsaturated Enones with Methanol

Complexes of the type [{(dippe)­Ni}_<i>n</i>(η²-C_α,C_β-1,4-dien-3-one)] (dippe = 1,2-bis­(diisopropylphosphino)­ethane); <i>n</i>= 1, 2; enone = aromatic 1,4-pentadien-3-ones) were synthesized. The “[(dippe)­Ni]” moiety derived from [(dippe)­Ni­(μ-H)]₂ η²-coordinated to the C,C double bonds of the corresponding α,β-unsaturated enone and was fully characterized using a variety of spectroscopic techniques, for instance, single-crystal X-ray diffraction, nuclear magnetic resonance (NMR), and mass spectrometry. The complexes were assessed in a catalytic transfer hydrogenation process using methanol (CH₃OH) as a hydrogen donor. This alcohol turned out to be a very efficient reducing and alkylating agent of 1,4-pentadien-3-ones, under neat conditions. The current methodology allowed the selective reduction of CC bonds in α,β-unsaturated enones to yield enones and saturated ketones by a homogeneous catalytic pathway, whereas by a heterogeneous pathway, the process leads to the formation of mono- and dimethylated ketones. In the latter case, the occurrence of nickel nanoparticles in the reaction media was found to participate in the catalytic alkylation of such dienones

Nickel-Catalyzed Alkylation and Transfer Hydrogenation of α,β-Unsaturated Enones with Methanol

Complexes of the type [{(dippe)­Ni}_<i>n</i>(η²-C_α,C_β-1,4-dien-3-one)] (dippe = 1,2-bis­(diisopropylphosphino)­ethane); <i>n</i>= 1, 2; enone = aromatic 1,4-pentadien-3-ones) were synthesized. The “[(dippe)­Ni]” moiety derived from [(dippe)­Ni­(μ-H)]₂ η²-coordinated to the C,C double bonds of the corresponding α,β-unsaturated enone and was fully characterized using a variety of spectroscopic techniques, for instance, single-crystal X-ray diffraction, nuclear magnetic resonance (NMR), and mass spectrometry. The complexes were assessed in a catalytic transfer hydrogenation process using methanol (CH₃OH) as a hydrogen donor. This alcohol turned out to be a very efficient reducing and alkylating agent of 1,4-pentadien-3-ones, under neat conditions. The current methodology allowed the selective reduction of CC bonds in α,β-unsaturated enones to yield enones and saturated ketones by a homogeneous catalytic pathway, whereas by a heterogeneous pathway, the process leads to the formation of mono- and dimethylated ketones. In the latter case, the occurrence of nickel nanoparticles in the reaction media was found to participate in the catalytic alkylation of such dienones

Selective <i>N</i>‑Methylation of Aliphatic Amines with CO₂ and Hydrosilanes Using Nickel-Phosphine Catalysts

A method using CO₂ and PhSiH₃ for the methylation of primary and secondary aliphatic amines catalyzed by Ni (0) complexes was developed, selectively producing the monomethylated products in moderate to good yields. For that purpose, two catalysts were used: [(dippe)­Ni­(μ-H)]₂ and the commercially available Ni­(COD)₂/dcype, both of which were rather efficient in this process. With a slight experimental modification, the reaction allowed the production of monomethylated ureas in good yields by using low amounts of PhSiH₃. On the basis of the experimental results, we propose a possible reaction mechanism for the formation of the new C–N bond

Nickel-Catalyzed Hydrosilylation of CO₂ in the Presence of Et₃B for the Synthesis of Formic Acid and Related Formates

The reaction of CO₂ with Et₃SiH catalyzed by the nickel complex [(dippe)­Ni­(μ-H)]₂ (<b>1</b>) afforded the reduction products Et₃SiOCH₂OSiEt₃ (12%), Et₃SiOCH₃ (3%), and CO, which were characterized by standard spectroscopic methods. Part of the generated CO was found as the complex [(dippe)­Ni­(CO)]₂ (<b>2</b>), which was characterized by single-crystal X-ray diffraction. When the same reaction was carried out in the presence of a Lewis acid, such as Et₃B, the hydrosilylation of CO₂ efficiently proceeded to give the silyl formate (Et₃SiOC­(O)­H) in high yields (85–89%), at 80 °C for 1 h. Further reactivity of the silyl formate to yield formic acid, formamides, and alkyl formates was also investigated

Hydrogenation of Biomass-Derived Levulinic Acid into γ‑Valerolactone Catalyzed by Palladium Complexes

The selective catalytic hydrogenation and cyclization of levulinic acid (LA) into valuable γ-valerolactone (GVL) catalyzed by different palladium compounds was achieved in water under mild conditions with high yields. Either formic acid (FA) or molecular hydrogen (H₂) was used as a hydrogen source. The precatalyst [(dtbpe)­PdCl₂] (dtbpe = 1,2-(bis-di-<i>tert</i>-butylphosphino)­ethane) (<b>1</b>) was highly active in the processes of LA hydrogenation (TON of 2100 and TOF of 2100 h^–1) and in the dehydrogenation of formic acid to produce H₂ and carbon dioxide. The catalytically active complexes [(dtbpe)­Pd­(H)­Cl)] (<b>2</b>) and [(dtbpe)₂Pd₂(μ-H)₃]⁺ (<b>3</b>) and the catalytically inactive complex [(dtbpe)₂Pd₂(μ-H) (μ-CO)]⁺ (<b>4</b>) all formed in situ and were identified as species resulting from FA decomposition

Mechanistic Insights into the C–S Bond Breaking in Dibenzothiophene Sulfones

The reactivity of Grignard reagents in the presence of nickel catalysts is known to be highly efficient in the deoxydesulfurization of dibenzothiophene sulfone (DBTO₂), 4-methyldibenzothiophene (4-MeDBTO₂), and 4,6-dimethyldibenzothiophene (4,6-Me₂DBTO₂), to yield sulfur-free biphenyls via cross-coupling reactions. However, the mechanistic details involved in the process remained unknown. In this report the reactivity of [(dippe)­Pt­(μ-H)]₂ with DBTO₂ turned out to be catalytically less efficient compared with [(dippe)­Ni­(μ-H)]₂, but the first allowed the isolation and full characterization of several reaction intermediates, such as [(dippe)­Pt­(κ²-<i>C</i>,<i>S</i>-DBTO₂)]. It was demonstrated that this is a key intermediate in all the deoxydesulfurization reactions of the above-mentioned aromatic sulfones (DBTsO₂)

Mechanistic Insights into the C–S Bond Breaking in Dibenzothiophene Sulfones

The reactivity of Grignard reagents in the presence of nickel catalysts is known to be highly efficient in the deoxydesulfurization of dibenzothiophene sulfone (DBTO₂), 4-methyldibenzothiophene (4-MeDBTO₂), and 4,6-dimethyldibenzothiophene (4,6-Me₂DBTO₂), to yield sulfur-free biphenyls via cross-coupling reactions. However, the mechanistic details involved in the process remained unknown. In this report the reactivity of [(dippe)­Pt­(μ-H)]₂ with DBTO₂ turned out to be catalytically less efficient compared with [(dippe)­Ni­(μ-H)]₂, but the first allowed the isolation and full characterization of several reaction intermediates, such as [(dippe)­Pt­(κ²-<i>C</i>,<i>S</i>-DBTO₂)]. It was demonstrated that this is a key intermediate in all the deoxydesulfurization reactions of the above-mentioned aromatic sulfones (DBTsO₂)

Selective CO Reduction in Phthalimide with Nickel(0) Compounds

The catalytic reduction of phthalimide was achieved using nickel catalysts. The use of catalytic amounts (20% mol) of [Ni­(COD)₂] or [(dippe)­Ni­(μ-H)]₂ (<b>1</b>) allowed the monoreduction of phthalimide to yield isoindolinone and benzamide, at 140–180 °C and 750 psi of H₂. When the N–H moiety of phthalimide was protected with a trimethylsilyl group, both CO groups were reduced to yield (trimethylsilyl)­isoindoline. However, when a methyl moiety was used as the protecting group, the CO groups and the aromatic ring were reduced, using rather similar reaction conditions, due to the formation of 5 Å (average) nickel nanoparticles

On the Catalytic Hydrodefluorination of Fluoroaromatics Using Nickel Complexes: The True Role of the Phosphine

Homogeneous catalytic hydrodefluorination (HDF) of fluoroaromatics under thermal conditions was achieved using nickel(0) compounds of the type [(dippe)­Ni­(η²-C₆F_6‑<i>n</i>H_<i>n</i>)] where <i>n</i> = 0–2, as the catalytic precursors. These complexes were prepared <i>in situ</i> by reacting the compound [(dippe)­Ni­(μ-H)]₂ with the respective fluoroaromatic substrate. HDF seems to occur homogeneously, as tested by mercury drop experiments, producing the hydrodefluorinated products. However, despite previous findings by other groups, we found that these HDF reactions were actually the result of direct reaction of the alkylphosphine with the fluoroaromatic substrate. This metal- and silane-free system is the first reported example of a phosphine being able to hydrodefluorinate on its own