Highly Electrophilic Mononuclear Cationic Aluminium Alkoxide Complexes: Syntheses, Reactivity and Catalytic Applications

Herein, we report the synthesis of β-diketiminate-supported aluminium complexes bearing terminal alkoxide and mono-thiol functional groups: LAlOMe(Et) (2), LAlOtBu(Et) (3), and LAlSH(Et) (4), (L=[HC{C(Me)N-(2,6-iPr2 C6 H3 )}2 ]). Complexes 2 and 3 are further used as synthons to generate the fascinating cationic aluminium alkoxide complexes, [LAlOMe(μ-OMe)-Al(Et)L][EtB(C6 F5 )3 ] (5), [LAlOMe(OEt2 )][EtB(C6 F5 )3 ] (6), and [LAlOtBu(OEt2 )][EtB(C6 F5 )3 ] (8). These electrophilic cationic species are well characterized by spectroscopic and crystallographic techniques. The assessment of Lewis acidity by the Gutmann-Beckett method revealed superior Lewis acidity of the cations substituted with electron-demanding alkoxy groups in comparison to the known methyl analogue [LAlMe][B(C6 F5 )4 ]. This has been further endorsed by computational calculations to determine the NBO charges and hydride ion affinity for complexes 6 and 8. These complexes are also capable of activating triethylsilane in stoichiometric reactions. The applicability of these complexes has been realized in the hydrosilylation of ethers, carbonyls, and olefines. Additionally, the solid-state structure of a new THF stabilized aluminium halide cation [LAlCl(THF)][B(C6 F5 )4 ] (11) has also been reported.

Cobalt Carbonyls Stabilized by N,P-Ligands: Synthesis, Structure, and Catalytic Property for Ethylene Oxide Hydroalkoxycarbonylation

Reactions of N,P-Ligands as Ph₂ P(o-NMe₂ C₆ H₄ ) (¹ L), 2,6-iPr₂ C₆ H₃ NHC(Ph)=NC₆ H₄ (o-PPh₂ ) (² L), and Ph₂ PN(R)PPh₂ (R=iPr (³ L), cyclo-C₆ H₁₁ (⁴ L), tBu (⁵ L), CH₂ C₄ H₇ O (⁶ L)) each with dicobalt octacarbonyl produced complexes [¹ LCo(CO)₃ ]₂ (1), [² LCo(CO)(μ-CO)₂ Co(CO)₃ ] (2), [³ LCo(CO)₃ ]⁺ [Co(CO)₄ ]^- (3), [³ LCo(CO)₂ ]₂ (4), [⁴ LCo(CO)₂ ]₂ (5), [⁵ LCo(CO)₂ ]⁺ [Co(CO)₄ ]^- (6), and [⁶ LCo(CO)₂ ]⁺ [Co(CO)₄ ]^- (7). Complexes 1-7 have all been structurally characterized by X-ray crystallography, IR and NMR spectroscopies, and elemental analysis. Catalytic tests on transformation of ethylene oxide (EO), CO and MeOH into methyl 3-hydroxypropionate (3-HMP) indicate that complexes 1-7 are active, where ion-pair complexes 3 and 6-7 behave more excellently (by achieving 88.4-93.6% 3-HMP yields) than the neutral species 1-2 and 4-5 (35.0-46.5% 3-HMP yields) when the reactions are all operated at 2 MPa CO pressure and 50 °C in MeOH solvent. Density functional theory (DFT) study by selecting 3 as a model suggests a cooperative catalytic reaction mechanism by [Co(CO)₄ ]^- and its counter cation [³ LCo(CO)₃ ]⁺ . The cobalt-homonuclear ion-pair catalyzed hydroalkoxycarbonylation of EO is present herein.

Cationic aluminum, gallium, and indium complexes in catalysis

The introduction of cationic charge allows cationic group 13 complexes to be excellent Lewis acid catalysts. Cationic aluminum, gallium, and indium complexes in catalysis are comprehensively reviewed based on the reaction type.

Cobalt-Catalyzed Alkoxycarbonylation of Epoxides to β-Hydroxyesters

Herein, we developed a new and practical catalytic system for the carbonylative synthesis of β-hydroxyesters. By using simple, cheap, and air-stable cobalt(II) bromide as the catalyst, combined with pyrazole and catalytic amount of manganese, active cobalt complex can be generated in situ and can catalyze various epoxides to give the corresponding β-hydroxyesters in moderate to excellent yields. Mechanism studies indicate that pyrazole plays a crucial role in this reaction. Moreover, with the addition of the catalytic amount of manganese, the active cobalt catalyst can be regenerated, which provides a possibility for reusing the cobalt catalyst.

A Metalated Porous Porphyrin Polymer with [Co(CO)4]- Anion as an Efficient Heterogeneous Catalyst for Ring Expanding Carbonylation

The synthesis of β-lactones from epoxides through ring-expanding carbonylation using homogeneous catalysts has received much attention. However, homogeneous catalysts suffer from difficulty in product separation and recycling of the catalyst, limiting their industrial usage. Herein, a novel heterogeneous catalyst, [Cr-metalated porous porphyrin polymer]⁺[Co(CO)₄]⁻, was prepared and used for the conversion of propylene oxide (PO) to β-butyrolactone; this catalyst presented superior catalytic activity and selectivity (99%) than our previous heterogeneous catalyst. In addition, the catalyst was readily separated from the product without significant loss of catalytic activity. A possible method to recover the original catalytic activity also was demonstrated.

Nucleophilic ring opening of trans-2,3-disubstituted epoxides to β-amino alcohols with catalyst-controlled regioselectivity

We report the nucleophilic ring opening of unsymmetrical trans-epoxides to β-amino alcohols with catalyst-controlled regioselectivity.

Tuning the Lewis acid phenol ortho-prenylation as a molecular diversity tool

A diversity-oriented approach for the synthesis of various structurally different prenylated alcohols from readily accessible and common precursors was developed. With varying approaches, this article describes some successful examples of a Friedel-Crafts alkylation using methoxyphenols and different prenyl alcohols (geraniol and (E,E)-farnesol). We demonstrated that just by varying the stoichiometry of the Lewis acid used, the course of the reaction can be shifted to produce the alkylated or the cyclized product. Eighteen unique products were obtained with good isolated yields by direct alkylation with or without a consecutive π-cationic cyclization.

Enantioselective cross-coupling of meso-epoxides with aryl halides

The first enantioselective cross-electrophile coupling of aryl bromides with meso-epoxides to form trans-β-arylcycloalkanols is presented. The reaction is catalyzed by a combination of (bpy)NiCl2 and a chiral titanocene under reducing conditions. Yields range from 57 to 99% with 78-95% enantiomeric excess. The 30 examples include a variety of functional groups (ether, ester, ketone, nitrile, ketal, trifluoromethyl, sulfonamide, sulfonate ester), both aryl and vinyl halides, and five- to seven-membered rings. The intermediacy of a carbon radical is strongly suggested by the conversion of cyclooctene monoxide to an aryl [3.3.0]bicyclooctanol.