SOMC@Periodic Mesoporous Silica Nanoparticles: Meerwein–Ponndorf–Verley Reduction Promoted by Immobilized Rare-Earth-Metal Alkoxides

Abstract

The Meerwein–Ponndorf–Verley (MPV) reduction is a reaction that offers a mild reduction of aldehydes and ketones to the corresponding alcohols. Although described as a catalytic reaction, its real-life applicability suffers from the necessity of using the standard catalyst [Al­(OiPr)3] in stoichiometric amounts or even in excess. Rare-earth-metal-based catalysts are capable of performing in these reactions in a truly catalytic fashion. The ceric alkoxide [Ce­(OiPr)4]3 has been synthesized via silylamine elimination from Ce­[N­(SiHMe2)2]4 with isopropyl alcohol, its trimetallic solid-state structure has been determined by X-ray diffraction, and its performance in the MPV reduction of 4-tBu-cyclohexanone has been examined and compared to that of cerous [Ce­(OCH2 tBu)3]4. Spherical mesoporous silica nanoparticles with an MCM-41-type honeycomb pore symmetry, termed MSN-MCM-41 (particle size, ca. 250 nm diameter; pore size, 2.6 nm diameter), are employed for grafting the molecular precursors Ce­[N­(SiHMe2)2]4, [Ce­(OiPr)4]3, Ce­[N­(SiMe3)2]3, and La­[N­(SiMe3)2]3 according to the methods of surface organometallic chemistry (SOMC). The MPV reductions carried out with the homogeneous and heterogeneous catalysts reveal (a) a better performance of Ce­(III) in comparison to Ce­(IV), (b) better performance of La­[N­(SiMe3)2]3@MSN-MCM-41 in comparison to Ce­[N­(SiMe3)2]3@MSN-MCM-41 (high sensitivity of Ce­(III)-grafted materials), and (c) reusability of the grafted catalyst systems. All hybrid materials were characterized by PXRD, N2 physisorption, and 1H/13C/29Si MAS NMR and FTIR spectroscopies as well as elemental analysis.

Tuning Organocerium Electrochemical Potentials by Extending Tris(cyclopentadienyl) Scaffolds with Terminal Halogenido, Siloxy, and Alkoxy Ligands

“…The cerium­(IV) alkoxides 5 (OR = OMe), 6 (OR = OEt), 7 (OR = OCH 2 t Bu), and 8 (OR = OCHMe 2 ) revealed characteristic NMR chemical shifts for the protons in the α-position of the alkoxy ligands (Ce–O–C– H ), as they appear heavily shifted downfield (Table ). Such a deshielding effect was also observed for the homoleptic cerium­(IV) alkoxides [Ce­(OCHMe 2 ) 4 ] 3 and [Ce­(OCH 2 t Bu) 4 ] 2 (thf), albeit to a minor extent. , The latter complexes show the relevant proton signals at 5.25 and 4.53 ppm, respectively, significantly shifted to lower field in comparison to 4.01 and 3.28 ppm in the free alcohol in CDCl 3 . ,, However, the 1 H NMR signals of the CH 2 moieties in neopentoxy derivatives Cp′ 3 Ce­(OCH 2 t Bu) ( 7a and 7b ) are detected at 5.87 and 6.09 ppm, respectively. This indicates a decrease in electron density at the respective protons close to the cerium­(IV) center, possibly caused by the +M effect of the alkoxy ligand.…”

“…Such a deshielding effect was also observed for the homoleptic cerium(IV) alkoxides [Ce(OCHMe 2 ) 4 ] 3 and [Ce(OCH 2 tBu) 4 ] 2 (thf), albeit to a minor extent. 22,39 The latter complexes show the relevant proton signals at 5.25 and 4.53 ppm, respectively, significantly shifted to lower field in comparison to 4.01 and 3.28 ppm in Scheme 1. Oxidation of Cp R 3 Ce(thf) (1), 14,32 to Form Cerium(IV) Halogenides 2-4 The ceric tris(cyclopentadienyl) complexes under study could be obtained as needlelike microcrystals via crystallization from n-hexane.…”

“…For further comparison, the terminal Ce–O distances in homoleptic [Ce­(O t Bu) 4 ] 2 and [Ce­(OCH 2 t Bu) 4 ] 3 range from 2.058(3) to 2.065(3) Å and from 2.029(1) to 2.030(2) Å, respectively, while the terminal Ce–O­(methoxy) distance in Ce­[OSi­(O t Bu) 3 ] 3 (OCH 3 )­(thf) 2 is 2.094(1) Å . Generally, the Ce–O distances span a wide range depending on the coordination number and terminal/μ x -bridging mode: e.g., as detected for ceric Ce 3 (μ 3 -O i Pr) 2 (μ s -O i Pr) 3 (O i Pr) 7 (2.061(4)–2.721(3) Å) . The Ce–O–C angles appear affected by both the sterics of the alkoxy and Cp ligands and the Cp alignment (minimum, OMe/Cp ( 5a ) 164.5(2)°; maximum, O i Pr/Cp ( 8b ) 176.6(2)°).…”

“…29 Generally, the Ce−O distances span a wide range depending on the coordination number and terminal/μ x -bridging mode: e.g., as detected for ceric Ce 3 (μ 3 -OiPr) 2 (μ s -OiPr) 3 (OiPr) 7 (2.061(4)−2.721(3) Å). 39 The Ce−O−C angles appear affected by both the sterics of the alkoxy and Cp ligands and the Cp alignment (minimum, OMe/Cp (5a) 164.5(2)°; maximum, OiPr/Cp (8b) 176.6(2)°). The average Ce−Cnt distances of the alkoxy complexes are very similar (2.503− 2.513 Å) but are longer than in the chlorido derivatives (2.460 and 2.482 Å) 15,21 and shorter than in the bromide complex 3b (2.567 Å).…”

Reduction of Amides to Amines with Pinacolborane Catalyzed by Heterogeneous Lanthanum Catalyst La(CH2C6H4NMe2-o)3@SBA-15

“…On the other hand, to overcome the drawbacks of the homogeneous catalysts in catalyst contamination in reaction outcomes and poor recyclability, increasing interest has been directed toward developing supported catalysts in surface organometallic chemistry (SOMC). − Such catalysts not only show the high activity of homogeneous catalysts but also display the ease of separation and recyclability of heterogeneous catalysts, allowing for better control of the whole synthetic process. Consequently, the immobilized heterogeneous rare-earth metal complexes have received much interest in SOMC and have demonstrated interesting performance in organic synthesis and polymerization. − Very recently, we developed the well-defined silica-grafted rare-earth metal benzyl materials Ln­(CH 2 C 6 H 4 NMe 2 - o ) 3 @SBA-15 (Scheme ) and found that they exhibited extremely high selectivity and good recyclable property in the transformation of primary arylsilanes into secondary arylsilanes through redistribution. , This interesting finding prompted us to explore their catalytic potential in other organic transformations. Inspired by the elegant work reported by the Marks group and the Chen group using rare-earth metal complexes Ln­[N­(SiMe 3 ) 2 ] 3 as catalysts in amide reduction, , it was envisaged that Ln­(CH 2 C 6 H 4 NMe 2 - o ) 3 @SBA-15 (Ln–C σ-bonded species) possesses promising catalytic performance in amide reduction owing to its bond disruption enthalpy comparable to the amide analogues (Ln–N σ-bonded complexes) .…”