Cationic Aluminium Complexes as Catalysts for Imine Hydrogenation

Abstract

Strongly Lewis acidic cationic aluminium complexes, stabilized by β–diketiminate (BDI) ligands and free of Lewis bases, have been prepared as their B(C6F5)4− salts and were investigated for catalytic activity in imine hydrogenation. The backbone (R1) and N (R2) substituents on the R1,R2BDI ligand (R1,R2BDI=HC[C(R1)N(R2)]2) influence sterics and Lewis acidity. Ligand bulk increases along the row Me,DIPPBDI<Me,DIPePBDI≈tBu,DIPPBDI<tBu,DIPePBDI; DIPP=2,6‐C(H)Me2‐phenyl, DIPeP=2,6‐C(H)Et2‐phenyl. The Gutmann‐Beckett test showed acceptor numbers of: (tBu,DIPPBDI)AlMe+ 85.6, (tBu,DIPePBDI)AlMe+ 85.9, (Me,DIPPBDI)AlMe+ 89.7, (Me,DIPePBDI)AlMe+ 90.8, (Me,DIPPBDI)AlH+ 95.3. Steric and electronic factors need to be balanced for catalytic activity in imine hydrogenation. Open, highly Lewis acidic, cations strongly coordinate imine rendering it inactive as a Frustrated Lewis Pair (FLP). The bulkiest cations do not coordinate imine but its combination is also not an active catalyst. The cation (tBu,DIPPBDI)AlMe+ shows the best catalytic activity for various imines and is also an active catalyst for the Tishchenko reaction of benzaldehyde to benzylbenzoate. DFT calculations on the mechanism of imine hydrogenation catalysed by cationic Al complexes reveal two interconnected catalytic cycles operating in concert. Hydrogen is activated either by FLP reactivity of an Al⋅⋅⋅imine couple or, after formation of significant quantities of amine, by reaction with an Al⋅⋅⋅amine couple. The latter autocatalytic Al⋅⋅⋅amine cycle is energetically favoured.

Synthesis, Structure, and Reactivity of a Superbulky Low-Valent β-Diketiminate Al(I) Complex

“…As described previously, the DIPeP BDI ligand can be prepared from 2,6-Et 2 (H)­C-aniline in 72% yield . The reaction of DIPeP BDI-H with Me 3 Al gave the complex ( DIPeP BDI)­AlMe 2 ( IV , Scheme ) in 96% yield, which has been previously reported and fully characterized . The reaction with 2 equivalents of I 2 gave ( DIPeP BDI)­AlI 2 ( 2 ) as a white powder in nearly quantitative isolated yield (90%) and was characterized by X-ray diffraction (Figure S32).…”

“…16 The reaction of DIPeP BDI-H with Me 3 Al gave the complex ( DIPeP BDI)AlMe 2 (IV, Scheme 2) in 96% yield, which has been previously reported and fully characterized. 22 The reaction with 2 equivalents of I 2 gave ( DIPeP BDI)AlI 2 (2) as a white powder in nearly quantitative isolated yield (90%) and was characterized by X-ray diffraction (Figure S32). Reduction of ( DIPeP BDI)AlI 2 with KC 8 (3 equiv) in benzene led to clean conversion in ( DIPeP BDI)Al (1).…”

“…Deuterated solvents were purchased from Deutero GmbH, degassed, and dried over molecular sieves (3 Å). The following compounds were synthesized according to literature procedures: ( DIPeP BDI)AlMe 2 , 22 KCH-(SiMe 3 ) 2 . 33 NMR spectra were measured on Bruker AVANCE III HD 400 MHz and Bruker AVANCE III HD 600 MHz spectrometers.…”

Chiral Bis(oxazoline) Ligand‐Supported Alkyl Aluminum Cations

“…[14] More recently, the Harder group fine-tuned the steric of methyl aluminum cation (B) to facilitate the hydrogenation of imines via the frustrated Lewis pair (FLP) mechanism. [15] The catalytic reactivity of these β-diketiminate-stabilized alkyl aluminum cations shows that they can activate non-polar bonds, like SiÀ H and HÀ H bonds. However, creating a chiral environment for alkyl cationic aluminum is rarely reported.…”

“…In 2016, the Nikonov group utilized the hydrido or alkyl aluminum cations ( A ) to promote hydrosilylation of olefins [14] . More recently, the Harder group fine‐tuned the steric of methyl aluminum cation ( B ) to facilitate the hydrogenation of imines via the frustrated Lewis pair (FLP) mechanism [15] . The catalytic reactivity of these β‐diketiminate‐stabilized alkyl aluminum cations shows that they can activate non‐polar bonds, like Si−H and H−H bonds.…”