CO₂ Activation with Bulky Neutral and Cationic Phenoxyalanes

Abstract

The sterically crowded aluminum diphenolate (2,6-Mes 2 C 6 H 3 O) 2 AlEt (2; Mes =2,4,6-Me 3 C 6 H 2 −) was converted into the ionic species [(2,6-Mes 2 C 6 H 3 O) 2 Al] + [CHB 11 Cl 11 ] − (6) by ethide abstraction with the silylium salt [Et 3 Si][CHB 11 Cl 11 ] or by a combination of β-hydride abstraction and concomitant ethylene elimination with the trityl salt [Ph 3 C][CHB 11 Cl 11 ]. Compound 6 consists of solvent-separated ions, and the cation features very short Al•••C contacts involving the flanking mesityl groups, leading to an overall distorted-tetrahedral coordination geometry around the aluminum center. Analogous reactions with the diphenolate (2,6-tBu 2 -4-MeC 6 H 2 O) 2 AlEt ( 1) led to tert-butyl transfer to the benzene solvent to afford tert-butylbenzene as the main product. A 1:1 mixture of the triphenolate (2,6-Ph 2 C 6 H 3 O) 3 Al and tri-tert-butylphosphine forms a frustrated Lewis pair, which reacts with CO 2 at room temperature to give the compound (2,6-Ph 2 C 6 H 3 O) 3 AlOC(O)Pt-Bu 3 (8). The high Lewis acidity of 6 catalyzes the scrambling of the subsituents of Et 3 SiH at room temperature to give Et 4 Si, Et 2 SiH 2 , and EtSiH 3 and also the reduction of CO 2 to d 5 -toluene (C 6 D 5 CH 3 ) and CH 4 at 82 °C in d 6 -benzene solution.

Elusive Silane–Alane Complex [SiH⋅⋅⋅Al]: Isolation, Characterization, and Multifaceted Frustrated Lewis Pair Type Catalysis

“…Theu tility of the present silane-alane complex has been examined for applications in four types of catalytic reactions. First, strong activation of silanes by the super-Lewis-acidic alane promotes silane ligand redistribution, [22] thus converting tertiary silanes into secondary and quaternary silanes (Scheme 2; see Table S1 in the Supporting Information). Thus,h eating as olution of Et 3 SiH in ortho-dichlorobenzene at 80 8 8Cf or 4hours with 5mol %o fA l(C 6 F 5 ) 3 led to 96 % conversion into Et 4 Si and Et 2 SiH 2 .I nt he case of PhSiMe 2 H, the reaction proceeded even at 25 8 8Ci nC 6 D 5 Br to produce Ph 2 SiMe 2 and Me 2 SiH 2 (95 %i n3h).…”

Transition‐Metal‐Free Catalytic Reduction of Carbon Dioxide

“…Using the analogous phenolate species [Al(2,6-Mes 2 C 6 H 3 O) 2 Al] + (Mes = 2,4,6,-trimethylphenyl), faster reaction rates were obtained (> 90 % conversion of Et 3 SiH in 25 h at 85 8C), but the reaction was still lacking selectivity. [19] Putting these results together, it is clear that carbon dioxide reduction can be promoted both by main group Lewis acids and bases. However, the efficiency of the catalytic process of CO 2 reduction remains to be optimised.…”

Recent Developments on the Use of Group 13 Metal Complexes in Catalysis

“…Given that Lewis acids catalyzet he hydrosilylationa nd hydroborationo fc arbonyl groups, the catalytic reduction of CO 2 using Al-based catalysts was recently developed. Strong cationic Lewis acids such as [Et 2 Al][CH 6 B 11 I 6 ], [50] [Et 2 Al][CHB 11 Cl 11 ] (11), [2,6-Ph 2 C 6 H 3 AlEt][CHB 11 Cl 11 ]( 12), [51] [(2,6-Ar*C 6 H 3 O) 2 Al] [CHB 11 Cl 11 ]( Ar* = 2,4,6-Me 3 C 6 H 2 , 13,S cheme 7) [52] catalyze the reduction of CO 2 with Et 3 SiH to methane and toluene.T he latter forms by Friedel-Craftsa lkylation of the benzene solvent with the methoxysilane Et 3 SiOMe. The strongn eutral Lewis acid Al(C 6 F 5 ) 3 performs only sluggishly,b ut is quite active in tandemw ith B(C 6 F 5 ) 3 .…”