Meerwein-Ponndorf-Verley-type Reduction Processes in Aluminum and Indium Isopropoxide Complexes of Imino-Phenolate Ligands.

International audienceUnexpected Meerwein-Ponndorf-Verley (MPV) reduction products have been obtained during attempts to prepare aluminum and indium isopropoxide complexes of an imino-phenolate ligand derived from 8-aminoquinoline ({ONNqui}−). Reaction of the imino-phenol proligand {ONNqui}H and Al(OiPr)3 in toluene/THF at 90 °C affords selectively [{ONNqui}{ONHNqui}Al] (1a), which contains a monoanionic{ONNqui}− ligand and another dianionic ligand ({ONHNqui}2-) that results from the reduction of the latter imino group into an amido moiety. The same and similar MPV reduction products 1a and [{ONNqui}{ONHNqui}In] (5a) were obtained from the treatment of {ONNqui}AlMe2 (2a) and dialkylindium compounds {ONNqui}InMe2 (3a) and {ONNqui}In(CH2SiMe3)2 (4a), respectively, with isopropyl alcohol in THF at room temperature. Related reactions of Al(OiPr)3 with imino-phenol(ate) (pro)ligands bearing N-benzyl- and N-methyl(2-pyridinyl) substituents or of iPrOH with {ONBn}AlMe2 (2b) and {ONNCH2pyr}AlMe2 (2c) did not lead to MPV reduction products but to the expected {ONBn}2Al(OiPr) (6b) and {ONNCH2Pyr}2Al(OiPr) (6c) compounds (and other unidentified products). The solid-state structures of 1a, 3a, 4a, and 5a were determined by X-ray diffraction studies. DFT computations performed on the putative complex {ONNqui}2Al(OiPr) ("6a" = Ia) and its isolated analogue {ONBn}2Al(OiPr) (6b) indicated an intramolecular MPV pathway with direct hydrogen transfer. Formation of 1a from the presumed alkoxide intermediate Ia is calculated to be a highly exergonic reaction which features a relatively low activation barrier. On the other hand, formation of the MPV reaction products from 6b appears to be highly unfavorable on both thermodynamic and kinetic grounds