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

Unexpected 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.