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 ({ONN qui } − ). Reaction of the imino-phenol proligand {ONN qui }H and Al(O i Pr) 3 in toluene/THF at 90 °C affords selectively [{ONN qui }{ON H N qui }Al] ( 1a ), which contains a monoanionic{ONN qui } − ligand and another dianionic ligand ({ON H N qui } 2– ) that results from the reduction of the latter imino group into an amido moiety. The same and similar MPV reduction products 1a and [{ONN qui }{ON H N qui }In] ( 5a ) were obtained from the treatment of {ONN qui }AlMe 2 ( 2a ) and dialkylindium compounds {ONN qui }InMe 2 ( 3a ) and {ONN qui }In(CH 2 SiMe 3 ) 2 ( 4a ), respectively, with isopropyl alcohol in THF at room temperature. Related reactions of Al(O i Pr) 3 with imino-phenol(ate) (pro)ligands bearing N -benzyl- and N -methyl(2-pyridinyl) substituents or of i PrOH with {ON Bn }AlMe 2 ( 2b ) and {ONN CH2pyr }AlMe 2 ( 2c ) did not lead to MPV reduction products but to the expected {ON Bn } 2 Al(O i Pr) ( 6b ) and {ONN CH2Pyr } 2 Al(O i Pr) ( 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 {ONN qui } 2 Al(O i Pr) (“ 6a ” = Ia ) and its isolated analogue {ON Bn } 2 Al(O i Pr) ( 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.