Sterically crowded aryloxide compounds of aluminum: Reduction of coordinated benzophenone

Article abstract of DOI:10.1021/om00041a017

The interaction of AlEt(BHT)₂ with benzophenone, O=CPh₂, in pentane or benzene yields as the sole product Al(BHT)₂(OCHPh₂)(O=CPh₂) (1). In diethyl ether, however, the Lewis acid-base complex AlEt(BHT)₂(O=CPh₂) (2) is isolated. Thermolysis of 2 yields Al(BHT)₂(OCHPh₂) (3), which reacts rapidly with Et₂O, THF, or O=CPh₂ to give the acid-base complexes Al(BHT)₂(OCHPh₂)(L) (L = Et₂O (4), THF (5), O=CPh₂ (1)). Interaction of AlEt₂(BHT)(OEt₂) with 1 equiv of benzophenone in diethyl ether produces the isolable complex AlEt₂(BHT)(O=CPh₂) (6). Solid-phase thermolysis of 6 yields the monomer AlEt(BHT)(OCHPh₂) (7), which dimerizes upon dissolution in organic solvents to give [AlEt(BHT)(μ-OCHPh₂)]₂ (8). In the presence of excess benzophenone in benzene solution, AlEt₂(BHT)(OEt₂) gives AlEt(BHT)(OCHPh₂)(O=CPh₂) (9), which rearranges when heated in hexane to the bridged dimer (BHT)(Et)Al(μ-OCHPh₂)₂Al(OCHPh₂)(Et) (10). Thermolysis of 7 in the presence of excess benzophenone results in the reduction of a second ketone to give Al(BHT) (OCHPh₂)₂(O=CPh₂) (11). Unlike the benzophenone derivatives, the reaction of acetophenone with AlEt(BHT)₂ and AlEt₂(BHT)(OEt₂) does not result in ketone reduction but rather in the formation of the thermally stable Lewis acid-base adducts AlEt(BHT)₂[O=C(Me)Ph] (12) and AlEt₂(BHT)[O=C(Me)Ph] (13), respectively. The solvent-dependent formation of the benzophenone adducts 2 and 6 has been related to the solution equilibria and the relative metal-ligand bond dissociation energies (the BDE's) of the methyl compounds AlMe(BHT)₂L (L = Et₂O, THF, py, O=CPh₂,O₂N-C₆H₄-p-Me) and 1 which have been obtained from variable-temperature ¹H NMR data. The kinetics of the conversion of 2 to 1 and 9 to 11 have been investigated and the ΔH^? and ΔS^? values determined. Interaction of 2,6-diphenylphenol (DPP-H) with AlR₃ in a 1:1 molar ratio allows for the isolation of the dimeric compounds [R₂Al(μ-DPP)]₂ (R = Me (14), Et (15)). The reaction of 15 with O=CPh₂ results after hydrolysis in the formation of approximately 1 equiv of HOCHPh₂ per aluminum; however, no intermediate could be isolated.