STERICALLY CROWDED ARYLOXIDE COMPOUNDS OF ALUMINUM - REDUCTION OF COORDINATED BENZOPHENONE

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)(mu-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(mu-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 H-1 NMR data. The kinetics of the conversion of 2 to 1 and 9 to 11 have been investigated and the DELTA-H double dagger and DELTA-S double dagger 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(mu-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.