Intramolecular C-H⋯O hydrogen bond controlling the conformation of heterocycles: Synthesis, structure and catalytic reactivity of aluminum aryloxides

The reactions of R₂AlX with one equivalent of 2,2′-methylene-bis-(4,6-di-tert-butylphenol) (MDBP-H₂) or 2,2′-methylene-bis-(4-methyl-6-tert-butylphenol) (MMBP-H₂) in oxygenated solvent afford monomeric four-coordinated compounds, XAl(MDBP)(S), 1-5 (1: R = Me, X = Br, S = Et₂O; 2: R = Et, X = Cl, S = Et₂O; 3: R = Me, X = Br, S = THF; 4: R = Et, X = Cl, S = THF; 5: R = X = Et, S = Et₂O) or XAl(MMBP)(S), 6-8 (6: R = X = Et, S = Et₂O; 7: R = X = ⁱBu, S = Et₂O; 8: R = Et, X = Cl, S = Et₂O). Crystal structures of 1, 2 and 7 demonstrate that the conformation of the eight-membered heterocycles containing aluminum is controlled by an unusual intramolecular C-H⋯O hydrogen bond. Et₂AlCl or ⁱBu₃Al reacts with one equivalent of 2,2′-ethylidene-bis-(4,6-di-tert-butylphenol) (EDBP-H₂) or MDBP-H₂ in toluene giving a dimeric four coordinate compound [CLAL(μ-EDBP]₂, 9 or [ⁱBuAl(μ-MDBP)]₂, 10. Some of these compounds have shown great catalytic activities toward the reaction of cyclopentadiene with methacrolein.