Interaction of organic carbonyls with sterically crowded aryloxide compounds of aluminum

Article abstract of DOI:10.1021/om00162a021

The interaction of AlR(BHT)₂ and AlR₂(BHT)(OEt₂) (R = Me, Et) with organic carbonyls O=C(X)R (X = H, R′, OR, NR₂) leads to the formation of the Lewis acid-base complexes AlR(BHT)₂[O=C(X)R] and AlR₂(BHT)[O=C(X)R], respectively. The aluminum-methyl ¹³C NMR chemical shifts for the methyl aryloxide compounds AlMe(BHT)₂[O=C(X)R] and AlMe₂(BHT)[O=C(X)R] are found to be dependent primarily on the steric bulk of the substituents, X and R, on the carbonyl ligand. In contrast, the decrease in the carbonyl stretching frequency in the IR spectrum, and the downfield shift in the ¹³C NMR spectrum for the carbonyl α-carbon of the carbonyl adducts, when compared to those of the "free" carbonyls, is dependent on the electron-donating ability of X. The molecular structures of AlMe(BHT)₂[O=C(H)Bu^t] (1), AlMe₂(BHT)(O=CPh₂) (8), and AlMe(BHT)₂[O=C(OMe)Ph] (17) have been determined by X-ray crystallography. The mode of coordination of the organic carbonyls to aluminum is discussed. Crystal data for 1: monoclinic, P2₁/n, a = 10.0765 (5) A?, b = 26.201 (2) A?, c = 14.0106 (4) A?, β = 103.638 (3)○, Z = 4, R = 0.0576, R_w = 0.0623. Crystal data for 8: orthorhombic, Pn2₁a, a = 19.133 (3) A?, b = 14.638 (2) A?, c = 10.022 (1) A?, Z = 4, R = 0.0552, R_w = 0.0550. Crystal data for 17: triclinic, P1?, a = 9.394 (2) A?, b = 9.741 (3) A?, c = 23.091 (7) A?, α = 91.43 (2)?, β = 91.93 (2)?, γ = 116.41 (2)?, Z = 2, R = 0.0599, R_w = 0.0689.