Conformation of Heterocycles Controlled by the Existence of Unusual C−H···X Hydrogen Bonds:  Syntheses and Structure Determination of Aluminum Aryloxides

The reactions of AlMe₃ in diethyl ether with 1 molar equiv of 2,2‘-methylenebis(4-methyl-6-<i>tert</i>-butylphenol) (MMBP-H₂), 2,2‘-methylenebis(4,6-di-<i>tert</i>-butylphenol) (MDBP-H₂), and 2,2‘-ethylidenebis(4,6-di-<i>tert</i>-butylphenol) (EDBP-H₂) afford series of four-coordinate monomeric aluminum aryloxides, MeAl(O∼O)(OEt₂), <b>1</b><b>−</b><b>3 </b>(<b>1</b>, (O∼O) = MMBP; <b>2</b>, (O∼O) = MDBP; <b>3</b>, (O∼O) = EDBP). In THF, 1 molar equiv of EDBP-H₂ reacts with AlMe₃ to provide the THF-coordinated complex MeAl(EDBP)(THF) <b>4</b>. However, in the absence of a coordinating solvent, the reaction of EDBP-H₂ with AlMe₃ yields the dimeric complex [MeAl(μ-EDBP)]₂ (<b>5</b>). Complex <b>5</b> further reacts with Et₄NCl, Et₄NBr, and Ph₃PO to afford the corresponding monomeric ionic complex [Et₄N][MeAl(EDBP)(X)] (<b>6</b>, X = Cl; <b>7</b>, X = Br) and the neutral complex [MeAl(EDBP)(OPPh₃)] (<b>8</b>), respectively. Complexes <b>1</b>, <b>2</b>,<b> 4</b> and <b>6</b>−<b>8</b> are subjected to X-ray structure analyses, and the solid state structures reveal that the conformations of the eight-membered heterocycles are governed by the formation of the unusual C−H···X hydrogen bonds.