Insertion, Reduction, and Carbon–Carbon Coupling Induced by Monomeric Aluminum Hydride Compounds Bearing Substituted Pyrrolyl Ligands

A monomeric aluminum hydride complex bearing substituted pyrrolyl ligands, AlH[C 4 H 3 N(CH 2 NMe 2 )-2] 2 (1), was synthesized and structurally characterized. To further confirm the presence of Al-H bonds, the compound AlD[C4H3N(CH2NMe2)-2]2 ([D]1) was synthesized by reacting LiAlD 4 with [C 4 H 4 N(CH 2 NMe 2 )-2]. Compound 1 and [D]1 react with phenyl isothiocyanate yielding Al[C 4 H 3 N(CH 2 NMe 2 )-2] 2 [η 3 -SCHNPh] (2) and Al[C 4 H 3 N(CH 2 NMe 2 )-2] 2 [η 3 -SCDNPh] ([D]2) by insertion. The reactions of 1 with 9-fluorenone and benzophenone generated the unusual aluminum alkoxide complexes 3 and 4, respectively, through intramolecular proton abstraction and C-C coupling. A mechanistic study shows that 9-fluorenone coordinates to [D]1 and releases one equivalent of HD followed by C-C coupling and hydride transfer to yield the final product. Reduction of benzil with 1 affords aluminum enediolate complex 5 in moderate yield. Mechanistic studies also showed that the benzil was inserted into the aluminum hydride bond of [D]1 through hydroalumination followed by proton transfer to generate the final product [D]5. All new complexes have been characterized by 1 H and 13 CNMR spectroscopy and X-ray crystallography.