Theoretical and Synthetic Investigations of Carbodiimide Insertions into Al−CH3 and Al−N(CH3)2 Bonds

Abstract

Carbodiimides are known to insert into aluminum−carbon bonds to form four-membered bidentate amidinate chelate rings. Insertions into Al−R and Al−NR‘₂ (R, R‘ = alkyl) have been reported in the literature. We have devised a mechanism for these insertions and modeled it using density functional theory (DFT) calculations. The calculated barrier heights for competitive insertions show the insertion into Al−N(CH₃)₂ goes through a lower barrier than the reaction with Al−CH₃ for diisopropyl carbodiimide due to the necessity of forming a pentavalent carbon intermediate in the Al−CH₃ case. However, insertion into Al−CH₃ has the lower barrier for the reaction with di-tert-butyl carbodiimide because of steric effects, which is consistent with the published experimental results. We have synthesized aluminum amidinates containing two and three acetamidinate rings via insertion of 2 and 3 equiv of diisopropylcarbodiimide into trimethylaluminum (TMA). The crystal structure for [CH₃C(NⁱPr)₂]₂AlCH₃ is reported. We have found that, although the first insertion is rapid at room temperature, the second and third insertions require refluxing above 70 °C. We have calculated the barrier heights for the first and second insertion and have found that this is due to a higher barrier for the migration of the methyl group in the second insertion. This higher barrier is the result of the lack of an exergic precoordination of the carbodiimide to the metal center, which facilitates the first insertion.