Towards the Next Generation of Lochmann-Schlosser Superbases: A Potassium Neopentyl/Alkoxy Aggregate used in the Tetra-Functionalization of Ferrocene.

Lochmann-Schlosser superbases are formed by mixing alkyllithium with potassium alkoxides. These reagents could prove their synthetic usefulness and reliability in many reactions over five decades. However, despite many efforts, the real source of the exceptional reactivity remained a secret. The seemingly manageable system of four components (lithium, potassium atoms, alkyl groups, and alkoxy groups) and their interaction is obscured by poor solubility and fierce reactivity. Recent progress was achieved by using neopentyllithium, leading to alkane-soluble aggregates with varying lithium/potassium content and a flexible alkyl/alkoxy ratio. Herein, we isolated two new alkane-soluble alkyl/alkoxy mixed aggregates, [Li4 KNp2 (OtBu)3 ] and [K4 Np(OtAm)3 ]. The latter compound is a thermally stable three-component potassium alkyl/alkoxy base with well-defined stoichiometry, in contrast to lithium-containing Lochmann-Schlosser bases with variable metal and alkyl/alkoxy content. In a simple protocol, this potassium-base gave tetrametalated ferrocene, which was converted into 1,1',3,3'-ferrocenetetracarboxylic acid by reaction with CO2 . A subsequent conversion into the methyl ester allowed its separation from accompanying di- and tri-substituted ferrocenes.