Structural studies of lithiated enaminones: the 1-oxa-5-azapentadienyllithium fluxional heterocubane [(PriNCMeCHCMeOLi)4] and its dimeric hexamethylphosphoric triamide complex [{PriNCMeCHCMeOLi·OP(NMe2)3}2]

Abstract

Lithiation of 4-isopropylaminopent-3-en-2-one in hexane gave a 1-oxa-5-azapentadienyl compound [(PrⁱNCMeCHCMeOLi)₄] 1, which comprises a tetrameric aggregate of Li₄ and O₄ interpenetrating tetrahedra in which chelating terminal nitrogens undergo a fluxional process around each face of the Li₄ tetrahedron in toluene solution. Addition of hexamethylphosphoric triamide (hmpa) produced a dimeric complex 2 in which oxygen bridges are retained. The crystal structures of both compounds were determined. Bond lengths indicate that an iminoenolate form of the ligand predominates, irrespective of aggregation state. The data are used to rationalise the selectivity of dimetallations of enaminones.