Synthesis and structures of four crystalline lithium β-diketiminates derived from [Li{CH(SiMe3)(SiMe2OMe)}]8 and PhCN or ButCN and PhCN

Treatment at ambient temperature in diethyl ether of one equivalent of [Li{CH(SiMe3)(SiMe2OMe)}]8 (A) with (i) two equivalents of PhCN, or (ii) successive equivalent portions of ButCN and PhCN gave from (i) (1) and (2) as well as the known lithium 1-azaallyl [Li{N(SiMe2OMe)C(Ph)CH(SiMe3)}]3, and from (ii) (3) The latter was also obtained from the known [Li{N(SiMe2OMe)C(But)CH(SiMe3)}]2 and PhCN under similar conditions. Recrystallisation of 2 from THF/hexane afforded (2′). X-ray diffraction data on 2, 2′ and 3 are presented; data for 1 were only adequate to confirm its gross tetrameric structure. The particularly novel feature of these transformations of the lithium alkyl A into lithium β-diketiminates 1, 2, 2′ or 3 is that while the first 1,3-carbon to nitrogen shift from the α-carbon of A is both silicotropic and regiospecific (in so far as SiMe2OMe > SiMe3 in migratory aptitude), the second migration is indiscriminate: silicotropy yielding 1 or 3 but prototropy giving 2 (or 2′). Consistent with these observations, the central carbon atom of the β-diketiminato ligand has significant carbanionic character in 2 or 2′, attributed to its stabilisation by the exocyclic Me3Si at C2, whereas in 1 or 3 there is π-delocalisation.