Synthesis of Superbulky Amide Ligands by Addition of Polar Reagents to Sila-Imine

The chemistry of extremely bulky amide ligands is troubled by difficulties in deprotonation of the parent amine. As an alternative route to superbulky amide reagents, the addition of polar reagents to a sila-imine has been investigated. Attempts to synthesize the superbulky amide anion (tBu₃Si)₂N^- by addition of tBuLi to tBu₂Si=N(SitBu₃) failed and gave tBu₃Si(tBu₂HSi)NLi and isobutene. Reaction of the sila-imine with KOtBu successfully led to tBu₃Si[tBu₂(tBuO)Si]NK which crystallized as a separated ion-pair. Reaction with the slightly bulkier KOAd (Ad=1-adamantyl) led in presence of THF to ether ring-opening. Reaction with tBuOH gave tBu₃Si[tBu₂(tBuO)Si]NH but this amine cannot be easily deprotonated. Reaction with (BDI*)MgnBu in presence of THF gave (BDI*)Mg⁺ ⋅ (THF)₂ and the non-coordinating anion tBu₃Si[tBu₂(nBu)Si]N^-; BDI*=ß-diketiminate ligand HC[C(tBu)N-DIPP]₂, DIPP=2,6-diisopropylphenyl. Reaction of Mg(nBu)₂ with tBu₂Si=N(SitBu₃) led to a Mg complex with one amide ligand: tBu₃Si[tBu₂(nBu)Si]N^-. The other superbulky amide anion isomerized by internal deprotonation of a tBu-substituent to give a primary carbanion that is also coordinated to Mg. Although the amide-to-carbanion isomerization is highly contrathermodynamic, it allows for coordination of both anions to a single Mg center. The new bulky amides are rare cases of halogen-free weakly coordinating anions.