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 (tBuSi)N by addition of tBuLi to tBuSi=N(SitBu) failed and gave tBuSi(tBuHSi)NLi and isobutene. Reaction of the sila-imine with KOtBu successfully led to tBuSi[tBu(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 tBuSi[tBu(tBuO)Si]NH but this amine cannot be easily deprotonated. Reaction with (BDI*)MgnBu in presence of THF gave (BDI*)Mg ⋅ (THF) and the non-coordinating anion tBuSi[tBu(nBu)Si]N; BDI*=ß-diketiminate ligand HC[C(tBu)N-DIPP], DIPP=2,6-diisopropylphenyl. Reaction of Mg(nBu) with tBuSi=N(SitBu) led to a Mg complex with one amide ligand: tBuSi[tBu(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.