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Keywords:

  • alkaline-earth metals;
  • alkenes;
  • amido complexes;
  • cyclohydroamination;
  • reaction mechanisms

Abstract

The behavior of the first aminophenolate catalysts of the large alkaline earth metals (Ae) [(LOi)AeN(SiMe2R)2(thf)x] (i=1–4; Ae=Ca, Sr, Ba; R=H, Me; x=0–2) for the cyclohydroamination of terminal aminoalkenes is discussed. The complexes [(BDI)AeN(SiMe2H)2(thf)x] (Ae=Ca, Sr, Ba, x=1–2; (BDI)H=H2C[C(Me)N-2,6-(iPr)2C6H3]2)) and [(BDI)CaN(SiMe3)2(thf)] supported by the β-diketiminate (BDI) ligand have also been employed for comparative and mechanistic considerations. The catalytic performances decrease in the order Ca>Sr≫Ba, which is the opposite trend to that previously observed during the intermolecular hydroamination of activated alkenes catalyzed by the same alkaline-earth metal complexes. Catalyst efficacy increases when the chelating and donating ability of the aminophenolate ligands decreases. For given metals and ancillary scaffolds, disilazide catalysts that incorporate the N(SiMe3)2 amido group outclass their congeners containing the N(SiMe2H)2 amide owing to the lower basicity of the N(SiMe2H)2 with respect to the N(SiMe3)2 group, and also because Ae–N(SiMe2H)2 catalysts suffer from irreversible deactivation through the dehydrogenative coupling of amine and hydrosilane moieties. This deactivation process takes place at 25 °C in the case of [(LOi)AeN(SiMe2H)2(thf)x] phenolate complexes and occurs even with the related [(BDI)AeN(SiMe2H)2(thf)x] complex, albeit under conditions harsher than those required for effective cyclohydroamination catalysis. A mechanistic scenario for cyclohydroamination catalyzed by [(LX)AeN(SiMe2H)2(thf)x] complexes ((LX)=(LOi) or (BDI)) is proposed. Although beneficial for the synthesis of Ae heteroleptic complexes able to resist deleterious Schlenk-type equilibria, the use of the N(SiMe2H)2 is prejudicial to catalytic activity in the case of catalyzed transformations that involve reactive amine (and potentially other) substrates. Mechanistic and kinetic investigations further illustrate the interplay between the catalytic activity, operative mechanism, and identity of the metal, ancillary ligand, and amido group. These studies suggest that the widely accepted mechanism for cyclohydroamination reactions cannot be extended systematically to all alkaline-earth catalysts. The [(BDI)CaN(SiMe2H)2{H2NCH2C(CH3)2CH2CH[DOUBLE BOND]CH2}2] complex, the first Ca–aminoalkene adduct structurally characterized, was prepared quantitatively and essentially behaves like [(BDI)CaN(SiMe2H)(thf)], thus serving as a model compound for mechanistic studies, as illustrated during stoichiometric reactions monitored by 1H NMR spectroscopy.