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Amino Ether–Phenolato Precatalysts of Divalent Rare Earths and Alkaline Earths for the Single and Double Hydrophosphination of Activated Alkenes

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posted on 02.08.2016, 18:10 by Ivan V. Basalov, Bo Liu, Thierry Roisnel, Anton V. Cherkasov, Georgy K. Fukin, Jean-François Carpentier, Yann Sarazin, Alexander A. Trifonov
A range of stable ytterbium­(II) amino ether–phenolato amido complexes of the type {LONxOy}­Yb­{N­(SiMe3)2}, together with a congeneric samarium­(II) and two calcium and strontium amido and alkyl derivatives, have been synthesized. They constitute very active, fully regioselective (anti-Markovnikov), and chemoselective precatalysts for the intermolecular hydrophosphination of styrene derivatives with PhPH2, achieving TONs up to 2150, TOFs up to 30 h–1 and chemoselectivities in the region of 95–99%. The ytterbium­(II) precatalysts, among which is most prominently {LONO4}­Yb­{N­(SiMe3)2} (3), where the ligand possesses a 15-c-5-aza-crown ether side arm, outperform their related calcium analogues, and the activity of the catalyst increases substantially with the denticity of the ligand. The rate law rate = k[p-tBu-styrene]­[3] was established following kinetic monitoring of the hydrophosphination of p-tBu-styrene and PhPH2 catalyzed by 3. The kinetic studies also revealed that the reactions are entropically driven and that reaction rates increase when electron-withdrawing groups are introduced at the para position of the styrenic substrate. It is proposed that these catalyzed hydrophosphination reactions proceed by rate-limiting olefin insertion into the [Yb]–phosphide bond. The chemoselective one-pot, two-step double hydrophosphination of PhPH2 with 2 equiv of styrene yields tertiary phosphines. It obeys an unusual kinetic profile where formation of the tertiary phosphine starts only when complete consumption of PhPH2 is first ensured. This sequence was used to obtain asymmetric tertiary phosphines with excellent selectivity.