Asymmetric Nickel-Catalyzed Hydrocyanation of Vinylarenes by Applying Homochiral Xantphos Ligands

Abstract

New homochiral xantphos-type diphosphonite ligands with binaphthoxy substituents have been prepared and characterized by NMR spectroscopy. These ligands have been applied in the nickel-catalyzed hydrocyanation of styrene and other vinylarenes. Enantioselectivities up to 63% ee have been obtained by using 4-isobutylstyrene as a substrate. Addition of an excess of ligand strongly inhibits the hydrocyanation reaction since the bis-chelate nickel complexes formed are highly stable and catalytically inactive.

Chiral (Diphosphonite)platinum Complexes in Asymmetric Hydroformylation

“…In fact, large-bite-angle diphosphine ligands with a rigid xanthene backbone give very stable and selective catalysts for the Pt-catalyzed hydroformylation of alkenes [16]. We already successfully prepared and applied chiral xanthene-based diphosphonite ligands in the nickel-catalyzed asymmetric hydrocyanation [17] and the rhodiumcatalyzed asymmetric hydroformylation [18]. Here, we report on the use of the chiral diphosphonite ligand (R,R)-XantBino ((R)-1) [19] in the Pt/Sn-catalyzed asymmetric hydroformylation of styrene (4a), 4-methylstyrene (4b), vinyl acetate (4c), and allyl acetate (4d).…”

Coordination Chemistry and Asymmetric Catalysis with a Chiral Diphosphonite.

“…[13] We have previously reported on the use of chiral xanthene-based diphosphonites in the asymmetric hydrocyanation of vinyl arenes that showed good enantioselectivities of up to 63% for 4-isobutylstyrene. [14] We recently published on the synthesis and use of sterically constrained achiral diphosphonites L 4 in the rhodium-catalyzed hydroformylation of alkenes and showed that these systems are wellsuited for the conversion of 2-butene into n-pentanal. [15] Together with our studies on the coordination chemistry of these xanthene-derived diphosphonites towards various transition metals, [16] this prompted us to explore the potential of chiral diphosphonite 1 in various asymmetric catalytic reactions.…”

“…), toluene, tetrazole, ∆T, 1 day erate to good yield. [14] Subsequently, the chiral binaphthyl groups were introduced to obtain the desired chiral diphosphonite. This reaction is catalyzed by tetrazole as protonation agent, which is a slight but important modification of the literature procedure.…”

“…Previously, only the bischelate Ni(1) 2 was described. [14] From the reaction of PdCl 2 (cod) with ligand (R,R)-1 at room temperature, a yellow compound was obtained, for which the 31 P NMR spectrum shows two doublets at δ ϭ 143.1 ppm and 137.4 ppm. The observed coupling constant J P-P is 46.5 Hz, apparently indicating that the phosphorus atoms are chemically inequivalent.…”

“…From the 1 H NMR spectrum, it appears that the methyl groups in the backbone are also inequivalent, indicating that the aromatic skeleton of the xanthene is bent. [14,17] Single crystals suitable for X-ray analysis were grown by layering a dichloromethane solution of complex 2 with acetonitrile. The molecular structure of cis-[PdCl 2 (1)] is presented in Figure 3, together with selected bond lengths and angles.…”