A Type of Structurally Adaptable Aromatic Spiroketal Based Chiral Diphosphine Ligands in Asymmetric Catalysis

Abstract

Metrics & MoreArticle Recommendations CONSPECTUS: While spectacular successes have been achieved in homogeneous catalysis with the use of achiral diphosphine ligands featuring a wide natural bite angle, such as XantPhos, chiral diphosphines that can induce a large P−M−P bite angle in their transition metal complexes are conspicuously less explored in asymmetric catalysis, probably due to the challenges in the identification and efficient construction of a suitable chiral backbone. In the past decade, a highly efficient synthesis of chiral aromatic spiroketals and the corresponding diphosphine ligands (SKPs) has been developed in this group.Based on a one-pot catalytic tandem double asymmetric hydrogenation−spiroketalization ring-closure reaction sequence, these SKP ligands featuring an extraordinarily long P•••P distance and a flexible backbone have been readily prepared in large scale. Remarkably versatile coordination modes have been found in the complexes of SKP with some catalysis-relevant transition metals, for example, Pd, Cu, Au, and Rh. Whereas SKP enforces an unusually large bite angle in [Pd(SKP)Cl 2 ] and [Cu(SKP)Cl] complexes (160.1°and 132.8°, respectively), it also allows for a bimetallic Au−Au interaction (3.254 Å) in the complex of [Au 2 (SKP)Cl 2 ] or a square-planar coordination geometry for the [Rh(SKP)(cod)]SbF 6 complex. Such an adaptable nature of SKP ligands for transition metal coordination has profound consequences in homogeneous asymmetric catalysis, as demonstrated by their unique performance in several types of catalytic asymmetric reactions. One of the most exciting examples is SKP/Pd-catalyzed asymmetric allylic amination of Morita−Baylis−Hillman (MBH) adducts, in which SKP/Pd complexes demonstrated excellent control of regio-and enantioselectivities and exhibited exceptionally high efficiency (with a TON up to 4750) in the catalysis. SKP ligands have also found a diversity of successful applications in Cu-, Au-, or Rh-catalyzed asymmetric reactions, further attesting their wide utilities in asymmetric catalysis. Overall, this class of readily accessible SKP ligands featuring a chiral aromatic spiroketal skeleton have demonstrated unique adaptable structures in a variety of transition metal complexes and provided outstanding performance in some difficult asymmetric transformations. The works delineated herein would be expected to stimulate further research efforts on the application of this type of chiral ligand and to provide useful clues in the design of new chiral diphosphine ligands with adaptable bite angles for transition metal catalyzed asymmetric reactions.

Enantioselective Radical SN2-Type Alkylation of Morita–Baylis–Hillman Adducts Using Dual Photoredox/Palladium Catalysis

“…Using a dual photoredox/palladium catalytic system, our group achieved the highly enantioselective allylic alkylation of allyl acetates . In addition, Ding and coworkers developed a unique spiroketal-based bis­(phosphine) (SKP) ligand and disclosed S N 2-type enantioselective substitutions of MBH adducts catalyzed by palladium . On the basis of these works, we expect that enantioselective radical S N 2-type alkylation of MBH adducts could be achieved using a dual photoredox and palladium/SKP catalytic system (Figure b).…”

The Implications of the Brønsted Acidic Properties of Crabtree-Type Catalysts in the Asymmetric Hydrogenation of Olefins

“…Dibrominated spiroketal 20c , prepared by the consecutive hydrogenation and in situ spiroketalization of α,α′-bis­(2-hydroxyarylidene) ketone 19c , was utilized to access a range of privileged aromatic spiroketal based diphosphine ligands (SKPs, Figure ). , An unusually large bite angle, a variable backbone, and large scale accessibility are prominent features of this versatile class of ligands. The SKP ligands have successfully been applied in a number of enantioselective metal-catalyzed reactions…”

Diastereo‐ and Enantioselective Synthesis of Bisbenzannulated Spiroketals and Spiroaminals by Ir/Ag/Acid Ternary Catalysis

“…Chiral spiroketals are ubiquitous motifs in many natural products and pharmacologically active substances. [8] Among them, bisbenzannulated [6,6]-spiroketal represents an important class, which behaves as a crucial pharmacophore in natural xerucitrinic acid A and B, [9] and also as a privileged skeleton in chiral ligands [10,11] (Scheme 2a). However, compared to the rich assortment of stereoselective synthetic routes to chiral aliphatic spiroketals, [7,12] catalytic asymmetric synthesis of aromatic spiroketals represents a considerable challenge.…”