Enantioselective Dual-Catalysis: A Sequential Michael Addition/Asymmetric Transfer Hydrogenation of α-Nitrosulfone and Enones

Abstract

Development of an efficient cocatalyst system to eliminate the intrinsic conflict of the cross-interactions in a pair of cocatalysts and to overcome the extrinsic conflict of the reaction conditions in an unmatched reaction environment has great significance in asymmetric dual catalysis. Herein, a compartmentalization method involving the integration of a cocatalyst system has been developed, which enables an efficient Michael addition/reduction enantioselective dual-catalysis process to be accomplished from a noncompatible system. In this process, the chiral squaramide species is encapsulated within the cavity of one hollow-shell-mesostructured silica support, whereas the chiral organoruthenium/diamine species is entrapped within the cavity of another water-soluble thermoresponsive polymer-coating hollow-shell-mesostructured silica support. This shielding feature together with the reversible on–off mode of the water-soluble thermoresponsive polymer not only controls the cross-interactions of dual species but also harmonizes the reaction conditions. As we envisioned, the one-pot sequential Michael addition of α-nitrosulfone and enones followed by asymmetric transfer hydrogenation provides various 1,4-distereocentered chiral δ-hydroxysulfones with enhanced yields and enantio/diastereoselectivities.

Phase Separation‐Promoted Redox Deracemization of Secondary Alcohols over a Supported Dual Catalysts System

“…In particular, the TEM image with a chemical mapping demonstrated that the rhodium centers in 5 were uniformly distributed (Figure 2c). Furthermore, the nitrogen adsorption‐desorption isotherms of 5 also confirmed its mesoporous structure, since a typical IV character with an H 1 hysteresis loop was observed that was the same as those reported for hollow‐shell‐structured mesoporous silica (see Figure S7 of ESI) [10c] …”

“…It was found that the thermoresponsive polymer‐coating layer in 5 was necessary because the cascade reaction either at 20 °C or 40 °C gave the final ( S )‐ 6 a with the obviously decreased yields and ee values. This was attributed to the swelling conformational state of the thermoresponsive polymer‐coating layer in 5 at >10 °C proven by a turbidity investigation of 5 (see Figure S8 of ESI) [10c,13a] . During this process, the first‐step oxidation and the second‐step reduction simultaneously proceeded.…”

“…Immobilization of the chiral rhodium/diamine‐species within the nanochannels of the silica shell in the water‐soluble thermoresponsive polymer‐coating hollow‐shell mesoporous silica nanoparticles, abbreviated as Polymer@[Rh]@HSMSNPs ( 5 ) (Polymer: [13] poly(ethene‐ co ‐acrylamide‐ co ‐acrylonitrile), [Rh]=Cp*RhArDPEN: [14] Cp*=pentamethyl cyclopentadiene, ArDPEN=( S , S )‐4‐((trimethoxysilyl)ethyl)phenylsulfonyl‐1,2‐diphenylethylene‐diamine), was performed via a sequential polymerization‐coordination procedure (Scheme 2). In the free‐radical polymerization step, the obtained as‐made 4 reacted with acrylamide and acrylonitrile to produce the polymer‐coating material Polymer@HSMSNPs ( 4′ ), [10c] wherein the polymer‐loading amounts were 2.01 %, as determined by the TG analysis (see Figure S4 of ESI). In the coordination step, the reaction of 4′ and (Cp*RhCl 2 ) 2 at 40 °C in CH 2 Cl 2 formed a coarse catalyst, wherein a strict Soxhlet extraction afforded a pure catalyst 5 as a light‐yellow powder (see Figure S5 of ESI).…”

“…Inspired by a compartmentalization‐featured dual catalysts system in our previous report, [10c] we hypothesize that the phase separation of the biphasic co‐solvents in a dual catalysts system can drive a redox deracemization of secondary alcohols to chiral alcohols, wherein two heterocatalysts have different hydrophilicity/hydrophobicity (Scheme 1B). During this transformation, the in‐situ generated organic ketones via an oxidation process of alcohols in the aqueous phase (H 2 O phase) could be pumped into the ATH process in the organic phase (CH 2 Cl 2 phase).…”

Integrated Cobaloxime and Mesoporous Silica-Supported Ruthenium/Diamine Co-Catalysis for One-Pot Hydration/Reduction Enantioselective Sequential Reaction of Alkynes

“…This superiority offers a practical opportunity for the one-pot hydration/reduction of alkynes to overcome the limitations of environmental issues, high cost, and/or the harsh conditions originating from noble metal/ligand catalysts. Due to the benefits of the compatibility of the mild catalysis condition of cobaloximes with the Rh/diamine catalyst, and taking into account our recent progress in silica-based chiral recyclable heterogeneous catalyst through a covalent-bonding method (Chang et al, 2020;Li et al, 2021;Zhao et al, 2021), we envision that via a one-pot hydration/ATH catalyzed with a combination of inexpensive cobaloximes (Schrauzer, and Windgassen, 1967;Bakac et al, 1986;Geno, and Halpern, 1987;Hou et al, 2017) and DMONs-based Rh/ diamine as co-catalysts (Hashiguchi et al, 1995;Hannedouche et al, 2004;Matharu et al, 2005;Hayes et al, 2005;Ohkuma et al, 2006;Cheung et al, 2007;Touge et al, 2011;Touge et al, 2016), the alkynes could be converted into chiral alcohols. As presented in this study, this sequential enantioselective organic transformation, an initial homogeneous cobaloxime-catalyzed hydration of alkynes followed by a subsequent heterogeneous Ru/diamine-catalyzed ATH transformation of in-situ generated ketones, provided various chiral alcohols in good yields and up to 99% ee.…”