Choon-Hong Tan

Novel mono glyceryl ethers (MGEs) were synthesized from biomass-based sources and used as diol monomers for synthesizing comb-shaped polyesters (comb-shaped polymers (CSPs)). MGEs with varied alkyl chain lengths (m values) were polymerized with biomass-based diacids of varied alkyl chain lengths (n values) via polycondensation, yielding a series of (32 kinds of) MGE-based CSPs with diverse m and n combinations. The resulting CSPs had molecular weights > 10 000 g mol⁻¹ (after purification) with a high reaction extent (≥91%). The melting temperatures (T_m) were systematically studied with respect to (1) the branch alkyl chain length (m), (2) the backbone alkyl chain length (n), (3) the branch position in MGE (Sn-1 or Sn-2 positions), and (4) the difference between ether-linked and ester-linked branch chains. Notably, several CSPs showed particularly low T_m (below 0 °C) or even no T_m (with no crystallinity), which is not attainable by linear polyesters. These CSPs may find unique future applications such as low-temperature lubricants.

ConspectusCatalysts drive asymmetric transformations by orchestrating a network of covalent and noncovalent interactions that precisely regulate the reactivity and stereoselectivity. Ion pair catalysis, developed based on the inherent strength and long-range nature of ionic interactions, has demonstrated high catalytic efficiency and broad applicability. While chiral cationic catalysts have long been central to this field, the critical roles of their counteranions have historically been overlooked. Over the past 15 years, we have developed a class of N-sp²-hybridized guanidinium chiral cation ion pair catalysts, which have been widely applied in enantioselective reactions. In this Account, we present new insights into these catalysts, revealing how the roles of anions, acting as substrates, reagents, and cocatalysts, can be strategically leveraged to achieve remarkable enantioselectivity across a wide range of organic transformations.When acting as substrates, anions, such as sulfinates, thiocarboxylates, and azides, are rendered reactive through intimate ion pairing with the chiral guanidinium moiety. This strategy facilitates desymmetrization processes, exemplified by the conversion of sulfinates to enantioenriched sulfinate esters and the remote desymmetrization of cis-dibromocyclohexanone via sequential S_N2 and acyl transfer steps. Mechanistically, halogenophilic S_N2X pathways (e.g., thiocarboxylate substitutions at sterically hindered tertiary carbons) bypass traditional steric limitations, while dynamic kinetic resolution of racemic bromides via azide substitution highlights the interplay between ion exchange and interfacial dynamics.Anions generated in situ from stoichiometric reagents give rise to highly reactive intermediates such as enolates, sulfenates, and hypervalent silicates, which form ion pairs with chiral cations, enabling enantioselective transformations. For instance, enolates displace tertiary bromides via an S_N2X mechanism (frontside attack), circumventing steric hindrance. Sulfur alkylation of sulfenamides yields chiral sulfilimines, while fluoride-activated acylsilanes undergo Brook-like rearrangements through penta-coordinate silicates. Silicon hydrides activated by fluoride form hydridosilicates, enabling enantioselective conjugate reductions of chromones and coumarins. The versatility of ion pairing is further illustrated by α-cyano carbanions in Pd-catalyzed decarboxylative allylic alkylations and is extended to a cooperative catalytic system, where DMAP-generated nucleophiles enable enantioselective phospha-Michael additions via dynamic cation-exchange activation.The utilization of inorganic anions as cocatalysts further expands the scope of chiral cation ion pair catalysis. Peroxytungstate anions synergize with chiral cations to enable the epoxidation of allylic amines, while peroxomolybdate facilitates the N-oxidation of tertiary amines with high enantioselectivity. Beyond inorganic systems, organic anions also serve as effective cocatalysts. Notably, preformed pentanidium pyridinyl-sulfonamide ion pairs have been shown to catalyze the enantioselective Steglich rearrangement with high efficiency. This strategy extends to bisguanidinium sulfonated-phosphine/Pd ion pair catalysis, achieving stereocontrol in allylic amination of Morita-Baylis-Hillman substrates.By systematically delineating the diverse roles of anions in guanidinium ion pair catalysis, this Account highlights new mechanistic insights and synthetic applications, paving the way for further advancements in asymmetric catalysis through precise control of ionic interactions.

Halogenophilic nucleophilic substitution (S_N2X) has a distinctly different reaction pathway compared to the bimolecular nucleophilic substitution reaction (S_N2), but they can lead to the same reaction product. However, their differences can be distinguished by studying stereoselective reactions in which both reaction pathways are possible. Herein, we utilize the stereospecific nature of S_N2 and the presence of a pro-chiral anion intermediate in S_N2X to conduct a quantitative study in which both S_N2 and S_N2X reactions exist. We developed a procedure supported by kinetic simulations to measure the halogenophilic percentage (X%). We also developed a new parameter, relative halogenophilicity (H), to quantify the intrinsic characteristics of S_N2X reactions and found it to correlate well with the Hammett and Mayr postulates. We studied the time dependency of X% under different reaction conditions, which led to the discovery of two new distinct mechanisms: chalcogenophilic nucleophilic substitution (S_N2Ch) reaction and bromide-catalyzed dynamic kinetic resolution. The S_N2 and S_N2X reactions exhibit similar behavior from both thermodynamic and kinetic perspectives, suggesting that they occur to varying degrees in most of the reactions. Consequently, they are inherently linked and should not be considered in isolation.

Antimony is a fifth-period element in the nitrogen family, a silver-white metalloid with weak conductivity and thermal conductivity. It is stable at room temperature and does not react easily with oxygen and water in the air. Natural minerals are found in the form of sulfides. Current research and applications are mostly concentrated on material modification, utilizing the properties of antimony in traditional chemical industries, helping alloys improve their flame retardancy, stability, increasing semiconductor performance, etc. For example, to enhance the electronic conductivity, after coating or embedding antimony or its derivatives in thin layers in photonic nanomaterials, the performance of the original material in photoelectrochemical catalysis can be effectively increased, thereby expanding the efficiency of oxidation-reduction reactions accounting for the degradation of organic matter in wastewater. However, the catalytic reaction between the derivatives of antimony and organic compounds beside the material is less studied, and the mechanism of the studies in organic synthesis is relatively unclear. The reported organic synthesis related to antimony is mainly in the form of Lewis acid catalysts or dual-metal catalytic systems combined with other metals. This Review will focus on the application of antimony in photocatalysis, electrocatalysis, and other organic syntheses in the past 10 years.

N-oxides play a pivotal role in natural products and emerging drug design, while also serving as valuable ligand scaffolds in organometallic chemistry. Among heteroatom oxidations, the conversion of amines to N-oxides is a critical and challenging facet. We present here a highly enantioselective N-oxidation methodology for both cyclic and acyclic amines. The method employs an ion-pair catalyst comprising a chiral bisguanidinium [BG]²⁺ cation and an achiral oxodiperoxomolybdosulfate anion [(µ-SO₄)₂Mo₂O₂(µ-O₂)₂(O₂)₂]^2-. Notably, the bisguanidinium cation undergoes modification through silyl group incorporation and is elucidated by X-ray crystallography. Our findings underscore the crucial role of the side chain in the determination of the chiral pocket size, allowing for the oxidation of diverse tertiary amines with enantioselectivities. Comprehensive mechanistic investigations are conducted to explain the catalytic system’s efficacy in achieving dynamic kinetic resolution (DKR) with high efficiency.

In this work, we describe an enantioselective reduction and sulfenylation of isoflavanone derivatives by an ion pair strategy. The chiral cationic catalyst bisguanidinium (BG) is capable of chiral induction in catalytic systems. Silane hydride works as a reductant and helps to form an anionic hypervalent silicate complex and intermediates with substrates to pair with chiral catalyst. A series of umpolung sulfur reagents accomplish electrophilic attack in the presence of a silicate anion. Both chemoselectivity and enantioselectivity are good to excellent to afford a wide scope of 4-oxo-4H-chromene-3-carbonitrile and S-electrophilic reagents. Further transformations were completed to introduce more applications.