Dr. Takashi Ooi has made a significant yet leading contribution to the field of organic molecular catalysis through the ingenious design of chiral organic cations and anions as novel molecular catalysts capable of directly and precisely controlling anions, cations, and ion radicals, fundamental reactive species in organic chemistry, enabling the development of synthetically relevant stereoselective carbon-carbon and carbon-heteroatom bond-forming reactions. His major achievements are summarized below. 　

1. Creation of catalytic functions based on the design of chiral organic cations 　
Anions are one of the most fundamental reactive species and are widely used to form new bonds for synthesizing the desired organic molecules. Therefore, "organic ion-pair catalysis" that allows the direct control of the reactive anionic intermediate by the pairing organic cation through electrostatic interaction becomes important. However, the chemistry of exploiting organic ion pairs as molecular catalysts has been very limited. To address this intrinsic problem, Dr. Ooi has designed and synthesized structurally rigid and readily modifiable organic cations for use as molecular catalysts. For instance, he has developed intramolecular ion-pairing, chiral ammonium betaines and demonstrated their functions as enantioselective organic base and anionic nucleophilic catalysts for the asymmetric construction of tetrasubstituted carbon stereocenters. Further, he has designed a series of P-spiro chiral tetraaminophosphonium salts. By the appropriate modifications on a single core structure, he has brought out the inherent ability of tetraaminophosphonium salt to exert four different asymmetric catalyses; these are (1) strong organic base catalysis of the conjugate base, triaminoiminophosphorane, (2) phase-transfer catalysis of fully N-alkylated aminophosphonium salts, (3) Brǿnsted acid catalysis of arylamine-derived aminophosphoniums bearing non-coordinating counterions, and (4) cooperative catalysis of supramolecularly assembled aminophosphonium phenoxides. Importantly, each catalysis has been utilized for the development of synthetically valuable stereoselective transformations, which provides the reliable access to structurally diverse, pharmaceutically attractive chiral building blocks such as optically active anti-1,2-amino alcohols, α-hydroxy phosphonates, unnatural oligopeptides, and 1,2-diamines. During this endeavor, Dr. Ooi uncovered the relevance of the precise control of the transition-state structures based on the recognition of anions or anion radicals by aminophosphonium cations to attaining requisite reactivity and selectivity for the target reactions. Moreover, he has succeeded in the molecular design of chiral 1,2,3-triazolium salts and elicited their inherent potential as an anion-recognizable molecular catalyst in developing asymmetric alkylation, aziridine ring opening, and amination.

2. Development and applications of ion-paired chiral ligands 　
Concurrently, Dr. Ooi has expanded the potential of his approach based on the molecular design of organic ions by developing an ion-paired chiral ligand consisted of an achiral phosphine having a cation site and a readily available chiral anion for transition-metal catalysis. The palladium complex of the ion-paired ligand exerted excellent catalytic activity and stereocontrolling ability in the allylic alkylation of α-nitrocarboxylates. This finding provides a solution to the long-standing problem associated with the conventional chiral ligands, whose complex synthesis hinders rapid and extensive screening. In addition, he has evolved the structure of the ion-paired ligands to realize a palladium-catalyzed asymmetric [3+2]-annulation for the construction of contiguous all-carbon quaternary stereocenters in a five-membered heterocyclic skeleton. 　

3. Creation of catalytic functions based on the design of non-nucleophilic chiral organic anions
Upon looking over the reaction systems with chiral ion-pair catalysts, strategies for controlling reactive cationic species by a catalytic amount of anionic molecules are extremely limited. This is simply because the highly reactive cationic intermediate readily reacts with the anionic catalyst, losing reactivity. Dr. Ooi conceived that this intrinsic problem stemmed from the nucleophilic nature of the anion used as a catalyst and embarked on the development of non-nucleophilic chiral anions for the direct and precise control of cationic intermediates to realize otherwise difficult organic transformations. Specifically, he has designed and synthesized a novel hexacoordinated chiral phosphate ion and demonstrated that the corresponding hydrogen phosphate effectively acts as a chiral Brǿnsted acid catalyst for Pictet-Spengler reaction, where the phosphate ion rigorously discriminates the prochiral iminium-ion intermediate. He further designed and assembled structurally robust borate ions featuring tetradentate chiral backbone and unveiled the ability of the hydrogen borate to differentiate the oxonium-ion intermediate in achieving Prins-type cyclization of vinylic ethers. These achievements underscore the significant potential of the catalysis of non-nucleophilic chiral anions as a general tool for dictating reactivity and selectivity of cationic reaction intermediates.

Dr. Ooi's designer chiral organic ions comprise universal elements and are stable and easy-to-handle as a form of ion pairs. It is important to emphasize that his catalysts are modular and thus appropriate structural manipulations allow for fine-tuning the catalytic activity and stereoselectivity for targeted transformations, greatly expanding the scope of asymmetric ion-pair catalysis. In this regard, he was one of the first to appreciate that hydrogen-bonding can be used as a strategic secondary interaction for defining the mutual disposition of a cation and an anion in the requisite ion-pairing. From this insight, he introduced a concept of "structured ion pair", through which otherwise ambiguous distance and direction of the orbital-free electrostatic interaction between two ions can be regulated in order to control the reaction of the chiral ion pair with various electrophiles and nucleophiles in a predictable manner. This conceptual framework has had a significant impact on the frontiers of chemistry in terms of developing a fundamental understanding of the relationship between the three-dimensional structure of a chiral organic ion-pair catalyst and its reactivity and selectivity in organic synthesis. Therefore, his achievements have been recognized worldwide and are worthy of the Chemical Society of Japan Award.