Prof. Kyoko Nozaki has consistently delivered very significant, highly creative, intellectually leading and industrially transformative contributions to the research field of homogeneous catalysis. She rationally designed metal complex catalysts based on the understanding of the reaction mechanism and accomplished efficient activation of polar small molecules such as carbon monoxide, carbon dioxide, and polar group-substituted ethylenes for various synthetic applications. Her major achievements are summarized below.
1. Alternating copolymerization of olefins with carbon monoxide
In collaboration with the late Prof. Takaya, Prof. Nozaki developed an unsymmteric bidentate phosphine-phosphite ligand BINAPHOS for highly enantioselective rhodium-catalyzed asymmetric hydroformylation of olefins. Prof. Nozaki applied this ligand to the asymmetric synthesis of optically active polymers. Using palladium complex of BINAPHOS as a catalyst, alternating copolymerization of carbon monoxide and mono-substituted ethenes such as propylene and styrene proceeded with almost complete regio and enantioselectivity (>95 % ee). Historically, since Natta succeeded in synthesizing isotactic polypropylene, tacticity control (control of relative configuration) has been mainstream in the field of synthetic polymers. Prof. Nozaki pioneered a field "asymmetric synthesis polymerization" in which chiral carbon centers were created with stereocontrol during the polymerization process.
2. Copolymerization of propylene with polar monomers
Although polyolefins such as polyethylene and polypropylene are inexpensive and widely used commodity polymers, their inherently nonpolar nature causes problems in surface properties such as adhesiveness, printability and miscibility. To solve this problem, palladium and nickel catalysts were developed over two decades for the copolymerization of ethylene with polar monomers such as methyl acrylate. Notably, however, there was no example for the use of propylene instead of ethylene until the report by Prof. Nozaki. Based on experimental and theoretical analyses of the reaction mechanism, Prof. Nozaki clarified why this polymerization was impossible with the conventional systems. That is, in the late transition metal catalyst, the polymerization is immediately terminated if the propylene insertion takes place in a 2,1-fashion (in the direction in which the metal is bonded to carbon inside propylene to a carbon-metal bond). Based on this analysis, Prof. Nozaki designed a carbene complex IzQO with bulky substituents to control the direction of propylene insertion to 1,2-mode. With its palladium complex, she achieved the first copolymerization of propylene and polar monomers. Furthermore, she expanded the research field to the synthesis of isotactic polypropylene with polar functional groups.
3. Efficient production of formic acid by hydrogenation of carbon dioxide
Carbon dioxide (CO2) is an easily available and inexpensive carbon source, and its effective use is highly desired. Formic acid synthesis by hydrogenation of CO2 has attracted attention as an alternative to the industrial process using carbon monoxide. Prof. Nozaki discovered that the iridium PNP pincer complex acts as an unprecedented high-activity catalyst (3.5 million catalyst turnover numbers) in the hydrogenation of CO2 in aqueous potassium hydroxide. The product is only formate and does not produce side-products such as carbon monoxide. Still now, this is the highest turnover number for the hydrogenation of CO2. Further modification of the pincer system allowed the catalyst highly active with organic bases, which allow the purification of formic acid by distillation.
4. Copolymerization of carbon dioxide with epoxide
The synthesis of aliphatic polycarbonate by alternating copolymerization of epoxide and carbon dioxide, discovered over half a century ago, is a promising use of carbon dioxide. Prof. Nozaki focused on the fact that, when the polymerization is mediated by a combination of cobalt complex and ammonium salt, the polymerization reaction rate was first order for both the cobalt catalyst and ammonium salt cocatalyst. Based on the hypothesis that the ammonium salt acts as a counter cation for the anionic growing polymer chain-end, she prepared a cobalt complex with an amino group in the molecule to achieve high conversion with high molecular weight. Her creation of this bifunctional cobalt complex led to the development of many bifunctional and bimetallic catalysts later.
5. Synthesis of copolymer of carbon dioxide and 1,3-butadiene
A long-standing goal in CO2 utilization has been to discover how to combine CO2 with alkenes to prepare polymers. Prof. Nozaki published a land-mark report of a catalytic route to circumvent the thermodynamic and kinetic barriers for copolymerization by using a metastable lactone intermediate. By exploiting known palladium catalysis, she made a lactone by the condensation of carbon dioxide and 1,3-butadiene. The subsequent free-radical polymerization of this lactone afforded polymers of high molecular mass and with carbon dioxide contents of up to 29 wt%.
In addition to above, Prof. Nozaki also discovered original catalyst systems potentially applicable to the effective use of renewable resources such as biomass. These achievements greatly contributed to the development of diverse fields not limited to homogeneous catalysis, but to organometallic chemistry, polymer synthesis, and organic synthesis. Therefore, Prof. Nozaki's work is highly deserving of the Japan Chemical Society award.