Professor Kazushi Mashima has focused his interest on the development of original metal-catalyzed reactions based on his deep understanding of reaction mechanisms obtained using comprehensive organometallic chemical approaches such as isolation and determination of key intermediate species as well as kinetic analysis. His achievements are highlighted by the following four topics.

1. Organosilicon Compounds for Salt-free Reduction of Metal Compounds, Generating Catalytically Active Species
Low-valent transition metal complexes have been utilized as reagents and catalysts to mediate bond forming reactions. Various reagents have been developed to reduce higher-oxidation state metal precursors such as metal halides; however, the interaction of the resulting salts with the generated low-valent or zero-valent metal species disrupted their intrinsic reactivity and catalytic performance. Prof. Mashima developed a conceptionally new methodology for generating low-valent catalytically active metal species in a salt-free manner using 3,6-bis(trimethylsilyl)-1,4-cyclohexadiene and 1,4-bis(trimethylsilyl)-1,4-dihydropyrazine as well as their derivatives as versatile reducing reagents. With these new reducing reagents, Prof. Mashima demonstrated that salt-contact-free low-valent TaCl₃ exhibited high catalytic activity for the trimerization of ethylene to 1-hexene (>1000 TOF) with excellent selectivity (>98%), and the salt-free tantalum system enabled us to spectroscopically observe the direct evidence for a metallacycle mechanism.
Prof. Mashima applied the salt-free reduction method to reduce well-defined monometallic tungsten oxo surface species for the generation of an active species of alkene metathesis at only 70 °C, a much lower temperature compared with that (>350 °C) used for industrial operation, and provided evidence for the formation of tungsten(IV) species that reacted with alkenes to form a metallacyclopentane intermediate followed by alkylidene species. 　

2. Mechanical Approaches using Triply-halide-bridged Dinuclear Iridium and Rhodium Catalysts for Asymmetric Hydrogenations of N-Heteroaromatics and Simple Olefins
Catalytic asymmetric hydrogenations of prochiral unsaturated compounds including C=C, C=O, and C=N bonds have been intensively investigated. In contrast, asymmetric hydrogenation of N-heteroaromatic compounds has been considered a challenging reaction because of the resonance stability of N-heteroaromatic compounds. Prof. Mashima developed asymmetric hydrogenation of 2-substituted quinoxalines 3 using the originally designed air-stable and handling-friendly chiral cationic dinuclear triply chloride-bridged iridium complexes [{Ir(H)[diphosphine]}₂(μ-Cl)₃]Cl [(S)-1a: diphosphine = (S)-BINAP; (S)-2a: diphosphine = (S)-SEGPHOS]. He found that an amine additive enhanced not only the catalytic activity but also the enantioselectivity. Mechanistic studies indicated a dual mechanism involving two individual catalytic cycles in equilibrium. In addition, Prof. Mashima revealed that the hydrogenated product had similar additive effects as well as positive feedback enhancement in which the presence of a partially enantiomerically-enriched hydrogenated product improved both the reaction rate and the enantioselectivity. This is the first report of positive feedback enhancement in asymmetric hydrogenation, which is rationalized by the proposed dual mechanism. Moreover, he demonstrated that these iridium dinuclear catalysts served as superior catalysts for asymmetric hydrogenations of isoquinolinium salts, quinazolinium salts, and pyridinium salts. Furthermore, Prof. Mashima found that chiral chloride-bridged dinuclear rhodium(III) complexes proficiently catalyzed the asymmetric hydrogenation of simple olefins together with allylic alcohols, alkenylboranes, and unsaturated cyclic sulfones. 　

3. Alkoxide-bridged Dinuclear Complexes of Cobalt(II) as a Key Intermediate through an Ordered Ternary Complex Mechanism for Hydroxy Group-Selective Transesterification in the Presence of an Amine Group
Catalytic transformation of ester moieties provides both key intermediates and protecting groups in organic chemistry. Prof. Mashima originally found that μ-oxo-tetranuclear zinc cluster Zn₄(OCOCF₃)₆O served as a unique and efficient catalyst for chemoselective acylation of hydroxy groups in the presence of an amino group. He recently found that such chemoselective acylation was achieved using not only zinc carboxylates, but also various carboxylates of first-row late-transition metals such as Mn, Fe, Co, and Cu. Prof. Mashima demonstrated that octanuclear cobalt carboxylate clusters [Co₄(OCOR)₆O]₂ supported by nitrogen-containing ligands such as 2,2'-bipyridine were an efficient catalyst system for the chemoselective transesterification, and confirmed that an alkoxide-bridged dinuclear cobalt (II) complex, Co₂(OCO^tBu)₂(bpy)₂(OCH₂-C₆H₄-4-CH₃)₂, was a key intermediate through an ordered ternary complex mechanism, similar to dinuclear metallo-enzymes, providing chemical insight into the reason why various esterases, peptidases, etc, favorably contain Mn, Co, and Zn as their dinuclear active sites.

4. σ-Bond Metathesis Mechanism for C-H Bond Activation and Functionalization of Pyridine Derivatives by Alkyl and Amide Complexes of Early-transition Metals
C-H bond activation followed by functionalization plays key roles for constructing C-C bond formations. Prof. Mashima paid focused on σ-bond metathesis reactions of pyridine derivatives using alkyl and amide complexes of early-transition metals to develop three new catalytic reactions based on the isolation of key intermediates and previous kinetic measurements. First, non-metallocene cationic hafnium alkyl complexes became catalysts for a novel oxidant-free cross dehydrogenative coupling reaction of 2,6-lutidine and internal alkynes to give five-membered carbocyclic compounds. Based on controlled experiments, he revealed that the reaction started by C(sp³)-H bond activation via σ-bond metathesis, followed by insertion, migration, and β-H elimination reactions around a coordinatively unsaturated cationic hafnium center.
Prof. Mashima found that yttrium alkyl complexes worked as catalysts for the living polymerization of 2-vinylpyridine. Noteworthy was that end-capping functional groups was introduced into poly(2-vinylpyridine)s via initial C-H bond activation of N-heteroaromatics and internal alkynes by yttrium alkyl precursors through an alkylyttrium-mediated s-bond metathesis reaction prior to starting the living polymerization of 2-vinylpyridine.
The third reaction was a catalytic aminoalkylation of pyridine derivatives, in which triamido complexes of group 3 metals such as yttrium and gadolinium activated the ortho C-H bond of pyridines and N-heteroaromatics, and the resulting metal-carbon bonds reacted with the C=N bond of imines to afford the corresponding aminomethylated products.

As mentioned above, with his strong background in organometallic chemistry, Prof. Mashima has conducted intensive approaches, including isolation and characterization of key intermediates, along with kinetic analyses to prepare original and precisely designed homogeneous catalysts to achieve extremely difficult substrate conversion reactions. His contributions have inspired many chemists worldwide, and therefore the Chemical Society of Japan has recognized that Prof. Mashima deserves the CSJ Award. 1