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This was the first demonstration that an SN2 type reaction is possible at cyclopropane carbons and stimulated research on use of ate complexes for geminal difucntionalization. Later, he extended the concept to synthesis of gem-dimetalated synthetic reagents, that allow multi step carbon framework construction in short steps, through the reaction of carbenoid species with inter-element compounds like diboron and silylborane. Meantime, he invented many fluorine-substituted carbenoid type synthetic reagents and applied these for synthesis of fluorine-containing artificial pyrethroids and biologically active agents. He was one of the pioneers in use of magnesium ester enolates and achieved cross Thorpe reaction for the first time by carrying out the reaction of the enolates with nitriles. This transformation is called Hiyama Reaction. Furthermore, he was successful in generating anhydrous CrCl2 from CrCl3 in THF and used the reagent for reduction of various organic halides. In particular, allylic chromium species, generated in situ from allylic halides, are found to add across aldehydes and ketones with high chemoselectivity and stereoselectivity. Worthy to note is extremely high functional group compatibility. He proposed chair-like 6-membered transition states to understand threo-selectivity observed in crotyl/aldehyde addition. This finding initiated intensive research on use of various low valent transition metal and lanthanide metal reagents for organic synthesis and opened a way for acyclic stereoselection with allylic metal reagents. Using commercially available CrCl2, he could achieve the Barbier-type vinyl addition to aldehydes. This particular transformation is catalyzed, as disclosed later by Takai and Kishi, by a nickel catalyst which was involved in the commercially impure reagent. The whole transformation proceeds under extremely mild conditions so that carbon-carbon bond formation is possible in late stages of total synthesis of structurally highly sophisticated natural products. The reaction is now named Nozaki-Hiyama-Kishi (NHK) Reaction.
Hiyama applied a nucleophilic activation of organosilicon compound with tetraalkylammonium fluoride to hydrosilanes, which would give pentacoordinate hydrosilicates. This hyper-coordinate species was found to reduce β-keto amides with high stereoselectivity. The naked carbenoid carbanions also are generated from the corresponding silicon agents and fluoride ion and employed for carbon-carbon bond formation at ambient temperatures. These findings led him to disclose the cross-coupling reaction of organosilicon compounds. What he found is that transmetalation from tetragonal silicon to palladium proceeds smoothly in the presence of a fluoride ion, and the resulting pentacoordinate silicates are responsible for successful transmetalation. Taking advantage of the configurational stability of organsilicon compounds, he uncovered stereochemical aspects of transmetalation and regio-cotrolled coupling of allylic silanes. The silicon-based cross-coupling reaction is now well-accepted as Hiyama Coupling. Recently he has invented a newer version of the silicon-based cross-coupling that involves intramolecular silicon activation and allows recovery of a silicon moiety. The feature of this new version is the reliability of the coupling position: ipso-coupling with ?-substituted vinylsilanes take place in contrast to the same transormation with other metallic reagents of main group elements. The intramolecular activation is shown to be effective also for Rh-catalyzed Michael addition of organosilicon agents.
In addition, Hiyama has invented many useful synthetic reactions including carbostannylation and carbocyanation reactions of alkynes and dienes, oxidative desulfurization fluorination of organosulfur compounds, and invented many ferroelectric and/or fluorine-containing liquid crystals useful for future display. He has been always playing a leading role in inventing novel synthetic methods and novel liquid crystal materials. His synthetic work is well-appreciated in worldwide communities.
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