Hiyama is best known for developing:
*The Nozaki-Hiyama-Kishi reaction (NHK reaction) is a nickel/chromium mediated cross-coupling reaction between an allyl,  vinyl or aryl halide and an aldehyde to form an alcohol upon aqueous workup. 
It was originally discovered in 1977, where Hiyama and Nozaki reported a chemospecific synthesis of homoallyl alcohols from an aldehyde and allyl halide using chromium(II) chloride.
In 1983, Hiyama and Nozaki published another paper extending the scope of the reaction to include aryl and vinyl halides.
In 1986, Nozaki and Kishi independently discovered that the reaction depended on the nickel impurities in the chromium(II) chloride salt. Since then, nickel(II) chloride has been used as a co-catalyst.
The NHK reaction demonstrates high chemoselectivity towards aldehydes, as it tolerates a range of functional groups, and has been used on the process scale.
*The Hiyama coupling is a palladium-catalyzed cross-coupling reaction between aryl, alkenyl or alkyl halides and an organosilicon compound to form a C-C bond.
:* : Aryl, Alkenyl or Alkynyl
:* : Aryl, Alkenyl, Alkynyl or Alkyl
:* : Cl, F or Alkyl
:* : Cl, Br, I or OTf
Hiyama developed this reaction in 1988. He says he developed this method in order to overcome the shortcomings of Grignard reagents. While Grignard reagents are powerful, Hiyama says, they can be hard to use in total synthesis as they are not as tolerant of other functional groups.