Naohiro Kirai

Efficient Synthesis of Diborylalkenes from Alkenes and Diboron by a New PSiP-Pincer Palladium-Catalyzed Dehydrogenative Borylation

The efficient synthesis of various diborylalkenes such as 1,1-, trans-1,2-, and cyclic 1,2-diborylalkenes from alkenes and diboron was achieved for the first time. Selective preparation of di- and monoborylalkenes was also realized by the appropriate choice of reaction conditions. The reaction was found to proceed via a new mechanism of dehydrogenative borylation through a monoborylpalladium complex bearing an anionic PSiP-pincer ligand as a key intermediate, which realized the efficient borylation without sacrificial hydroboration or hydrogenation of the alkene.

Synthesis of 4-arylcoumarins via Cu-catalyzed hydroarylation with arylboronic acids.

In the presence of 2-4 mol % of CuOAc, methyl phenylpropiolates having a MOM-protected hydroxy group at the ortho position underwent hydroarylation with various arylboronic acids in MeOH at ambient temperature, resulting in the formation of 4-arylcoumarins in high yields after the acidic workup. This method was effectively used for the synthesis of biologically active natural and artificial compounds.

Two reversible σ-bond metathesis pathways for boron-palladium bond formation: selective synthesis of isomeric five-coordinate borylpalladium complexes.

Two reversible σ-bond metathesis pathways for B-B bond activation to give borylpalladium complexes are demonstrated in the reaction of η(2)-(Si-H)Pd(0) complexes with B(2)pin(2). These two pathways are connected by fluxional behavior of the Si-H bond and can be efficiently controlled by the appropriate choice of phosphine ligand, enabling the selective synthesis of two types of five-coordinate borylpalladium complexes.

Synthesis of 4-aryl-substituted butenolides and pentenolides by copper-catalyzed hydroarylation

We have investigated the effect of the type of protecting group used and the tether length of 4- and 5-hydroxy-substituted 2-alkynoates on the yields and selectivity of the product of hydroarylation. The use of methyl, methoxymethyl, and tert-butyldimethylsilyl (TBS) as protecting groups increased the total yield of the products. While the protected 4-hydroxy-2-butynoates afforded products formed from two or more alkyne substrates, 5-hydroxy-2-pentynoate derivatives exclusively yielded 1:1 adducts. These facts suggest that the product selectivity depends on the distance of the alkoxy group from the alkyne moiety rather than the type of the protecting group. Among the protecting groups used in this study, the TBS group was found to produce butenolides and pentenolides in good total yields via the one-pot hydroarylation/lactonization.