| Research Abstract |
1)Catalytic Hydroboration of Terminal Alkynes The complex in situ generated from [M(cod)Cl]_2(M = Rh or Ir) and P'Pr_3 catalyzed a stereo- and regioselective trans-hydroboration of terminal alkynes with catecholborane or pinacolborane in the presence of Et_2N. The reaction yielded the (Z)-l-alkenylboronates with more than 96% selectivity for various terminal alkynes.
2)Synthesis of Organoboronic Esters using Diboron The metal-catalyzed borylation of alkenes, alkynes, and organic electrophites with B-B compounds was developed for the synthesis of organoboronic esters from simple organic substrates. The platinum(0)-catalyzed addition of bis(pinacolato)diboron to alkenes and alkynes stereoselectively yielded cis-bis(boryl)alkanes or cis-bis(boryl)alkenes. The addition of diboron to I.3-dienes with a platinum(0) complex afforded a new access to the cis-1, 4-bis(boryl)butene derivatives which are a versatile reagent for diastereoselective allylboration of carbonyl compounds. The cross-coupli
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ng reaction of diborons with aryl and vinyl halides or triflates, and allyl chlorides or acetates was found to yield aryl-, vinyl-, and allylboronates in high yields in the presence of a base and a palladium catalyst, which provides the first one-step procedure for the synthesis of organoboronic esters from organic electrophites.
Iridium complexes comprised of [Ir(Ome)(cod)]_2 and 4, 4'-di-tert-butyl-2, 2'-bipyridine are highly active catalysts for the aromatic C-H borylation by bis(pinacolato) diboron. This high activity allowed for the first room temperature borylation of arenes with a stoichiometric amount of arene to produce the corresponding arylboronates in high yields. With this advance, the aromatic C-H borylation provides a practical tool for preparing arylboronates. The catalytic C-H borylation of other hydrocarbons is being actively investigated. The reactions of five-membered substrates such as thiophene, furan, pyrrole, and their benzo-fused derivatives exclusively produced 2-borylated products, whereas those of six-membered heterocycles including pyridine and quinoline selectively occurred at the 3-position. Regioselective synthesis of bis(boryl)heteroaromatics was also achieved by using an almost equimolar amount of substrates and the diboron.
3)Asymmetric Carbon-Carbon Bond Formation using the Rhodium Catalyst Addition Reaction Various rhodium(1)complexes catalyze the 1, 4-addition of organoboronic acids to α, β-unsaturated carbonyl compounds or the Grignard-type addition to aldehydes in an organic solvent or a mixture of an organic solvent and water. The reaction provides a convenient method for the asymmetric carbon-carbon bond formation. The rhodium-catalyzed conjugate addition of tolylboronic acid to α, β-unsaturated ketones, esters or amides was carried out in water. [RhCl(cod)]2 and 2 β-cyclodextrin-[Rh(OH)(cod)]_2complexes were found to catalyze the reaction efficiently in a single aqueous medium.
The conjugate addition of arylboronic acids to α, β-unsaturated amides was carried out in the presence of a chiral rhodium catalyst and an aqueous base. The catalyst prepared in situ from Rh(acac)(CH_2=CH_2)_2 and (S)-BINAP providedR-N-benzyl-3-phenylbutanamide with 93% ee in the addition of phenylboronic acid to N-benzyl crotonamide. The reaction suffered from incomplete conversion resulting in moderate yields, but addition of an aqueous base, such as K_2CO_3(10-50 mol%) was found to be highly effective to improve the chemical yields. The role of the base giving a RhOH species active for transmetalation with arylboronic acids was discussed.
4)Conjugate Addition Reaction of Heteroatom-Boron Bond The Michael type addition reaction of B-N, B-O, and B-S bond by the rhodium catalyst is under development
5)Iridium Catalyzed Reaction 3-Alkoxy-1-alkynes were hydroborated with pinacolborane(Hbpin) to give 3-alkoxy-1-alkenylboronates. The latter gave (E)-γ-alkoxyallylboronates((E)-(MeO)_2CHCH_2(CH_2)_nCH_2OCH=CHCH_2Bpin, n=t-3) when they were subjected to iridium-catalyzed isomerization of the double bond. The corresponding(Z)-isomers were synthesized by nickel-catalyzed isomerization. Both allylboronates underwent intramolecular allylboration leading to the formation of trans-2-(ethenyl)tetrahydropyran-3-ol or 2-(ethenyl)oxepan-3-ol and the corresponding cis-isomers in the presence of Yb(Otf)_3(20% mol%) in aqueous acetonitrile at 90℃. Less
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