Toshimichi Ohmura

Stereospecific Suzuki−Miyaura Coupling of Chiral α-(Acylamino)benzylboronic Esters with Inversion of Configuration

The first invertive B-alkyl Suzuki-Miyaura coupling has been achieved. The coupling of enantioenriched α-(acylamino)benzylboronic esters with aryl bromides and chlorides took place efficiently in toluene at 80 °C in the presence of Pd(dba)(2) (5 mol %), XPhos (10 mol %), K(2)CO(3) (3 equiv), and H(2)O (2 equiv). The reaction proceeded with inversion of configuration to give diarylmethanamine derivatives in high yields with high conservation of enantiomeric excesses.

Inversion or Retention? Effects of Acidic Additives on the Stereochemical Course in Enantiospecific Suzuki–Miyaura Coupling of α-(Acetylamino)benzylboronic Esters

The stereochemical course of the stereospecific Suzuki-Miyaura coupling of enantioenriched α-(acetylamino)benzylboronic esters with aryl bromides can be switched by the choice of acidic additives in the presence of a Pd/XPhos catalyst system. Highly enantiospecific, invertive C-C bond formation takes place with the use of phenol as an additive. In contrast, high enantiospecificity for retention of configuration is attained in the presence of Zr(Oi-Pr)(4)·i-PrOH as an additive.

Regioselective Synthesis of 1,2-Dihydropyridines by Rhodium-Catalyzed Hydroboration of Pyridines

Pyridine undergoes addition of pinacolborane at 50 °C in the presence of a rhodium catalyst, giving N-boryl-1,2-dihydropyridine in a high yield. The selective 1,2-hydroboration also takes place in the reactions of substituted pyridines. In the reaction of 3-substituted pyridines, 3-substituted N-boryl-1,2-dihydropyridines are formed regioselectively.

Switch of Regioselectivity in Palladium-Catalyzed Silaboration of Terminal Alkynes by Ligand-Dependent Control of Reductive Elimination

The regioselectivity in the addition of silylboronic esters to terminal alkynes can be switched by the choice of phosphorus ligands on the palladium catalysts. The silaboration proceeds with normal regioselectivity in the presence of (eta(3)-C(3)H(5))Pd(PPh(3))Cl (1.0 mol %) to give 1-boryl-2-silyl-1-alkenes in high yields. In sharp contrast, selective formation of the inverse regioisomers, 2-boryl-1-silyl-1-alkenes, takes place when the reaction is carried out with a palladium catalyst bearing P(t-Bu)(2)(biphenyl-2-yl). A reaction mechanism for the change of regioselectivity that involves reversible insertion/beta-boryl elimination steps is proposed.

Palladium-catalyzed regioselective silaboration of pyridines leading to the synthesis of silylated dihydropyridines.

The addition of silylboronic esters to pyridine takes place in toluene at 50 °C in the presence of a palladium catalyst to give N-boryl-4-silyl-1,4-dihydropyridines in high yield. The regioselective 1,4-silaboration also proceeds in the reaction of 2-picoline and 3-substituted pyridines, whereas 4-substituted pyridines undergo 1,2-silaboration to give N-boryl-2-silyl-1,2-dihydropyridines regioselectively.

Nickel-Catalyzed Ring-Opening Hydroacylation of Methylenecyclopropanes: Synthesis of γ,δ-Unsaturated Ketones from Aldehydes

A nickel-catalyzed intermolecular hydroacylation of methylenecyclopropanes (MCPs) has been developed. The reaction proceeds with stereospecific cleavage of the proximal C-C bond of the cyclopropane ring to give gamma,delta-unsaturated ketones with high diastereoselectivities. A nickel catalyst generated in situ from Ni(cod)(2) and P(n-Bu)(3) with a P/Ni ratio of 1:1 is effective for the hydroacylation, in which benzaldehyde derivatives, heteroaryl aldehydes, and aliphatic aldehydes react with MCPs at 60-100 degrees C to afford the corresponding ketones in high yields.

Catalytic Functionalization of Methyl Group on Silicon: Iridium-Catalyzed C(sp3)–H Borylation of Methylchlorosilanes

A methyl group of methylchlorosilanes undergoes C-H borylation in an iridium-catalyzed reaction with bis(pinacolato)diboron in cyclohexane at 80 °C, giving (borylmethyl)chlorosilanes selectively.

Silylboranes Bearing Dialkylamino Groups on Silicon as Silylene Equivalents: Palladium-Catalyzed Regioselective Synthesis of 2,4-Disubstituted Siloles

Silylpinacolboranes bearing dialkylamino groups on the silicon atom served as synthetic equivalents of silylene in palladium-catalyzed reactions with terminal alkynes, leading to the formation of 2,4-disubstituted siloles in high yield. It was found that the amino group on the silicon atom was critically important for the reaction; no silole products were found in reactions using silylpinacolboranes carrying aryl, chloro, or alkoxy groups on the silicon atoms. Site-selective bromination of 1,1-dimethyl-2,4-diphenylsilole followed by Migita−Kosugi−Stille coupling with (arylalkynyl)tributylstannanes gave novel π-conjugated siloles with good total yields.

Synthesis of 1-borylisoindoles via palladium-catalyzed dehydrogenation/C-H borylation of isoindolines.

1-Boryl- and 1,3-diborylisoindoles were synthesized by the palladium-catalyzed reaction of isoindolines with pinacolborane via sequential dehydrogenation and C-H borylation. Selective formation of the borylated isoindoles was achieved using Pd(dba)(2)/Me(2)S as the catalyst, in which Me(2)S suppressed the formation of 1-boryl-4,5,6,7-tetrahydroisoindoles. Diborylisoindole was cross-coupled with iodobenzene, giving 1,3-diphenylisoindole in high yield.

Palladium-Catalyzed Silylene-1,3-Diene [4+1] Cycloaddition with Use of (Aminosilyl)boronic Esters as Synthetic Equivalents of Silylene

Silylboronic esters bearing a dialkylamino group on the silicon atoms reacted with 1,3-dienes in the presence of a palladium catalyst to give silacyclopent-3-enes (i.e., 2,5-dihydrosiloles) in high yields via efficient silylene transfer from the silylboronic ester to the 1,3-dienes. The [4 + 1] cycloaddition was applicable to the parent 1,3-butadiene and various mono-, di-, and trisubstituted 1,3-dienes having silyloxy, cyano, and ester groups. Stereospecific ring formation took place in the reaction with either stereoisomer of 5,7-dodecadiene: the (E,E)-diene gave the cis product, whereas selective formation of the trans product was observed in the reaction of the isomeric (E,Z)-diene. The [4 + 1] cycloaddition followed by dehydrogenation with DDQ or chloranil afforded 2,4- and 2,5-diarylsiloles.