Aromatic C-H silylation of arenes with 1-hydrosilatrane catalyzed by an iridium(I)/2,9-dimethylphenanthroline (dmphen) complex.

Aromatic C-H silylation of neat arenes with 1-hydrosilatrane was found to be efficiently catalyzed by iridium catalysts composed of 1/2[Ir(OMe)(cod)]2 and 2,9-dimethyl-1,10-phenanthroline at 120 °C to afford the corresponding silylated products in high yields. The silylated products can be used for the Hiyama cross-coupling reaction.

Me-BIPAM for the synthesis of optically active 3-aryl-3-hydroxy-2-oxindoles by ruthenium-catalyzed addition of arylboronic acids to isatins.

A chiral O-linked C(2)-symmetric bidentate phosphoramidite (Me-BIPAM) was found to be efficient for the ruthenium-catalyzed addition of arylboronic acids to isatins. Asymmetric synthesis of 3-aryl-3-hydroxy-2-oxindoles by 1,2-addition of arylboronic acids to isatins was carried out in the presence of [RuCl(2)(PPh(3))(3)]/(R,R)-Me-BIPAM and KF, resulting in an enantioselectivity as high as 90 % ee. It was found that the reaction with N-protected isatins proceeds with high yields and good enantioselectivities. The best protective groups on the nitrogen atom were different depending on the substituents on the aromatic ring. The use of a N-benzyl group resulted in excellent enantioselectivities in many substrates compared with other groups.

Asymmetric addition of arylboronic acids to glyoxylate catalyzed by a ruthenium/Me-BIPAM complex.

The enantioselective synthesis of α-hydroxy esters by ruthenium-catalyzed 1,2-addition of arylboronic acids to tert-butyl glyoxylate is described. The use of RuCl(2)(PPh(3))(3) with (R,R)-Me-BIPAM gave optically active mandelic acids of up to 99% ee. Addition of a fluoride salt such as potassium fluoride (KF) or caesium fluoride (CsF) was effective for achieving high enantioselectivities.

Cross-coupling reaction with lithium methyltriolborate.

We newly developed lithium methyltriolborate as an air-stable white solid that is convenient to handle. The good performance of this triolborate for metal-catalyzed bond-forming reactions was demonstrated in palladium-catalyzed cross-coupling reactions with haloarenes. Cross-coupling reaction of [MeB(OCH2)3CCH3]Li with aryl halides occurred in the presence of Pd(OAc)2/RuPhos complex in refluxing MeOH/H2O and the absence of bases.

An N-linked bidentate phosphoramidite ligand (N-Me-BIPAM) for rhodium-catalyzed asymmetric 1,4-addition of arylboronic acids to α,ß-unsaturated ketones.

A new bidentate phosphoramidite (N-Me-BIPAM) based on Shibasaki's N-linked BINOL was synthesized. This ligand appears to be highly effective for rhodium-catalyzed asymmetric conjugated addition of arylboronic acids to α,ß-unsaturated enones. The reaction of ortho-substituted arylboronic acid with acyclic and cyclic enones provides the corresponding products in good yields and enantioselectivities.

Ru/Me-BIPAM-catalyzed asymmetric addition of arylboronic acids to aliphatic aldehydes and α-ketoesters.

A ruthenium-catalyzed asymmetric arylation of aliphatic aldehydes and α-ketoesters with arylboronic acids has been developed, giving chiral alkyl(aryl)methanols and α-hydroxy esters in good yields. The use of a chiral bidentate phosphoramidite ligand (Me-BIPAM) achieved excellent enantioselectivities.

Rhodium-catalyzed 1,4-addition of lithium 2-furyltriolborates to unsaturated ketones and esters for enantioselective synthesis of γ-oxo-carboxylic acids by oxidation of the furyl ring with ozone.

Rhodium-catalyzed 1,4-addition of lithium 5-methyl-2-furyltriolborate ([ArB(OCH(2))(3)CCH(3)]Li, Ar = 5-methyl-2-furyl) to unsaturated ketones to give ß-furyl ketones was followed by ozonolysis of the furyl ring for enantioselective synthesis of γ-oxo-carboxylic acids. [Rh(nbd)(2)]BF(4) (nbd = 2,5-norbornadiene) chelated with 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl (binap) or 2,3-bis(diphenylphosphino)butane (chiraphos) gave high yields and high selectivities in a range of 91-99% ee at 30 °C in a basic dioxane/water solution. The corresponding reaction of unsaturated esters, such as methyl crotonate, had strong resistance under analogous conditions, but the 1,4-adduct was obtained in 70% yield and with 94% ee when more electron-deficient phenyl crotonate was used as the substrate.

Ortho-C-H borylation of benzoate esters with bis(pinacolato)diboron catalyzed by iridium-phosphine complexes.

Iridium complexes generated from [Ir(OMe)(COD)](2) and tris[3,5-bis(trifluoromethyl)phenyl]phosphine efficiently catalyzed the ortho-C-H borylation of benzoate esters with bis(pinacolato)diboron in octane at 80 degrees C to produce the corresponding arylboronates in high yields with excellent regioselectivities.

Me-bipam for enantioselective ruthenium(II)-catalyzed arylation of aldehydes with arylboronic acids.

Ligand design by-pam: A ruthenium-catalyzed asymmetric arylation of aldehydes with arylboronic acids has been developed, giving chiral diarylmethanols in good yields. The use of a chiral bidentate phosphoramidite ligand ((R,R)-Me-bipam) led to excellent enantioselectivities.

Vinylic C--H borylation of cyclic vinyl ethers with bis(pinacolato)diboron catalyzed by an Iridium(I)-dtbpy complex.

Borylation of the vinylic C--H bond of 1,4-dioxene, 2,3-dihydrofuran, 3,4-dihydro-2H-pyran and their gamma-substituted analogs was carried out in the presence of bis(pinacolato)diboron (B(2)pin(2)) and a catalytic amount of Ir(I)-dtbpy (dtbpy=4,4'-di-tert-butyl-2,2'-bipyridine) complex. The two boron atoms in B(2)pin(2) participated in the coupling, thus giving two equivalents of the coupling product from one equivalent of B(2)pin(2). The borylation of 1,4-dioxene in hexane resulted in 81 % yield at room temperature. The borylation of 2,3-dihydrofurans at 80 degrees C in octane suffered from low regioselectivity, and gave a mixture of alpha- and beta-coupling products even for hindered gamma-disubstituted analogs, but gamma-substituted analogs of 3,4-dihydro-2H-pyran achieved high alpha-selectivity, giving single coupling products. This protocol was applied to the syntheses of a key precursor of vineomycinone B2 methyl ester and other C-substituted D-glucals by borylation of protected D-glucals with B(2)pin(2) to give alpha-boryl glucal followed by cross-coupling with haloarenes, benzyl bromide, and allyl bromide. A catalytic cycle that involves the oxidative addition of sp(2) C--H bond to iridium(III)-trisboryl intermediate as the rate-determining step has been proposed.

Aryl triolborates: novel reagent for copper-catalyzed N arylation of amines, anilines, and imidazoles.

The N arylation of primary and secondary aliphatic amines, anilines, and imidazoles with novel potassium aryl triolborates was carried out in the presence of a reoxidant and a catalytic amount of Cu(OAc)(2) (10 mol %). Aryl triolborates were found to be better reagents than aryl boronic acids or potassium aryl trifluoroborates as the former achieved high yields under mild conditions. Coupling of primary and secondary aliphatic amines to give N-aryl amines in excellent yields was performed under oxygen atmosphere. The reactions of anilines and imidazoles to provide N-aryl anilines and N-aryl imidazoles in good yields proceeded smoothly when trimethylamine N-oxide was used as an oxidant.

Regioselective aromatic C-H silylation of five-membered heteroarenes with fluorodisilanes catalyzed by iridium(I) complexes.

The aromatic C-H silylation of five-membered heteroarenes with 1,2-di-tert-butyl-1,1,2,2-tetrafluorodisilane regioselectively proceeded at 120 degrees C in octane in the presence of a catalytic amount of iridium(I) complexes generated from 1/2[Ir(OMe)(COD)]2 and 2-tert-butyl-1,10-phenanthroline.

Mechanism of the mild functionalization of arenes by diboron reagents catalyzed by iridium complexes. Intermediacy and chemistry of bipyridine-ligated iridium trisboryl complexes.

This paper describes mechanistic studies on the functionalization of arenes with the diboron reagent B(2)pin(2) (bis-pinacolato diborane(4)) catalyzed by the combination of 4,4'-di-tert-butylbipyridine (dtbpy) and olefin-ligated iridium halide or olefin-ligated iridium alkoxide complexes. This work identifies the catalyst resting state as [Ir(dtbpy)(COE)(Bpin)(3)] (COE = cyclooctene, Bpin = 4,4,5,5-tetramethyl-1,3,2-dioxaborolanyl). [Ir(dtbpy)(COE)(Bpin)(3)] was prepared by independent synthesis in high yield from [Ir(COD)(OMe)](2), dtbpy, COE, and HBpin. This complex is formed in low yield from [Ir(COD)(OMe)](2), dtbpy, COE, and B(2)pin(2). Kinetic studies show that this complex reacts with arenes after reversible dissociation of COE. An alternative mechanism in which the arene reacts with the Ir(I) complex [Ir(dtbpy)Bpin] after dissociation of COE and reductive elimination of B(2)pin(2) does not occur to a measurable extent. The reaction of [Ir(dtbpy)(COE)(Bpin)(3)] with arenes and the catalytic reaction of B(2)pin(2) with arenes catalyzed by [Ir(COD)(OMe)](2) and dtbpy occur faster with electron-poor arenes than with electron-rich arenes. However, both the stoichiometric and catalytic reactions also occur faster with the electron-rich heteroarenes thiophene and furan than with arenes, perhaps because eta(2)-heteroarene complexes are more stable than the eta(2)-arene complexes and the eta(2)-heteroarene or arene complexes are intermediates that precede oxidative addition. Kinetic studies on the catalytic reaction show that [Ir(dtbpy)(COE)(Bpin)(3)] enters the catalytic cycle by dissociation of COE, and a comparison of the kinetic isotope effects of the catalytic and stoichiometric reactions shows that the reactive intermediate [Ir(dtbpy)(Bpin)(3)] cleaves the arene C-H bond. The barriers for ligand exchange and C-H activation allow an experimental assessment of several conclusions drawn from computational work. Most generally, our results corroborate the conclusion that C-H bond cleavage is turnover-limiting, but the experimental barrier for this bond cleavage is much lower than the calculated barrier.

Asymmetric 1,4-addition of triarylbismuths to enones catalyzed by dicationic palladium(II) complexes.

A 1,4-addition of triarylbismuths to cyclic and acyclic enones was carried out in aqueous methanol in the presence of a chiral phosphine-dicationic palladium(ii) complex. Palladium complexes of (S,S)-chiraphos or (S,S)-dipamp gave optically active beta-arylketones of up to 95% ee.

Metal-catalyzed reactions of diborons for synthesis of organoboron compounds.

Metal-catalyzed borylation of alkenes, alkynes, arenes, and organic halides with B-B or H-B compounds has been developed for the synthesis of organoboron compounds from simple organic substrates. The platinum(0)-catalyzed addition of bis(pinacolato)diboron to alkenes and alkynes provided a method for the stereoselective synthesis of cis-bis(boryl)alkanes or cis-bis(boryl)alkenes. The addition of diboron to 1,3-dienes with platinum(0) complexes provided a new access to cis-1,4-bis(boryl)-2-butene derivatives, which are versatile reagents for diastereoselective allylboration of carbonyl compounds. The first one-step procedure for the syntheses of aryl-, vinyl-, and allylboronates was achieved via crosscoupling reactions of diborons with aryl and 1-alkenyl halides or triflates and allyl acetates. Direct C-H borylation of arenes catalyzed by a transition metal complex was studied as an economical protocol for the synthesis of a variety of arylboron derivatives. Ir-catalyzed C-H borylation of arenes, heteroarenes, and benzylic positions of alkylarenes by bis(pinacolato)diboron or pinacolborane furnished aryl-, heteroaryl-, and benzylboron compounds. This article discusses the mechanisms of these reactions and their synthetic applications.

Room temperature borylation of arenes and heteroarenes using stoichiometric amounts of pinacolborane catalyzed by iridium complexes in an inert solvent.

Aromatic C-H borylation of arenes and heteroarenes using stoichiometric amounts of pinacolborane was catalyzed by an iridium complex generated from 1/2[Ir(OMe)(COD)]2 and 4,4'-di-tert-butyl-2,2'-bipyridine at room temperature in hexane and afforded the corresponding aryl- and heteroarylboronates in high yields with excellent regioselectivities.

Rhodium-catalyzed 1,4-addition of arylboronic acids to alpha,beta-unsaturated carbonyl compounds: large accelerating effects of bases and ligands.

The effects of ligands and bases in the rhodium(I)-catalyzed 1,4-addition of arylboronic acids to alpha,beta-unsaturated carbonyl compounds were reinvestigated to carry out the reaction under mild conditions. Rhodium(I) complexes possessing a 1,5-cyclooctadiene (cod) and a hydroxo ligand such as [RhOH(cod)](2) exhibited excellent catalyst activities compared to those of the corresponding rhodium-acac or -chloro complexes and their phosphine derivatives. The reaction was further accelerated in the presence of KOH, thus allowing the 1,4-addition even at 0 degrees C. A cationic rhodium(I)-(R)-binap complex, [Rh(R-binap)(nbd)]BF(4), catalyzed the reaction at 25-50 degrees C in the presence of Et(3)N with high enantioselectivities of up to 99% ee for alpha,beta-unsaturated ketones, 92% for aldehydes, 94% for esters, and 92% for amides.