Iridium‐Catalyzed Silylation of Five‐Membered Heteroarenes: High Sterically Derived Selectivity from a Pyridyl‐Imidazoline Ligand

The steric effects of substituents on five‐membered rings are less pronounced than those on six‐membered rings because of the difference in bond angles. Thus, the regioselectivities of reactions of five‐membered heteroarenes that occur with selectivities dictated by steric effects, such as the borylation of C?H bonds, have been poor in many cases. We report that the silylation of five‐membered‐ring heteroarenes occurs with high sterically derived regioselectivity when catalyzed by the combination of [Ir(cod)(OMe)]₂ (cod=1,5‐cyclooctadiene) and a phenanthroline ligand or a new pyridyl‐imidazoline ligand that further increases the regioselectivity. The silylation reactions with these catalysts produce high yields of heteroarylsilanes from functionalization at the most sterically accessible C?H bonds of these rings under conditions that the borylation of C?H bonds with previously reported catalysts formed mixtures of products or products that are unstable. The heteroarylsilane products undergo cross‐coupling reactions and substitution reactions with ipso selectivity to generate heteroarenes that bear halogen, aryl, and perfluoroalkyl substituents.     

Regioselective iridium(I)-catalysed remote borylation of oxygenated naphthalenes

We present our investigations into the regioselective borylation and halogenation of polyoxygenated naphthalene systems that are key precursors to dimeric pyranonaphthoquinone natural products. A variety of oxygenated naphthalenes were successfully borylated with pinacolborane in high yield and excellent regioselectivity using [Ir(COD)OMe]₂ and di-tert-butylbipyridine (dtbpy). The boronates were also readily transformed into the corresponding halides, opening new avenues for the preparation of advanced Suzuki coupling substrates useful for the synthesis of natural products.     

Iridium Complexes of 2,2′‐Bidipyrrins

The reaction of [{Ir(cod)(μ‐Cl)}₂] and K₂CO₃ or of [{Ir(cod)(μ‐OMe)}₂] alone with the non‐natural tetrapyrrole 2,2′‐bidipyrrin (H₂BDP) yields, depending on the stoichiometry, the mononuclear complex [Ir(cod)(HBDP)] or the homodinuclear complex [{Ir(cod)}₂(BDP)]. Both complexes react readily with carbon monoxide to yield the species [Ir(CO)₂(HBDP)] and [{Ir(CO)₂}₂(BDP)], respectively. The results from NMR spectroscopy and X‐ray diffraction reveal different conformations for the tetrapyrrolic ligand in both complexes. The reaction of [{Ir(coe)₂(μ‐Cl)}₂] with H₂BDP proceeds differently and yields the macrocyclic [4e^?,2H⁺]‐oxidized product [IrCl₂(9‐Meic)] (9‐Meic = monoanion of 9‐methyl‐9,10‐isocorrole), which can be addressed as an iridium analog of cobalamin.     

Iridium-Catalyzed Silylation of Five-Membered Heteroarenes: High Sterically Derived Selectivity from a Pyridyl-Imidazoline Ligand

The steric effects of substituents on five-membered rings are less pronounced than those on six-membered rings because of the difference in bond angles. Thus, the regioselectivities of reactions of five-membered heteroarenes that occur with selectivities dictated by steric effects, such as the borylation of C−H bonds, have been poor in many cases. We report that the silylation of five-membered-ring heteroarenes occurs with high sterically derived regioselectivity when catalyzed by the combination of [Ir(cod)(OMe)]₂ (cod=1,5-cyclooctadiene) and a phenanthroline ligand or a new pyridyl-imidazoline ligand that further increases the regioselectivity. The silylation reactions with these catalysts produce high yields of heteroarylsilanes from functionalization at the most sterically accessible C−H bonds of these rings under conditions that the borylation of C−H bonds with previously reported catalysts formed mixtures of products or products that are unstable. The heteroarylsilane products undergo cross-coupling reactions and substitution reactions with ipso selectivity to generate heteroarenes that bear halogen, aryl, and perfluoroalkyl substituents.     

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.     

Ligand‐Centred Reactivity of Bis(picolyl)amine Iridium: Sequential Deprotonation,Oxidation and Oxygenation of a “Non‐Innocent” Ligand

Treatment of [Ir(bpa)(cod)]⁺ complex [ 1 ]⁺ with a strong base (e.g., tBuO^?) led to unexpected double deprotonation to form the anionic [Ir(bpa?2H)(cod)]^? species [ 3 ]^?, via the mono‐deprotonated neutral amido complex [Ir(bpa?H)(cod)] as an isolable intermediate. A certain degree of aromaticity of the obtained metal–chelate ring may explain the favourable double deprotonation. The rhodium analogue [ 4 ]^? was prepared in situ. The new species [M(bpa?2H)(cod)]^? (M=Rh, Ir) are best described as two‐electron reduced analogues of the cationic imine complexes [M^I(cod)(Py‐CH₂‐N?CH‐Py)]⁺. One‐electron oxidation of [ 3 ]^? and [ 4 ]^? produced the ligand radical complexes [ 3 ]^. and [ 4 ]^.. Oxygenation of [ 3 ]^? with O₂ gave the neutral carboxamido complex [Ir(cod)(py‐CH₂‐N‐CO‐py)] via the ligand radical complex [ 3 ]^. as a detectable intermediate.     

Synthesis of masked haloareneboronic acids via iridium-catalyzed aromatic C-H borylation with 1,8-naphthalenediaminatoborane (danBH)

“Masked” areneboronic acids have been prepared by Ir-catalyzed C-H borylation of arenes. A [Ir(OMe)(cod)]₂ complex with a DPPE ligand showed the highest catalytic activity in the C-H borylation of benzene at 80 °C. The reaction system can be applied to substituted arenes, including halogen-substituted arenes. 1,3-Dihalobenzenes undergo the C-H borylation at their 5-positions in a regioselective fashion, affording 3,5-dihaloareneboronic acid derivatives, which serve as useful coupling modules for the convergent dendrimer synthesis.     

Rhodium‐Catalyzed ortho‐Selective CF Bond Borylation of Polyfluoroarenes with BpinBpin

An ortho‐selective C? F bond borylation between N‐heterocycle‐substituted polyfluoroarenes and Bpin‐Bpin with simple and commercially available [Rh(cod)₂]BF₄ as a catalyst is now reported. The reaction proceeds under mild reaction conditions with high efficiency and broad substrate scope, even toward monofluoroarene, thus providing a facile access to a wide range of borylated fluoroarenes that are useful for photoelectronic materials. Preliminary mechanistic studies reveal that a Rh^III/V catalytic cycle via a key intermediate rhodium(III) hydride complex [(H)Rh^IIIL_n(Bpin)] may be involved in the reaction.     

Ir-catalyzed C-H activation in the synthesis of borylated ferrocenes and half sandwich compounds

The complex generated from 1/2[Ir(OMe)(cod)]2 and 4,4'-di-tert-butyl-2,2'-bipyridine catalyzes the regioselective borylation of ferrocenes, CpMn(CO)3 and CpMo(CO)3CH3 with a stoichiometric amount of B2pin2.     

Rhodium‐Catalyzed ortho‐Selective C?F Bond Borylation of Polyfluoroarenes with Bpin?Bpin

An ortho‐selective C F bond borylation between N‐heterocycle‐substituted polyfluoroarenes and Bpin‐Bpin with simple and commercially available [Rh(cod)₂]BF₄ as a catalyst is now reported. The reaction proceeds under mild reaction conditions with high efficiency and broad substrate scope, even toward monofluoroarene, thus providing a facile access to a wide range of borylated fluoroarenes that are useful for photoelectronic materials. Preliminary mechanistic studies reveal that a Rh^III/V catalytic cycle via a key intermediate rhodium(III) hydride complex [(H)Rh^IIIL_n(Bpin)] may be involved in the reaction.     

Kinetic Studies Prove High Catalytic Activity of a Diene–Rhodium Complex in 1,4‐Addition of Phenylboronic Acid to α,β‐Unsaturated Ketones

In the 1,4‐addition of phenylboronic acid to α,β‐unsaturated ketones, [Rh(OH)(cod)]₂ has a much higher catalytic activity than [Rh(OH)(binap)]₂ (cod=1,5‐cyclooctadiene, binap=2,2′‐bis(diphenylphosphanyl)‐1,1′‐binaphthyl). Kinetic studies revealed that the rate‐determining transmetalation step in the catalytic cycle has a large rate constant when [Rh(OH)(cod)]₂ is used.     

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.     

Cationic Ir/Me‐BIPAM‐Catalyzed Asymmetric Intramolecular Direct Hydroarylation of α‐Ketoamides

Asymmetric intramolecular direct hydroarylation of α‐ketoamides gives various types of optically active 3‐substituted 3‐hydroxy‐2‐oxindoles in high yields with complete regioselectivity and high enantioselectivities (84–98 % ee). This is realized by the use of the cationic iridium complex [Ir(cod)₂](BAr^F₄) and the chiral O‐linked bidentate phosphoramidite (R,R)‐Me‐BIPAM.     

Synthesis,Structure, and Reactivity of Hydridoiridium Complexes Bearing a Pincer‐Type PSiP Ligand

A series of iridium tetrahydride complexes [Ir(H)₄(PSiP‐R)] bearing a tridentate pincer‐type bis(phosphino)silyl ligand ([{2‐(R₂P)C₆H₄}₂MeSi]⁻, PSiP‐R, R=Cy, iPr, or tBu) were synthesized by the reduction of [IrCl(H)(PSiP‐R)] with Me₄N ⋅ BH₄ under argon. The same reaction under a nitrogen atmosphere afforded a rare example of thermally stable iridium(III)–dinitrogen complexes, [Ir(H)₂(N₂)(PSiP‐R)]. Two isomeric dinitrogen complexes were produced, in which the PSiP ligand coordinated to the iridium center in meridional and facial orientations, respectively. Attempted substitution of the dinitrogen ligand in [Ir(H)₂(N₂)(PSiP‐Cy)] with PMe₃ required heating at 150 °C to give the expected [Ir(H)₂(PMe₃)(PSiP‐Cy)] and a trigonal bipyramidal iridium(I)–dinitrogen complex, [Ir(N₂)(PMe₃)(PSiP‐Cy)]. The reaction of [Ir(H)₄(PSiP‐Cy)] with three equivalents of 2‐norbornene (nbe) in benzene afforded [Ir^I(nbe)(PSiP‐Cy)] in a high yield, while a similar reaction of [Ir(H)₄(PSiP‐R)] with an excess of 3,3‐dimethylbutene (tbe) in benzene gave the C H bond activation product, [Ir^III(H)(Ph)(PSiP‐R)], in high yield. The oxidative addition of benzene is reversible; heating [Ir^III(H)(Ph)(PSiP‐Cy)] in the presence of PPh₃ in benzene resulted in reductive elimination of benzene, coordination of PPh₃, and activation of the C H bond of one aromatic ring in PPh₃. [Ir^III(H)(Ph)(PSiP‐R)] catalyzed a direct borylation reaction of the benzene C H bond with bis(pinacolato)diboron. Molecular structures of most of the new complexes in this study were determined by a single‐crystal X‐ray analysis.     

Anilines as C‐Nucleophiles in Ir‐Catalyzed Intramolecular Asymmetric Allylic Substitution Reactions

Anilines generally act as N‐nucleophiles in transition‐metal‐catalyzed allylic substitution reactions. In this paper, a highly enantioselective intramolecular Friedel–Crafts‐type allylic alkylation of aniline derivatives was realized by using an iridium catalyst derived from [Ir(cod)Cl]₂ and (R _a)‐BHPphos. Various tetrahydroisoquinilin‐5‐amines were obtained in moderate to good yields, excellent enantioselectivity and regioselectivity under mild reaction conditions. BHPphos=N ‐benzhydryl‐N ‐phenyldinaphthophosphoramidite.     

Practical synthesis of pinacolborane for one-pot synthesis of unsymmetrical biaryls via aromatic C-H borylation-cross-coupling sequence

A method for practical preparation of pinacolborane from borane-diethylaniline and pinacol was newly developed. Aromatic C-H borylation of arenes with pinacolborane or bis(pinacolato)diboron catalyzed by 1/2[Ir(OMe)(COD)]₂-(4,4′-di-tert-butyl-2,2′-bipyridine) at 25 °C in hexane to give arylboronic esters was directly followed by cross-coupling with aromatic bromides at 60 °C in the presence of PdCl₂(dppf) (3.0 mol %) and K₃PO₄ in DMF. This one-pot, two-step procedure provided a variety of unsymmetrical biaryls in high yields.     

Terminal and Bridging Parent Amido 1,5‐Cyclooctadiene Complexes of Rhodium and Iridium

The ready availability of rare parent amido d⁸ complexes of the type [{M(μ‐NH₂)(cod)}₂] (M=Rh ( 1 ), Ir ( 2 ); cod=1,5‐cyclooctadiene) through the direct use of gaseous ammonia has allowed the study of their reactivity. Both complexes 1 and 2 exchanged the di‐olefines by carbon monoxide to give the dinuclear tetracarbonyl derivatives [{M(μ‐NH₂)(CO)₂}₂] (M=Rh or Ir). The diiridium(I) complex 2 reacted with chloroalkanes such as CH₂Cl₂ or CHCl₃, giving the diiridium(II) products [(Cl)(cod)Ir(μ‐NH₂)₂Ir(cod)(R)] (R=CH₂Cl or CHCl₂) as a result of a two‐center oxidative addition and concomitant metal–metal bond formation. However, reaction with ClCH₂CH₂Cl afforded the symmetrical adduct [{Ir(μ‐NH₂)(Cl)(cod)}₂] upon release of ethylene. We found that the rhodium complex 1 exchanged the di‐olefines stepwise upon addition of selected phosphanes (PPh₃, PMePh₂, PMe₂Ph) without splitting of the amido bridges, allowing the detection of mixed COD/phosphane dinuclear complexes [(cod)Rh(μ‐NH₂)₂Rh(PR₃)₂], and finally the isolation of the respective tetraphosphanes [{Rh(μ‐NH₂)(PR₃)₂}₂]. On the other hand, the iridium complex 2 reacted with PMe₂Ph by splitting the amido bridges and leading to the very rare terminal amido complex [Ir(cod)(NH₂)(PMePh₂)₂]. This compound was found to be very reactive towards traces of water, giving the more stable terminal hydroxo complex [Ir(cod)(OH)(PMePh₂)₂]. The heterocyclic carbene IPr (IPr=1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene) also split the amido bridges in complexes 1 and 2 , allowing in the case of iridium to characterize in situ the terminal amido complex [Ir(cod)(IPr)(NH₂)]. However, when rhodium was involved, the known hydroxo complex [Rh(cod)(IPr)(OH)] was isolated as final product. On the other hand, we tested complexes 1 and 2 as catalysts in the transfer hydrogenation of acetophenone with iPrOH without the use of any base or in the presence of Cs₂CO₃, finding that the iridium complex 2 is more active than the rhodium analogue 1 .     

Catalytic polymerization of phenylacetylene with dimeric [Rh(OMe)(cod)]2 complex in ionic liquids

Dimeric rhodium(I) complex [Rh(OMe)(cod)]₂ was found to be an active catalyst of phenylacetylene polymerization to poly(phenylacetylene) (PPA) in ionic liquids containing imidazolium or pyridinium cations. The highest yield of PPA (92%) was obtained in 1‐butyl‐4‐methylpyridinium tetrafluoroborate as reaction medium. The yield of PPA in imidazolium ionic liquids containing BF₄^? or PF₆^? anions increased to 83–99% when Et₃N or cycloocta‐1,5‐diene were added as co‐catalysts. In 1‐methyl‐3‐octylimidazolium chloride (MOI · Cl) polymerization rate was much lower than in other ionic liquids, although the highest M_w (72 400) was obtained. Spectroscopic studies confirmed that [Rh(OMe)(cod)]₂ reacted with MOI · Cl forming new carbene Rh(I) complex, which can participate in the polymerization process. Copyright © 2006 John Wiley & Sons, Ltd.     

Silyl-directed, iridium-catalyzed ortho-borylation of arenes. A one-pot ortho-borylation of phenols, arylamines, and alkylarenes

The regioselectivity of the borylation of arenes catalyzed by the combination of 4,4'-di-tert-butylbipyridine (dtbpy) and [Ir(cod)Cl]2 has typically been governed by steric effects. We describe a strategy that makes use of a new substituent for ortho-functionalization to overcome this bias. We show that arenes containing hydrosilyl substituents on an atom attached to the arene ring undergo borylation at the position ortho to the hydrosilyl group. Using iridium-catalyzed formation of silyl ethers and silylamines from silanes and either phenols or arylamines, we have developed the ortho-borylation into a one-pot conversion of free phenols and monoprotected anilines into hydroxy- and amino-substituted organoboron derivatives.     

New Rhodium Complexes Coordinated by Bidentate Imine Ligands with Benzothiazolyl Functionalities

The pseudo‐square‐planar complexes [Rh(cod)(Hbbtm)]BF₄ ( 3 ), [Rh(bbte)(cod)]BF₄ ( 4 ), [Rh(CO)₂(Hbbtm)]BF₄ ( 5 ), [Rh(bbte)(CO)₂]BF₄ ( 6 ), [Rh(bbtm)(cod)] ( 7 ) and [Rh(bbtm)(CO)₂] ( 8 ) (Hbbtm=bis(benzothiazol‐2‐yl)methane=2,2′‐methylenebis[benzothiazole], bbte=bis(benzothiazol‐2‐yl)ethane=2,2′‐(ethane‐1,2‐diyl)bis[benzothiazole], and cod=cycloocta‐1,5‐diene) were synthesized and characterized. Diastereotopic protons were observed for the protons at the bridge in the ¹H‐NMR of 3 and 5 . Twisting of the ethane‐1,2‐diyl bridge in 4 and 6 effects chemical equivalence of the CH₂ groups in solution. Unusually large downfield shifts occur on coordination of the deprotonated ligand Hbbtm as the negative charge is delocalized in 7 and 8 . The NMR signals of the cod ligand in 4 could be differentiated. The X‐ray crystal structures of 3, 4 , and 6 are reported.