Aromatic C-H borylation of arenes and heteroarenes by 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.Aryl-and heteroarylboron derivatives have been applied to various fields of chemistry.1 Traditional methods for their synthesis are based on the reactions of trialkylborates with arylmagnesium or -lithium reagents derived from haloarenes, which are most common and convenient for large-scale preparations. Previous work has demonstrated the reaction of arenes and heteroarenes with bis(pinacolato)diboron (pin 2 B 2 , pin = Me 4 C 2 O 2 ) or pinacolborane (pinBH) in the presence of various transition metal complexes to form arylboronates. [5][6][7][8][9][10][11] Most of these studies have been conducted with excess of substrate, and the resulting methods have required the use of pin 2 B 2 instead of pinBH to obtain high yields. No reactions of a 1:1 ratio of substrate and the readily accessible and inexpensive pinBH, instead of the expensive pin 2 B 2 , have been reported. We disclose the culmination of several steps toward the development of such an aromatic C-H borylation process. With modest loadings of 1/2[Ir(OMe)(COD)] 2 (COD = 1,5-cyclooctadiene) and 4,4'-ditert-butyl-2,2'-bipyridine (dtbpy), the reaction of pinBH with many arenes and heteroarenes in a 1:1 ratio occurs in an inert solvent at room temperature (Scheme 1). To achieve the borylation of arenes and heteroarenes at room temperature with equimolar amounts of pinBH and substrate, several combinations of Ir(I) precursors (0.03 mmol of Ir) and ligands (0.03 mmol) were investigated as catalysts for the reaction of pinBH (1.1 mmol) with 1,3-dichlorobenzene (1.0 mmol) in hexane (6 mL) at 25 °C for 8 h. Of the precursors and ligands examined, the combination of 1/2[Ir(OMe)(COD)] 2 and dtbpy 12 efficiently catalyzed the borylation to form isomerically pure 5-boryl-1,3-dichlorobenzene in 86% yield.The choice of catalyst precursor was crucial to observe room temperature reactions. Although the combination of 1/2[Ir(OAc)(COD)] 2 and dtbpy produced the borylated product in 42% yield after 8 h, the combination of dtbpy and either 1/2[IrCl(COD)] 2 or [Ir(COD) 2 ]BF 4 formed no borylated product.The effects of steric and electronic properties of bipyridine ligands were evaluated with 1/2[Ir(OMe)(COD)] 2 as a catalyst precursor. Catalysts bearing 2,2'-bipyridine (bpy), 4,4'-di-Mebpy, and 5,5'-di-Me-bpy displayed moderate or good reactivity, but catalysts bearing 3,3'-di-Me-bpy or 6,6'-di-Me-bpy displayed little activity. These results indicated the importance of a parallel arrangement of two pyridine rings and a relatively unhindered coordination sphere at iridium. Reactions catalyzed by complexes containing electron-rich derivatives of bpy generated more active catalysts than those containing electronpoor derivatives. Catalysts c...
Aromatic C-H borylation of arenes and heteroarenes by 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.Aryl-and heteroarylboron derivatives have been applied to various fields of chemistry.1 Traditional methods for their synthesis are based on the reactions of trialkylborates with arylmagnesium or -lithium reagents derived from haloarenes, which are most common and convenient for large-scale preparations. Previous work has demonstrated the reaction of arenes and heteroarenes with bis(pinacolato)diboron (pin 2 B 2 , pin = Me 4 C 2 O 2 ) or pinacolborane (pinBH) in the presence of various transition metal complexes to form arylboronates. [5][6][7][8][9][10][11] Most of these studies have been conducted with excess of substrate, and the resulting methods have required the use of pin 2 B 2 instead of pinBH to obtain high yields. No reactions of a 1:1 ratio of substrate and the readily accessible and inexpensive pinBH, instead of the expensive pin 2 B 2 , have been reported. We disclose the culmination of several steps toward the development of such an aromatic C-H borylation process. With modest loadings of 1/2[Ir(OMe)(COD)] 2 (COD = 1,5-cyclooctadiene) and 4,4'-ditert-butyl-2,2'-bipyridine (dtbpy), the reaction of pinBH with many arenes and heteroarenes in a 1:1 ratio occurs in an inert solvent at room temperature (Scheme 1). To achieve the borylation of arenes and heteroarenes at room temperature with equimolar amounts of pinBH and substrate, several combinations of Ir(I) precursors (0.03 mmol of Ir) and ligands (0.03 mmol) were investigated as catalysts for the reaction of pinBH (1.1 mmol) with 1,3-dichlorobenzene (1.0 mmol) in hexane (6 mL) at 25 °C for 8 h. Of the precursors and ligands examined, the combination of 1/2[Ir(OMe)(COD)] 2 and dtbpy 12 efficiently catalyzed the borylation to form isomerically pure 5-boryl-1,3-dichlorobenzene in 86% yield.The choice of catalyst precursor was crucial to observe room temperature reactions. Although the combination of 1/2[Ir(OAc)(COD)] 2 and dtbpy produced the borylated product in 42% yield after 8 h, the combination of dtbpy and either 1/2[IrCl(COD)] 2 or [Ir(COD) 2 ]BF 4 formed no borylated product.The effects of steric and electronic properties of bipyridine ligands were evaluated with 1/2[Ir(OMe)(COD)] 2 as a catalyst precursor. Catalysts bearing 2,2'-bipyridine (bpy), 4,4'-di-Mebpy, and 5,5'-di-Me-bpy displayed moderate or good reactivity, but catalysts bearing 3,3'-di-Me-bpy or 6,6'-di-Me-bpy displayed little activity. These results indicated the importance of a parallel arrangement of two pyridine rings and a relatively unhindered coordination sphere at iridium. Reactions catalyzed by complexes containing electron-rich derivatives of bpy generated more active catalysts than those containing electronpoor derivatives. Catalysts c...
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 PdCl 2 (dppf) (3.0 mol%) and K 3 PO 4 in DMF. This one-pot, two-step procedures provided a variety of unsymmetrical biaryls in high yields.
Bis(η 4 ‐1,5‐cyclooctadiene)‐di μ‐methoxy‐diiridium(1) 4,4′‐Di‐ tert‐ butyl‐2,2′‐bipyridine Bis(pinacolato)diboron 1‐Chloro‐3‐iodobenzene Pinacolborane Indole
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