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.     

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.     

One-pot synthesis of arylboronic acids and aryl trifluoroborates by Ir-catalyzed borylation of arenes

[reaction: see text] The synthesis of arylboronic acids and aryl trifluoroborates in a one-pot sequence by Ir-catalyzed borylation of arenes is reported. To prepare the arylboronic acids, the Ir-catalyzed borylation is followed by oxidative cleavage of the boronic ester with NaIO4. To prepare the aryltrifluoroborate, the Ir-catalyzed borylation is followed by displacement of pinacol by KHF2. These two-step sequences give products that are more reactive toward subsequent chemistry than the initially formed pinacol boronates.     

Site‐Differentiated Polyboron Arenes Prepared by Direct C?H Borylation and Their Highly Selective Suzuki–Miyaura Cross‐Coupling Reactions

Di‐ and polyboron (hetero)arenes, site‐differentiated with MIDA boronyl (MIDA=N‐methyliminodiacetic acid) and pinacolato boronyl (Bpin), were prepared by an iridium‐catalyzed direct C H borylation of readily available (hetero)aryl MIDA boronates. The excellent synthetic uses of these multisite nucleophiles were demonstrated by the high‐yield production of a variety of multifunctionalized poly(hetero)arenes with the highly chemoselective Suzuki–Miyaura coupling (SMC) of the Bpin moiety being an essential step.     

Site‐Differentiated Polyboron Arenes Prepared by Direct C?H Borylation and Their Highly Selective Suzuki–Miyaura Cross‐Coupling Reactions

Di‐ and polyboron (hetero)arenes, site‐differentiated with MIDA boronyl (MIDA=N‐methyliminodiacetic acid) and pinacolato boronyl (Bpin), were prepared by an iridium‐catalyzed direct C H borylation of readily available (hetero)aryl MIDA boronates. The excellent synthetic uses of these multisite nucleophiles were demonstrated by the high‐yield production of a variety of multifunctionalized poly(hetero)arenes with the highly chemoselective Suzuki–Miyaura coupling (SMC) of the Bpin moiety being an essential step.     

Aromatic borylation/amidation/oxidation: a rapid route to 5-substituted 3-amidophenols

[reaction: see text] 5-Substituted 3-amidophenols are prepared by subjecting 3-substituted halobenzenes to an Ir-catalyzed aromatic borylation, followed by a Pd-catalyzed amidation, and finally an oxidation of the boronic ester intermediate. The entire C-H activation borylation/amidation/oxidation sequence can be accomplished without isolation of any intermediate arenes. Usefully, amide partners can include lactams, carbamates, and ureas.     

Mild iridium-catalyzed borylation of arenes. High turnover numbers, room temperature reactions, and isolation of a potential intermediate.

The borylation of arenes leads to the formation of synthetically versatile products from unactivated arene reagents. We report that Ir(I) precursors in conjunction with bipyridine ligands catalyze in high yields the borylation of arenes under mild conditions. These reactions encompase arenes bearing both electron-withdrawing and electron-donating substituents. The temperatures required for the transformation are much lower than those previously reported for direct arene borylation. The combination of [Ir(COE)2Cl]2 and (4,4-di-t-butyl)bipyridine even allows for reaction at room temperature. The same catalyst system at 100 degrees C provides remarkably high turnover numbers for a hydrocarbon functionalization process. Mechanistic studies show that the reactions involve uncommon, Ir(II) tris-boryl complexes. An example of this type of complex ligated by di-t-butylbipyridine was isolated and structurally characterized. It reacted rapidly at room temperature to produce aryl boronate esters in high yields.     

Regioselectivity of the borylation of alkanes and arenes

The borylation of alkanes and arenes has become some of the most practical C-H bond functionalization chemistry. Most striking is the high regioselectivity of these reactions. Rhodium and ruthenium complexes catalyze with exquisite selectivity the borylation of methyl C-H bonds over methylene or methine C-H bonds. Iridium complexes catalyze, with high steric control, the borylation of one aromatic C-H bond over another. In contrast, iridium-catalyzed borylation of heteroaromatic C-H bonds is more controlled by electronic effects. Detailed information on these selectivities and mechanistic information on the origins of this regioselectivity will be described in this critical review (95 references).     

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.     

Transition metal-catalyzed borylation of alkanes and arenes via C---H activation

Since 1999, direct borylation of hydrocarbons catalyzed by a transition metal complex has been extensively studied by several groups and has become an economical, efficient, elegant, and environmentally benign protocol for the synthesis of a variety of organoboron compounds. Re-, Rh-, Ir- and Pd-catalyzed C---H borylation of alkanes, arenes and benzylic positions of alkylarenes by bis(pinacolato)diboron or pinacolborane provide alkyl-, aryl-, heteroaryl- and benzylboron compounds, respectively. In this review, seminal early works and exciting recent developments in the area of direct borylation of hydrocarbons via C---H activation are summarized.     

Ruthenium-catalyzed regiospecific borylation of methyl C-H bonds

We report the regiospecific, ruthenium-catalyzed borylation of saturated terminal C-H bonds. Alkylboronates were obtained in 78-98% yields. The borylations of alkanes, trialkylamines, protected alcohols, and fluoroalkanes occurred regiospecifically at the methyl group that is least sterically hindered. In contrast to most organometallic C-H activation, the reactions of alkanes occurred in higher yields than the reactions of arenes. Reactions were conducted that probed steric and electronic effects on the alkyl borylation. These reactions showed that the borylation occurred preferentially at the methyl group that is least sterically hindered and most electron-deficient. Ruthenium compounds containing boryl ligands were synthesized, and one was characterized by X-ray crystallography. One of these compounds contained a rare bridging boryl ligand and served as a catalyst precursor for the borylation of octane.     

Conversion of 1,3-disubstituted arenes to chiral α,α-diaryl methylammonium chlorides using arene borylation

A two-step conversion of 1,3-disubstituted arenes to chiral α,α-diaryl methylammonium chlorides is described. In this procedure, arenes are converted to aryl boronic esters by iridium-catalyzed borylation, and the aryl boronic esters are converted to enantioenriched amines by subsequent rhodium-catalyzed addition to chiral tert-butanesulfinimes.     

Synthesis and metal ion complexation studies of proton-ionizable calix

A series of novel N-chromogenic calix[4]arene azacrown ethers were synthesized as selective extractants of potassium ion. 1,3-Alternate calix[4]arene azacrown ethers were prepared by reacting 25,27-dipropyloxy-26,28-bis(5-chloro-3-oxapentyloxy) calix[4]arenes with p-toluenesulfonamide in the presence of potassium carbonate. The coupling reaction of calix[4]arene azacrown ether with 2-hydroxy-5-nitrobenzyl bromide in the presence of triethylamine in THF gave the chromogenic calix[4]arene azacrown ether in moderate yield. These compounds show high potassium selectivity over other metal ions as shown by two-phase extraction, bulk liquid membrane, and 1H NMR studies on a ligand-metal complex. It is assumed that the OH of the chromogenic group attached on nitrogen can assist the complexation by encapsulation of the metal.     

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.     

Regioselective Conversion of Arenes to N‐aryl‐1,2,3‐triazoles Using CH Borylation

A one‐pot protocol for the synthesis of N‐aryl 1,2,3‐triazoles from arenes by an iridium‐catalyzed C?H borylation/copper catalyzed azidation/click sequence is described. 1 mol % of Cu(OTf)₂ was found to efficiently catalyze both the azidation and the click reaction. The applicability of this method is demonstrated by the late‐stage chemoselective installation of 1,2,3‐triazole moiety into unactivated molecules of pharmaceutical importance.     

Rh-catalyzed ortho-selective C-H borylation of N-functionalized arenes with silica-supported bridgehead monophosphine ligands

Supported phosphine-Rh systems, prepared in situ from silica-supported bridgehead monophosphines and [Rh(OH)(cod)](2), have enabled ortho-selective C-H borylation for a range of arenes containing nitrogen-based directing groups. The regioselectivity was excellent with various N-directing groups, including saturated and unsaturated N-heterocycles, tert-aminoalkyl groups, and imine-type C-N double bonds. The reaction showed significant tolerance toward steric repulsion around the reacting C-H bond. This Rh catalysis complements the Ir-catalyzed ortho-borylation, which is effective for arenes with oxygen-based directing groups.     

Retention in gas-liquid chromatography with a polyethylene oxide stationary phase: molecular simulation and experiment

Configurational-bias Monte Carlo simulations in the isobaric-isothermal Gibbs ensemble were carried out to investigate the partitioning of normal alkanes, primary and secondary alcohols, symmetric alkyl ethers and arenes between a helium vapor phase and a polyethylene oxide stationary phase (M(W)=382 g mol(-1)). The united-atom version of the transferable potentials for phase equilibria force field was used to model all solutes, polyethylene oxide and helium. The Gibbs free energies of transfer and Kovats retention indices of the solutes were calculated directly from the partition constants at two different temperatures, 353 and 393 K. Chromatographic experiments on a Carbowax 20M retentive phase were performed for the same set of solutes and temperatures ranging from 333 to 413 K. The predicted retention indices for alcohols, ethers and arenes are overestimated by about 120, 70 and 20 retention index units, respectively, pointing to an overestimation of the first-order electrostatic interactions in the model system. Molecular-level analysis shows that hydrogen-bonding and dipole-dipole interactions lead to orientational ordering for the alcohol and ether analytes, whereas the weaker dipole-quadrupole interactions for the arene solutes are not sufficient to induce orientational ordering. The retention indices of alcohols and ethers decrease with increasing temperature because of the large entropic cost of hydrogen-bonding and orientational ordering. In contrast, the retention indices for arenes increase with increasing temperature because the entropic cost of cavity formation is smaller for arenes than for comparable alkanes.     

Iridium-catalyzed borylation of secondary C-h bonds in cyclic ethers

The borylation of secondary C-H bonds, specifically secondary C-H bonds of cyclic ethers, with a catalyst generated from tetramethylphenanthroline and an iridium precursor is reported. This borylation occurs with unique selectivity for the C-H bonds located β to the oxygen atoms over the weaker C-H bonds located α to oxygen atoms. Mechanistic studies imply that the C-H bond cleavage occurs directly at the β position rather than at the α position followed by isomerization of a reaction intermediate.     

Cleavage of C(aryl)−CH3 Bonds in the Absence of Directing Groups under Transition Metal Free Conditions

Organic chemists now can construct carbon–carbon σ‐bonds selectively and sequentially, whereas methods for the selective cleavage of carbon–carbon σ‐bonds, especially for unreactive hydrocarbons, remain limited. Activation by ring strain, directing groups, or in the presence of a carbonyl or a cyano group is usually required. In this work, by using a sequential strategy site‐selective cleavage and borylation of C(aryl)?CH₃ bonds has been developed under directing group free and transition metal free conditions. Methyl groups of various arenes are selectively cleaved and replaced by boryl groups. Mechanistic analysis suggests that it proceeds by a sequential intermolecular oxidation and coupling of a transient aryl radical, generated by radical decarboxylation, involving a pyridine‐stabilized persistent boryl radical.     

Straightforward synthesis of phenanthrenes from styrenes and arenes

Semi-one-pot synthesis of phenanthrenes from styrenes and arenes was developed through cross-dehydrogenative coupling. A sequence of Heck-type coupling and photo-cyclization were involved and a variety of functionalities were tolerated. This method provides an effective and practical protocol towards the synthesis of substituted phenanthrenes.