Pd(0)-catalyzed diarylation of sp3 C-H bond in (2-azaaryl)methanes

A highly efficient and selective palladium-catalyzed diarylation of (2-azaaryl)methanes at the methyl group is described. Aryl chlorides proved reactive enough. A palladium η(3)-azaallyl intermediate has been identified on the basis of DFT studies.

Practical C-H functionalization of quinones with boronic acids

A direct functionalization of a variety of quinones with several boronic acids has been developed. This scalable reaction proceeds readily at room temperature in an open flask using inexpensive reagents: catalytic silver(I) nitrate in the presence of a persulfate co-oxidant. The scope with respect to quinones is broad, with a variety of alkyl- and arylboronic acids undergoing efficient cross-coupling. The mechanism is presumed to proceed through a nucleophilic radical addition to the quinone with in situ reoxidation of the resulting dihydroquinone. This method has been applied to complex substrates, including a steroid derivative and a farnesyl natural product.

Rh-catalyzed oxidative coupling between primary and secondary benzamides and alkynes: synthesis of polycyclic amides

A methodology for the high yield and facile synthesis of isoquinolones from benzamides and alkynes via the oxidative ortho C-H activation of benzamides has been developed. Ag(2)CO(3) proved to be an optimal oxidant when MeCN was used as a solvent, and [RhCp*Cl(2)](2) was utilized as an efficient catalyst. Both N-alkyl and N-aryl secondary benzamides can be applied as effective substrates. Furthermore, primary benzamides react with two alkyne units, leading to tricyclic products via double C-H activation and oxidative coupling. The reactivity of the structurally related 1-hydroxyisoquinoline was also demonstrated, where both N- and O-containing rhodacyclic intermediates can be generated, leading to the construction of different O- or N-containing heterocycles.

Allylic C-H acetoxylation with a 4,5-diazafluorenone-ligated palladium catalyst: a ligand-based strategy to achieve aerobic catalytic turnover

Pd-catalyzed C-H oxidation reactions often require the use of oxidants other than O(2). Here we demonstrate a ligand-based strategy to replace benzoquinone with O(2) as the stoichiometric oxidant in Pd-catalyzed allylic C-H acetoxylation. Use of 4,5-diazafluorenone (1) as an ancillary ligand for Pd(OAc)(2) enables terminal alkenes to be converted to linear allylic acetoxylation products in good yields and selectivity under 1 atm O(2). Mechanistic studies have revealed that 1 facilitates C-O reductive elimination from a π-allyl-Pd(II) intermediate, thereby eliminating the requirement for benzoquinone in this key catalytic step.

On the mechanism of palladium-catalyzed aromatic C-H oxidation

The mechanism of Pd-catalyzed aromatic C-H oxidation chemistry continues to be vigorously discussed. Historically, Pd(II)/Pd(IV) catalysis cycles have been proposed. Herein, we present a detailed study of Pd(OAc)(2)-catalyzed aromatic C-H chlorination and propose dinuclear Pd(III) complexes as intermediates. We have identified a succinate-bridged dinuclear Pd(II) complex, which self-assembles during catalysis, as the catalyst resting state. In situ monitoring of catalysis has revealed that chlorination proceeds with turnover-limiting oxidation of a dinuclear resting state, and that acetate ions, liberated during the formation of the catalyst resting state, catalyze the bimetallic oxidation. Informed by reaction kinetics analysis, relevant dinuclear Pd(III) complexes have been prepared and observed to undergo selective C-Cl reductive elimination. Based on the combination of kinetic data obtained during catalysis and explicit structural information of relevant intermediates, we propose a Pd(II)(2)/Pd(III)(2) catalysis cycle for Pd(OAc)(2)-catalyzed aromatic C-H chlorination.

Palladium-catalyzed indole, pyrrole, and furan arylation by aryl chlorides

The palladium-catalyzed direct arylation of indoles, pyrroles, and furans by aryl chlorides has been demonstrated. The method employs a palladium acetate catalyst, 2-(dicyclohexylphosphino)-biphenyl ligand, and an inorganic base. Electron-rich and electron-poor aryl chlorides as well as chloropyridine coupling partners can be used, and arylated heterocycles are obtained in moderate to good yields. Optimization of base, ligand, and solvent is required for achieving best results.

Oxidative coupling of aromatic substrates with alkynes and alkenes under rhodium catalysis

Aromatic substrates with oxygen- and nitrogen-containing substituents undergo oxidative coupling with alkynes and alkenes under rhodium catalysis through regioselective C-H bond cleavage. Coordination of the substituents to the rhodium center is the key to activate the C-H bonds effectively. Various fused-ring systems can be constructed through these reactions.

Cross-coupling of C(sp)-H Bonds with Organometallic Reagents via Pd(II)/Pd(0) Catalysis**

Palladium-catalyzed C-H activation/C-C bond-forming reactions have emerged as a promising class of synthetic tools in organic chemistry. Among the many different means of forging C-C bonds using Pd-mediated C-H activation, a new horizon in this field is Pd(II)-catalyzed cross-coupling of C-H bonds with organometallic reagents via a Pd(II)/Pd(0) catalytic cycle. While this type of reaction has proven to be effective for the selective functionalization of aryl C(sp(2))-H bonds, the focus of this review is on Pd(II)-catalyzed C(sp(3))-H activation/C-C cross-coupling, a topic of particular importance because reactions of this type enable fundamentally new methods for bond construction. Since our laboratory's initial report on cross-coupling of C-H bonds in 2006, this area has expanded rapidly, and the unique ability of Pd(II) catalysts to cleave and functionalize alkyl C(sp(3))-H bonds has been exploited to develop protocols for forming an array of C(sp(3))-C(sp(2)) and C(sp(3))-C(sp(3)) bonds. Furthermore, enantioselective C(sp(3))-H activation/C-C cross-coupling has been achieved through the use of chiral amino acid-derived ligands, offering a novel technique for producing enantioenriched molecules. Although this nascent field remains at an early stage of development, further investigations hold the potential to revolutionalize the way in which chiral molecules are synthesized in industrial and academic laboratories.

Synthesis of 2-pyridones and iminoesters via Rh(III)-catalyzed oxidative coupling between acrylamides and alkynes

Catalytic oxidative coupling between acrylamides and alkynes was achieved using 0.5 mol % loading of [RhCp*Cl(2)](2) with Cu(OAc)(2) as an oxidant. 2-Pyridones, iminoesters, and substituted indoles could be obtained as a result of the electronic and steric effects of the substituents in the acrylamides.

Total synthesis of diptoindonesin G via a highly efficient domino cyclodehydration/intramolecular Friedel-Crafts acylation/regioselective demethylation sequence

A highly efficient total synthesis of diptoindonesin G is described employing a domino dehydrative cyclization/intramolecular Friedel-Crafts acylation/regioselective demethylation reaction of aryloxyketone 7 by the action of BCl(3) wherein the tetracyclic 6H-anthra[1,9-bc]furan-6-one skeleton was constructed via the 3-arylbenzofuran in a one-pot manner. This is the first example of the strategic combination of these three reactions in a cascade fashion. The routes presented here allow for direct access to diptoindonesin G and its analogues.

An efficient organocatalytic method for constructing biaryls through aromatic C-H activation

The direct functionalization of C-H bonds has drawn the attention of chemists for almost a century. C-H activation has mainly been achieved through four metal-mediated pathways: oxidative addition, electrophilic substitution, σ-bond metathesis and metal-associated carbene/nitrene/oxo insertion. However, the identification of methods that do not require transition-metal catalysts is important because methods involving such catalysts are often expensive. Another advantage would be that the requirement to remove metallic impurities from products could be avoided, an important issue in the synthesis of pharmaceutical compounds. Here, we describe the identification of a cross-coupling between aryl iodides/bromides and the C-H bonds of arenes that is mediated solely by the presence of 1,10-phenanthroline as catalyst in the presence of KOt-Bu as a base. This apparently transition-metal-free process provides a new strategy with which to achieve direct C-H functionalization.