CH Activation as a Shortcut to Conjugated Polymer Synthesis.

Direct (hetero)arylation polymerization exploits the palladium-catalyzed activation of aromatic CH bonds for the atom-economical synthesis of conjugated polymers for a wide range of applications. This account outlines how direct arylation methodologies overcome many of the limitations of contemporary polymerization techniques at both the research and production scale, and explains how monomer design and reaction conditions must be tailored to ensure high polymer molecular weight, yield, and structural integrity. Current research aims to improve further this reaction's profile as a sustainable methodology while at the same time making it competitive with the Migita-Stille and Miyaura-Suzuki polymerizations both in scope of accessible structures and synthetic efficiency. This feature article charts the recent developments and future directions of CH activation research as it moves toward becoming at once an industrially feasible, environmentally friendly, and synthetically powerful polymerization technique.

Rhodium-Catalyzed C-S and C-N Functionalization of Arenes: Combination of C-H Activation and Hypervalent Iodine Chemistry.

Rhodium-catalyzed sulfonylation, thioetherification, thiocyanation, and other heterofunctionalizations of arenes bearing a heterocyclic directing group have been realized. The reaction proceeds by initial Rh(III) -catalyzed C-H hyperiodination of arene at room temperature followed by uncatalyzed nucleophilic functionalization. A diaryliodonium salt is isolated as an intermediate, which represents umpolung of the arene substrate, in contrast to previous studies that suggested umpolung of the coupling partner.

Domino Carbopalladation/C-H Functionalization Sequence: An Expedient Synthesis of Bis-Heteroaryls through Transient Alkyl/Vinyl-Palladium Species Capture.

A microwave-assisted highly efficient intermolecular domino carbopalladation/C-H functionalization sequence has been developed to access bis-heteroaryl frameworks in a single operation. The reaction involves carbopalladation of the halogenated acrylamides or phenylpropiolamides by the Pd(0) catalysis, followed by the direct (hetero)arylation to give products with good to excellent yields. The synthetic utility of this method was also extended towards the application of the Ugi-adduct as the starting material.

Stereoselective Synthesis of 1,3-Diaminotruxillic Acid Derivatives: An Advantageous Combination of C-H-ortho-Palladation and On-Flow [2+2]-Photocycloaddition in Microreactors.

The stereoselective synthesis of ε-isomers of dimethyl esters of 1,3-diaminotruxillic acid in three steps is reported. The first step is the ortho-palladation of (Z)-2-aryl-4-aryliden-5(4H)-oxazolones 1 to give dinuclear complexes 2 with bridging carboxylates. The reaction occurs through regioselective activation of the ortho-CH bond of the 4-arylidene ring in carboxylic acids. The second step is the [2+2]-photocycloaddition of the CC exocyclic bonds of the oxazolone skeleton in 2 to afford the corresponding dinuclear ortho-palladated cyclobutanes 3. This key step was performed very efficiently by using LED light sources with different wavelengths (465, 525 or 625 nm) in flow microreactors. The final step involved the depalladation of 3 by hydrogenation in methanol to afford the ε-1,3-diaminotruxillic acid derivatives as single isomers.

Transition Metal-Catalyzed Carbonylative C-H Bond Functionalization of Arenes and C(sp(3))-H Bond of Alkanes.

In this article, we present the progress made in the area of carbonylative C-H functionalization, with special emphasis on arenes and alkanes. The importance of directing group assistance and C-H functionalization using CO surrogates is also included. The budding development in the area of transition metal-catalyzed C(sp(3))-H activation makes us feel it necessary to file a summary on the past, as well as current, contributions and a prospective outlook on the transition metal-catalyzed carbonylative transformation of C-H bonds, which is the focus of this review.

Structure of the Dioxygenase AsqJ: Mechanistic Insights into a One-Pot Multistep Quinolone Antibiotic Biosynthesis.

Multienzymatic cascades are responsible for the biosynthesis of natural products and represent a source of inspiration for synthetic chemists. The Fe(II)/α-ketoglutarate-dependent dioxygenase AsqJ from Aspergillus nidulans is outstanding because it stereoselectively catalyzes both a ferryl-induced desaturation reaction and epoxidation on a benzodiazepinedione. Interestingly, the enzymatically formed spiro epoxide spring-loads the 6,7-bicyclic skeleton for non-enzymatic rearrangement into the 6,6-bicyclic scaffold of the quinolone alkaloid 4'-methoxyviridicatin. Herein, we report different crystal structures of the protein in the absence and presence of synthesized substrates, surrogates, and intermediates that mimic the various stages of the reaction cycle of this exceptional dioxygenase.

A Highly Reactive Seven-Coordinate Osmium(V) Oxo Complex: [Os(V)(O)(qpy)(pic)Cl](2+).

Seven-coordinate ruthenium oxo species have been proposed as active intermediates in catalytic water oxidation by a number of highly active ruthenium catalysts, however such species have yet to be isolated. Reported herein is the first example of a seven-coordinate group 8 metal-oxo species, [Os(V)(O)(qpy)(pic)Cl](2+) (qpy = 2,2':6',2'':6'',2'''-quaterpyridine, pic = 4-picoline). The X-ray crystal structure of this complex shows that it has a distorted pentagonal bipyramidal geometry with an Os=O distance of 1.7375 Å. This oxo species undergoes facile O-atom and H-atom-transfer reactions with various organic substrates. Notably it can abstract H atoms from alkylaromatics with C-H bond dissociation energy as high as 90 kcal mol(-1). This work suggests that highly active oxidants may be designed based on group 8 seven-coordinate metal oxo species.

Tunable Cascade Reactions of Alkynols with Alkynes under Combined Sc(OTf)3 and Rhodium Catalysis.

Two tunable cascade reactions of alkynols and alkynes have been developed by combining Sc(OTf)3 and rhodium catalysis. In the absence of H2O, an endo-cycloisomerization/C-H activation cascade reaction provided 2,3-dihydronaphtho[1,2-b]furans in good to high yields. In the presence of H2O, the product of alkynol hydration underwent an addition/C-H activation cascade reaction with an alkyne, which led to the formation of 4,5-dihydro-3H-spiro[furan-2,1'-isochromene] derivatives in good yields under mild reaction conditions. Mechanistic studies of the cascade reactions indicated that the rate-determining step involves C-H bond cleavage and that the hydration of the alkynol plays a key role in switching between the two reaction pathways.

Rhodium-Catalyzed Regioselective C7-Functionalization of N-Pivaloylindoles.

An efficient rhodium-catalyzed method for direct C-H functionalization at the C7 position of a wide range of indoles has been developed. Good to excellent yields of alkenylation products were observed with acrylates, styrenes, and vinyl phenyl sulfones, whereas the saturated alkylation products were obtained in good yield with α,ß-unsaturated ketones. Both the N-pivaloyl directing group and the rhodium catalyst proved to be crucial for high regioselectivity and conversion.

Ligand-Promoted Borylation of C(sp(3))-H Bonds with Palladium(II) Catalysts.

A quinoline-based ligand effectively promotes the palladium-catalyzed borylation of C(sp(3))-H bonds. Primary ß-C(sp(3))-H bonds in carboxylic acid derivatives as well as secondary C(sp(3))-H bonds in a variety of carbocyclic rings, including cyclopropanes, cyclobutanes, cyclopentanes, cyclohexanes, and cycloheptanes, can thus be borylated. This directed borylation method complements existing iridium(I)- and rhodium(I)-catalyzed C-H borylation reactions in terms of scope and operational conditions.

Ruthenium Oxidase Catalysis for Site-Selective C-H Alkenylations with Ambient O2 as the Sole Oxidant.

Ruthenium(II) oxidase catalysis by direct dioxygen-coupled turnover enabled step-economical oxidative C-H alkenylation reactions at ambient pressure. Versatile ruthenium(II) biscarboxylate catalysts displayed ample substrate scope and proved applicable to weakly coordinating and removable directing groups. The twofold C-H functionalization strategy was characterized by exceedingly mild reaction conditions as well as excellent positional selectivity.

Contrasting electronic requirements for C-H binding and C-H activation in d(6) half-sandwich complexes of rhenium and tungsten.

A computational study of the interaction half-sandwich metal fragments (metal = Re/W, electron count = d(6)), containing linear nitrosyl (NO(+) ), carbon monoxide (CO), trifluorophosphine (PF3 ), N-heterocyclic carbene (NHC) ligands with alkanes are conducted using density functional theory employing the hybrid meta-GGA functional (M06). Electron deficiency on the metal increases with the ligand in the order NHC < CO < PF3 < NO(+). Electron-withdrawing ligands like NO(+) lead to more stable alkane complexes than NHC, a strong electron donor. Energy decomposition analysis shows that stabilization is due to orbital interaction involving charge transfer from the alkane to the metal. Reactivity and dynamics of the alkane fragment are facilitated by electron donors on the metal. These results match most of the experimental results known for CO and PF3 complexes. The study suggests activation of alkane in metal complexes to be facile with strong donor ligands like NHC.

Cobalt-catalyzed C-H arylations with weakly-coordinating amides and tetrazoles: expedient route to angiotensin-II-receptor blockers.

Cobalt-catalyzed C-H arylations enabled the synthesis of biaryl tetrazoles, which are key structural motifs in antihypertensive angiotensin-II-receptor blockers. Thus, weakly-coordinating benzamides were employed for step-economical C-H arylations with ample scope. Further, a low-valent NHC complex enabled first cobalt-catalyzed C-H functionalization by tetrazole assistance.

Rh(V) -Nitrenoid as a Key Intermediate in Rh(III) -Catalyzed Heterocyclization by C-H Activation: A Computational Perspective on the Cycloaddition of Benzamide and Diazo Compounds.

A mechanistic study of the substituent-dependent ring formations in Rh(III) -catalyzed C-H activation/cycloaddition of benzamide and diazo compounds was carried out by using DFT calculations. The results indicated that the decomposition of the diazo is facilitated upon the formation of the five-membered rhodacycle, in which the Rh(III) center is more electrophilic. The insertion of carbenoid into Rh-C(phenyl) bond occurs readily and forms a 6-membered rhodacycle, however, the following C-N bond formation is difficult both kinetically and thermodynamically by reductive elimination from the Rh(III) species. Instead, the Rh(V) -nitrenoid intermediate could be formed by migration of the pivalate from N to Rh, which undergoes the heterocyclization much more easily and complementary ring-formations could be modulated by the nature of the substituent at the α-carbon. When a vinyl is attached, the stepwise 1,3-allylic migration occurs prior to the pivalate migration and the 8-membered ring product will be formed. On the other hand, the pivalate migration becomes more favorable for the phenyl-contained intermediate because of the difficult 1,3-allylic migration accompanied by dearomatization, thus the 5-membered ring product was formed selectively.

Ruthenium-Catalyzed Oxidative Coupling of Primary Amines with Internal Alkynes through C-H Bond Activation: Scope and Mechanistic Studies.

The oxidative coupling of primary amines with internal alkynes catalyzed by Ru complexes is presented as a general atom-economy methodology with a broad scope of applications in the synthesis of N-heterocycles. Reactions proceed through regioselective C-H bond activation in 15 minutes under microwave irradiation or in 24 hours with conventional heating. The synthesis of 2,3,5-substituted pyridines, benzo[h]isoquinolines, benzo[g]isoquinolines, 8,9-dihydro-benzo[de]quinoline, 5,6,7,8-tetrahydroisoquinolines, pyrido[3,4g]isoquinolines, and pyrido[4,3g]isoquinolines is achievable depending on the starting primary amine used. DFT calculations on a benzylamine substrate support a reaction mechanism that consists of acetate-assisted C-H bond activation, migratory-insertion, and C-N bond formation steps that involve 28-30 kcal mol(-1) . The computational study is extended to additional substrates, namely, 1-naphthylmethyl-, 2-methylallyl-, and 2-thiophenemethylamines.

Rhodium(III)-Catalyzed [4+1] Annulation of Aromatic and Vinylic Carboxylic Acids with Allenes: An Efficient Method Towards Vinyl-Substituted Phthalides and 2-Furanones.

A highly regio- and stereoselective synthesis of 3,3-disubstituted phthalides from aryl carboxylic acids and allenes using a rhodium(III) catalyst has been demonstrated. The reaction features broad functional group tolerance and provides a simple and straightforward route to the synthesis of various 3-vinyl-substituted phthalides. Furthermore, the catalytic reaction can also be applied to the synthesis of biologically active 5-vinyl-substituted 2-furanones from α,ß-unsaturated carboxylic acids and allenes. The reactions proceed through a carboxylate-assisted ortho-CH activation and [4+1] annulation. The preliminary mechanistic studies suggest that a CH cleavage is the rate-determining step.

Catalyst-controlled regiodivergent C-H borylation of multifunctionalized heteroarenes by using iridium complexes.

The regiodivergent C-H borylation of 2,5-disubstituted heteroarenes with bis(pinacolato)diboron was achieved by using iridium catalysts formed in situ from [Ir(OMe)(cod)]2 /dtbpy (cod=1,5-cyclooctadiene, dtbpy: 4,4'-di-tert-butyl-2,2'-bipyridine) or [Ir(OMe)(cod)]2 /2 AsPh3 . When [Ir(OMe)(cod)]2 /dtbpy was used as the catalyst, borylation at the 4-position proceeded selectively to afford 4-borylated products in high yields (dtbpy system A). The regioselectivity changed when the [Ir(OMe)(cod)]2 /2 AsPh3 catalyst was used; 3-borylated products were obtained in high yields with high regioselectivity (AsPh3 system B). The regioselectivity of borylation was easily controlled by changing the ligands. This reaction was used in the syntheses of two different bioactive compound analogues by using the same starting material.

Iron-catalyzed C(sp(2))-H and C(sp(3))-H methylations of amides and anilides.

The so-called magic methyl effect significantly boosts the bioactivities and physical properties of pharmacologically active drugs. Direct introduction of the methyl group by CH activation was accomplished with a versatile iron catalyst, which enabled the CH methylation of (hetero)benzamides, anilides, alkenes, and even alkanes by triazole assistance in a chemo-, site- and diastereo-selective fashion.

Divergent reactivity in palladium-catalyzed annulation with diarylamines and α,ß-unsaturated acids: direct access to substituted 2-quinolinones and indoles.

A palladium-catalyzed CH activation strategy has been successfully employed for exclusive synthesis of a variety of 3-substituted indoles. A [3+3] annulation for synthesizing substituted 2-quinolinones was recently developed by reaction of α,ß-unsaturated carboxylic acids with diarylamines under acidic conditions. In the present work, an analogous [3+2] annulation is achieved from the same set of starting materials under basic conditions to generate 1,3-disubstituted indoles exclusively. Mechanistic studies revealed an ortho-palladation-π-coordination-ß-migratory insertion-ß-hydride elimination reaction sequence to be operative under the reaction conditions.

Pd-catalyzed Csp(2) -H functionalization of heteroarenes via isocyanide insertion: concise synthesis of di-(hetero)aryl ketones and di-(hetero)aryl alkylamines.

We report herein an efficient Pd-catalyzed direct C-H bond functionalization of heteroarenes via an isocyanide insertion strategy for the synthesis of di-(hetero)aryl ketones and di-(hetero)aryl alkylamines. The methodology involves a three component reaction between an azole, a haloarene and an isocyanide resulting in the formation of an imine, which in turn is either hydrolyzed or reduced to get the desired product.