Programmed Synthesis of Tetraarylthiophenes through Sequential C−H Arylation

Ligand-enabled methylene C(sp3)-H bond activation with a Pd(II) catalyst

Pd(II) insertion into β-methylene C(sp(3))-H bonds was enabled by a mutually repulsive and electron-rich quinoline ligand. Ligand tuning led to the development of a method that allows for installation of an aryl group on a range of acyclic and cyclic amides containing β-methylene C(sp(3))-H bonds.

Pd-catalyzed direct C-H bond functionalization of spirocyclic σ1 ligands: generation of a pharmacophore model and analysis of the reverse binding mode by docking into a 3D homology model of the σ1 receptor

To explore the hydrophobic binding region of the σ(1) receptor protein, regioisomeric spirocyclic thiophenes 9-11 were developed as versatile building blocks. Regioselective α- and β-arylation using the catalyst systems PdCl(2)/bipy/Ag(2)CO(3) and PdCl(2)/P[OCH(CF(3))(2)](3)/Ag(2)CO(3) allowed the introduction of various aryl moieties at different positions in the last step of the synthesis. The increasing σ(1) affinity in the order 4 < 5/6 < 7/8 indicates that the positions of the additional aryl moiety and the S atom in the spirocyclic thiophene systems control the σ(1) affinity. The main features of the pharmacophore model developed for this class of σ(1) ligands are a positive ionizable group, a H-bond acceptor group, two hydrophobic moieties, and one hydrophobic aromatic group. Docking of the ligands into a σ(1) 3D homology model via molecular mechanics/Poisson-Boltzmann surface area calculations led to a very good correlation between the experimentally determined and estimated free energy of receptor binding. These calculations support the hypothesis of a reverse binding mode of ligands bearing the aryl moiety at the "top" (compounds 2, 3, 7, and 8) and "left" (compounds 4, 5, and 6) positions, respectively.

Palladium-catalyzed C-H activation taken to the limit. Flattening an aromatic bowl by total arylation

All 10 C-H positions on the rim of corannulene can be arylated by repetitive palladium-catalyzed C-H activation. To relieve congestion among the 10 tightly packed aryl substituents in the product, the central corannulene adopts a nearly planar geometry.

Decarbonylative C-H coupling of azoles and aryl esters: unprecedented nickel catalysis and application to the synthesis of muscoride A

A nickel-catalyzed decarbonylative C-H biaryl coupling of azoles and aryl esters is described. The newly developed catalytic system does not require the use of expensive metal catalysts or silver- or copper-based stoichiometric oxidants. We have successfully applied this new C-H arylation reaction to a convergent formal synthesis of muscoride A.

C-H activation: a complementary tool in the total synthesis of complex natural products

The recent advent of transition-metal mediated C-H activation is revolutionizing the synthetic field and gradually infusing a "C-H activation mind-set" in both students and practitioners of organic synthesis. As a powerful testament of this emerging synthetic tool, applications of C-H activation in the context of total synthesis of complex natural products are beginning to blossom. Herein, recently completed total syntheses showcasing creative and ingenious incorporation of C-H activation as a strategic manoeuver are compared with their "non-C-H activation" counterparts, illuminating a new paradigm in strategic synthetic design.

Catalytic arylation of a C-H bond in pyridine and related six-membered N-heteroarenes using organozinc reagents

Despite significant advances in the catalytic direct arylation of heteroarenes, the application of this reaction to pyridines has been met with limited success. An oxidative nucleophilic arylation strategy has been developed to overcome this problem. Pyridine, pyrazine, quinolone, and related electron-deficient N-heteroarenes can be arylated at the most electrophilic site using the developed nickel-catalyzed reaction. This protocol serves as a complementary method to catalytic direct arylation reactions.

Toward controlled synthesis of carbon nanotubes and graphenes

A bottom-up synthesis of structurally uniform carbon nanotubes (CNTs) and graphenes is recognized as one of the greatest challenges of primary importance in nanocarbon science. This paper highlights our efforts to address these challenges since 2005. These endeavors have led to (i) modular, size-selective, and scalable synthesis of [n]cycloparapheneylenes (CPPs), the shortest segment of armchair CNTs, (ii) design and synthesis of the shortest segment of chiral CNTs, and (iii) efficient synthesis of carbon nanosheets through catalytic C–H bond arylation of polycyclic aromatic hydrocarbons (PAHs). We consider these works as a possible first step toward a controlled synthesis of structurally uniform CNTs and nanographenes.

Pd(OAc)2/o-chloranil/M(OTf)n: a catalyst for the direct C-H arylation of polycyclic aromatic hydrocarbons with boryl-, silyl-, and unfunctionalized arenes

Pd(OAc)(2)/o-chloranil/M(OTf)(n) can effectively promote the C-H arylation of fluoranthene with arylboron compounds or arylsilanes. The reaction takes place with high regioselectivity at the C3 position of fluoranthene. Moreover, the new catalytic system allows the use of unfunctionalized arenes as coupling partners in the arylation of polycyclic aromatic hydrocarbons.

Palladium-catalysed direct arylation of a tris-cyclometallated Ir(III) complex bearing 2,2'-thienylpyridine ligands: a powerful tool for the tuning of luminescence properties

Palladium-catalysed direct 5-arylation of metallated thiophenes of fac-Ir(N^C(3')-thpy)(3) with aryl bromides via C-H bond functionalisation allows the synthesis of a variety of new Ir complexes in only one step (thpyH = 2,2'-thienylpyridine). The method offers simple modification of the nature of the ligand and hence of the photophysical properties of such complexes.

Nickel-catalyzed C-H/C-O coupling of azoles with phenol derivatives

The first nickel-catalyzed C-H bond arylation of azoles with phenol derivatives is described. The new Ni(cod)(2)/dcype catalytic system is active for the coupling of various phenol derivatives such as esters, carbamates, carbonates, sulfamates, triflates, tosylates, and mesylates. With this C-H/C-O biaryl coupling, we synthesized a series of privileged 2-arylazoles, including biologically active alkaloids. Moreover, we demonstrated the utility of the present reaction for functionalizing estrone and quinine.

Mechanistic origin of ligand-controlled regioselectivity in Pd-catalyzed C-H activation/arylation of thiophenes

The use of ligands to control regioselectivity in transition-metal-catalyzed C-H activation/functionalization is a highly desirable but challenging task. Recently, Itami et al. reported an important finding relating to Pd-catalyzed ligand-controlled α/β-selective C-H arylation of thiophenes. Specifically, the use of the 2,2'-bipyridyl ligand resulted in α-arylation, whereas the use of the bulky fluorinated phosphine ligand P[OCH(CF(3))(2)](3) resulted in β-arylation. Understanding of this surprising ligand-controlled α/β-selectivity could provide important insights into the development of more efficient catalyst systems for selective C-H arylation, and so we carried out a detailed computational study on the problem with use of density functional theory methods. Three mechanistic possibilities--S(E)Ar and migration, metalation/deprotonation, and Heck-type arylation mechanisms--were examined. The results showed that the S(E)Ar and migration mechanism might not be plausible, because the key Wheland intermediates could not be obtained. On the other hand, our study indicated that the metalation/deprotonation and Heck-type arylation mechanisms were both involved in Itami's reactions. In the metalation/deprotonation pathway the α-selective product (C5-product) was preferred, whereas in the Heck-type arylation mechanism the β-selective product (C4-product) was favored. The ligands played crucial roles in tuning the relative barriers of the two different pathways. In the 2,2'-bipyridyl-assisted system, the metalation/deprotonation pathway was energetically advantageous, leading to α-selectivity. In the P[OCH(CF(3))(2)](3)-assisted system, on the other hand, the Heck-type arylation mechanism was kinetically favored, leading to β-selectivity. An interesting finding was that P[OCH(CF(3))(2)](3) could produce a C-H···O hydrogen bond in the catalyst system, which was crucial for stabilization of the Heck-type transition state. In comparison, this C-H···O hydrogen bond was absent with the other phosphine ligands [i.e., P(OMe)(3), PPh(3), PCy(3)] and these phosphine ligands therefore favored the metalation/deprotonation pathway leading to α-selectivity. Furthermore, in this study we have provided theoretical evidence showing that the Heck-type arylation reaction could proceed through an anti-β-hydride elimination process.

Selective mono- to perarylations of tetrabromothiophene by a cyclobutene-1,2-diylbisimidazolium preligand

(Cyclobut-1-ene-1,2-diyl)bis(1-methylimidazolium)tetrafluoroborate is applied as preligand in palladium-catalyzed cross-coupling reactions starting from tetrabromothiophene for the synthesis of mono-, bi-, tri-, and tetraaryl-substituted thiophenes bearing up to four different aryl rings. A synthetic kit for preparations of nine different substitution patterns of arylated thiophenes is presented by application of only one single catalyst system. In agreement with DFT calculations, which predict energetically low rotational barriers in triaryl-3-bromothiophenes and tetraarylthiophenes, no NOE effects between adjacent aryl rings are detectable. The regioselectivity of their syntheses has therefore been elucidated by reduction of triaryl-3-bromothiophene to 2,3,5-triarylthiophene followed by HMBC, HSQC, and NOESY NMR measurements. Additionally, results of an X-ray single structure analysis are presented.