Palladium-catalyzed rearrangement/arylation of 2-allyloxypyridine leading to the synthesis of N-substituted 2-pyridones

[reaction: see text] The Pd-catalyzed one-pot rearrangement/arylation of 2-allyloxypyridine is described. The catalyst/base combination of Pd[P(t-Bu)(3)](2)/Ag(2)CO(3) was found to be optimal for this one-pot rearrangement/arylation. The initial rearrangement of 2-allyloxypyridine was found to be catalyzed by both Pd(0) and Pd(II) complexes with different mechanisms.     

New homoleptic organometallic derivatives of vanadium(III) and vanadium(IV): synthesis, characterization, and study of their electrochemical behaviour

The arylation of [VCl3(thf)3] with LiR(Cl), where R(Cl) is a polychlorinated phenyl group [C6Cl5, 2,4,6-trichlorophenyl(tcp), or 2,6-dichlorophenyl (dcp)] gives four-coordinate, homoleptic organovanadium(III) derivatives with the formula [Li(thf)(4)][V(III)(R(Cl))(4)] (R(Cl) = C(6)Cl(5) (1), tcp (2), dcp (3)). The anion [V(III)(C6Cl5)4]- has an almost tetrahedral geometry, as observed in the solid-state structure of [NBu4][V(C6Cl5)4] (1') (X-ray diffraction). Compounds 1-3 are electrochemically related to the neutral organovanadium(IV) species [V(IV)(R(Cl))4] (R(Cl) = C6Cl5 (4), tcp (5), dcp (6)). The redox potentials of the V(IV)/V(III) semisystems in CH2Cl2 decrease with decreasing chlorination of the phenyl ring (E(1/2) = 0.84 (4/1), 0.42 (5/2), 0.25 V (6/3)). All the [V(IV)(R(Cl))4] derivatives involved in these redox couples could also be prepared and isolated by chemical methods. The arylation of [VCl(3)(thf)(3)] with LiC6F5 also gives a homoleptic organovanadium(III) compound, but with a different stoichiometry: [NBu4]2[V(III)(C6F5)5] (7). In this five-coordinate species, the C6F5 groups define a trigonal bipyramidal environment for the vanadium atom (X-ray diffraction). EPR spectra for the new organovanadium compounds 1-6 are also given and analysed in terms of an elongated tetrahedral structure with C(2v) local symmetry. It is suggested that the R(Cl) groups exert a protective effect towards the vanadium centre.     

Mechanistic analysis of iridium(III) catalyzed direct C-H arylations: a DFT study

In a recent experimental work the Ir complex [Ir(cod)(py)(PCy(3))](PF(6)) (that is, Crabtree's catalyst) has been shown to catalyze the C-H arylation of electron-rich heteroarenes with iodoarenes using Ag(2)CO(3) as base. For this process, an electrophilic metalation mechanism, (S(E)Ar) has been proposed as operative mechanism rather than the concerted metalation-deprotonation (CMD) mechanism, widely implicated in Pd-catalyzed arylation reactions. Herein we have investigated the C-H activation step for several (hetero)arenes catalyzed by a Ir(III) catalyst and compared the data obtained with the results for the Pd(II)-catalyzed C-H bond activation. The calculations demonstrate that, similar to Pd(II)-catalyzed reactions, the Ir(III)-catalyzed direct C-H arylation occurs through the CMD pathway which accounts for the experimentally observed regioselectivity. The transition states for Ir(III)-catalyzed direct C-H arylation feature stronger metal-C((arene)) interactions than those for Pd(II)-catalyzed C-H arylation. The calculations also demonstrate that ligands with low trans effect may decrease the activation barrier of the C-H bond cleavage.     

A new series of homoleptic,paramagnetic organochromium derivatives: synthesis,characterization, and study of the magnetic properties

The synthesis and characterization of the triad of organochromium derivatives [Cr(C(6)Cl(5))(4)](n-) (n=0, 1, 2) are described. By treating [CrCl(3)(thf)(3)] with LiC(6)Cl(5) in 1:5 molar ratio, the salt [Li(thf)(4)][Cr(III)(C(6)Cl(5))(4)] (1) was obtained as a violet solid in 57 % yield. Oxidation of 1 with [N(4-BrC(6)H(4))(3)][SbCl(6)] yielded the neutral complex [Cr(IV)(C(6)Cl(5))(4)] (2) as a brown solid in 71 % yield. The arylation of [CrCl(2)(thf)] with LiC(6)Cl(5) under similar conditions as above gave [[Li(thf)(3)](2)(mu-Cl)](2)[Cr(II)(C(6)Cl(5))(4)] (3) as an extremely air- and water-sensitive red solid in 47 % yield. The crystal and molecular structures of 1 and 3 have been established by X-ray diffraction methods. Complex 3 contains the unusual cation [[Li(thf)(3)](2)(mu-Cl)](+) with an almost linear Li-Cl-Li unit (174.2(6)degrees). All four C(6)Cl(5) groups are sigma-bonded to the Cr(II) center, which is located in a square-planar environment. The local geometry around the Cr(III) center in 1 is, in turn, pseudo-octahedral, since two of the C(6)Cl(5) groups act as standard sigma-bonded monodentate ligands, while the other two act as small-bite didentate ligands coordinated through both the ipso-C and one of the ortho-Cl atoms. Compounds 1-3 are paramagnetic with maximum spin multiplicity each (EPR and magnetization measurements).     

First Synthesis of 5-Chloro-7-[1,3]oxazolo[4,5-b]pyridin-2-ylquinolin-8-ol by Pd-Catalyzed Arylation

5-Chloro-7-[1,3]oxazolo[4,5-b]pyridin-2-ylquinolin-8-ol was synthesized by a Pd-catalyzed arylation, which proceeds efficiently with 2 equivalents of benzylated clioquinol and 1 equivalent of oxazolo[4,5-b]pyridine. The product was smoothly debenzylated by boron trichloride in dichloromethane.     

Direct arylation of polycyclic aromatic hydrocarbons through palladium catalysis

We have discovered that the combination of Pd(OAc)(2)/o-chloranil can catalyze the direct C-H bond arylation of polycyclic aromatic hydrocarbons (PAHs) with arylboroxins that occurs selectively at the K-region. The sequential integration of Pd-catalyzed direct arylation of PAHs and FeCl(3)-mediated cyclodehydrogenation is effective in rapidly extending a parent PAH π-system with high directionality.     

Direct arylation of simple azoles catalyzed by 1,10-phenanthroline containing palladium complexes: an investigation of C4 arylation of azoles and the synthesis of triarylated azoles by sequential arylation

Direct triarylation and sequential triarylation reactions of simple azoles catalyzed by [Pd(phen)(2)]PF(6) are described. Simple azoles, such as N-methylimidazole, thiazole, and oxazole, were observed to undergo triaryaltion reactions even at their C4 positions when treated with aryl iodides in the presence of [Pd(phen)(2)]PF(6) as a catalyst and a stoichiometric amount of Cs(2)CO(3) in DMA at 150 °C. Using excess amounts of azoles, selective C5 monoarylation was achieved by using the same catalytic system. Subsequent efforts demonstrated that C5 arylated azoles undergo exclusive C2 arylation using [Pd(phen)(2)]PF(6) as the catalyst with galvinoxyl as an additive. Finally, unprecedented C4 arylation reactions of 2,5-diaryl-azoles occur by using the new catalytic system to give the corresponding triarylated products in good to excellent yields. The results of mechanistic studies suggest that the C2 arylation process takes place by way of an electrophilic aromatic substitution (S(E)Ar) palladation pathway, while arylation reactions at the C4 position occur via a S(E)Ar palladation and/or radical mechanism. Finally, a concise, three-step synthesis of the Tie-2 Tyrosine Kinase Inhibitor has been executed starting with commercially available N-methylimidazole by a route that employs the new sequential arylation process.     

Rapid construction of multisubstituted olefin structures using vinylboronate ester platform leading to highly fluorescent materials

[reaction: see text] A catalytic one-pot triarylation on the C=C core of vinylboronate pinacol ester (1) produces extended pi-systems based on a multisubstituted olefin structure very rapidly. We established an efficient protocol for the Pd-catalyzed double C-H arylation of 1 with aryl halides, which was successfully integrated with C-B arylation with aryl halides (cross-coupling) in one pot. By using this method, several highly fluorescent materials with a wide range of color variations (blue to red) were discovered.     

Intramolecular arylation reactions: first efficient synthesis of novel fused pyridoimidazoquinolinones or pyridoimidazoazepinones libraries

An efficient method for the synthesis of new polycyclic skeletons: pyrido[2′,1′:2,3]imidazo[5,4-c]quinolin-6(5H)-ones and pyrido[2′,1′:2,3]imidazo[5,4-c]azepin-7(6H)-ones libraries is described via Pd-catalyzed intramolecular arylation involving C(sp2)-H activation. This method permits the synthesis of polycyclic derivatives in good yield. The process tolerates a variety of aryl substituents as well as alkyl imidazo[1,2-a]pyridine-2-carboxamide structures. The resultant compounds, 10(11)-chloro-pyrido[2′,1′:2,3]imidazo[5,4-c]quinolinones or azepinones are functionalized under Suzuki cross-coupling conditions to give polyfunctional compounds.     

Synthesis of 6,8,9-tri- and 2,6,8,9-tetrasubstituted purines by a combination of the Suzuki cross-coupling, N-arylation, and direct C-H arylation reactions

Novel practical methodology of synthesis of a several types of di-, tri-, and tetraarylpurine derivatives by a combination of regioselective Suzuki cross-coupling reactions and/or Cu-catalyzed N-arylation with direct C-H arylations was developed. 6,8-Diaryl- and 2,6,8-triaryl-9-isopropylpurines were prepared by one or two cross-couplings of 6-chloro- or 2,6-dichloro-9-isopropylpurine with arylboronic acids followed by Pd-catalyzed C-H arylation by aryl halides to position 8. 6-Chloropurine and adenine underwent Cu-catalyzed N-arylation to position 9 with boronic acids, followed by cross-coupling with AlMe3 and/or C-H arylation to obtain 8,9-diaryl-6-methylpurines or 8,9-diaryladenines (accompanied by products of partial N-arylation of adenine in position 6). The methodology is suitable for construction of small libraries of modified purines.     

Silver-mediated palladium-catalyzed direct C-H arylation of 3-bromoisothiazole-4-carbonitrile

Silver(I) fluoride-mediated Pd-catalyzed C-H direct arylation/heteroarylation of 3-bromoisothiazole-4-carbonitrile (1a) gives twenty-four 5-aryl/heteroaryl-3-bromoisothiazole-4-carbonitriles. The reaction was partially optimized with respect to catalyst, ligand, and base. During this study 3,3'-dibromo-5,5'-biisothiazole-4,4'-dicarbonitrile (3a) was isolated as a byproduct and subsequently prepared via the silver-mediated Pd-catalyzed oxidative dimerization of 3-bromoisothiazole-4-carbonitrile in 67% yield. The analogous phenylation and oxidative dimerization of 3-chloroisothiazole-4-carbonitrile (1b) gave 3-chloro-5-phenylisothiazole-4-carbonitrile (4) and 3,3'-dichloro-5,5'-biisothiazole-4,4'-dicarbonitrile (3b) in 96% and 69% yields, respectively.     

Completely N1-selective palladium-catalyzed arylation of unsymmetric imidazoles: application to the synthesis of nilotinib

The completely N(1)-selective Pd-catalyzed arylation of unsymmetric imidazoles with aryl halides and triflates is described. This study showed that imidazoles have a strong inhibitory effect on the in situ formation of the catalytically active Pd(0)-ligand complex. The efficacy of the N-arylation reaction was improved drastically by the use of a preactivated solution of Pd(2)(dba)(3) and L1. From these findings, it is clear that while imidazoles can prevent binding of L1 to Pd, once the ligand is bound to the metal, these heterocycles do not displace it. The utility of the present catalytic system was demonstrated by the regioselective synthesis of the clinically important tyrosine kinase inhibitor nilotinib.     

Synthesis of unsymmetrical diarylureas via Pd-catalyzed C-N cross-coupling reactions

A facile synthesis of unsymmetrical N,N'-diarylureas is described. The utilization of the Pd-catalyzed arylation of ureas enables the synthesis of an array of diarylureas in good to excellent yields from benzylurea via a one-pot arylation-deprotection protocol, followed by a second arylation.     

Ligand-promoted C3-selective arylation of pyridines with Pd catalysts: gram-scale synthesis of (±)-preclamol

The first example of Pd-catalyzed, C3-selective arylation of unprotected pyridines has been developed by employing a catalytic system consisting of Pd(OAc)(2) and 1,10-phenanthroline. This protocol provides an expeditious route to an important class of 3-arylpyridines and 3-arylpiperidines frequently found in bioactive compounds. A brief synthesis of the drug molecule (±)-preclamol is also reported.     

Function of an N-heterocyclic carbene ligand based on concept of chiral mimetic

Function of a new N-heterocyclic carbene ligand based on the concept of a chiral mimetic is described. With (4R,5R)-4,5-diphenyl-1,3-dialkyl-4,5-dihydro-3H-imidazol-1-ium tetrafluoroborates as N-heterocyclic carbene precursors, Rh-catalyzed enantioselective arylation (up to 27% ee) of aromatic aldehydes with arylboronic acids and Pd-catalyzed enantioselective intramolecular alpha-arylation (up to 66% ee) of N-(2-bromoaryl)-N-alkyl-2-arylpropanamides are investigated.     

Synthesis of 6-methyl-6H-indolo[3,2-c]isoquinoline and 6-methyl-6H-indolo[2,3-c]isoquinoline: two new unnatural isoquinoline isomers of the cryptolepine series

11H-indolo[3,2-c]isoquinoline has been synthesized in two steps starting from 4-bromoisoquinoline and 2-bromoaniline via a selective Buchwald-Hartwig reaction followed by a Pd-catalyzed intramolecular direct arylation involving C(sp²)-H activation. The synthesis of 7H-indolo[2,3-c]isoquinoline was achieved by a combination of a Suzuki reaction with an intramolecular nitrene insertion reaction starting from 4-bromoisoquinoline and {2-[(2,2-dimethylpropanoyl)amino]phenyl}boronic acid. Selective methylation of the tetracyclic skeletons yielded the title compounds 6-methyl-6H-indolo[3,2-c]isoquinoline and 6-methyl-6H-indolo[2,3-c]isoquinoline, which have never been described in the literature before.     

From imidates to vinyl-1,2,4-triazoles: Synthesis,mechanistic aspects and first issues of their reactivity

The reaction of variously substituted acylimidates with hydrazine derivatives represents an efficient and easy to set synthetic entry towards 5-vinyl-1,2,4-triazole derivatives. The construction of the triazole ring allows the installation of variety of substituent combination at the N(1), C(3) and C(5) positions of the five-membered heterocycle in good to high yields. The method reveals selective towards 5-vinyl-1,2,4-triazoles avoiding the potential formation of seven- and five-membered side-products as supported by theoretical calculations and NMR experiments. First lines of Pd-catalyzed arylation of the vinyl fragment towards 5-styryl-1,2,4-triazoles and Cu-catalyzed arylation at the N(1) site are finally described.     

Acetonimine and 4-imino-2-methylpentan-2-amino platinum(II) complexes: synthesis and in vitro antitumor activity

The reaction of [Pt(dmba)(PPh3)Cl] [where dmba = N,C-chelating 2-(dimethylaminomethyl)phenyl] with aqueous ammonia in acetone in the presence of AgClO4 gives the acetonimine complex [Pt(dmba)(PPh3)(NH=CMe2)]ClO4 (1). The reaction of [Pt(dmba)(DMSO)Cl] with aqueous ammonia in acetone in the presence of AgClO4 gives a mixture of [Pt(dmba)(NH=CMe2)2]ClO4 (2) and [Pt(dmba)(imam)]ClO4 (3a) (where imam = 4-imino-2-methylpentan-2-amino). [Pt(dmba)(DMSO)Cl] reacts with [Ag(NH=CMe2)2]ClO4 in a 1:1 molar ratio to give [Pt(dmba)(DMSO)(NH=CMe2)]ClO4 (4). The reaction of [Pt(dmba)(DMSO)Cl] with 20% aqueous ammonia in acetone at 70 degrees C in the presence of KOH gives [Pt(dmba)(CH2COMe)(NH=CMe2)] (5), whereas the reaction of [Pt(dmba)(DMSO)Cl] with 20% aqueous ammonia in acetone in the absence of KOH gives [Pt(dmba)(imam)]Cl (3b). The reaction of [NBu4]2[Pt2(C6F5)4(mu-Cl)2] with [Ag(NH=CMe2)2]ClO4 in a 1:2 molar ratio produces cis-[Pt(C6F5)2(NH=CMe2)2] (6). The crystal structures of 1 x 2 Me2CO, 2, 3a, 5, and 6 have been determined. Values of IC50 were calculated for the new platinum complexes against a panel of human tumor cell lines representative of ovarian (A2780 and A2780 cisR) and breast cancers (T47D). At 48 h incubation time complexes 1, 4, and 5 show very low resistance factors against an A2780 cell line which has acquired resistance to cisplatin. 1, 4, and 5 were more active than cisplatin in T47D (up to 30-fold in some cases). The DNA adduct formation of 1, 4, and 5 was followed by circular dichroism and electrophoretic mobility.     

Pd-catalyzed direct arylation of tautomerizable heterocycles with aryl boronic acids via C-OH bond activation using phosphonium salts

The first direct arylation via C-OH bond activation of tautomerizable heterocycles has been achieved using phosphonium salts, on the basis of a combination of the phosphonium coupling and Suzuki-Miyaura cross-coupling conditions. Optimal reaction condition is obtained through screening of phosphonium salts, Pd catalysts, and bases. The direct arylation via C-OH bond activation tolerates a variety of tautomerizable heterocycles and aryl boronic acids. The mechanism of the Pd-catalyzed phosphonium coupling is proposed to proceed via a domino seven-step process including the unprecedented heterocycle-Pd(II)-phosphonium species. Application of the Pd-catalyzed direct arylation via C-OH bond activation using PyBroP leads to the most efficient synthesis of the biologically important 6-arylpurine ribonucleoside in a single step from unactivated and unprotected inosine.     

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.