Improved Catalysts for the Palladium-Catalyzed Synthesis of Oxindoles by Amide α-Arylation. Rate Acceleration, Use of Aryl Chloride Substrates, and a New Carbene Ligand for Asymmetric Transformations

Palladium-catalyzed direct intermolecular α-arylation of amides with aryl chlorides

An efficient catalytic system for the direct intermolecular α-arylation of acetamide derivatives with aryl chlorides is presented. Chemoselectivities up to 10:1 in the mono- and diarylation of acetamides were achieved by careful selection of bases, solvents, and stoichiometry. Bis-arylated amides were prepared in up to 95% yield.

Preparation of Aryl-Susbstituted 2-Oxyindoles by Superelectrophilic Chemistry

A series of pyridyl-substituted 3-hydroxy-2-oxyindoles have been prepared and reacted with arenes in superacid promoted Friedel-Crafts reactions. The product aryl-substituted 2-oxyindoles are formed in generally good yields. With substituted arenes such as toluene, bromobenzene, or ethyl salicylate, the Friedel-Crafts reactions occur with excellent regioselectivity. A mechanism is proposed involving dicationic, superelectrophilic species leading to the electrophilic aromatic substitution chemistry.

Rotational isomers of N-methyl-N-arylacetamides and their derived enolates: implications for asymmetric Hartwig oxindole cyclizations

The rotational preferences of N-(2-bromo-4,6-dimethylphenyl)-N-methyl 2-phenylpropanamide were studied as a model of precursors for Hartwig asymmetric oxindole cyclizations. The atropisomers of this compound were separated by flash chromatography, and then the enantiomers were resolved and the interconversions of the stereocenter and the N-Ar axis were studied. Under thermal conditions, the axis is very stable. Under the basic conditions of the Hartwig cyclization, both the stereocenter and the chiral axis equilibrate via enolate formation. The N-Ar rotation barrier of a 2-phenylacetamide analogue was reduced from 31 kcal mol(-1) in the precursor to 17 kcal mol(-1) in the enolate. Reasons for this dramatic barrier reduction and implications of both N-Ar and amide C-N rotations for Hartwig cyclizations are discussed.

Transition-metal-free synthesis of oxindoles by potassium tert-butoxide-promoted intramolecular α-arylation

Potassium tert-butoxide-mediated intramolecular α-arylations of fluoro- and chloro-substituted anilides provide oxindoles in DMF at 80 °C. In this manner, diversely substituted products have been obtained in moderate to high yields.

Asymmetric Pd-NHC*-catalyzed coupling reactions

Very high asymmetric inductions result in the Pd-catalyzed intramolecular arylation of amides to give 3,3-disubstituted oxindoles when new in situ-generated chiral N-heterocyclic carbene (NHC*) ligands are employed. Structural studies show that conformational locking to minimize allylic strain is the key to understanding the function of these ligands. New applications of these ligands in the frontier area of asymmetric coupling reactions involving C(sp3)–H bonds are detailed. Highly enantioenriched fused indolines are accessible using either preformed- or in situ-generated Pd-NHC* catalysts. Remarkably, this occurs at high temperature (140–160 °C) via excellent asymmetric recognition of an enantiotopic C–H bond in an unactivated methylene unit.

Synthesis of tetracyclic indoles via intramolecular α-arylation of ketones

An intramolecular palladium(0)-mediated α-arylation of ketones applied to the synthesis of various substituted tetracyclic indoles is reported. Most significantly, the efficiency of the transformation was enhanced by the use of monoligated Pd(0) complexes. This methodology was extended to double α-arylation of ketones using one-pot reactions with either simultaneous addition or sequential addition of two aryl halides for producing aryl substituted tetracyclic indoles.

3,3'-[1,2-Phenyl-enebis(methyl-ene)]bis-(1-propyl-benzimidazolium) dibromide hemihydrate

The asymmetric unit of the title compound, C(28)H(32)N(4) (2+)·2Br(-)·0.5H(2)O, contains one 3,3'-[1,2-phenyl-enebis(methyl-ene)]bis-(1-propyl-benzimidazolium) cation, two bromide anions and one half-mol-ecule of water. In the crystal, the whole 3,3'-[1,2-phen-yl-enebis(methyl-ene)]bis-(1-propyl-benzimidazolium) cation and one of the bromide anions are each disordered over two positions with site-occupancy ratios of 0.751 (6):0.249 (6) and 0.680 (8):0.320 (8). For the major component of the disordered cation, the central benzene ring forms dihedral angles of 83.6 (5) and 88.2 (4)° with its adjacent imidazole rings, while these angles for the minor component are 89.2 (15) and 84.9 (13)°. In the crystal, the cations and anions are linked by C-H⋯Br hydrogen bonds into dimers and then stacked along the c axis. The crystal packing is further stabilized by C-H⋯π and π-π inter-actions [shortest centroid-centroid distance = 3.646 (4) Å].