Exploring the oxidative cyclization of substituted N-aryl enamines: Pd-catalyzed formation of indoles from anilines

The direct Pd-catalyzed oxidative coupling of two C-H-bonds within N-aryl-enamines 1 allows the efficient formation of differently substituted indoles 2. In this cross-dehydrogenative coupling, many different functional groups are tolerated and the starting material N-aryl-enamines 1 can be easily prepared in one step from commercially available anilines. In addition, the whole sequence can also be run in a one-pot fashion. Optimization data, mechanistic insight, substrate scope, and applications are reported in this full paper.     

Unexpected hydrobromic acid-catalyzed C-C bond-forming reactions and facile synthesis of coumarins and benzofurans based on ketene dithioacetals

Hydrobromic acid was found to be a unique catalyst in C? C bond‐forming reactions with ketene dithioacetals. Distinctly different from other acids (including Lewis and Brønsted acids), the remarkable catalytic performance of hydrobromic acid in catalytic amounts was observed in the “acid”‐catalyzed reactions of readily available functionalized ketene dithioacetals 1 with various electrophiles. Under the catalysis of 0.1 equivalents of hydrobromic acid, the reaction of 1 with carbonyl compounds 2 a – l gave polyfunctionalized penta‐1,4‐dienes 3 or conjugated dienes 4 in good to excellent yields. The reaction tolerated a broad range of substituents on both the ketene dithioacetals 1 and the carbonyl compounds 2 . Application of this efficient C? C bond‐forming method generated coumarins 5 and benzofurans 7 under mild, metal‐free conditions by hydrobromic acid‐catalyzed reactions of 1 with salicylaldehydes 2 m – o and p‐quinones 6 a – d , respectively. A new reactive species, a sulfur‐stabilized carbonium ylide, formed depending on the nature of the counterion, and this was proposed as the key intermediate in the unique catalysis of hydrobromic acid.     

C-H activation and C=C double bond formation reactions in iridium ortho-methyl arylphosphane complexes

The Vaska-type iridium(I) complex [IrCl(CO){PPh(2)(2-MeC(6)H(4))}(2)] (1), characterized by an X-ray diffraction study, was obtained from iridium(III) chloride hydrate and PPh(2)(2,6-MeRC(6)H(3)) with R=H in DMF, whereas for R=Me, activation of two ortho-methyl groups resulted in the biscyclometalated iridium(III) compound [IrCl(CO){PPh(2)(2,6-CH(2)MeC(6)H(3))}(2)] (2). Conversely, for R=Me the iridium(I) compound [IrCl(CO){PPh(2)(2,6-Me(2)C(6)H(3))}(2)] (3) can be obtained by treatment of [IrCl(COE)(2)](2) (COE=cyclooctene) with carbon monoxide and the phosphane in acetonitrile. Compound 3 in CH(2)Cl(2) undergoes intramolecular C-H oxidative addition, affording the cyclometalated hydride iridium(III) species [IrHCl(CO){PPh(2)(2,6-CH(2)MeC(6)H(3))}{PPh(2)(2,6-Me(2)C(6)H(3))}] (4). Treatment of 2 with Na[BAr(f) (4)] (Ar(f)=3,5-C(6)H(3)(CF(3))(2)) gives the fluxional cationic 16-electron complex [Ir(CO){PPh(2)(2,6-CH(2)MeC(6)H(3))}(2)][BAr(f) (4)] (5), which reversibly reacts with dihydrogen to afford the delta-agostic complex [IrH(CO){PPh(2)(2,6-CH(2)MeC(6)H(3))}{PPh(2)(2,6-Me(2)C(6)H(3))}][BAr(f)(4)] (6), through cleavage of an Ir-C bond. This species can also be formed by treatment of 4 with Na[BAr(f)(4)] or of 2 with Na[BAr(f)(4)] through C-H oxidative addition of one ortho-methyl group, via a transient 14-electron iridium(I) complex. Heating of the coordinatively unsaturated biscyclometalated species 5 in toluene gives the trans-dihydride iridium(III) complex [IrH(2)(CO){PPh(2)(2,6-MeC(6)H(3)CH=CHC(6)H(3)Me-2,6)PPh(2)}][BAr(f) (4)] (7), containing a trans-stilbene-type terdentate ligand, as result of a dehydrogenative carbon-carbon double bond coupling reaction, possibly through an iridium carbene species.