The combined use of cationic palladium(II) with a surfactant for the C–H functionalization of indoles and pyrroles in water

Abstract

The utility of electrophilic palladium(II) species was demonstrated for C-H bond functionalization of indoles and pyrroles in water. The system displayed attractive features that are reminiscent of both precious-metal catalysis and micellar catalysis.

Catalytic Organic Reactions in Water toward Sustainable Society

“…Recently, Kobayashi and co-workers discovered the unique nature of palladium(II) catalysis for the 1,4-addition of indoles to enones with the aid of an anionic surfactant and an additive (Scheme 133). 587,588 Interestingly, the palladium(II) catalyst did not respond to commonly recognized phosphine-based ligands but rather to chiral 2,2′-bipyridine. It is also noteworthy that only anionic surfactants could promote the reactions in good yield with high enantioselectivity.…”

Surfactant-Aided Chiral Palladium(II) Catalysis Exerted Exclusively in Water for the C–H Functionalization of Indoles

“…Since the improved catalytic performance of cationic palladium­(II) hinges on the properties of the ligand scaffolds, − , our investigations commenced with identifying a suitable chiral palladium­(II) complex in the presence of 2,6-di- tert -butylpyridine ,, as a sterically hindered base (Table ). In the reaction of 1a with indole 2a , well-known phosphine-based bidentate ligands that are known to stabilize the indolyl-palladium­(II) complex − were found to completely thwart the desired pathway (entries 1–3). , In contrast, the use of a chiral 2,2′-bipyridine L1 − resulted in quantitative formation of the desired product with 83% ee (entry 4).…”

“…31,32 Previous work has underlined the beneficial role of an anionic surfactant to stabilize electrophilic palladium-(II) intermediate I (Scheme 1) for C−H bond functionalization of indoles, and has also demonstrated an enantioselective example. 33 Indolyl-palladium(II) intermediate I would react with α,β-unsaturated carbonyl compounds to form C-bonded palladium(II) enolate II or III. In spite of the intrinsic propensity of III to undergo β-hydride elimination, 34,35 intermediate II is expected to undergo remarkable stabilization in the Stern layer spread on a micellar surface.…”

“…Functionalized indoles represent ubiquitous structural motifs with biological activities, including bioactive indole alkaloids known as hapalindoles and other 3-substituted indoles. − In general, C–H bond functionalization of heteroarenes has relied on the transition-metal–heteroarene interaction through low-hapticity modes because of the high reactivity and ease of stereocontrol . Among the myriad interactions, an indolyl-palladium­(II) intermediate − is one of the most popular and alluring. , In contrast to the nucleophilic character of palladium(0) complexes, palladium­(II) complexes are relatively electrophilic and tend to react with a wide range of electron-rich substrates. , Previous work has underlined the beneficial role of an anionic surfactant to stabilize electrophilic palladium­(II) intermediate I (Scheme ) for C–H bond functionalization of indoles, and has also demonstrated an enantioselective example . Indolyl-palladium­(II) intermediate I would react with α,β-unsaturated carbonyl compounds to form C-bonded palladium­(II) enolate II or III .…”

Highly Efficient Tetranuclear ZnII2LnIII2 Catalysts for the Friedel–Crafts Alkylation of Indoles and Nitrostyrenes

“…For the reaction between indole and β-nitrostyrene, various catalytic systems have been proposed. These include hydrogen-bond-based compounds [11][12][13][14][15][16] such as thiourea [11,14,15,[17][18][19][20], phosphoric acid [16], silanediols [21,22], sulfamic acid [23] and 2,6-bis(amido)benzoic acid [24]; metal based compounds [25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41] such as Al III [25,39], Cu II [26][27][28][29][30]41], Zn II [31][32][33][34][35]40,…”