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

Abstract

The electrophilic palladation of indoles through C–H bond functionalization with the aid of surfactant was achieved in water in a highly enantioselective manner. The system displayed attractive features that are reminiscent of both precious-metal catalysis and micellar catalysis. The palladium­(II) catalyst entangled with a surfactant did not respond to commonly recognized phosphine-based ligand but to 2,2′-bipyridine L1. The insights gained from this unique set of palladium­(II) catalysts demonstrate the potential of further applications toward this novel mode of chemical transformation.

Facile, highly efficient and environmentally friendly transesterification mediated by platinum dioxide and nickel oxide under essentially neutral conditions

“…On the basis of these discussions, a plausible mechanistic pathway for the transesterification process is proposed and depicted in Scheme 4. Supported by the previous literature on Pd [18][19][20][21] and other transition-metal catalysts, [22][23][24][25] On the basis of the possible mechanism mentioned above and the experimental results shown in Scheme 3, we speculated that the Pt-alcohol intermediate formed from low alcohols was more stable than those formed from high alcohols. Therefore, in the intermediate generation stage (first stage), low alcohols showed stronger reactivity than high alcohols, resulting in the production of a low-alcohol intermediate.…”

Hydrogen‐Bonding‐Assisted Cationic Aqua Palladium(II) Complex Enables Highly Efficient Asymmetric Reactions in Water

“…According to our definition, [6a] this reaction is classified as Type IIIa (Figure S2). The distinguished catalytic activity of L Pd(OTf) 2 was apparent by comparing it with the micellar system previously reported as Type IIb reaction, [11] with the latter showing significantly retarded kinetics and erosion of enantioselectivity (entry 2). Likewise, the reaction was sluggish and less selective in the presence of the polyoxyethanyl‐α‐tocopheryl succinate‐based amphiphile designed by Lipshutz and Ghorai, [22] although the field of transition‐metal‐catalyzed chemistry has flourished in water with the aid of this surfactant (entry 3).…”

“…The problem may lie with the insolubility and inherent ease of hydrolysis of these complexes in water because soft acids are reluctant to coordinate with the hard base H 2 O and furthermore they exhibit low hydrolysis constants (e.g. 2.3 for Pd 2+ ), [11] irreversibly leading to the less soluble μ‐hydroxo‐bridged dimer or oligomer [1c,h–m,o, 2e] (Scheme 1 a). Lured by the pluripotency of chiral aqua precious metal complexes for functionalizing carbon‐hydrogen bonds in water, we herein describe the development of a chiral palladium complex that is highly stable and active in water.…”

“…Applications of chiral palladium complexes to reactions in aqueous media include examples of complexes embedding into a protein scaffold, [6a, 8] a robust PEPPSI complex, [9] a complex supported on an amphiphilic resin, [10] and a micellar system [11] . We envisaged a complementary approach wherein a chiral hydrogen‐bond donor associated with trifluoromethanesulfonate (triflate) anion would furnish a charge‐separated palladium aqua complex with enhanced Lewis acidity (Scheme 1 b).…”

“…Conditions are adopted from Ref. [11] with reduced catalyst loading. [e] Run in the presence of 2 wt % aqueous TPGS‐750‐M.…”