σ–π Continuum in Indole–Palladium(II) Complexes

Abstract

The intrinsic features of (hetero-arene)-metal interactions have been elusive mainly because the systematic structure analysis of non-anchored hetero-arene-metal complexes has been hampered by their labile nature. We report successful isolation and systematic structure analysis of a series of non-anchored indole-palladium(II) complexes. It was revealed that there is a σ-π continuum for the indole-metal interaction, while it has been thought that the dominant coordination mode of indole to a metal center is the Wheland-intermediate-type σ-mode in light of the seemingly strong electron-donating ability of indole. Several factors which affect the σ- or π-character of indole-metal interactions are discussed.

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

“…1.8 Å). As anticipated, the occupancy of the triflate anions within the second shell of the anhydrous L Pd(OTf) 2 complex provides only an outer‐sphere site for the approaching indole, which may impede the formation of the σ‐indolylpalladium complex considering that a C‐Pd bond with a bond length of 2.2 Å or shorter is required unless coordination bonds around Pd 2+ are broken [14] . The complex in solution was characterized based on UV‐vis and 1 H NMR spectroscopic studies.…”

“…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). [13] The high Lewis acidity and the vacant coordination site of such a complex make it more electrophilic than the corresponding neutral complex in coordination with carbon‐carbon or carbon‐heteroatom multiple bonds [14] . The pseudo‐square‐planar coordination of a chiral hydrogen‐bond donor bound to the palladium core would preclude the presence of equatorially bound water molecules that invoke hydrolysis, resulting in enhanced longevity in water.…”

Pyridylmethylamine–Palladium Catalytic Systems: A Selective Alternative in the C−H Arylation of Indole

“…In a first trial, the reaction between indole and iodobenzene by using ligand L1 and Pd(OAc) 2 as the catalytic system under conventional conditions (Cs 2 CO 3 , toluene/H 2 O=8:2) led to encouraging results, which demonstrated that pma‐based catalysts were not able only to promote C‐arylation but also N‐arylation, as shown in Table (entry 1) . Interestingly, C3, C2, and N1 arylation products were obtained in a 2.5:1:1.5 ratio.…”

“…In the latter case, the selectivity reached a value of 15:1 for C3 arylated product 3 a . The highly electrophilic Pd center bearing electron‐withdrawing L5 and two additional tfa ligands is assumed to generate a σ(η 1 ‐C3) bonded indolinium–palladium complex of increasing stability in the first step of the electrophilic palladation process, which likely contributes to the observed selectivity; this is in good agreement with recent reports …”

“…In af irst trial, the reactionb etween indole and iodobenzene by using ligand L1 and Pd(OAc) 2 as the catalytic system under conventional conditions (Cs 2 CO 3 ,t oluene/H 2 O = 8:2) led to encouraging results, whichd emonstrated that pma-based catalysts weren ot able only to promote C-arylation but also N-arylation,a ss hown in Ta ble 1( entry 1). [12] Interestingly,C 3, C2, and N1 arylation products wereo btainedi na2.5:1:1.5 ratio. SelectiveC -arylation could be obtained by switching either the solventt o1 ,1,1,3,3,3-hexafluoropropan-2-ol (HFIP)/H 2 O (Table 1, entry 2) or the Pd source to Pd(tfa) 2 (tfa = trifluoroace- In stark contrast, the presence of an EWG had ab eneficial effect on the selectivity.I ndeed, the best resultsw ere obtained by using ligands L4 and L5 bearing sulfonyl groups.I nt he latter case, the selectivity reached av alue of 15:1 for C3 arylated product 3a.T he highly electrophilic Pd center bearing electron-withdrawing L5 andt wo additional tfa ligands is assumed to generate a s(h 1 -C3) bondedi ndolinium-palladium complex of increasing stability in the first step of the electrophilic palladation process, which likely contributest ot he observeds electivity;this is in good agreementw ith recent reports.…”