Phase‐Transfer Activation of Transition Metal Catalysts

Abstract

With metal-based catalysts, it is quite common that a ligand (L) must first dissociate from a catalyst precursor (L'n M-L) to activate the catalyst. The resulting coordinatively unsaturated active species (L'n M) can either back react with the ligand in a k-1 step, or combine with the substrate in a k2 step. When dissociation is not rate determining and k-1 [L] is greater than or comparable to k2 [substrate], this slows the rate of reaction. By introducing a phase label onto the ligand L and providing a suitable orthogonal liquid or solid phase, dramatic rate accelerations can be achieved. This phenomenon is termed "phase-transfer activation". In this Concept, some historical antecedents are reviewed, followed by successful applications involving fluorous/organic and aqueous/organic liquid/liquid biphasic catalysis, and liquid/solid biphasic catalysis. Variants that include a chemical trap for the phase-labeled ligands are also described.

Impact of the Long‐Range Electronic Effect of a Fluorous Ponytail on Metal Coordination during Solvent Extraction

“…Furthermore, the four-unit spacer did not lead to the same energy as with the Me-H14 compound. This is in agreement with the previously reported computational studies, [11,13] in which af our-unit spacer appeared to screen only half of the inductive effect of the F-chain. This energy differenceb etween Me-H4F8 and Me-H14 could also result from an on-negligible difference in solvent effect between the F-chain and the H-chain.…”

“…131 8C; 1 HNMR (400 MHz, MeOD): d = 3.49 (t, J = 7.1 Hz, 1H), 2.32 (m, 2H), 2.14 ppm (m, 2H); 13 CNMR (101 MHz, MeOD): d = 170.7 (CO), 120.9-108.3 (CF 2 ), 50.2 (CH), 28.1 (CH 2 ), 19.4 ppm (CH 2 ); 19 FNMR (376 MHz,MeOD): d = À82.37, À115.69, À122.71, À122.89, À123.72, À124.52, À127.26 ppm;I R( ATR): ñ = 2950 (broad), 1692, 1198, 1146, 653, 559, 511cm À1 ;H RMS-ES À : m/z calcd for C 13 H 6 F 17 O 4 [M] À :5 48.9995; found:549.0002. 4,5,5,6,6,7,7,8,8,9,9,10,10,11,11, ChemPhysChem 2017, 18,3583 -3594 www.chemphyschem.org 2- (5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12, Example Synthesis of SubstitutedM alonamides (7-12): F(CF 2 ) 8 (CH 2 ) 2 CH(CONMe 2 ) 2 (7) F(CF 2 ) 8 (CH 2 ) 2 CH(COOH) 2 (4;2g, 3.63 mmol, 1equiv.) was ground to af ine powder and then suspended in toluene (60 mL;[ 4] = 0.06 m) and DMF (40 mL, 0.55 mmol, 0.15 equiv.)…”

“…This property has been exploited, for example, for the easy separation of organic compounds, for the preparation of superhydrophobic surfaces, to increase solubility in supercritical CO 2 , in the preparation of original ionic liquids, or for original materials synthesis . The major application to date remains the stabilization of metal coordination complexes with perfluorinated ligands into a distinct fluorous phase, which enables straightforward catalyst recovery during homogeneous catalysis processes and new catalyst activation strategies . The need to attenuate the strong electron‐withdrawing character of the F‐ponytail to maintain the reactivity of the system was quickly observed, and the beneficial insulation effect of a spacer located between the reactive site and the F‐ponytail of the molecule was reported as early as 1996 .…”

“…[9] The major application to date remains the stabilization of metal coordination complexes with perfluorinated ligandsi nto ad istinct fluorous phase, which enables straightforward catalyst recovery during homogeneous catalysis processes [10] and new catalyst activation strategies. [11] The need to attenuate the strong electron-withdrawing character of the F-ponytail to maintain the reactivity of the system was quickly observed, and the beneficial insulation effect of a spacer located betweent he reactive site and the F-ponytail of the molecule was reporteda se arly as 1996. [12] This article reported that the presence of as pacero ft wo methylene units had as trong impact on the physicochemical properties of both the ligand and the resulting metal complex, and also on reactionm echanisms.…”

Latent curing systems stabilized by reaction equilibrium in homogeneous mixtures of benzoxazine and amine

“…The use of the latent reaction system, which is dormant in storage conditions (often at room temperature) but rapidly reacts in curing condition (commonly >100 °C), is one of the most powerful methods used to solve this dilemma in the polymer industries45678. Specifically, latent epoxide formulations have led to profound developments in the applications of epoxy resins relevant for adhesion, coating, and composites910.…”