| Autor: |
Platt, George M.H., Aguiar, Pedro M., Athavan, Gayathri, Bray, Joshua T.W., Scott, Neil W.J., Fairlamb, Ian J.S., Perutz, Robin N. |
| Zdroj: |
Organometallics; September 2023, Vol. 42 Issue: 17 p2378-2394, 17p |
| Abstrakt: |
Direct C–H functionalization reactions have opened new avenues in catalysis, removing the need for prefunctionalization of at least one of the substrates. Although C–H functionalization catalyzed by palladium complexes in the presence of a base is generally considered to proceed by the CMD/AMLA-6 mechanism, recent research has shown that silver(I) salts, frequently used as bases, can function as C–H bond activators instead of (or in addition to) palladium(II). In this study, we examine the coupling of pentafluorobenzene 1to 4-iodotoluene 2a(and its analogues) to form 4-(pentafluorophenyl)toluene 3acatalyzed by palladium(II) acetate with the commonplace PPh3ligand, silver carbonate as base, and DMF as solvent. By studying the reaction of 1with Ag2CO3/PPh3and with isolated silver (triphenylphosphine) carbonate complexes, we show the formation of C–H activation products containing the Ag(C6F5)(PPh3)nunit. However, analysis is complicated by the lability of the Ag–PPh3bond and the presence of multiple species in the solution. The speciation of palladium(II) is investigated by high-resolution-MAS NMR (chosen for its suitability for suspensions) with a substoichiometric catalyst, demonstrating the formation of an equilibrium mixture of Pd(Ar)(κ1-OAc)(PPh3)2and [Pd(Ar)(μ-OAc)(PPh3)]2as resting states (Ar = Ph, 4-tolyl). These two complexes react stoichiometrically with 1to form coupling products. The catalytic reaction kinetics is investigated by in situIR spectroscopy revealing a two-term rate law and dependence on [Pdtot/nPPh3]0.5consistent with the dissociation of an off-cycle palladium dimer. The first term is independent of [1], whereas the second term is first order in [1]. The observed rates are very similar with Pd(PPh3)4, Pd(Ph)(κ1-OAc)(PPh3)2, and [Pd(Ph)(μ-OAc)(PPh3)]2catalysts. The kinetic isotope effect varied significantly according to conditions. The multiple speciation of both AgIand PdIIacts as a warning against specifying the catalytic cycles in detail. Moreover, the rapid dynamic interconversion of AgIspecies creates a level of complexity that has not been appreciated previously. |
| Databáze: |
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