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Thermal and Electrochemical C−X Activation (X = Cl, Br, I) by the Strong Lewis Acid Pd3(dppm)3(CO)2+ Cluster and Its Catalytic Applications

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journal contribution
posted on 10.04.2003 by Frédéric Lemaître, Dominique Lucas, Katherine Groison, Philippe Richard, Yves Mugnier, Pierre D. Harvey
The stoichiometric and catalytic activations of alkyl halides and acid chlorides by the unsatured Pd3(dppm)3(CO)2+ cluster (Pd32+) are investigated in detail. A series of alkyl halides (R−X; R = t-Bu, Et, Pr, Bu, allyl; X = Cl, Br, I) react slowly with Pd32+ to form the corresponding Pd3(X)+ adduct and “R+”. This activation can proceed much faster if it is electrochemically induced via the formation of the paramagnetic species Pd3+. The latter is the first confidently identified paramagnetic Pd cluster. The kinetic constants extracted from the evolution of the UV−vis spectra for the thermal activation, as well as the amount of electricity to bring the activation to completion for the electrochemically induced reactions, correlate the relative C−X bond strength and the steric factors. The highly reactive “R+” species has been trapped using phenol to afford the corresponding ether. On the other hand, the acid chlorides react rapidly with Pd32+ where no induction is necessary. The analysis of the cyclic voltammograms (CV) establishes that a dissociative mechanism operates (RCOCl → RCO+ + Cl-; R = t-Bu, Ph) prior to Cl- scavenging by the Pd32+ species. For the other acid chlorides (R = n-C6H13, Me2CH, Et, Me, Pr), a second associative process (Pd32+ + RCOCl → Pd32+.....Cl(CO)(R)) is seen. Addition of Cu(NCMe)4+ or Ag+ leads to the abstraction of Cl- from Pd3(Cl)+ to form Pd32+ and the insoluble MCl materials (M = Cu, Ag) allowing to regenerate the starting unsaturated cluster, where the precipitation of MX drives the reaction. By using a copper anode, the quasi-quantitative catalytic generation of the acylium ion (“RCO+”) operates cleanly and rapidly. The trapping of “RCO+” with PF6- or BF4- leads to the corresponding acid fluorides and, with an alcohol (R‘OH), to the corresponding ester catalytically, under mild conditions. Attempts were made to trap the key intermediates "Pd3(Cl)+...M+" (M+ = Cu+, Ag+), which was successfully performed for Pd3(ClAg)2+, as characterized by 31P NMR, IR, and FAB mass spectrometry. During the course of this investigation, the rare case of PF6- hydrolysis has been observed, where the product PF2O2- anion is observed in the complex Pd3(PF2O2)+, where the substrate is well-located inside the cavity formed by the dppm-Ph groups above the unsatured face of the Pd32+ center. This work shows that Pd32+ is a stronger Lewis acid in CH2Cl2 and THF than AlCl3, Ag+, Cu+, and Tl+.