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
journal contributionposted on 10.04.2003 by Frédéric Lemaître, Dominique Lucas, Katherine Groison, Philippe Richard, Yves Mugnier, Pierre D. Harvey
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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+.