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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- 作者：Fré ; dé ; ric Lemaî ; tre ; Dominique Lucas ; Katherine Groison ; Philippe Richard ; Yves Mugnier ; and Pierre D. Harvey
- 刊名：Journal of the American Chemical Society
- 出版年：2003
- 出版时间：May 7, 2003
- 年：2003
- 卷：125
- 期：18
- 页码：5511 - 5522
- 全文大小：229K
- 年卷期：v.125,no.18(May 7, 2003)
- ISSN：1520-5126
文摘
The stoichiometric and catalytic activations of alkyl halides and acid chlorides by the unsaturedPd3(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+". Thisactivation can proceed much faster if it is electrochemically induced via the formation of the paramagneticspecies Pd3+. The latter is the first confidently identified paramagnetic Pd cluster. The kinetic constantsextracted from the evolution of the UV-vis spectra for the thermal activation, as well as the amount ofelectricity to bring the activation to completion for the electrochemically induced reactions, correlate therelative C-X bond strength and the steric factors. The highly reactive "R+" species has been trapped usingphenol 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 adissociative mechanism operates (RCOCl 
RCO+ + Cl-; R = t-Bu, Ph) prior to Cl- scavenging by thePd32+ 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 ofCl- from Pd3(Cl)+ to form Pd32+ and the insoluble MCl materials (M = Cu, Ag) allowing to regenerate thestarting 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. Thetrapping 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 keyintermediates "Pd3(Cl)+...M+" (M+ = Cu+, Ag+), which was successfully performed for Pd3(ClAg)2+, ascharacterized by 31P NMR, IR, and FAB mass spectrometry. During the course of this investigation, therare case of PF6- hydrolysis has been observed, where the product PF2O2- anion is observed in the complexPd3(PF2O2)+, where the substrate is well-located inside the cavity formed by the dppm-Ph groups abovethe unsatured face of the Pd32+ center. This work shows that Pd32+ is a stronger Lewis acid in CH2Cl2 andTHF than AlCl3, Ag+, Cu+, and Tl+.
RCO+ + Cl-; R = t-Bu, Ph) prior to Cl- scavenging by thePd32+ 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 ofCl- from Pd3(Cl)+ to form Pd32+ and the insoluble MCl materials (M = Cu, Ag) allowing to regenerate thestarting 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. Thetrapping 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 keyintermediates "Pd3(Cl)+...M+" (M+ = Cu+, Ag+), which was successfully performed for Pd3(ClAg)2+, ascharacterized by 31P NMR, IR, and FAB mass spectrometry. During the course of this investigation, therare case of PF6- hydrolysis has been observed, where the product PF2O2- anion is observed in the complexPd3(PF2O2)+, where the substrate is well-located inside the cavity formed by the dppm-Ph groups abovethe unsatured face of the Pd32+ center. This work shows that Pd32+ is a stronger Lewis acid in CH2Cl2 andTHF than AlCl3, Ag+, Cu+, and Tl+.