Electronic and Steric Effects on the Reductive Elimination of Diaryl Ethers from Palladium(II)

Abstract

Arylpalladium aryloxide complexes containing sterically and electronically varied phosphine ligands were prepared, and the rates for reductive elimination of diaryl ethers from these complexes were studied to determine the ligand properties that most strongly accelerate this unusual reaction. Electronic and steric effects were probed by preparing monomeric palladium complexes of the type (L)Pd(Ar)(OAr‘), in which L = DPPF (1,1‘-bis-diphenylphosphinoferrocene), CF3-DPPF (1,1‘-bis[di(4-(trifluoromethyl)phenyl)phosphino]ferrocene), and D-t-BPF (1,1‘-bis(di-tert-butylphosphino)ferrocene) and Ar = electron-deficient and electron-neutral aryl groups. Direct C−O bond-forming reductive elimination to form diaryl ethers in high yield was observed after warming the complexes that contained an electron-deficient aryl group bound to palladium. The rate constant for C−O bond-forming reductive elimination from the CF3-DPPF-ligated palladium complex was twice that obtained for the analogous DPPF-ligated complex. Reductive elimination of diaryl ether from the more bulky D-t-BPF complex occurred roughly 100 times faster than from the DPPF complex. Thermolysis of DPPF and CF3-DPPF complexes containing an electron-neutral aryl group did not form diaryl ether. Thermolysis of (D-t-BPF)Pd(Ph)(OC6H4-4-OMe) also did not form diaryl ether and generated the two monophosphines PhP(t-Bu)2 and FcP(t-Bu)2 (di-tert-butylphosphinoferrocene). However, heating of a FcP(t-Bu)2-ligated aryloxide complex containing an electron-neutral, palladium-bound aryl group generated diaryl ether in 10−25% yield. Moreover, heating of this complex in the presence of an excess of P(t-Bu)3 or Ph5FcP(t-Bu)2 or 1 equiv of 2-di-tert-butylphosphino-1,1‘-binaphthyl generated diaryl ether in higher, 58−95%, yields. The effect of ligand concentrations on reaction yields implied that exchange of the bulkier ligands with FcP(t-Bu)2 induced the reductive elimination of diaryl ether.

Palladium Complexes of N‐Heterocyclic Carbenes as Catalysts for Cross‐Coupling Reactions—A Synthetic Chemist's Perspective

“…Even though a number of metals have been used to mediate this process, the versatility of palladium compounds has remained unsurpassed. [7] The advantages of using NHCs as ligands in Pd mediated reactions are: 1) the strong s-donating ability of NHCs results in a Pd center capable of oxidative addition into bonds traditionally considered resistant, for example, in chloroarenes [132] or alkyl halides; [133,134] 2) the steric bulk of NHCs facilitates reductive elimination in a manner analogous to bulky phosphanes; [135,136] and 3) the strong Pd-NHC bond and limited decomposition pathways available ensure that the metal is kept in a soluble, catalytically active state even when only a single NHC is attached. As a consequence of the special complexation properties of NHCs to Pd, well-defined complexes that are stable, easy to synthesize, yet readily activated under the reaction conditions offer definite advantages over catalysts formed in situ.…”

“…As the oxidative addition intermediate is some 25 ) than the oxidative addition (29 kJ mol À1 ). [136] authors concluded that chloromethane would not undergo oxidative addition in the gas phase. However, cross-coupling reactions of alkyl halides, including chlorides, [147] are facile with Pd-NHC catalysts in polar solvents (Scheme 54).…”

Chemoselectivity and the Curious Reactivity Preferences of Functional Groups

“…The rate of reductive elimination is relatively fast with the sterically encumbered ligand L3, thus precluding efficient conversion of 64 into 67 and leading to the reaction following path a. The rate of reductive elimination with L1 is slower, [44] thereby allowing enough time for conversion into the more stable species 67 and ultimately to the reaction following path b. Thus, the reason for the difference in the findings made by the research groups of Beletskaya and Rouden may lie in the ability of rigid cyclic diamines to efficiently interconvert between 64 and 67.…”

Aus der Sicht des Synthetikers: Palladiumkomplexe N‐heterocyclischer Carbene als Katalysatoren für Kreuzkupplungen

“…Methode A: 1 oder Chloralkanen; [133,134] 2) wegen des Platzbedarfs der NHCs ist ebenso wie bei sperrigen Phosphanen die reduktive Eliminierung erleichtert; [135,136] 3) wegen der starken Pd-NHC-Bindung und den wenigen möglichen Zerfallswegen bleibt das Metall, selbst wenn nur ein einziger NHC-Ligand koordiniert ist, sicher in einem löslichen, katalytisch aktiven Zustand. Durch die besonderen Komplexierungseigenschaften von NHCs an Pd sind also definierte Komplexe synthetisierbar, die leicht aktiviert werden können und gegenüber in situ hergestellten Komplexen viele Vorteile bieten.…”