The Ionic Palladium Porphyrin as a Highly Ef?cient
and Recyclable Catalyst for Heck Reaction in Ionic Liquid Solution Under Aerobic Conditions
Qing-Xia Wan ?Ye Liu
Received:16September 2008/Accepted:11November 2008/Published online:26November 2008óSpringer Science+Business Media,LLC 2008
Abstract The ionic palladium porphyrin featured with pyridinium tags at four meso -positions,palladium tetrakis-(N -methyl-4-pyridinium)-porphyrin iodide ([Pd(II)TM-Py)P][I]4(3),embedded in the similar structured ionic liquid of N -butyl pyridinium tetra?uoroborate ([Bpy]BF 4)was found to be a highly ef?cient and recyclable catalytic system for the Heck cross-coupling of iodobenzene (derivatives)and ethyl acylate,with high turnover fre-quency of 56,000h -1under aerobic conditions.The active charge transfer transition occurred in 3-[Bpy]BF 4system during the Heck reaction could account for the ef?ciency in the catalytic coupling,which was observed in the UV–visible spectra.
Keywords Palladium porphyrin áHeck reactions áIonic liquids áUV–visible spectrophotometer
The Heck reaction,a palladium-catalyzed carbon–carbon bond formation between aryl halides and ole?ns,is one of the most important tools for the synthesis of elaborated styrene derivatives due to its wide variety of functional groups on both reactants [1,2].In the past decades,the considerable progresses have been made in palladium-catalyzed Heck reactions.[3–5]The development of homogeneous/heterogeneous catalysts led to a variety of different ligand systems,among which phosphine-
containing palladacycles [6–9],bulky and electron-rich monodentate phosphine ,and carbene donors like N -heterocyclic carbenes (NHCs)which are strong r -donors with negligible p -accepting ability [11–15],are exempl-arily attractive owing to the high activities of the derived Pd catalysts.The N-containing ligands recently became preferable in considerations of non-toxicity and insensi-tivity to oxygen/moisture,having the advantage of performing the reactions under aerobic conditions;how-ever the rapid deactivation of the derived palladium catalysts in the recycling use was still a frustrated problem [16–19].Recently,the phosphine-free systems of the CNC-pincer palladium complexes have been found to be the highly ef?cient and recyclable catalysts for the Suzuki reaction .
Porphyrin free bases are important N -pincer ligands to which most of the metal elements in the periodic table can coordinate.Palladium porphyrins have found applications as luminescent markers ,oxygen sensor ,sensi-tizers for singlet oxygen formation ,and photo-induced protein cross-linking agents ,due to their high yield of intersystem crossing and long lifetime of the resulting triplet state in diverse media [25,26].Anyway,the examples of palladium porphyrins as phosphine-free cata-lysts in C–C coupling reactions were few,except for the ?rst case in the Suzuki reaction reported very recently .In contrast to the common N-ligand,the porphyrin base as a tetradentate pincer ligand can provide more stable Pd complexes suitable for Heck reactions and thereby avoid the use of exogenous ligands and high loadings of palla-dium.However,the conventional palladium porphyrins give poor solubility even in polar organic solvents (DMF,DMSO)limited their use in homogeneous catalysis,which is a highly ef?cient process but with disadvantage of dif-?cult separation from the catalyst with the reaction mixture
Q.-X.Wan áY.Liu (&)
Shanghai Key Laboratory of Green Chemistry and Chemical Processes,Chemistry Department,East China Normal University,200062Shanghai,China e-mail:yliu@http://m.wendangku.net/doc/333375d7240c844769eaee5e.html
Catal Lett (2009)128:487–492DOI 10.1007/s10562-008-9780-2