[23586-53-0] · C6F9O6Tl · Thallium(III) Trifluoroacetate--Palladium(II) Acetate · (MW 543.44)
[3375-31-3] · C4H6O4Pd · Thallium(III) Trifluoroacetate--Palladium(II) Acetate · (MW 224.52)
(catalyst for aryl couping, carbonylation, and alkenation)
Physical Data: see entries Thallium(III) Trifluoroacetate and Palladium(II) Acetate.
Solubility: sol common organic solvents; limited sol hexane.
Form Supplied in: white solid (Tl(OCOCF3)3); orange-brown solid (Pd(OAc)2); commercially available.
Handling, Storage, and Precautions: all thallium compounds are extremely toxic to inhalation, skin contact, and ingestion. Toxicity is cumulative. Extreme caution should be used when handling these materials. Use in a fume hood.
Thallium(III) salts are efficient reagents for the conversion of aryls to biaryls under mild conditions.2 The major drawback of these reactions is the requirement of at least one electron-donating substituent on the aromatic ring. A practical alternative for the preparation of biaryls containing both electron-donating and mild electron-withdrawing groups has been developed.3 The reaction involves the coupling of arylthallium bis(trifluoroacetates) in the presence of catalytic palladium(II) acetate (eq 1).
The reaction proceeds in good to high yields, with thallium(III) functioning as a reoxidant for palladium. These reactions are homogeneous and product isolation is simple. Lithium tetrachloropalladate has been used as the catalyst in similar coupling reactions to produce good yields of biaryls (eq 2).4 It is interesting to note that the palladium(II) acetate-catalyzed reaction with chlorobenzene gives a higher yield of the biphenyl as compared to the reaction with lithium tetrachloropalladate (66% vs. 20%).
A tandem thallation-carbonylation process has been developed for arenes en route to aromatic esters, phthalides, anhydrides, imides, and 3,4-dihydroisocoumarins.5 The reaction involves the carbonylation of arylthallium bis(trifluoroacetates) in the presence of catalytic Palladium(II) Chloride, Carbon Monoxide, and Lithium Chloride at room temperature and atmospheric pressure. The preparation of a variety of esters and phthalides are shown in eqs 3 and 4, respectively.
Palladium(II)-catalyzed alkenation of arylthallium bis(trifluoroacetates) has been explored by several groups.6 An interesting Heck-type alkenation of arylthallium bis(trifluoroacetate) with Methyl Vinyl Ketone has been reported.7 This reaction uses lithium tetrachloropalladate as the catalyst and proceeds under mild reaction conditions (eq 5).
Mukund P. Sibi
North Dakota State University, Fargo, ND, USA
Nancy E. Carpenter
University of Minnesota, Morris, MN, USA