Arylation of Car bon-Car bon Double Bonds Catalyzed by Palladium Salts H e l h e Laurm, Jean-Maurice Mallet, Helene Mestdagh, and Guy Vllle' Agregation de Chimie, Ecole Normale Superieure, 24 Rue Lhomond, F-75005 Paris, France
A variety of organic reactions promoted by palladium compounds has been discovered in the last decades.28 Among them the Wacker process has found industrial applications. For the synthetic organic chemist, a reaction leading to the formation of carbon-carbon honds is a precious tool. Such a reaction is the palladium-catalyzed arylation of carbon-carbon double bonds, also known as Heck reaction, which is a good example of homogeneous organometallic catalysis. This reaction has been shown to proceed in a twostage process: cis addition of an organopalladium complex followed by a palladium hydride ci~-@-elimination.~~ The required catalyst is a palladium(0) complex that is formed in the reaction mixture by reduction of a readily available palladium(I1) salt such as palladium acetate. It has been suggested that the olefin itself serves as a reducing agentzb."
.
a vinyiic substitution reaction at the doublr bond of styrene snd an addition reaction to the double bond crf an ailgiic alroho.. Ieadmg to arylatrd ketones
Experimental Procedures Synthesis of trans-Stilbene PhI
+ Ph-CH=CH2 + nBusN
+
Ph
\
+
+ nBu3NH IdC="., /H
A mixture of palladium acetate (45 mg, 0.2 mmol), iodobenzene (4.08 g, 20 mmol), styrenes (2.9 mL, 25 mmol), and 4.15 mL (20 mmal) of tri-n-hutylamine,is stirred magnetically at 100 'C for 2 h in a 25mL round-hottomed flask fitted with a reflux can dense^.^ Thereaction mixture is cooled,and the solid mass is broken up with a spatula and triturated with 50 mL of water. The resulting solid is collected by filtration and recrystallized from boiling ethanol, with hot filtration to remove palladium metal. The yield is 60-70% of shining plates: mp 122-124 'C. Synthesis of Phenylbutanones
where L = two-electron-donating ligand such as EON or Ph3P. The formed palladium(0) complex then reads with the organic halide to form the active organopalladium halide intermediate. Cis addition to the olefin gives rise to a shortlived addition complex, which undergoes spontaneous cis-@ elimination under the reaction conditions used. The catalyst is regenerated by the base, which also serves as a ligand in the formation of the organopalladium halide complex.
1
catalmt
A,X
regeneration
\
PdLz + Base. H+X-
Ar
\
C=C
/
/ \
where X = Br or I. The reaction is generally regioselective, with the aryl group appearing predominantly a t the less substituted carbon atom.3 In the case of a terminal alkene as starting material, a predominantly trans relationship between the aryl group and the bulkier group of the alkene is observed. Both the regioselectivity and stereoselectivity are consistent with cis addition and elimination processes proceeding in one stepviacyclic transition states. Thus, i t is the stereoelectronic requirement of the transition state of palladium hydride cis elimination that favors a trans arrangement hetween groups having the largest steric requirements. We now report simple procedures that have been sucessfully carried out in our undergraduate students' laboratory in the recent past that illustrate:
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Journal of Chemical Education
+ PhI + Et3N +
CHBCHOH-CH=CH1
CH3-CO-CHrCHyPh 1 (90%)
+ CH3-CO-CHPh-CH3
+
Z (10%)
EtsNH IA magnetically stirred mixture of hut-3-en-2-01(1.44 g, 20 mmol), 4.08 g (20 mmal) of iodobenzene, 45 mg of palladium acetate (0.2 mmol) and 2 e f20mmoll of triethvlamine is heatedat 90% in 5 mL dimethvlfor&ide for 1- h.Vhe geaction mixture is cooled. diluted with water ( 2 5 r n ~and ) extracted with ether (3 X 20 mL). After the ether phase is washed with water (3 X 10 mL), it is dried over anhydrous magnesium sulfate and filtered. The solvent is then evaporated in vacuo at rmm temperature and the resulting liquid residur is analyred hy (;LC (ID: 90 101. Thp 1.2 ratio can also he d~rrrminrdusinr -HNMR: comptrund I shows a multiplet at 2.65 ppm for H-3 and^-4 while compound 2 shows a quadriplet signal Dom at 3.55 DDm for H-3 and a doublet at 1.25 . . for H-4. The infrared spectrum shuns the presence of a carhmyl group luc-o = 1721) cm ' J borne our by a pusitive test with 2,4-dinitruphenylhydrnzine. A mrchaniam for the formation of L and 2 rhuuld be proposed by the student ~~~
~~
~
We are grateful to M. E. Munk (Arizona State University) for helpful discussions.
' Author to whom correspondence should be addressed.
(a) For a review. see Hegedus. L. S. Tetrahedron 1984, 4413). 2415. (b) Heck, R. F. Organic Reactions 1982, 27,345. (a)PurceII. K. F.; Kotz, J. C. An Introduction to Inorganic Chemis try: Saunders: Philadelphia. 1980; pp 499-500. 552-553, and 560562. (b) Pearson. A. J. Metallo-Organic Chemistry: Wiley. 1985; pp 128-130. (c) Since the vinyl acetate is produced in the catalyst formation reaction, and not in the catalyst dependent reaction, it is obtained in inconsequential yield (
