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dejoarder-suzuki - A REVIEW PAPER ON SUZUKI COUPLING...

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A REVIEW PAPER ON SUZUKI COUPLING REACTION DRIPTA DE JOARDER 3/12/2008
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2 Introduction: The Suzuki cross coupling reaction is an extremely versatile methodology for generation of carbon carbon bonds. This is a reaction of an aryl- or vinyl-boronic acid with an aryl- , vinyl- or an alkyl-halide catalyzed by palladium. It is widely used to synthesize poly-olefins, styrenes and substituted biphenyls. The reaction was first reported by Akira Suzuki and his group in 1979. They reported that in presence of base such as sodium methoxide, ethoxide, acetate and hydroxide reaction of an equimolar amount of (E)- 1-hexenyldisiamyl borane in THF with (E)-1-bromo-2-phenylethene with 1 mol% of tetrakis (triphenylphosphine) palladium gave (E,E)-1-phenyl-1,3-octadiene in decent yields. 1 (Scheme 1.). They also applied the same reaction for the synthesis of (E, Z)-dienes. Although the reactions proceeded smoothly but the results were unsatisfactory because the initially formed (E, Z) isomer was isomerized to the more stable (E, E) isomer (Scheme 2.). A novel route for the synthesis of (E)-enynes was also reported in the same paper (Scheme 3.).
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3 Reaction mechanism: By analogy to the other cross coupling reactions, the catalytic cycle of Suzuki coupling reaction involves three basic steps: 1) Oxidative addition, 2) Transmetallation and 3) Reductive elimination. 2 A general catalytic cycle for the Suzuki coupling reaction is given in scheme 4. 3 The efficiency of palladium originates from its ability, when it is zerovalent, to activate C-X bonds (X=I, Cl, Br, O) by an oxidative addition which provides an organopalladium (II) complex prone to react with nucleophiles. 4,5 A large variety of palladium(0) catalysts or precursors can be used for this reaction. Pd (0) L 4 where L=phosphine is most commonly used since they are air stable. Palladium(II) complexes along with a reducer are also used. 6 Oxidative addition of 1-alkenyl,1-alkynyl, allyl, benzyl and aryl halides to a Pd(0) complex gives a stable trans- б -palladium (II) complex. 7 The reaction proceeds with complete retention of stereochemistry for alkenyl halide and with inversion for allylic and benzylic halides. Oxidative addition is often the rate limiting step in the catalytic cycle. 3 The mechanism of the oxidative addition step is characterized by means of electrochemical techniques(as the metal is oxidized) such as steady state voltametry, transient voltametry, cyclic voltametry and reaction kinetics. 6 The transmetallation step between organopalladium(II) complex and organoboron compound does not usually proceed in absence of base due to low nucleophilicity of organic group on boron atom. 3 However the nucleophilicity can be enhanced by quarternization of the boron with negatively charged bases giving the corresponding “ate” complex.
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