DOC

Coupling Reactions - LSU chemistry

By Marjorie Rice,2014-02-22 22:50
46 views 0
Coupling Reactions - LSU chemistry

    Prof. Lionel Delaude, University of Liege in Belgium Cross-Coupling 1 Coupling Reactions

    Organometallic chemistry has provided important new methods to carry out

    carbon-carbon or carbon-heteroatom bond formation. Such processes, termed

    coupling reactions, now have a central place in organic synthesis. The most

    important ones are summarized below. They often bear the name of their

    discoverer (organic tradition). Stille coupling:

    organotinPd(0)

    R-R' + XSnRR-X + R'-SnR33

    aryl or

    vinyl halide Negishi coupling:

    organozincPd(0)

    R-R' + XZnRR-X + R'-ZnR

    aryl or

    vinyl halide Suzuki coupling:

    organoboronPd(0)

    R-R' + XB(OH)R-X + R'-B(OH)22

    aryl or

    vinyl halide Heck reaction:

    alkeneRPd(0)

    R-X ++ HX

    R'aryl orR'

    vinyl halide Sonogashira coupling:

    alkynePd(0)

    + HXRR'R-X +R'

    aryl or

    vinyl halide

Prof. Lionel Delaude, University of Liege in Belgium Cross-Coupling 2

    Cross-Coupling of Organometallics and Halides

    Pd(0)

    R-R' + MXR-X + R'-M

    M = MgX, ZrCpCl, ZnX, SnR, B(OR), AlMe, SiR, Cu,… 23223

    The mechanism involves oxidative addition of the halide or triflate to the initial Pd(0) phosphine complex to form a Pd(II) species. The key slow step is a transmetallation, so called because the nucleophile (R') is transferred

    from the metal in the organometallic reagent to the palladium and the counterion (X = halide or triflate) moves in the opposite direction. The new Pd(II) complex with two organic ligands undergoes reductive elimination to give the coupled product and the Pd(0) catalyst ready for another cycle.

    LPdL

    R'-R

    + R-X

    LLRR

    PdPd

    LR'LX

    R'-MM-X

    transmetallation

    The halide partner (RX) must be chosen with care, as -hydride elimination

    would decompose the first intermediate during the slow transmetallation step. The choice for R is restricted to substituents without -hydrogen atoms: vinyl,

    allyl, benzyl, and polyfluoroalkyl halides, triflates, and phosphates have all been coupled successfully.

Prof. Lionel Delaude, University of Liege in Belgium Cross-Coupling 3

    The organometallic reagent (R'M) can be based on Mg, Zn, Cu, Sn, Si, Zr, Al, or B and the organic fragment can have a wide variety of structures as

    coupling is faster than -hydride elimination.

Formation of the active species may conveniently be carried out in situ by

    reduction of a Pd(II) precursor, for example, PdCl, Pd(OAc), or NaPdCl. 2224

    The reduction of Pd(II) to Pd(0) can be achieved with amines, phosphines,

    alkenes, and organometallics such as DIBAL-H, butyl lithium, or

    trialkylaluminium.

Reduction with EtN: 3

    reductive-hydrideligandH

    eliminationeliminationexchangePdLXX2+ HXPdLXXPdLHPdL2222NEt2

    -XNEt2

    Reduction with PPh: 3

    ligandreductivePhPPPh33exchangeeliminationPdOAcOPdPd(OAc)Pd-PPh23PPhPOAc3AcOPh3+ AcO2OAcOPO+ Ph3

    Reduction with ethylene:

    -hydridereductivemigratoryHeliminationeliminationinsertion+ HXPdLXXXPdLHPdLXPdL22222

    -X

    Reduction with an organometallic:

    reductive

    Relimination2 R-M

    PdLX+ R-RPdL+ 2 MXPdL2222R

    Usually, a stoichiometric excess of an amine such as EtN is part of the 3

    reaction mixture, serving both as a base to trap the HX formed and as a reducing agent for Pd.

Prof. Lionel Delaude, University of Liege in Belgium Cross-Coupling 4

    Stille Coupling

    The Stille coupling uses organotin compounds (called stannanes) as organometallic components. Since its first reported use in the late 1970's, the reaction has been widely used for the coupling of both aromatic and vinylic systems. The Stille coupling represents over half of all current cross-coupling reactions, however, due to their high toxicity, stannanes tend to be replaced more and more with organozinc and organoboron compounds.

    The reaction may be carried out intramolecularly and with alkynyl stannanes instead of the more usual aryl or vinyl stannanes to form medium-sized rings. For example, the reaction below forms a 10-membered ring containing two alkynes.