Mechanistic investigation of platinum and palladium catalysed cross- coupling reactions. Implications for the Stille and Heck reactions

University dissertation from Department of Chemistry

Abstract: The present thesis deals with various types of palladium(II) and platinum(II) complexes and their use in homogeneous catalysis. The focus has been devoted to mechanistic investigations concerning the Stille and Heck reactions and studies under both catalytic and stoichiometric conditions were conducted. As a stoichiometric probe for studying the transmetallation step in the Stille reaction the platinum(II) complexes trans-[PtPhCl(PMe2Ph)2] (8), trans-[PtPhCl(PPh3)2] (9), trans-[PtPh(OTf)(PMe2Ph)2] (10), trans-[PtPh(OTf)(PPh3)2] (11), trans-[PtPhF(PMe2Ph)2], (12) and trans-[PtPhF(PPh3)2] (13) were treated with the tetraorganotin compound Me3SnPh. The reactivity of the Pt?F complexes is very high especially in comparison to the corresponding Pt-Cl complexes, where no reactivity is observed under the reaction conditions used. The overall reactivity decreases in the order 13 > 12 > 11 > 10 >> 8, 9 and can be rationalised in terms of the lability of the leaving group and the steric demand of the ancillary phosphorus ligands. The product distribution is strongly dependent on the reaction conditions. Thus, the sterically more demanding PPh3 ligand directs the transmetallation to occur over the Sn?Me bond instead of over the Sn?Ph bond, which is normally the case. The mechanism for the transmetallation is proposed to involve parallel equilibria to both cis and trans products with associative activation in all steps. In this mechanism the initial attack always takes place trans to the phenyl group giving trans products via an open transition state. In a possible subsequent step another molecule of stannane can give cis products via a cyclic transition state. The formation of a unreactive tris(phosphine) complex [PtPh(PMe2Ph)3]+ (15), could explain the rate retardation found in Stille reactions when free ligand is added. Also, the reversibility of the transmetallation reactions implicates that all organic product could be funnelled out via a cis complex although there is substantial amounts of trans complex initially formed. In conclusion this means that the organotin reagent catalyses the cis to trans isomerisation. For the investigations under catalytic conditions two types of PCP palladium(II) complexes have been used, {2,6-bis[(diphenylphosphino)methyl]-benzene}palladium(II)trifluoroacetate (23) and {cis-1,3-bis[(di-tertbutyl-phosphino)methyl]cyclohexane}palladium(II)iodide (24). The complexes serve as Pd(II) pre-catalysts forming small amounts of catalytically active colloidal Pd species. The nature of the catalyst was confirmed by the inhibition seen on addition of elementary mercury to the reaction solutions and clearly speaks in favour of a heterogeneous form of the active catalysts. In the presence of 23 as pre-catalyst for the Stille reaction between 4-bromoanisole and Me3SnPh the mechanism is suggested to consist of a reversible oxidative addition of the aryl halide followed by rate determining irreversible transmetallation. For the Heck reaction between styrene and 4-iodoanisole, with 24 as the pre-catalyst, the kinetics display saturation behaviour with respect to the olefin concentration. This is compatible with a mechanism consisting of a pre-equilibrium step involving association of the olefin followed by a rate determining oxidative addition step of the aryl halide.

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