Martin-Luther-Universität Halle-Wittenberg


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3. Homogeneously Catalyzed C-C and C-Y Bond Formation Reactions

C-C and C-Y bond formation reactions (Y = heteroatom), catalyzed by metal complexes are of fundamental interest not only from the point of view of reaction theory, but also for synthetic organic chemistry and chemical industry. The palladium-catalyzed vinylation of organic halides RX (Heck reaction) both with conventional catalyst systems and under hydro-
organylation (reductive) conditions with Larock catalyst systems is one of the most important C-C bond formation reactions in synthetic organic chemistry. Nevertheless, there are some open questions in the mechanism of Heck reactions.

  • We are interested in the catalytic addition of nucleophiles to alkynes. Our investigations of the Heck reaction are directed to contribute to a deeper understanding of the mechanism and to the reactivity of 1-haloalk-1-ynes scarcely used in the Heck reaction so far. These reactions open up the possibility to synthesize enynes (C(sp2)-C(sp) bond  formation).
  • Establishing a phase-transfer catalytic variant of the "classical" Reppe synthesis of vinyl ethers and vinyl thioethers (RYH + HCCH ----> H2C=CHYR; Y = O, S) by using KOR (KSR) as catalyst in less polar aprotic solvents (like xylene) with 18-crown-6 as phase-transfer catalyst. These catalytic systems are much more active than the classical Reppe catalysts. For platinum-catalyzed acetal synthesis, see previous point.
  • The Heck reaction between 1-chloro-2-phenylacetylene and cycloalkenes or cycloalkadienes affords phenylethynyl-substituted cycloalkenes as "normal" Heck  products as well as 1,3-diphenylprop-2-ynylidene-substituted and (cycloalkenyl)phenylmethylidene-substituted bicyclic compounds as tandem products by reaction of ClCCPh and the cycloolefin in the ratio 1 : 2 and 2 : 1, respectively.
  • RCCI reacts with phosphine palladium(0) complexes to give in an oxidative addition reaction alkynyl palladium(II) complexes  [Pd(CCR)I(PR3)2], see the structure shown in Fig. 9 as an example. Furthermore, palladium complexes with β-hydrogen containing alkyl ligands are synthesized as model complexes for intermediates in the Heck reaction, see Fig. 10 as an example.
Fig. 09

Fig. 09

Figure 9: Structure of [Pd(CCPh)I(PPh3)2].

Fig. 10

Fig. 10

Figure 10: Structure of cis-[Pd(CH2CH2Ph)2(dppe)], a palladium complex with two 2-phenylethyl ligands having β-H atoms.

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