Carbon-heteroatom bonds formation

Transition-Metal-Based Carbon-Carbon and Carbon-Heteroatom Bond Formation for the Synthesis and Decoration of Heterocycles... 

Maes BUW (2006) Transition-Metal-Based Carbon-Carbon and Carbon-Heteroatom Bond Formation for the Synthesis and Decoration of Heterocycles. 1 155-211 Maiti M, Kumar GS (2007) Protoberberine Alkaloids Physicochemical and Nucleic Acid Binding Properties. lO. 155-210... 

Transition Metal-Catalyzed Carbon-Heteroatom Bond Formations... 

Relatively soon after the discovery that aqueous solutions containing PtCl - and PtClg- can functionalize methane to form chloromethane and methanol, a mechanistic scheme for this conversion was proposed (16,17). As shown in Scheme 4, a methylplatinum(II) intermediate is formed (step I), and this intermediate is oxidized to a methylplatinum(IV) complex (step II). Either reductive elimination involving the Pt(IV) methyl group and coordinated water or chloride or, alternatively, nucleophilic attack at the carbon by an external nucleophile (H20 or Cl-) was proposed to generate the functionalized product and reduce the Pt center back to Pt(II) (step III) (17). This general mechanism has received convincing support over the last two decades (comprehensive reviews can be found in Refs. (2,14,15)). Carbon-heteroatom bond formation from Pt(IV) (step III) has been shown to occur via nucleophilic attack at a Pt-bonded methyl, as discussed in detail below (Section V. A). 

Very few transition-metal catalyzed electroreductive carbon-heteroatom bond formations have been described. The electrochemical silylation of allylic acetates was carried out in the presence of Pd-PPha [131]. The electrosynthesis of arylthioethers from thiophenol and aryl halides [132] and the coupling of bromobenzene with dichlorophenylphosphine [133] were performed with Ni-bpy as catalyst. 

An example of C—Si bond formation concludes this overview of carbon heteroatom bond formation. Reflux of bromide 62 in benzene and in the presence of small amounts of (TMS)3SiH and AIBN afforded the silabicycle 63 in 88 % yield (Reaction 7.64) [76]. The key step for this transformation is the intramolecular homolytic substitution at the central silicon atom, which occurred with a rate constant of 2.4 x 10 s at 80 °C (see also Section 6.4). The reaction has also been extended to the analogous vinyl bromide (Reaction 7.65) [49]. 

Carbon-heteroatom bond formation by free-radical chain additions to... 

Besides the successful development of asymmetric syntheses with carbon-carbon bond formation, methods for carbon-heteroatom bond formation were also investigated intensively. In this context we developed several practical protocols for carbon-nitrogen, carbon-oxygen, and carbon-phosphorus bond formation. 

The opening step of the Buchwald-Hartwig reaction, similarly to the previous cases, is the oxidative addition of an aryl halide or sulfonate onto a low oxidation state metal. Although the term Buchwald-Hartwig reaction is usually reserved for palladium catalyzed processes, carbon-heteroatom bond formation also proceeds readily with nickel and copper. The nickel catalyzed processes follow a similar mechanism, while the distinctly different copper catalyzed reactions will be discussed in Chapter 2.5. 

The transition metal catalyzed synthesis of seven membered and larger heterocycles attracted considerably less attention than the preparation of their five and six membered analogues. Typical examples in this chapter include the formation of heterocycles in insertion reactions, or through carbon-heteroatom bond formation. Although the formation of some macrocyclic natural products was also achieved in cross-coupling reactions they will not be discussed in detail. 

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