Ruthenium catalysis annulations

Related cyclocotrimerizations of two alkyne molecules with isocyanates have also been achieved using cobalt and nickel catalysts [9], With respect to intramolecular versions, two examples of the cobalt(I)-catalyzed cycloaddition of a,co-diynes with isocyanates have been reported to afford bicyclic pyridones only in low yields, although 2,3-dihydro-5(lH)-indolizinones were successfully obtained from isocyana-toalkynes and several silylalkynes with the same cobalt catalysis [50], On the other hand, the cycloaddition of 1,6-diynes 20 with 4 equiv. of isocyanates proceeded in refluxing DCE under the ruthenium catalysis to afford bicyclic pyridones 58 in 58-93% yield (Scheme 4.21) [51], Both aryl and aliphatic isocyanates can be widely employed in this pyridone annulation. 

In this chapter we outline advances in the ruthenium-mediated alkyne [2 + 2 + 2] cycloaddition reactions. These can be classified into two major categories in terms of the types of products (1) syntheses of benzene derivatives via alkyne [2 + 2 + 2] cycloadditions and (2) syntheses of heteroaromatics via [2 + 2 + 2] cocycloadditions of alkynes with nitriles or heterocumulenes. Benzene ring-forming reactions are essentially prototypes of the corresponding heteroaromatic annulations. Therefore, the first class of reactions is reviewed in the next section and followed by a discussion of the second class of reactions. The mechanistic aspects and synthetic applications of ruthenium-catalyzed [2 + 2 + 2] cycloadditions are also described to exemplify the scientific and practical significance of ruthenium catalysis. 

Su et al. reported the annulation of A -phenylmethacrylamide (98) with 2c using [Cp RhCl2]2 and Cu(OAc)2-H20 as catalyst and oxidant, respectively, to produce 2-pyridone derivative 99 (Scheme 25.48) [42], The same reaction can be conducted under ruthenium catalysis [43]. 

Coupling reactions of aromatic, heteroaromatic, and vinylic substrates with alkynes via sp C—H bond cleavage under rhodium, iridium, ruthenium, palladium, or nickel catalysis to produce a wide range of cyclic compounds have been described in this chapter. These annulation reactions provide powerful methods for constructing a variety of -rr-conjugated molecules containing fused aromatic and heteroaromatic nuclei from simple, readily available substrates. Extensive efforts will continue to be made to extend the scope of starting materials for this catalysis. 

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