Palladium-Catalyzed Carbon-Oxygen Bond Formation

Palladium-Catalyzed Carbon-Oxygen Bond Formation 

In an interesting extension. Oh has recently shown that palladium complexes can be used to catalyze the cascade cyclization of 23 to furan derivatives [56]. This reaction presumably involves the initial generation of a palladium-hydride bond under the reaction conditions, which can mediate sequential insertion of the alkyne units to 

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Palladium-Catalyzed Carbon-Oxygen Bond Formation... 

The exchange of a halogen to a classical nitrogen or oxygen nucleophile usually proceeds readily on the purine skeleton, without the necessity of using a transition metal catalyst. There are certain cases, however, where the palladium catalyzed carbon-heteroatom bond formation might take preference over noncatalysed methods. Inosine derivatives, for example,... 

The palladium-catalyzed synthesis of functionalized indoles through intramolecular nitrogen-carbon(sp ) bond formation has been reported (Scheme 3.55) [61], This was an oxidative cycloisomerization process, and the authors found that molecular oxygen (1 atm) was an effective oxidant for the process. The substrate scope for this reaction was quite... 

The palladium catalyzed iminoannulation and carboxyannulation of alkynes and an appropriate aryl/vinyl halide is an efficient tool to construct six membered nitrogen and oxygen heterocycles. The process encompasses the concomitant formation of a carbon-carbon and a carbon-heteroatom bond. 

The last example focuses not on the functionalization of heterocycles by a transition metal mediated carbon-heteroatom bond forming reaction, but the palladium catalyzed conversion of primary amines, including amino-heterocycles, into urea derivatives. A representative example, shown in 8.38., includes the reaction of an amino-carbazole derivative with morpholine, carbon monoxide and oxygen in the presence of catalytic amounts of palladium(II) iodide. The formation of the urea moiety proceeds with great selectivity and in high yield.49 The reaction works equally well for primary aliphatic and aromatic amines. 

It is worth mentioning that some precursors easily catalyze the reductive carbonylation of alkynes from the C0/H20 couple. Here, the main role of water is to furnish hydrogen through the water-gas-shift reaction, as evidenced by the co-production of CO2. In the presence of Pd /KI terminal alkynes have been selectively converted into furan-2-(5H)-ones or anhydrides when a high concentration in CO2 is maintained. Two CO building blocks are incorporated and the cascade reactions that occur on palladium result in a cyclization together with the formation of an oxygen-carbon bond [37,38]. Two examples are shown in Scheme 4. 

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