Palladium oxygen containing heterocycles

Hydroalkoxylation of alkynes, or the addition of alcohol to alkynes, is a fundamental reaction in organic chemistry that allows the preparation of enol ethers and a variety of oxygen-containing heterocycles such as furan, pyran, and benzofuran derivatives. Bergbreiter et al. found that a Mnear poly-(A-isopropylacrylamide) (PNIPAM) polymer exhibited inverse temperature solubility in water (i.e., soluble in cold water but insoluble in hot water). A recoverable homogeneous palladium catalyst was prepared based on the polymer. The PNIPAM-bound Pd(0) catalyst was effective for the reaction of 2-iodophenol with phenylacetylene in aqueous THE media to give the target product... 

Synthesis of Oxygen-Containing Heterocycles Using Palladium(II) Catalysts. 

Palladium-catalyzed synthesis of oxygen-containing heterocycles involving a C-H activation/C-C formation process has been substantially investigated in the past few decades. Palladium-catalyzed arylation, olefination, alkylation. 

Palladium-catalyzed C-H olefination is also a very useful protocol to synthesize oxygen-containing heterocycles, since it allows to functionalize the substrates with an unsaturated olefinic moiety which would constitute a Jt-conjugated structure or be elaborated further. Two pathways are often encountered in this approach (i) addition of the resulted arylpalladium complex to alkynes and subsequent protonation or transformation and (ii) addition of the arylpalladium complex to olefins and following palladium hydride elimination [7b, 20]. The current methods are mainly focused on the functionalization of aromatic C-H bonds to form benzo-oxacycles. 

Phenol and its derivatives are the most common substrates for the construction of oxacycles in this approach. In 2000, Fujiwara and coworkers reported a palladium-catalyzed intramolecular addition reaction of aryl alkynoates to construct coumarins in a mixture of trrfluoroacetic acid and CH2CI2 at room temperature (Scheme 3.6) [21]. This method is simple and efficient to give the desired oxygen-containing heterocycles in high yields. However, this palladium-catalyzed C-H olefination reaction is only constrained to the electron-rich aromatic compounds owing to the inherent limitation of electrophilic metalation process. 

It is known that alkynes could readily interact with transition metals to form q -metal complexes and then undergo further elaboration [27] thus alkynes should be potentially useful in the ortho-aroimtic C-H olefination to form oxygen-containing heterocycles. In 2012, Minami and Hiyama reported a palladium-catalyzed cycloaddition of alkynyl aryl ethers to give internal alkynes... 

Based on the rapid development of C-H activation in the past few decades, palladium-catalyzed synthesis of oxygen-containing heterocycles via C-H activation has been obtained a significant progress in turn, these achievements also promoted the advance of transition metal-catalyzed C-H functionalization. A broad scope of oxacycles has been synthesized via this approach. Some of these structures are even difficult to access through the conventional methods and hence highlight the palladium-catalyzed C-H activation a powerful and efficient method to achieve novel transformations. 

More recently, reductive elimination of aryl ethers has been reported from complexes that lack the activating substituent on the palladium-bound aryl group (Equation (55)). These complexes contain sterically hindered phosphine ligands, and these results demonstrate how steric effects of the dative ligand can overcome the electronic constraints of the reaction.112,113 Reductive elimination of oxygen heterocycles upon oxidation of nickel oxametallacycles has also been reported, but yields of the organic product were lower than they were for oxidatively induced reductive eliminations of alkylamines from nickel(II) mentioned above 215-217... 

Heteroatom-containing aryl iodides react with 1,3-dienes in the presence of a palladium catalyst and an appropriate base to afford a variety of oxygen and nitrogen heterocycles. Mechanistically, heteroannulation proceeds via aryl-and TC-aUylpaUadium intermediates. Similar results have been obtained using either Pd(OAc)2 or Pd(dba)2 as catalysts (Scheme 29). The yield of... 

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