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Iridium-Catalyzed [2+2+2] Cycloaddition of α,ω-Diynes with Isocyanates

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posted on 20.01.2012, 00:00 by Gen Onodera, Mari Suto, Ryo Takeuchi
[Ir­(cod)­Cl]2/BINAP was found to be an efficient catalyst for the [2+2+2] cycloaddition of α,ω-diynes with isocyanates to give 2-pyridones. A wide range of isocyanates can be used for this reaction. Both aliphatic and aromatic isocyanates smoothly reacted with α,ω-diynes to give 2-pyridones in high yields. Aliphatic isocyanates were more reactive than aromatic isocyanates. For aromatic isocyanates, the electronic properties of the substituents affected the reactivity: electron-donating substituents enhanced the reaction. The reaction of unsymmetrical α,ω-diynes possessing two different internal alkyne moieties with isocyanates was regiospecific and gave a single product. This regioselectivity could be explained by the reaction of iridacyclopentadiene with a nitrogen–carbon double bond. The regioselectivity of the reaction of malonate-derived diyne was controlled by a steric effect, while that of the reaction of ester-tethered diyne was controlled by an electronic effect. [Ir­(cod)­Cl]2/chiral diphosphine catalyst could be used for the enantioselective synthesis of C–N axially chiral 2-pyridone. The reaction of diyne 1a with o-methoxyphenyl isocyanate (7a) gave C–N axially chiral 2-pyridone (R)-8aa in 78% yield with 94% ee.