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

[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.