Popis: |
Cationic iridium complex [Ir(CH 3 )(OTf)(CO)(OH 2 )(PPh 3 ) 2 ](OTf) ( 1 ) catalyzes addition of diols (HO (CH 2 ) n OH ( n = 2–6)) to terminal alkynes (RC CH: R H, CH 2 (CH 2 ) 2 CH 3 , C 6 H 5 , p -CH 3 C 6 H 4 ) to produce cyclic acetals exclusively in the absence of H 2 O at room temperature. While complex 1 does not catalyze the hydration of alkynes to produce the carbonyl compounds (RCOCH 3 ), the cyclic acetals rapidly undergo hydrolysis to give RCOCH 3 and regenerate diols in the presence of 1 . A deuterium-labeling study (MeO D + n- BuC CH → CH D 2 C(OMe) 2 ( n -Bu)) suggests a reaction pathway involving a π-alkyne complex, η 2 -(RC CH)Ir which is attacked by alcohol (R′O D ) to give a β-alkoxy-alkenyl complex, Ir CH CRO + D R′. Proton transfer and attack by another alcohol molecule on the intermediate, Ir CH D C + R(OR′) to produce Ir CH D CR(OR′) O + D R′ that finally yields the acetal CH D 2 CR(OR′) 2 . It has been found that bulky substituents R on RC CH cause a decrease in the rate of diol addition, and that the production of six- and seven-membered acetals is faster than that of five- and eight-membered ones. |