Cyclic acetals from catalytic addition of diols to terminal alkynes with a cationic iridium complex containing two labile ligands

Abstract

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) 2CH 3, C 6H 5, p-CH 3C 6H 4) to produce cyclic acetals ▪ exclusively in the absence of H 2O 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 2C(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 2CR(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.