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

Cationic iridium complex [Ir(CH3)(OTf)(CO)(OH2)(PPh3)2](OTf) (1) catalyzes addition of diols (HO(CH2)nOH (n = 2–6)) to terminal alkynes (RCCH: RH, CH2(CH2)2CH3, C6H5, p-CH3C6H4) to produce cyclic acetals exclusively in the absence of H2O at room temperature. While complex 1 does not catalyze the hydration of alkynes to produce the carbonyl compounds (RCOCH3), the cyclic acetals rapidly undergo hydrolysis to give RCOCH3 and regenerate diols in the presence of 1. A deuterium-labeling study (MeOD + n-BuCCH → CHD2C(OMe)2(n-Bu)) suggests a reaction pathway involving a π-alkyne complex, η2-(RCCH)Ir which is attacked by alcohol (R′OD) to give a β-alkoxy-alkenyl complex, IrCHCRO+DR′. Proton transfer and attack by another alcohol molecule on the intermediate, IrCHDC+R(OR′) to produce IrCHDCR(OR′)O+DR′ that finally yields the acetal CHD2CR(OR′)2. It has been found that bulky substituents R on RCCH 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.

Cationic iridium complex [Ir(CH3)(OTf)(CO)(OH2)(PPh3)2](OTf) catalyzes addition of diols (HO(CH2)nOH (n = 2–6)) to terminal alkynes (RCCH: RH, CH2(CH2)2CH3, C6H5, p-CH3C6H4) to produce cyclic acetals excl usively in the absence of H2O at room temperature. Figure optionsDownload as PowerPoint slideFigure optionsDownload as PowerPoint slide