Cationic iridium complex [Ir(CH3)(OTf)(CO)(OH2)(PPh3)(2)](OTf) (1) catalyzes addition of diols (HO-(CH2)(n)-OH (n = 2-6)) to terminal alkynes (RC CH: R = H, CH2(CH2)(2)CH3, C6H5, p-CH3C6H4) to produce cyclic acetals (CH3C(R)O(CH2)(n)O) 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-BuC CH -> CHD2COMe)(2)(n-Bu)) suggests a reaction pathway involving a pi-alkyne complex, eta(2)-(RC CH)Ir which is attacked by alcohol (R'OD) to give a beta-alkoxy-alkenyl complex, Ir-CH = CRO+DR'. Proton transfer and attack by another alcohol molecule on the intermediate, Ir-CHD-C+R(OR') to produce Ir-CHD-CR(OR')-O+DR' that finally yields the acetal CHD2CR(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. (c) 2006 Elsevier B.V. All rights reserved.