Iridium-catalyzed addition of methanol to internal alkynes

Abstract

The 18-crown-6 (18C6) ether adduct of sodium hexachloroiridate [Na(18C6)] 2[IrCl 6]· xH 2O ( 1) was found to catalyze an addition of methanol to a wide variety of internal alkynes RC CR′ (R/R′ = Et/Et, Me/Et, Me/ nPr, Me/ nBu, Me/ tBu, Me/Ph, Et/ nPr, Et/Ph) yielding the corresponding ketals and, due to the presence of water traces, their hydrolysis products (ketones). The regioselectivity of the addition of methanol to unsymmetrically disubstituted internal alkynes is governed by steric and electronic factors, being the highest in the case of pent-2-yne, hex-2-yne and hept-2-yne (80–85%). In the case of MeC CCO 2Me, an alkyne having an electron-withdrawing substituent, the addition was found to be 100% regioselective, with two methoxy groups going to the side away from an ester group. Furthermore, in the analogous addition of methanol to MeO 2CC CCO 2Me, besides the vinyl ethers ( E)/( Z)-MeO 2CC(OMe) CHCO 2Me, a cyclotrimerization product (C 6(CO 2Me) 6) was also observed. A comparison of the catalytic potential of other iridium compounds in the addition of methanol to hex-3-yne revealed that all examined ionic hexachloroiridates ([Na(18C6)] 2[IrCl 6]· xH 2O, Na 2[IrCl 6]·6H 2O, H 2[IrCl 6]·6H 2O, Na 3[IrCl 6]· xH 2O) were catalytically active, whereas [IrCl(CO)(PPh 3) 2] and [(IrCl 2Cp*) 2] were found to be almost inactive (degree of conversion <10%). However, the best results were obtained for the crown ether adduct 1. Moreover, in the addition of CD 3OD to hex-3-yne, [Na(18C6)] 2[IrCl 6]· xH 2O ( 1) was also found to catalyze a H/D exchange of protons in the neighborhood of a keto group or a quaternary carbon of a ketal with a degree of deuteration >97%.