Iridium-catalyzed addition of methanol to internal alkynes

The 18-crown-6 (18C6) ether adduct of sodium hexachloroiridate [Na(18C6)]2[IrCl6]·xH2O (1) was found to catalyze an addition of methanol to a wide variety of internal alkynes RCCR′ (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 MeCCCO2Me, 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 MeO2CCCCO2Me, besides the vinyl ethers (E)/(Z)-MeO2CC(OMe)CHCO2Me, a cyclotrimerization product (C6(CO2Me)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[IrCl6]·xH2O, Na2[IrCl6]·6H2O, H2[IrCl6]·6H2O, Na3[IrCl6]·xH2O) were catalytically active, whereas [IrCl(CO)(PPh3)2] and [(IrCl2Cp*)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 CD3OD to hex-3-yne, [Na(18C6)]2[IrCl6]·xH2O (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%.

The 18-crown-6 (18C6) ether adduct of sodium hexachloroiridate [Na(18C6)]2[IrCl6]·xH2O (1) was found to catalyze effectively an addition of methanol to a wide variety of internal alkynes RCCR′ yielding the corresponding ketals. The regioselectivity, governed by steric and electronic factors, is discussed in terms of the nature of R/R′ (n-alkyls versus branched alkyls versus aryls). Furthermore, the catalytic potential of other iridium compounds in the addition of methanol to hex-3-yne is reported. Figure optionsDownload as PowerPoint slide