Iridium(I)-Catalyzed Aerobic Oxidation Of Alcohols To Aldehydes Or Lactones

Oxidation of alcohols to the corresponding carbonyl compounds plays very important position in organic chemistry and industrial application. Recently, much research has been developed to overcome the disadvantages due to the rise of the environment problem. An efficient approach was developed to oxide alcohols to carbonyl compounds using chloro(1,5-cyclooctadiene)iridium(Ⅰ) dimmer [Ir(cod)Cl]2in1,4-dioxane under air. The oxidation has high yield in the presence of [Ir(cod)Cl]2as catalyst, air as oxidant, and Cs2CO3as base in dioxane at110℃. Various conditions, including catalyst loading, temperature, base and reaction time were examined. Reaction time prolongation and shortage or reaction temperature minus all reduced the yield. The optimal conditions were obtained with10%catalyst loading and40%Cs2CO3as base at110℃for1.5h in dioxane under air. Other analogues of cinnamyl alcohol were also evaluated in the presence of catalyst. The electron-withdrawing groups at the phenyl ring of substrates accelerated the reaction rate; however, the yields are slightly decreased. Taking into account hydrogen processes, borrowing hydrogen or hydrogen auto-transfer, occurred simultaneously. A proposed possible mechanism may be (i) hydrogen transfer from alcohol to iridium giving an alkoxy-iridium complex Ⅰ,(ⅱ) then β-elimination giving an aldehyde-iridium hydride complex Ⅱ,(ⅲ) intramolecular six-membered ring hydrogen auto-transfer give the saturated aldehyde.Unsaturated or saturated1,4-diols were also oxidized to saturated lactones. Oxidation of Benzenedimethanols gave isobenzofuranones with good yields. Methoxyl groups at the phenyl ring of substrates reduced yield. To obtain the chiral lactones, we experienced the oxidative desymmetric lactonization of1,4-mesodiols with diphosphines as chiral ligands. The enantioselectivity was modest (12%ee).A convenient and highly effective green catalytic methodology for activation of allyl alcohols to saturated aldehydes or lactones by catalytic iridium(Ⅰ) system has been developed. This catalytic aerobic method is green catalytic and readily available.