Abstract
Homogeneous palladium-catalyzed (Pd-catalyzed) cyclocarbonylation of unsaturated allylic alcohols and alkynols in the presence of hydrogen forms lactone products with important applications in the food, perfume, and polymer industry. In this work, the cyclocarbonylation of 2-methyl-3-buten-2-ol was studied for the first time using a very active Pd-DPEphos (bis[(2-diphenylphosphino)phenyl]ether) catalyst in the presence of the ionic liquid (IL) [BMIM]Cl (1-butyl-3-methylimidazolium chloride) in dichloromethane to selectively produce 4,4-dimethyl-γ-butyrolactone. The effect of different parameters such as temperature, gas partial pressures, time of reaction, substrate and ligand concentrations were investigated and found to provide optimal conditions for lactonization (95 °C, 28 bar (CO/H2/N2: 20/5/3)), 18 h, 0.1 M substrate, and 16 mol% DPEphos), which were significantly milder than previously reported systems for cyclocarbonylation. Importantly, the study further showed that presence of the IL in the reaction mixture provided stabilization of the catalyst system and prevented formation of Pd-black, which allowed reuse of the catalytic system in consecutive reactions after intermediate extraction of the lactone product.
Highlights
Metal-catalyzed cyclocarbonylations of unsaturated allylic alcohols or alkynols with carbon monoxide are atom efficient synthetic routes for producing important chemicals like lactones and indanones [1]
Cyclocarbonylation with 2-methyl-3-buten-2-ol was performed in the absence of ionic liquids (ILs) with
Comparison of the 1 H nuclear magnetic resonance (NMR) spectra of the substrate and the crude reaction mixture (Figure S1) confirmed that the substrate was successfully converted to the desired 4,4-dimethyl-γ-butyrolactone product in high yield with only a low amount of byproduct formed
Summary
Metal-catalyzed cyclocarbonylations of unsaturated allylic alcohols or alkynols with carbon monoxide are atom efficient synthetic routes for producing important chemicals like lactones and indanones [1]. The formation of five- and six-membered (γ and δ) lactones is facile and has been widely investigated under acidic or neutral conditions using palladium (Pd) complex-based catalysts in homogeneous liquid-phase reactions [2,3,4,5,6]. Several reviews are available in the literature on reactions and mechanisms of transition-metal catalyzed carbonylation and cyclocarbonylation for lactone production using Group 8–10 catalysts, including [Fe(CO)5]−, [Co2(CO)8], [Ni(cod)2PCy3] (cod: 1,5-cyclooctadiene, PCy3: tricyclohexyl-phosphine),. Materials 2020, 13, 2093 almost exclusive role in lactonization reactions as it generally provides higher yields of lactones compared to other catalysts [13,14]. An interesting cyclocarbonylation catalyst system reactions it generally provides higher yields of lactones compared towas other catalysts [13,14]
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