Zeolites of *BEA framework topology containing isomorphously substituted Lewis acidic metal centers catalyze the liquid-phase intramolecular Prins cyclization of citronellal with outstanding catalytic activity and (di...Zeolites of *BEA framework topology containing isomorphously substituted Lewis acidic metal centers catalyze the liquid-phase intramolecular Prins cyclization of citronellal with outstanding catalytic activity and (dia-)stereoselectivity to the commercially most valuable product, isopulegol (IPL). Effects of the metal-center identity and solvent type were occasionally noted, yet without systematic studies hitherto reported. Here, characteristic dependences of catalytic activities and stereoselectivities on solvent and metal identity were uncovered over four M(IV)-Beta catalysts (M = Sn, Ti, Zr and Hf) in four distinct solvents (i.e., acetonitrile, tert-butanol, cyclohexane and n-hexane). Zr^(-) and Hf-Beta were the most active in acetonitrile and the most selective (> 90% to IPL) in tert-butanol, though their activities were generally lower than Ti- and Sn-Beta in solvents other than acetonitrile. By comparison, Ti-Beta was inferior to other catalysts in terms of both activity and IPL selectivity (as previously shown) in acetonitrile but became the most active in other solvents, with markedly increased IPL selectivity from 60% to 70%?80%. Combining multiple site discrimination and quantification techniques, turnover frequencies were determined for the first time in this reaction;such site-based activities, coupled with comprehensive kinetic interrogations, not only enabled a rigorous comparison of catalytic activities across M-Beta catalysts but also provided deeper insights into the free energy driving forces as solvent and metal identity are varied. The activity and selectivity trends, as well as those for the adsorption and intrinsic activation parameters are caused by solvent co-binding at the active site (acetonitrile and tert-butanol) and less quantifiable crowding effects (cyclohexane) due to the limited pore space and the need to accommodate relatively bulky reactant-derived moieties. This work exemplifies how the interplay of metal identity and solvent determines the reactivities and selectivities in Lewis-acid-catalyzed reactions within confined spaces.展开更多
An unprecedented protocol has been developed for the efficient synthesis of substituted tetrahydropyrans via a bismuth-promoted Prins cyclization of imines with homoallyl alcohols.In the presence of 40 mol%BiCl_(3),a ...An unprecedented protocol has been developed for the efficient synthesis of substituted tetrahydropyrans via a bismuth-promoted Prins cyclization of imines with homoallyl alcohols.In the presence of 40 mol%BiCl_(3),a wide variety of imines react smoothly with homoallyl alcohols at room temperature to give the corresponding 4-chlorotetrahydropyran derivatives in good to excellent yields.展开更多
The Prins cyclization of enol ethers has been realized by employing BiX3 （or FeX3） as catalyst and TMSX （X = Br, C1） as the halogen source. The presence of a tiny amount of water in the solvent dichloromethane pla...The Prins cyclization of enol ethers has been realized by employing BiX3 （or FeX3） as catalyst and TMSX （X = Br, C1） as the halogen source. The presence of a tiny amount of water in the solvent dichloromethane played a key role for the reaction to proceed. The reaction is believed to be catalyzed by Lewis acid-assisted Bronsted acids, which were generated in situ from MX3 and water in the solvent.展开更多
文摘Zeolites of *BEA framework topology containing isomorphously substituted Lewis acidic metal centers catalyze the liquid-phase intramolecular Prins cyclization of citronellal with outstanding catalytic activity and (dia-)stereoselectivity to the commercially most valuable product, isopulegol (IPL). Effects of the metal-center identity and solvent type were occasionally noted, yet without systematic studies hitherto reported. Here, characteristic dependences of catalytic activities and stereoselectivities on solvent and metal identity were uncovered over four M(IV)-Beta catalysts (M = Sn, Ti, Zr and Hf) in four distinct solvents (i.e., acetonitrile, tert-butanol, cyclohexane and n-hexane). Zr^(-) and Hf-Beta were the most active in acetonitrile and the most selective (> 90% to IPL) in tert-butanol, though their activities were generally lower than Ti- and Sn-Beta in solvents other than acetonitrile. By comparison, Ti-Beta was inferior to other catalysts in terms of both activity and IPL selectivity (as previously shown) in acetonitrile but became the most active in other solvents, with markedly increased IPL selectivity from 60% to 70%?80%. Combining multiple site discrimination and quantification techniques, turnover frequencies were determined for the first time in this reaction;such site-based activities, coupled with comprehensive kinetic interrogations, not only enabled a rigorous comparison of catalytic activities across M-Beta catalysts but also provided deeper insights into the free energy driving forces as solvent and metal identity are varied. The activity and selectivity trends, as well as those for the adsorption and intrinsic activation parameters are caused by solvent co-binding at the active site (acetonitrile and tert-butanol) and less quantifiable crowding effects (cyclohexane) due to the limited pore space and the need to accommodate relatively bulky reactant-derived moieties. This work exemplifies how the interplay of metal identity and solvent determines the reactivities and selectivities in Lewis-acid-catalyzed reactions within confined spaces.
基金We are grateful for the financial support from the National Natural Science Foundation of China(No.21202154)Nanjing Institute of Technology Fund for Talents(No.YKJ201329)Nanjing Institute of Technology and the University of Science and Technology of China.
文摘An unprecedented protocol has been developed for the efficient synthesis of substituted tetrahydropyrans via a bismuth-promoted Prins cyclization of imines with homoallyl alcohols.In the presence of 40 mol%BiCl_(3),a wide variety of imines react smoothly with homoallyl alcohols at room temperature to give the corresponding 4-chlorotetrahydropyran derivatives in good to excellent yields.
基金Supporting information for this article is available on the WWW under http：//dx.cioi.org/10. 1002/cjoc.201100659 or from the author.Acknowledgement The authors thank the National Natural Science Foundation of China （No. 20772053） and the Fundamental Research Funds for the Central Universities forfinancial support.
文摘The Prins cyclization of enol ethers has been realized by employing BiX3 （or FeX3） as catalyst and TMSX （X = Br, C1） as the halogen source. The presence of a tiny amount of water in the solvent dichloromethane played a key role for the reaction to proceed. The reaction is believed to be catalyzed by Lewis acid-assisted Bronsted acids, which were generated in situ from MX3 and water in the solvent.
