Supported protic acid-catalyzed synthesis of 2,3-disubstituted thiazolidin-4-ones: enhancement of the catalytic potential of protic acid by adsorption on solid supports
作者:Dinesh Kumar、Mukesh Sonawane、Brahmam Pujala、Varun K. Jain、Srikant Bhagat、Asit K. Chakraborti
DOI:10.1039/c3gc41218k
日期:——
The catalytic potential of various protic acids has been assessed for the one pot tandem condensation–cyclisation reaction involving an aldehyde, an amine, and thioglycolic acid to form 2,3-disubstituted thiazolidin-4-ones. The catalytic potential of the various protic acids that follows the order TfOH > HClO4 > H2SO4 ∼ p-TsOH > MsOH ∼ HBF4 > TFA ∼ AcOH is improved significantly by adsorption on solid supports, in particular using silica gel (230–400 mesh size), with the resulting relative catalytic potential following the order HClO4–SiO2 > TfOH–SiO2 ≫ H2SO4–SiO2 > p-TsOH–SiO2 > MsOH–SiO2 ∼ HBF4–SiO2 > TFA–SiO2 ∼ HOAc–SiO2. The better catalytic potential of HClO4–SiO2 as compared to that of Tf–SiO2, although TfOH is a stronger protic acid than HClO4, can be rationalised through a transition state model depicting the interaction of the individual protic acid with SiO2. The catalytic efficiency of HClO4 adsorbed on various solid supports was in the order HClO4–SiO2 ≫ HClO4–K10 > HClO4–KSF > HClO4–TiO2 ∼ HClO4–Al2O3. The catalytic system HClO4–SiO2 is compatible with different variations of aldehydes (aryl/heteroaryl/alkyl/cycloalkyl) and the amines (aryl/heteroaryl/arylalkyl/alkyl/cycloalkyl) affording the desired 2,3-disubstituted thiazolidin-4-ones in 70–87% yields (43 examples). The electronic and the steric factors associated with the aldehydes and the amines provide a handle for selective thiazolidinone formation and were found to be dependent on the extent of imine formation. No significant amount of thiazolidinone formation took place during the reaction of the preformed amide (synthesised from the amine and thioglycolic acid) with benzaldehyde suggesting that the reaction proceeds through the initial reversible imine formation followed by cyclocondensation of the preformed imine with thioglycolic acid, the reversible imine formation being the determining step to control selectivity of thiazolidinone formation in competitive environments. The feasibility of a large scale reaction and catalyst recycling/reuse is demonstrated.
已对各种质子酸的催化潜能进行了评估,这些质子酸用于涉及醛、胺和巯基乙酸的一锅法串联缩合-环化反应,以形成2,3-二取代的噻唑烷-4-酮。通过吸附在固体载体上,特别是使用硅胶(230-400目大小),可以显著提高各种质子酸的催化潜能,这些质子酸的催化潜能顺序为:TfOH > HClO4 > H2SO4 ∼ p-TsOH > MsOH ∼ HBF4 > TFA ∼ AcOH。所得相对催化潜能的顺序为:HClO4–SiO2 > TfOH–SiO2 ≫ H2SO4–SiO2 > p-TsOH–SiO2 > MsOH–SiO2 ∼ HBF4–SiO2 > TFA–SiO2 ∼ HOAc–SiO2。与Tf–SiO2相比,HClO4–SiO2具有更好的催化潜能,尽管TfOH是一种比HClO4更强的质子酸,这可以通过描述单个质子酸与SiO2相互作用的过渡态模型来合理化。HClO4吸附在各种固体载体上的催化效率顺序为:HClO4–SiO2 ≫ HClO4–K10 > HClO4–KSF > HClO4–TiO2 ∼ HClO4–Al2O3。催化系统HClO4–SiO2与不同变体的醛(芳基/杂芳基/烷基/环烷基)和胺(芳基/杂芳基/芳基烷基/烷基/环烷基)兼容,以70-87%的产率(43个例子)提供所需的2,3-二取代噻唑烷-4-酮。与醛和胺相关的电子和空间因素为选择性噻唑烷酮的形成提供了依据,并发现它们依赖于亚胺形成的程度。在预形成的酰胺(由胺和巯基乙酸合成)与苯甲醛的反应中,没有发生显著量的噻唑烷酮形成,这表明反应通过初始的可逆亚胺形成,然后是预形成的亚胺与巯基乙酸的环化缩合,可逆的亚胺形成是控制竞争环境中的噻唑烷酮形成选择性的决定步骤。大规模反应的可行性以及催化剂的回收/再利用得到了证明。