Catalytic Intramolecular Cycloaddition Reactions by Using a Discrete Molecular Architecture

A discrete tetragonal tube-shaped complex (MT-1) has been synthesized by coordination-driven self-assembly of a carbazole-based tetraimidazole donor L and a Pd(II) 90° acceptor, viz. [cis-(dppf)Pd(OTf)2] (dppf = diphenylphosphino ferrocene, OTf = CF3SO3-). Complex MT-1 was characterized by multinuclear NMR, ESI-MS and single crystal X-ray diffraction analysis (SCXRD), which showed its symmetrical tetrafacial tube-shaped architecture possessing a large cavity described by four aromatic walls. This coordination cage was successfully utilized as a molecular vessel to perform intramolecular cycloaddition reactions of O-allylated benzylidinebarbituric acid derivatives inside its confined nanospace. The presence of catalytic amount of MT-1 promoted the [4+2] cycloaddition reactions in regio- and stereo-selective manner yielding the corresponding penta-/tetra-cyclouracil derivatives in good yields under mild reaction condition. This protocol is interesting compared to the literature reports for the synthesis of similar chromenopyran pyrimidinedione derivatives under high temperature reflux conditions or solid-state melt reaction (SSMR).