Chain-Conformation-Directed Polymerization Cyclization for Effective Synthesis of Macrocycles in Bulk

Biological cyclization is highly efficient, and this can be attributed to the conformation of the backbone of the biopolymer. Taking advantage of metal-coordination geometry, we developed a method for conformation-directed polymerization cyclization through rational design of metal carbonyl monomers that could be used to produce cyclic macromolecules, even in bulk. ^P FpR [^P Fp=(PPh₂ (CH₂ )₃ Cp)Fe(CO)₂ with the phosphine group tethered on the cyclopentadiene (Cp) ring; R=CH₃ or (CH₂ )₅ CH₃ ] was designed and synthesized for migration insertion polymerization to generate P(^P FpR) with the polymer backbone containing Cp-Fe bonds. Growth of the backbone led to a cyclic conformation with close end-to-end distances, which facilitated the cyclization. This conformation-directed cyclization was attributed to the piano-stool metal-coordination geometry of the repeating units and the low rotational barrier of the Cp-Fe bonds in the backbone. The produced macrocycles, which contain a metal carbonyl coordination structure in their backbones, are rigid, unlike many organic macrocycles. The macrocycles thus have a large excluded volume. This new type of metal carbonyl macrocycle will be of interest as a building block for supramolecular chemistry and in the exploration of novel materials.