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

Abstract

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. FpR [ Fp=(PPh (CH ) Cp)Fe(CO) with the phosphine group tethered on the cyclopentadiene (Cp) ring; R=CH or (CH ) CH ] was designed and synthesized for migration insertion polymerization to generate 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.

Competition between Ring-Closing Migratory Insertion Polymerization and Monomer Cyclization

“…The polymerization behavior for the two monomers ( 1a and 1b ) is identical, because the pendent R groups are not involved in the migratory insertion reaction of the Fp group. As has been reported, the polymerization of 1b generated a small fraction of unknown products in addition to 2b , which is illustrated by the 31 P NMR (CDCl 3 ) spectrum of a crude product (Figure a). As shown in Figure a, the two signals at 72.3 and 71.5 ppm are attributed to 2b and the unknown product, respectively .…”

“…As has been reported, the polymerization of 1b generated a small fraction of unknown products in addition to 2b , which is illustrated by the 31 P NMR (CDCl 3 ) spectrum of a crude product (Figure a). As shown in Figure a, the two signals at 72.3 and 71.5 ppm are attributed to 2b and the unknown product, respectively . To remove the unknown product, we added hexane to a THF solution of the crude products to precipitate 2b .…”

“…2b has been fully characterized as a macrocycle in our previous work . The 1 H and 31 P NMR spectroscopies of 2a (Figure ), like those for 2b , only show the Cp group (4.0 ppm in Figure a) and the coordinated phosphorus (72.3 ppm in Figure b) in the main chain.…”

“…The 1 H and 31 P NMR spectroscopies of 2a (Figure ), like those for 2b , only show the Cp group (4.0 ppm in Figure a) and the coordinated phosphorus (72.3 ppm in Figure b) in the main chain. The absence of detectable NMR signals for the end-groups is attributed to the formation of a cyclic structure . To further confirm the structure of 2a , transmission electron microscopy (TEM) was subsequently performed.…”

“…The backbiting of Cp anions replaces the Me 2 bpy ligands generating polyferrocenylsilane (PFS) macrocycles . We have previously developed migratory insertion polymerization (MIP) and reported the MIP in bulk of P FpR ( 1 ) [ P Fp = (PPh 2 (CH 2 ) 3 Cp)­Fe­(CO) 2 , R = CH 3 or (CH 2 ) 5 CH 3 ], resulting in a new type of iron–carbonyl macrocycles P­( P FpR), 2 . The ring-closing process is facilitated by the nonlinear piano-stool metal coordination geometry and the conformational flexibility of the polymer chain due to the low rotation barrier of the Cp–Fe bond in the backbone. − During the synthesis of 2 , a small fraction of unknown molecules was produced and removed by a precipitation process .…”