Chiral phospha[n]ferrocenophanes: New Metallopolymers through Ring-Opening Polymerizations
Date
2018-04-24
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
0000-0003-2588-8707
Type
Thesis
Degree Level
Doctoral
Abstract
Chiral polymers have many important applications, including separation of racemic compounds and catalysts in asymmetric syntheses. There are many examples of chiral natural polymers, such as proteins, DNA, or cellulose. However, synthetic chiral polymers are rather rare. On the other hand, the incorporation of metal centers into synthetic polymer chains allows the preparation of new functional materials with unique properties that complement those of organic macromolecular materials. Recently, Müller’s group developed a synthetic route to strained ferrocenophanes (FCPs) with planar chirality, and via ring-opening polymerization (ROP), new chiral metallopolymers could be obtained. In this thesis, the syntheses of new chiral phospha[n]FCPs are described, as well as their behaviors towards ROP.
New enantiomerically pure phospha[n]FCPs (n = 1, 2) equipped with two iPr groups in α positions were prepared. The molecular structure of diphospha[2]FCP (Sp,Sp,R,R)-90CH2SiMe3 was determined by single-crystal X-ray analysis showing a tilt angle α = 12.2°. Furthermore, the new chiral (Sp,Sp,S,S)-1,1′-(NiPr2)2fciPr 88NiPr2 (fciPr = (Sp,Sp)-[(Me2CH)–(C5H3)]2Fe) was also selectively synthesized and isolated, which gave suitable crystals for single-crystal X-ray analysis.
Attempted anionic ROPs were performed on phospha[1]FCPs, however, no polymeric materials could be obtained from these reactions. Detailed studies on phospha[1]FCPs with different substituents on phosphorus atom were carried out, showing that the initiation step in the anionic ROP worked, but the ring-propagation did not occur. In attempts to perform this type of reaction, one of the phospha[1]FCP (Sp,Sp)-20Ph showed the ability to be cleanly ring-opened when using 1 equiv of nBuLi and afforded a 3 to 1 diastereomeric mixture of products. This mixture was sulfurized and a single isomer, (Sp,R)-95S, was isolated and characterized by single-crystal X-ray analysis. In addition, based on the clean anionic ring-opening reaction of the compound (Sp,Sp)-20Ph with 1 equiv of nBuLi, the new chiral ferrocene-based phosphine ligand (Sp,S)-99 was successfully synthesized.
In order to measure the amount of strain in [1]FCPs, differential scanning calorimetry (DSC) studies were performed on a series of phospha[1]FCPs having different substituent groups. On the other hand, thermal ROP of compound (Sp,Sp)-21Ph afforded both linear polymer 102n and cyclic oligomers 103. Gel permeation chromatography (GPC) of the sulfurized polymer resulted in a molecular weight of 19 kDa (Mw) and a dispersity of 1.3 (Ð). Mass spectroscopic analysis of the oligomers showed the presence of cyclic species from dimers to hexamers. After sulfurization, preparative thin layer chromatography (PTLC) led to the separation of five isomeric dimers. Structural characterization of one of the dimers (syn-C2-103S-3) by single-crystal X-ray analysis revealed that Fe–Cp bond broke during the thermal ROP process.
Description
Keywords
chiral polymer, phospha[n]ferrocenophanes, metallopolymers, ring-opening polymerization
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Chemistry
Program
Chemistry