A Synthetic Route to Polymeric Carbohelicenes and Cyclic Derivatives
Loading...
Authors
Thompson, William Bailey
Issue Date
2014
Type
Thesis
Language
Keywords
Alternative Title
Abstract
The King group's interest is in the synthesis of polycylic aromatic hydrocarbons (PAHs) for their wide range of applications in materials, electronics, and mechanical properties. The primary focus of my own synthetic efforts in PAHs involves the production of polymeric carbohelicenes and their cyclic derivatives. Traditional methods for synthesizing helicenes structure use cross coupling of two phenanthrenes with desired functional groups. The final synthetic step usually involves a single reaction to close the ring and complete the helicenes such as Diels-Alder, carbenoid couplings, or photocyclisation. Our own approach to synthesizing carbohelicenes is to synthesize a polymer precursor, poly{4,6-di((1E)-(2-2H)-propenyl])-m-phenylene), followed by a ring closing metathesis reaction (RCM) to stitch up the helicene. Our route is eight synthetic steps starting from commercially available m-xyene. The sequential steps involve bromination of m-xylene by electrophilic aromatic substitution, with a 70-74% yield, followed by a benzylic bromination with a 75-78% yield. Next a hydrolysis is performed using ZnBr2 in acetic acid yielding the dialdehyde in 75-80%. A double Wittig reaction follows using ethyltriphenylphosphonium bromide to obtain on average 65% yield of a mixture of EE, EZ, and ZZ isomers. Through optimization, we obtained a 95:5:0 ratio of our Wittig product (EE, EZ, and ZZ, checked by GC-MS). The key to this excellent stereoisomeric composition is the application of applying an excess of base. Investigating this further we found a general and practical method for the isomerization of cis-β-methyl styrenes to their trans-isomers using potassium tert-butoxide. Our reaction conditions can be applied across a range of β-methyl styrenes and halogen substituted β-methyl styrene compounds that are difficult to isomerize by other methods. Additionally the reaction settings can be applied to a one-pot Wittig synthesis to obtain predominately trans-isomer. The reaction involves a deprotonation of the terminal carbon of the phenyl alkene to form an allylic carbanion as an intermediate. The allylic intermediate then reforms to the more thermodynamically stable trans-isomer. With this serendipitous discovery we were able to skip our isomerization step of our mixture and obtain predominately our desired EE configuration. With our desired stereoisomer in hand we borylated via lithium halogen exchange and trapping with isopropylpinacol borate to give monomer for a Suzuki polymerization. Using Pd(P(o-Tol)3)4 as our catalyst we obtained a polymer of Mw = 1.38 x 103 Daltons and PDI = 1.51 analyzed by gel permeation chromatography (GPC). However, with Suzuki polymerization there are competing side reactions that can occur, shortening the polymer chain. Common complications include aryl-aryl exchange, hydrolytic deboronation, and dehalogenation. To help reduce these unwanted side products our catalysts were freshly synthesized. To avoid the side products of Suzuki polymerization we employed a different polymerization method by using Yamamoto coupling. Using the dibromo monomer, 1,5-dibromo-2,4-di[(1E)-2-2H]propenylbenzene. This reaction allowed us to skip borylating our monomer and go straight to polymerization. The Yamamoto coupling reactions yielded similar results to the Suzuki polymerization with Mw = 1.30 x 103 Daltons and PDI = 1.35. Matrix-assisted laser desorption ionization (MALDI) mass spectrometry of the polymers showed that both linear and cyclic polymers formed up to thirty repeat units. To separate the larger oligomers we used a preparatory recycling gel-permeation chromatography and removed the smaller oligomers from our product. In a final step we did an ring-closing metathesis RCM reaction with our longer oligomers in an attempt to synthesize our polymeric carbohelicenes. Based on our past research and past literature precedence, we felt that using Grubbs 2nd generation catalysts would be sufficient to close the rings in our polymers. Other catalysts, like Grubbs 1st and 3rd generation or Schrock's catalyst were considered but were not used because the Grubbs 2nd generation is able to undo stilbene-like defects that can occur in the final product. The RCM reaction was monitored by MALDI TOF mass spectrometery. We looked for a repeat unit of 100 daltons and 56 daltons which is the loss of butene. The loss of butene is from the RCM catalytic cycles reacting with one of the propenyl groups. However, after four days the reaction can no longer be monitored by MALDI spectrometery even when using a various selection of matrixes and cation salts. This is probably due to the increased difficulty in ionizing our product. Normal phase UV-HPLC is able to detect similar UV-vis spectra compared to past helicenes and cyclic derivatives, but do not conclusively confirm product formation. We believe that most of our products may also be insoluble in even our best solvent 1,2,4 trichlorobenzene, which makes standard workups difficult.
Description
Citation
Publisher
License
In Copyright(All Rights Reserved)