Synthesis of A [5] Helicene-Based Chiral Polymer

precursor, an alkoxy-functionalized 3,12-diethyenyl [5] helicene, was prepared in seven consecutive steps. A Pd (0)-catalyzed reaction between this helicene derivative and p-diiodo benzene gave polymer 11, which is soluble in many common organic solvents. Peaks in the UV spectrum of 11 are shifted to the red when compared with the spectrum of monomeric analogue 12, presumably because of extended conjugation between the helicene comonomers and the aryl connector units.


Introduction
Helicenes are polycyclic structures composed of n ortho-fused aromatic rings, where n refers to the number of rings. When n reaches five or more, the molecule experiences steric repulsion between the terminal rings, causing the molecule to twist into a helix [1]. However, some phenanthrenes and benzo phenanthrene were optically resolved and conformationally stable since the early days of helicene chemistry (n = 3 & 4). The inherent chirality of these molecules gives rise to large optical rotations (several thousand degrees) [2] and high circular dichroism values. [3] As chiral conjugated compounds, helicenes and their derivatives have found applications, including semiconductors [4,5] organic light-emitting diodes (OLEDs), [6,7] and nonlinear optical (NLO) materials [8].
The above reports have all dealt with helicenes on the molecular level, i.e. small molecules. The synthesis of the corresponding polymeric helicenes is well known not to be feasible as the steric congestion has been shown to prevent the formation of helicenes longer than [14] helicene [9]. However, the synthesis of [16] helicenes containing alternating copolymers having a vinylene [10] or aryl [11] connector "connector" is possible [11]. Such structures could feature both extended partial conjugation and the helicene chirality. Pu and co-workers synthesized many polymers in which 1,1'-binaphthyls alternated with aryl groups to produce structures with main-chain chirality [12]. However, extended conjugation across the binaphthyl 1,1'-moiety in a polymer is essentially absent given the large dihedral angle between the two naphthyl groups [13].
Unlike 1,1'-binaphthalene, partial conjugation in helicenes can extend across the entire molecule [11] Moreover, copoly merization with a benzylidine or similar arylidene connector group should allow increased delocalization. However, helicenes with polymerization potential represent a more synthetically challenging target [11,14,15]. Hence, the synthesis of new and more effective helicene polymer precursors is of interest.
The route toward a new helicene monomer was inspired by a 2007 report by Kamikawa and co-workers. The focus of this work was the one-step conversion of Z, Z-stilbenes into helicenes, via a palladium-mediated C-H arylation reaction ( Figure  1) [16]. Upon replacement of the fluorine in this reported helicene with an acetylene group, reaction with an aryl halide to generate a fully conjugated polymer under Sonogashira reaction conditions would be possible [17,18]. While the direct conversion of aryl fluorides into acetylenes is difficult, replacing fluorine with chlorine should make this reaction possible [19].

Academic Journal of Polymer science
planar polymers, π-π interactions in helicenes could render these polymers insoluble in the common organic solvents used for processing and characterization. This can be addressed by including solubilizing groups, such as long alkyl chains [20]. Cleavage of the methyl ether, followed by alkylation with a longer chain will help to ensure that the resulting polymer will stay dissolved during its polymerization and subsequent processing, if needed. Additionally, the methoxy functional group serves as an electron-donor (D). The general D-π-A (where π is a conjugated linker, and A is an electron-acceptor) structure has been determined to be effective in NLO polymers, [21] and is also desirable for modification of electronic properties through tuning of the band gap [22]. An aryl group functionalized with A groups and then polymerized with a D-containing helicene could give an extended structure of these D-π-A units. Here we report our route toward helicene derivative 1. However, the racemization barrier for [5] helicene (24.1 kcal/mol at 27 °C) [23] is low enough that it racemizes at room temperature within a few hours [24]. Because of this, the synthesis of a [5] helicene-based chiral (but not necessarily configurationally stable) polymer is not the main objective, but rather, a demonstration of a viable synthetic route Figure 2.

Experimental section
All reactions, unless otherwise noted, were conducted using commercially available solvents and reagents as received, without additional purification, in ordinary glassware under an inert argon atmosphere. Veratrole and TEA were distilled after being stirred over CaH2 for several hours. Dry solvents, such as: DCM, DMF, and THF were obtained from a DriSolv® bottle. 1H & 13C NMR spectra were recorded on Mercury 400 or Varian 400-MR (400MHz) NMR spectrometers, using residual 1H or 13C signals of deuterated solvents as internal reference standards. Reactions were monitored by TLC carried out on 0.200mm analytical layer Baker-flex® plates using UV light (254nm) as the visualizing agent. Silica gel (60Å, 40-63μm; Alfa Aesar) was used as a sorbent for flash column chromatography.

Characterization
SEC was performed on a Shimadzu HPLC system consisting of: a Shimadzu LC-20AT HPLC pump, a Rheodyne 7725i injector, Phenogel 5u 50 x 7.8mm Guard Column, Polymer Laboratories PLgel 5μm MIXED-C column x 2, and a Shimadzu RID 10-A detector, at ambient temperature, using THF (HPLC grade) as the elu-tion solvent with a flow rate of 1 ml/min. Polystyrene standards were used for calibration.
The absorbance spectra were recorded on an Agilent UV-Visible Spectrophotometer using a 1cm trajectory and a blank cell (CHCl3) for each sample. Monomer 2.1 (4.4mg) was dissolved in 5ml of CHCl3 to make a stock solution (3.69x10-4 M), which was then diluted to achieve an appropriate concentration for measurement (1.42x10-5M). Model monomer 2.14 (1.6mg) was dissolved in 4 ml of CHCl3 to make a stock solution (5.45x10-4M), which was then diluted to achieve an appropriate concentration for measurement (4.95x10-5M M). Polymer 2.15 (0.4mg) was dissolved in 4ml of CHCl3 to make a stock solution (2.01x10-5M), which was then diluted to achieve an appropriate concentration for measurement (1.82x10-6M).

Results and Discussion
The synthesis of target structure 1 begins with the assembly of a Z, Z-stilbene (Figure 3), which is, in turn, the precursor for the C-H arylation reaction ( Figure 4). The required triphenylphosphine salt 3 was synthesized ( Figure 5) by reaction of commercially available 2 with triphenylphosphine in near quantitative yield [25,26].   Dialdehyde 5 was prepared according to previous reports, via double directed ortho-lithiation of veratrole [4] [27] The relatively low yield of this reaction could be was likely due to the reduced solubility of the aryllithium intermediate or, possibly, secondary reactions, thus compromising the second lithiation [28]. Varying the conditions have not improved this low yield Figure 6. With both aldehyde 5 and triphenylphosphine salt 3 in hand, the two were combined under basic conditions to generate stilbene 6 ( Figure 3) in a ratio of 6:1 Z, Z: Z,E. 15 The subsequent conversion of 6 into 7, via a Pd (0)-catalyzed double C-H arylation reaction was accomplished in 63% yield (Figure 4). Because helicene 7 has limited solubility in many solvents, this intermediate was transformed into 9, having two 2-ethylhexyl groups [29]. This was carried out by cleavage of the methoxy ether functionality using boron tribromide [30,31] The resulting diol [8] was subsequently alkylated with 2-ethylhexyl bromide. This reaction proceeded slowly, but with the inclusion of dibenzo-18crown-6 as a catalyst, a moderate yield (38%) of the alkylated product [9] was obtained as a viscous oil [32] Compound 9 was significantly more soluble in common organic solvents in comparison with its precursordecessor, 7. As pointed out above, the ability to vary the type of alkoxy group allows for a degree of leverage with respect to physical properties [8,22]    The last step in completing the helicene monomer 1 was the substitution of the two chlorinegs of 9 with alkyne groups that should allow copolymerization using Sonogashira conditions. Using the method reported by Buchwald,18 a Pd (II)-catalyzed substitution gave 10 having two TIPS-protected acetylene groups ( Figure 8). In this case, doubling the ratio of ligand to metal (6:1 rather than 3:1) was necessary for optimal yields. A tetrabutylammonium fluoride-mediated deprotection of the TIPS groups gave monomer 1 in near quantitative yield [33].

Academic Journal of Polymer science
Monomer 1 was subjected to a Pd (0)-catalyzed Sonogashira reaction with p-diiodobenzene in the presence of catalytic amounts of Pd (PPh3)4Cl and CuI in THF and TEA to afford polymer 11 in 80% yield [17]. This polymer was soluble in common organic solvents, including THF, DCM, and CHCl3. The number-average molecular weight (Mn) was determined to be 4,411g/mol (PDI = 1.48) by SEC, using polystyrene standards in THF [34,35] Figure 9. In addition, a model analogue 12 was synthesized by end-capping of 1 using similar reaction conditions,16 allowing comparisons with polymer 11 (Figure 10).  As shown in Figure 11, the absorption maxima at 286, 348, &382nm of polymer 11 are red shifted compared with model monomer 12 having maxima at 283, 332, and 372nm, with red shifts of 3, 16, & 12nm, respectively. These spectral differences are consistent with the extended partial π conjugation [36]. This is significant, because extended conjugation is a desired characteristic in the design of many NLO and other organic materials [37,38].

Conclusion
A 3,12-diethyenyl [5] helicene with electron-donating ether groups was prepared and copolymerized with an aryl connector to produce a polymer with extended conjugation. This is the first example of a polymerization of a helicene with modifiable electron-donating functionality. This variability opens up helicenes as potential tunable components in organic electronic materials. Efforts in our laboratory toward more conformationally stable helicene monomers are currently underway.