Letter Cite This: ACS Macro Lett. 2017, 6, 1357−1361
pubs.acs.org/macroletters
Functionalized Rigid Ladder Polymers from Catalytic AreneNorbornene Annulation Polymerization Holden W. H. Lai, Yew Chin Teo, and Yan Xia* Department of Chemistry, Stanford University, Stanford, California 94305, United States S Supporting Information *
ABSTRACT: Rigid ladder polymers represent a unique polymer architecture but have limited synthetic accessibility and structural diversity. Using catalytic arene-norbornene annulation (CANAL) polymerization, we synthesized ladder polymers consisting of rigid and kinked norbornyl benzocyclobutene backbones and bearing various functional groups, such as alcohol, amine, ester, carbamate, amide, benzyl bromide, azide, and heterocycles. The incorporation of functional groups was achieved by either copolymerization of functionalized ladder-type dinorbornenes or postpolymerization functionalization. Functionalization of ladder polymers allows modification of their solubility, compatibility, and other properties, expanding their utilities. These ladder polymers remain microporous and highly glassy, which are desirable for separation and high-temperature applications.
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bromobenzenes were used to direct selective annulation (Scheme 1a).20,21 However, the possibility of using other functional groups to selectively direct annulation and introduce functionalities into the CANAL polymers has remained largely unexplored. Incorporating functional groups in CANAL ladder polymers can significantly expand their function and utility, such as modifying polymer solubility and compatibility, tuning interactions with penetrants for more selective separation, and providing functional handles for cross-linking and grafting. Both copolymerization of functionalized monomers and postpolymerization functionalization may be pursued to incorporate functional groups in CANAL ladder polymers (Scheme 1b). Since many functionalized dibromobenzenes are readily available as potential monomers for CANAL polymerization, we first sought to investigate the effect of different functionalities in dibromobenzenes on CANAL reactivity. Substituents at the R1 and/or R2 positions could coordinate unfavorably with Pd(II) in intermediates I, II, or III of the catalytic cycle to affect CANAL reactivity (Scheme 1a). We first investigated CANAL of a range of dibromobenzene derivatives 1a−k with excess (5 equiv) NBD (Scheme 2). These model reactions serve to not only assess the reaction efficiency and annulation selectivity but also provide a new family of functionalized ladder-type dinorbornenes (diNBEs) that can be used as monomers for CANAL polymerization, olefin metathesis, and many other reactions. The reactions were carried out using 1 mol % Pd(OAc)2, 2 mol % PPh3, and 1 equiv of Cs2CO3 in 1,4-dioxane at 150 °C. As a benchmark
adder polymers represent a unique and intriguing type of polymer architecture, consisting of an uninterrupted series of conformationally restrictive rings.1,2 Such highly restricted chain conformations often result in insolubility without long aliphatic side chains.3 Recently, rigid yet soluble contorted ladder polymers, termed polymers of intrinsic microporosity (PIM),4,5 received significant attention as promising microporous materials for gas separation. Despite the unique properties and exciting applications of contorted soluble ladder polymers, their synthetic methods and structural diversity remain limited. Currently, the predominant methods for the synthesis of contorted ladder polymers is based on double nucleophilic aromatic substitution6−8 and Trö ger’s base formation,9,10 developed by McKeown and co-workers. Significant efforts have been devoted to functionalize PIMs in order to tune their interactions with gases.11−19 These efforts have been focused almost exclusively on the postpolymerization modifications of the nitrile moiety on PIM-1.13−19 These modifications can dramatically alter the solubility of polymers or, in some cases, lead to insoluble ladder polymers.17,19 We recently developed a new ladder polymerization method based on efficient catalytic arene-norbornene annulation (CANAL), 20,21 where palladium complex catalyzes the annulation of aryl bromides and norbornene via ortho-C−H activation followed by reductive elimination to form a fourmembered ring (Scheme 1a). Using optimized conditions and substrates, CANAL reaction is highly efficient and selective and allowed the synthesis of a new family of shape-persistent ladder polymers with contorted fused norbornyl benzocyclobutene backbones.20,21 A notable advantage of CANAL polymerization is that readily available dibromoarenes and norbornadiene (NBD) can be used as monomers. In our initial reports, methyl substituents ortho (R1) and meta (R2) to the bromide on © XXXX American Chemical Society
Received: October 11, 2017 Accepted: November 20, 2017
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DOI: 10.1021/acsmacrolett.7b00806 ACS Macro Lett. 2017, 6, 1357−1361
Letter
ACS Macro Letters Scheme 1. (a) Plausible Mechanism of Pd-Catalyzed AreneNorbornene Annulation (CANAL) and (b) Strategies to Incorporate Functional Groups in CANAL Polymers
Scheme 2. Synthesis of Functionalized Dinorbornenes via CANAL between NBD and Dibromobenzene Derivatives
experiment, p-dimethyl-p-dibromobenzene (1a) gave >90% yield of the diNBE product, and only a trace amount of oligomers was formed. The same reaction with p-diisopropyl-pdibromobenzene (1b) led to a lower yield of 75%. Gratifyingly, the reaction yield was increased to 92% when 10 equiv of NBD was used. We next probed the effect of functional groups on CANAL reactivity. 1c with o-methoxy groups gave 79% yield to 2c, slower than the o-methyl analogue 1a. Tetrahydropyran (THP) protected p-xylylene glycol (1d) gave 54% yield with 5 equiv of NBD, but the yield was increased to 75% using 10 equiv of NBD. The effect of potentially coordinating substituents on CANAL reactivity depends on their positions relative to the bromide. When placed ortho to the bromide as in substrates 1e and 1f, both the acetoxy and amino groups hindered CANAL. While CANAL of 1e resulted in complex mixtures with 90% yields. Without protection of the secondary aliphatic amine, 1i led to very low yield of 2i and complex side products. Using p-dichloro-p-dibromobenzene 1k as a substrate, we observed exclusive halogen selectivity, leading to annulation only on bromide to yield 2k in 87% yield. 2k can then be used
a Ten equiv of NBD used. bTwo mol % of Pd(OAc)2 used, 80 h. cAll conversions were determined by 1H NMR spectroscopy of crude reaction mixture using dimethyl terephthalate as an internal standard.
as a substrate for cross-coupling reactions, such as Suzuki coupling with functionalized aryl boronic acids. This provides an additional way to install functional groups that may interfere with CANAL when placed ortho to the bromide and also forms rigid terphenyl motifs in diNBEs. The presence of NBE moiety on 2k did not interfere with the Suzuki reaction, yielding diNBEs 2l−m with aryl amines and esters (Scheme 2). All these ladder-type diNBEs underwent CANAL polymerization with 1a in complete conversion (Table 1). THF was used as the solvent for polymerization because of its good solubility for the resulting polymers. Polymerizations were performed using 1 mol % Pd(OAc)2 and 2 mol % PPh3 in sealed pressure tubes at 115 °C. We also demonstrated that the bulkier 1b can be used to polymerize with diNBEs to obtain high MW ladder polymers, although 150 °C was required for the more sterically hindered monomer. The resulting ladder polymers were isolated after simple filtration through Celite and precipitation into MeOH or EtOAc, depending on their solubility. Ligand and oligomers were effectively removed during the simple one-time precipitation and washing with the precipitation solvent. Rigid ladder polymers without flexible long substituents are notoriously known for their poor solubility,2 but the CANAL ladder polymers P(1a-co-2b), P(1a-co-2d), P(1b-co-2d), and P(1a-co-2j) were all soluble in 1358
DOI: 10.1021/acsmacrolett.7b00806 ACS Macro Lett. 2017, 6, 1357−1361
Letter
ACS Macro Letters Table 1. Synthesis of CANAL Ladder Polymersa
polymer
time (h)
Mn, MALLSb (kDa)
DPc
ĐM
P(1a-co-2b) P(1a-co-2d) P(1b-co-2d)d P(1a-co-2h-Boc) P(1a-co-2i-Boc) P(1a-co-2j) P(1a-co-2l-amide)
9 24 14 24 24 24 24
31 15 36 37 10 66 13
140 51 112 151 36 287 35
2.0 1.9 1.5 2.6 1.3 3.8 1.3
presumably adopt kinked conformations in approximately two dimensions, resembling the shape of a bending ribbon. Complementary to the copolymerization of functional monomers, we also investigated postpolymerization functionalization of CANAL ladder polymers. Conveniently, we took advantage of the abundant benzylic C−H bonds on the methyl goups of these ladder polymers, such as the simplest P1. Simple bromination of benzylic positions was achieved using Nbromosuccinimide (NBS) and a catalytic amount of AIBN. The extent of bromination can be controlled by the stoichiometry of NBS: 1, 2, 3, and 4 equiv of NBS led to 30%, 55%, 73%, and 88% of the benzylic methyl groups on P1 brominated, respectively. Subsequent reaction with sodium azide yielded azide-functionalized ladder polymer P1−N3, which can be then functionalized via the efficient “click” reaction with alkynes as demonstrated by attaching benzylaldehyde groups (Scheme 3a). Alternatively, the incorporated functional groups can be further derivatized. For example, the THP-protected benzyl alcohols in P(1b-co-2d) were deprotected to obtain P2. The isopropyl groups on P2 rendered the polymer soluble in THF. The benzyl alcohol groups can then be coupled with carboxylic acids to introduce heterocycles, such as picolinic acid and pyridazine-4-carboxylic acid to yield polymers P3 and P4, respectively (Scheme 3b). Nitrogen-rich heterocycles are desirable for tuning the sorption of acidic gases, such as CO2, in polymers.14,15 We next investigated surface area (SA) of the functionalized CANAL ladder polymers using gas sorption and BET analysis (Table 2 and Figures S31−32).23 Frustrated packing of the rigid and contorted backbones led to high SA (200−600 m2/g) in the CANAL polymers. CANAL polymers without strongly polar functionalities, such as P1, P(1a-co-2b), P(1a-co-2h-Boc), P(1a-co-2i-Boc), and P(1a-co-2j), had high N2 uptake at 77 K and BET SA > 300 m2 g−1. Interestingly, small cyclic or branched substituents, such as cyclic saturated heterocycles, isopropyl, and Boc groups (entries 1−3, 5 in Table 2), did not decrease the BET SA much compared to P1, which has only methyl substituents. On the other hand, CANAL polymers containing polar or more flexible substituents (entries 6−10 in Table 2) exhibited low BET SA of
