Letter Cite This: ACS Catal. 2018, 8, 5323−5327
pubs.acs.org/acscatalysis
External Regulation of Cobalt-Catalyzed Cycloaddition Polymerization with Visible Light Benjamin D. Ravetz,† Kyle E. Ruhl,†,‡ and Tomislav Rovis*,†,‡ †
Department of Chemistry, Columbia University, New York, New York 10027, United States Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, United States
‡
S Supporting Information *
ABSTRACT: Coordination polymerizations have been extensively used in commercial plastic and rubber industries since 1956, because of their ability to produce polymeric material with well-defined structure, tunable molecular weight (Mn), narrow dispersity (Đ), and useful chain ends. Typically, these processes are initiated by chemical reagents that sacrifice temporal and spatial control of catalysis. Recent advances in photoredox catalysis have sparked an emergence of photocontrolled ionic and radical polymerizations; meanwhile, photocontrolled coordination polymerizations remain relatively unexplored. Herein, we report a light-regulated cobalt-catalyzed [2+2+2] cycloaddition polymerization. Using this methodology, block copolymers are generated while regulating their Mn with visible light. KEYWORDS: block copolymerization, chain-growth, cobalt catalysis, cycloaddition polymerization, photocontrol
E
polymerizations were susceptible to external regulation, it would greatly increase its potential use in materials applications, commensurate with emerging radical and ionic counterparts.1 Therefore, integrating externally controlled catalysis into coordination polymerizations has emerged as a significant goal for organic chemists. Current strategies for temporally controlled polymerizations utilize chemical reagents, applied voltage, mechanical force, or light to externally dictate catalytic activity.2 Among these stimuli, visible light has emerged as an ideal reagent because its intensity, duration, and location can be easily modulated to direct when and where a catalyst is activated. Photosensitization is commonly used to reversibly activate a monomer functional group, such as an alkyl halide, to generate a radical or ionic species capable of polymer initiation or propagation;3 however, photocontrolled polymerization of monomers that lack leverageable functional groups remains challenging. This limitation can be addressed by expanding light-regulated polymerization to transition-metal catalysis. Contrary to ionic and radical polymerization, metal-catalyzed polymerization enables the activation of inert structural features, which broadens the scope of viable monomers and resulting polymers. To obtain photocontrol in metal-catalyzed polymerization, the operative mechanism must reversibly, and rapidly, activate and deactivate at the will of the external stimulus, and undergo a catalyst chain-walking event.4 Expanding photocontrolled polymerizations beyond existing ionic or radical bond
xternal regulation of catalytic activity has emerged as a powerful tool in the creation of new materials and
Figure 1. (A) Photoinitiation vs photocontrol, (B) radical and ionic polymerizations, and (C) metal-catalyzed polymerization.
polymers. Specifically, transition-metal catalysis remains a dominant means, because of ligand modularity, which enables precise control of molecular structure, tunable molecular weight (Mn), and narrow dispersity (Đ). If transition-metal-catalyzed © XXXX American Chemical Society
Received: April 11, 2018 Revised: April 27, 2018
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DOI: 10.1021/acscatal.8b01431 ACS Catal. 2018, 8, 5323−5327
Letter
ACS Catalysis Table 1. Optimizationa
a
entry
solvent
[M]
1 2 3 4 5 6 7
MeCN/DCE(1:5) MeCN/DCE(1:5) MeCN/DCE(1:5) DMF MeCN/DCE(1:1) MeCN/DCE(1:1) MeCN/DCE (1:1)
0.3 0.3 0.1 0.1 0.1 0.1 0.1
additive i1 i1 i1 i1 i1 i2
(10%) (10%) (10%) (10%) (10%) (3%)
time (h)
Mn (kD)
Đ
yield (%)
16 16 3 3 3 16 3
4.7 2.7 2.8 3.4 3.2 3.0 3.3
2.17 1.39 1.38 1.52 1.35 1.38 1.28
90 78 76 71 72 85 82
Upon completion, reaction is concentrated in vacuo, precipitated, and washed with MeOH for GPC analysis.
Figure 2. (A) On−off study; (B) Mn, as a function of monomer conversion; (C) initiator affects the operative mechanism.
constructions would significantly expand our ability to generate new polymers and materials with externally tuned structural features (see Figure 1). We have recently reported a photocontrolled [2+2+2] cycloaddition of alkynes using Co(II) complexes and photoredox catalysts in the presence of visible light.5 The reaction demonstrates on/off behavior with the external light as a stimulus, ascribed to an energy transfer event in the catalytic cycle rescuing a dormant catalyst. We were drawn to a recent
report by Okamoto, who had demonstrated Co-catalyzed polymerization of triyne monomers, with selective crosstrimerization enabled by the judicious choice of alkyne partner.6 We aimed to adapt our methodology to a coordination polymerization and wondered if the previously observed photocontrol would be maintained. We began our investigation with monomer 1a, a phosphine ligated Co(II) precatalyst, a fluorinated polypyridyl iridium(III) photocatalyst (PC), and diisopropylethylamine (DIPEA) as a sacrificial reductant. 5324
DOI: 10.1021/acscatal.8b01431 ACS Catal. 2018, 8, 5323−5327
Letter
ACS Catalysis
Figure 3. (A) Proposed chain walking event; (B) block copolymerization via sequential addition of monomer; (C) decomposition study to determine ratio of HTT with a complex mixture of HTH oligomers branching with and without initiator; (D) acetal hydrolysis of block copolymerization; (E) solvent ratio impacts the Mn; and (F) proposed mechanism.
decorated with two phenyl-pyridines containing strong electron-withdrawing groups to reach an oxidizing overpotential of the excited state, with respect to DIPEA. Furthermore, an electron-rich and sterically shielded bipyridine is responsible for maintaining a reduction potential capable of Co(II/I) reduction. After noticing ambiguities in Đ when varying the reaction conditions (Table 1, entries 1 and 2), we focused our attention toward understanding the mechanism of growth. From the outset, we suspected that a step-growth mechanism was operative, because of a Đ value of >2.0 (Table 1, entry 1). Furthermore, the monomer contains two reactive ends, which is characteristic of step-growth polymerization.9 Despite our hypothesis, we were fascinated to see a linear relationship between Mn and monomer conversion, which is suggestive of chain-growth character (Figure 2B). The proposed mechanism implicates the regeneration of I after [4+2] and reductive elimination (RE) events. If so, we expect to observe a purely step-growth mechanism where Co(I) can initiate a new chain after every RE event. To rationalize the unanticipated results, a plausible propagation transition state was proposed for catalyst transfer (Figure 3A).10 We envision two likely orientations in which monomer can initially react: head-to-head (HTH) and
Gratifyingly, initial results illustrated that oligomers and a lowmolecular-weight polymer were made in 90% yield (Table 1, entry 1), as evidenced by gel permeation chromatography (GPC). The introduction of an exogenous alkyne additive leads to sharply reduced Đ (Table 1, entry 2). Further manipulation of solvent leads to improved Mn. Importantly, longer reaction times result in no change in Mn or Đ (Table 1, entry 5 vs entry 6). Notably, the most efficient polymerization occurs when CoBr2(PCy3)2, PC, DIPEA, and i2 are irradiated for 15 min prior to the addition of monomer (Table 1, entry 7). Presumably, this period of preactivation generates a highly reactive Co(I) species prior to the addition of monomer leading to a more controlled polymerization. After optimization of the reaction, an on−off study (Figure 2A) showed Mn growing in periods of irradiation and stalling in periods of darkness. Concurrently, the monomer is only consumed in the presence of light. Such precise photocontrol was a pleasant find, especially when considering that a single Co(II/I) reduction can catalyze the entire reaction.7 This behavior is consistent with our previous work and can be explained if PC has two roles in catalysis: a reductant of Co(II)8 and an energy transfer reagent for reversible ligand dissociation (Figure 3F). Key to efficient initiation, PC is 5325
DOI: 10.1021/acscatal.8b01431 ACS Catal. 2018, 8, 5323−5327
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ACS Catalysis head-to-tail (HTT). In the HTT orientation, the Co(I)benzene complex, formed after reductive elimination, has a proximal diyne to transfer to, which causes the catalyst to remain on the end of the growing chain. However, the HTH orientation lacks an adjacent diyne which leads to the release of the catalyst from the chain and, in turn, initiation of a new chain. To take advantage of the intriguing chain-growth behavior, we synthesized block copolymers by sequential addition of monomers (Figure 3B). After consumption of 1a, acetal 1b was added and the Mn and Đ were measured before and after by gel permeation chromatography (GPC). A significant increase in Mn was observed after adding 1b, indicating that the monomer was added primarily to the ends of existing chains, rather than starting new ones. Upon treating the resulting polymer 2ab with acid (Figure 3D), half of the copolymer decomposed via acetal hydrolysis; analogously, the molecular weight was halved, reaffirming two separate blocks were formed.11 To determine if the presence of i2 was responsible for the observed reactivity, we monitored the polymerization with and without i2 in the preirradiation period (Figure 2C). Without i2, the growth of Mn, as a function of conversion, becomes exponential, which is characteristic of a step-growth mechanism. Higher-molecular-weight polymers (∼6000 g/mol) can be made when the initiator is excluded from the polymerization, albeit with a significant increase in Đ (∼2.0), which suggests a shift toward a mechanism of inferior control. Decomposing the resulting polymer made with and without i2 in the preirradiation period reveals a larger amount of HTT product when i2 is used (Figure 3C). Taken together, these data support the existence of two distinct catalytically viable species in these reactions: one that delivers step-growth polymerization and a second that proceeds via enhanced chain-growth control. We propose that the two species are due to a Co(0)/Co(II) vs a Co(I)/Co(III) catalytic cycle. The triyne monomer reacts with Co to generate cobaltacyclopentadiene I, which can react with alkyne to begin polymerization in step-growth fashion. In the presence of excess monomer, this pathway is dominant (Figure 3F, stepgrowth cycle). If the system is deficient in monomer, an analogous structure I is generated from the initiator i2; this Co(III) intermediate is slowly reduced to a Co(II) complex, which can then engage monomer to begin polymerization, but now by chain-growth mechanism (Figure 3F, chain-growth cycle).12 Importantly, the reduction potential of the excited state of PC matches that required for reduction of CoBr2(PCy3)2 to Co(I) but falls short of that required for fast reduction to Co(0).5 In conclusion, we have developed a photocontrolled cycloaddition polymerization. The Mn and monomer conversion can be controlled with visible-light irradiation. Mechanistic studies reveal that the addition of an alkyne additive in the preirradiation period leads to a different mechanism of growth. We propose that the mechanistic dichotomy between step-growth and chain-growth results from divergence from a Co(I)/Co(III) to a Co(0)/Co(II) catalytic cycle.
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Experimental procedures; mass, GPC, and NMR data (PDF)
AUTHOR INFORMATION
Corresponding Author
*Tel.: 1-212-854-4162. E-mail:
[email protected]. ORCID
Tomislav Rovis: 0000-0001-6287-8669 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was performed in association with the Catalysis Collaboratory for Light-activated Earth Abundant Reagents (CCLEAR), which is supported by the National Science Foundation and the Environmental Protection Agency through the Network for Sustainable Molecular Design and Synthesis program (No. NSFCHE-1339674). We thank Travis Bailey (CSU) for access to instrumentation and Eugene Y.-X. Chen (CSU) for helpful discussions.
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ABBREVIATIONS HTH, head-to-head; HTT, head-to-tail; RE, reductive elimination; GPC, gel permeation chromatography; Mn, molecular weight; Đ, dispersity
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.8b01431. 5326
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ACS Catalysis Barton, B. E.; Read de Alaniz, J.; Fors, B. P.; Hawker, C. J. Metal-Free Atom Transfer Radical Polymerization. J. Am. Chem. Soc. 2014, 136, 16096−16101. (d) Michaudel, Q.; Chauvire, T.; Kottisch, V.; Supej, M. J.; Stawiasz, K. J.; Shen, L.; Zipfel, W. R.; Abruna, H. D.; Freed, J. H.; Fors, B. P. Mechanistic Insight into the Photocontrolled Cationic Polymerization of Vinyl Ethers. J. Am. Chem. Soc. 2017, 139, 15530− 15538. (e) Kottisch, V.; Michaudel, Q.; Fors, B. P. Photocontrolled Interconversion of Cationic and Radical Polymerizations. J. Am. Chem. Soc. 2017, 139, 10665−10668. (4) Michaudel, Q.; Kottisch, V.; Fors, B. P. Cationic Polymerization: From Photoinitiation to Photocontrol. Angew. Chem., Int. Ed. 2017, 56, 9670−9679. (5) Ruhl, K. E.; Rovis, T. Visible Light-Gated Cobalt Catalysis for a Spatially and Temporally Resolved [2+2+2] Cycloaddition. J. Am. Chem. Soc. 2016, 138, 15527−15530. (6) Sugiyama, Y.-K.; Kato, R.; Sakurada, T.; Okamoto, S. ChainGrowth Cycloaddition Polymerization via a Catalytic Alkyne [2+2+2] Cyclotrimerization Reaction and Its Application to One-Shot Spontaneous Block Copolymerization. J. Am. Chem. Soc. 2011, 133, 9712−9715. (7) (a) Aalbersberg, W. G. L.; Barkovich, A. J.; Funk, R. L.; Hillard, R. L.; Vollhardt, K. P. C. Transition Metal Catalyzed Acetylene Cyclizations. 4,5-Bis(trimethylsilyl)benzocyclobutene, a Highly Strained, Versatile Synthetic Intermediate. J. Am. Chem. Soc. 1975, 97, 5600. (b) Vollhardt, K. P. C. Cobalt-Mediated [2+2+2]Cycloadditions: A Maturing Synthetic Strategy. Angew. Chem., Int. Ed. Engl. 1984, 23, 539−556. (c) Bönnemann, H. Organocobalt Compounds in the Synthesis of Pyridines−An Example of StructureEffectivity Relationships in Homogeneous Catalýsis. Angew. Chem., Int. Ed. Engl. 1985, 24, 248−262. (d) Saino, N.; Amemiya, F.; Tanabe, E.; Kase, K.; Okamoto, S. A Highly Practical Instant Catalyst for Cyclotrimerization of Alkynes to Substituted Benzenes. Org. Lett. 2006, 8, 1439−1442. (8) (a) Buriez, O.; Labbe, E.; Périchon, J. Unexpected Stabilization of a Simple Cobalt(I) Salt in Acetonitrile at a Glassy Carbon Electrode. J. Electroanal. Chem. 2006, 593, 99−104. (b) Thullen, S. M.; Rovis, T. A Mild Hydroaminoalkylation of Conjugated Dienes Using a Unified Cobalt and Photoredox Catalytic System. J. Am. Chem. Soc. 2017, 139, 15504−15508. (9) Stille, J. K. Step-growth polymerization. J. Chem. Educ. 1981, 58, 862. (10) (a) Bryan, Z. J.; McNeil, A. J. Conjugated Polymer Synthesis via Catalyst-Transfer Polycondensation (CTP): Mechanism, Scope, and Applications. Macromolecules 2013, 46, 8395−8405. (b) Sheina, E. E.; Liu, J. S.; Iovu, M. C.; Laird, D. W.; McCullough, R. D. Chain Growth Mechanism for Regioregular Nickel-Initiated Cross-Coupling Polymerizations. Macromolecules 2004, 37, 3526−3528. (c) Miyakoshi, R.; Yokoyama, A.; Yokozawa, T. Synthesis of Poly(3-hexylthiophene) with a Narrower Polydispersity. Macromol. Rapid Commun. 2004, 25, 1663−1666. (d) Miyakoshi, R.; Yokoyama, A.; Yokozawa, T. J. Catalyst-Transfer Polycondensation. Mechanism of Ni-Catalyzed Chain-Growth Polymerization Leading to Well-Defined Poly(3hexylthiophene). J. Am. Chem. Soc. 2005, 127, 17542−17547. (11) Interestingly, MALDI and NMR revealed that acetonitrile is incorporated into the polymer. We propose that slow pyridine formation, from reaction with the solvent, leads to termination of polymer growth. Decreasing the equivalents of acetonitrile can increase molecular weights by slowing a mechanism of termination; however, the simultaneous increase in Đ alludes to a shift toward a step growth mechanism (see Figure 3E). (12) The use of 3 mol % of 1a in place of the initiator results in identical results with chain-growth control, highlighting the importance of cobaltacyclopentadiene I and a concentration effect of monomer rather than the absence of aryl alkyne per se.
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