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J Am Chem Soc
J Am Chem Soc
ja
jacsat
Journal of the American Chemical Society
0002-7863
1520-5126
American Chemical Society

39186110
10.1021/jacs.4c09012
Communication
Synthesis and Photochemical Uncaging of Alkene-Protected, Polymer-Bound Vicinal Frustrated Lewis Pairs
https://orcid.org/0000-0003-0298-8318
Latif Emily A.
https://orcid.org/0000-0001-5142-3079
Hilgar Jeremy D.
https://orcid.org/0000-0001-8127-4617
Romero Nathan A. *
Department of Chemistry & Biochemistry, University of California San Diego, La Jolla, California 92093, United States
* Email: naromero@ucsd.edu.
26 08 2024
11 09 2024
146 36 2476424769
03 07 2024
22 08 2024
15 08 2024
© 2024 The Authors. Published by American Chemical Society
2024
The Authors
https://creativecommons.org/licenses/by-nc-nd/4.0/ Permits non-commercial access and re-use, provided that author attribution and integrity are maintained; but does not permit creation of adaptations or other derivative works (https://creativecommons.org/licenses/by-nc-nd/4.0/).

Polymeric materials bearing Frustrated Lewis Pair (FLP) functionality are promising candidates for use as heterogeneous catalysts and adaptive materials, but synthetic access to FLP-functional polymers remains limited due to the incompatibility of FLPs with standard polymerization chemistries. Herein, we describe a synthetic approach that “cages” highly reactive vicinal phosphine-borane FLPs as covalent alkene adducts, which are stable to Ni-mediated vinyl addition polymerization. We discovered that the caged FLP adducts can be photochemically activated to liberate vicinal FLPs, enabling spatiotemporally controlled release of FLPs from polymeric precursors.

Air Force Office of Scientific Research 10.13039/100000181 FA9550-23-1-0079 document-id-old-9ja4c09012
document-id-new-14ja4c09012
ccc-price
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pmcBuilding on established applications of molecular Frustrated Lewis Pairs (FLPs) in metal-free small molecule activation,1−5 catalysis,6−9 and controlled polymerization,10,11 there is emerging interest in polymeric systems that contain FLPs (Figure 1a).12,13 Frustrated LA or LB functionality can be incorporated into polymeric scaffolds12,13 to construct heterogeneous FLP catalyst systems, which may derive enhanced catalytic activity or reusability from the bulk properties of the macromolecular material.14−18 Reciprocally, the macromolecular properties of FLP polymers can be regulated through reversible covalent reactions of FLPs with small molecules, as showcased by Shaver19−22 and Yan.23−25 Collectively, FLP polymers hold vast potential as materials whose dynamic molecular reactivity and multi-length-scale structure could be leveraged in tandem to achieve advanced properties and functions.26

Figure 1 (a) Emerging applications of FLP polymers. (b) Synthetic challenges in accessing bifunctional FLP polymers.

Yet, bifunctional FLP polymers have proven to be synthetically elusive, precluding access to FLP polymers capable of triggered FLP release or FLP polymers that reversibly bind small molecules while maintaining constant topology (Figure 1b). The primary obstacle to these advanced FLP polymer architectures is the incompatibility of FLPs with conventional monomer functionalities and polymerization chemistries. New protecting group strategies are needed for orthogonal protection–deprotection of both LA and LB components of the FLP. To this end, our work seeks to develop synthetic tools for controllably generating FLPs from inert precursors, as both a gateway to studying new FLP polymer architectures and a means of triggering FLP release on-demand. Unmasking latent FLP functionality with (spatio)temporal precision could ultimately be harnessed to externally regulate catalytic FLP reactivity within polymeric matrices,27 or to achieve localized release of FLP payloads, akin to drug delivery polymer systems.28,29

Herein, we demonstrate that FLP-alkene adducts serve as protected FLP precursors, enabling the synthesis of polymer-bound “caged” FLPs that can be controllably “uncaged” (Figure 2). Fontaine and co-workers followed a similar approach to polymerize styrene-based aminoborane-functional monomers by masking the aminoborane FLPs as adducts with HF,30 but this system lacks a mechanism for discrete external control of uncaging. The focus of our work is FLP-alkene adducts 2, which are derived from ethylene-bridged phosphine-borane 1. Vicinal FLP 1 was first reported by Erker and co-workers31 and exhibits potent small molecule reactivity due to the proximity of the LA and LB.31−34 Caged FLP polymers 3 have two possible uncaging pathways: “uncaging mode A” releases molecular vicinal FLP 1, leaving an alkene within the polymer backbone, while “uncaging mode B” generates a bifunctional, vicinal FLP polymer through the release of ethylene. Since both uncaging modes represent promising avenues toward new polymeric FLP-based materials, we sought to develop a synthetic route to caged FLP polymers 3 and to study their uncaging behavior.

Figure 2 Strategy used in this work to prepare caged FLP polymers 3 capable of uncaging vicinal FLPs.

We designed norbornadiene-derived FLP adduct 5 as a caged monomer (Figure 3), which features a strained cyclic alkene primed for either ring-opening metathesis polymerization35 or vinyl addition (VA) polymerization.36,37 Monomer 5 was synthesized by addition of excess norbornadiene (4) to FLP 1 and isolated as a mixture of diastereomers (ca. 4:1 exo-5/endo-5). Exo-5 was assigned as the major diastereomer based on X-ray diffraction (XRD) analysis (Figure S56). We also isolated diaddition product 6 (XRD analysis in Figure S57; additional discussion in Figure S13), which—although undesired—could be efficiently converted to monoadduct 5 by heating 6 at 90 °C with excess norbornadiene (Figure S14). While thermally reversible at elevated temperatures, adducts 5 and 6 can be purified by silica gel column chromatography, demonstrating their kinetic stability under ambient conditions.

Figure 3 Synthesis of caged monomer 5 and thermal conversion of diaddition product 6 to monomer 5.

We have initially pursued VA polymerization of monomer 5 to simplify characterization of the uncaging process, as the resultant VA polymers possess a fully saturated backbone (Table 1). We used (η6-C6H5CH3)Ni(C6F5)2 as a convenient initiator that is highly active in VA polymerization without requiring exogenous activating agents.38−40 Caged polymers 8a–d were obtained successfully when employing pentylnorbornene 7 as a comonomer (Table 1, entries 1–4). 1H, 31P, 19F, and 11B NMR spectra confirm that the key phosphonium-borate functionality remains in-tact in polymers 8 (Figures S15–S19). Based on differential conversion rates observed for exo-5 and endo-5, polymers 8 are likely enriched in or consist exclusively of caged FLP adducts with exo-configurations (Figure S24).41,42 The actual degree of functionalization for polymers 8 plateaus at higher feed ratios (n:m) of monomer 5, reflecting sluggish incorporation of the sterically bulky monomer 5 relative to 7. Decreased molar mass and lower isolated yields observed at higher feed ratios are also consistent with inefficient incorporation of the bulky monomer 5. The fact that monomer 5 does not undergo homopolymerization to 8e (Table 1, entry 5) suggests sequential insertion of 5 is unfavorable. Thus, a random or gradient-like distribution of the caged FLP units is most likely for polymers 8a–d, rather than block-like structures. Whereas trace diene impurities from 7 led to poor uniformity and partial cross-linking in 8a and 8b (see SI, Section 3 for additional details), these undesired reactivities appear minimal for polymers 8c and 8d, which showed dispersities (Đ) of 2.1 and 1.5, respectively, that are typical of VA polynorbornenes prepared using (η6-C6H5CH3)Ni(C6F5)2 and similar Ni(II) initiators.40,43−45 Although as yet unoptimized, this VA polymerization system afforded the opportunity to study the uncaging of caged FLP polymers 8.

Table 1 Vinyl Addition Polymerization of Caged Monomer 5 and Comonomer 7

entry	sample	isolated yield	theor. n:ma	exp. n:mb	Mn (Da)c	Đc	wt % at 175 °Cd	
1	8a	46%	5:95	5:95	3.7 × 106	7.1	95%	
2	8b	61%	10:90	7:93	9.3 × 105	4.2	93%	
3	8c	56%	20:80	9:91	3.2 × 105	2.1	89%	
4	8d	<20%	50:50	13:87	5.1 × 104	1.5	85%	
5	8e	0%	100:0	–	–	–	–	
a Estimated from feed ratio of monomers 5:7.

b Actual comonomer content estimated from 1H NMR spectra.

c Mn and dispersity (Mw/Mn) determined by SEC.

d Weight percent at 175 °C determined by TGA.

To elucidate the uncaging reactivity of alkene-caged FLPs, we used adduct 9(46) as a molecular model system for the caged FLP repeat units in polymers 8, facilitating analysis by NMR spectroscopy (Figure 4). We initially probed for thermal uncaging of adduct 9. After heating at 80 °C, we observed partial conversion to norbornene (NBE) and FLP 1, with no ethylene present to suggest uncaging via mode B (Figure 4a). However, vinylphosphine 10, formed by retro-hydroboration of 1, was also present after heating 9 at 80 °C, and extended heating above 100 °C resulted in complete decomposition to 10 (Figures S26 and S27). Polymer 8c exhibited similar behavior at 80 and 100 °C, with no FLP 1 observed at 100 °C (Figure S47). Broad alkene resonances observed in 1H NMR spectra at these temperatures likely correspond to norbornenyl repeat units in the resulting polymer. TGA data show significant weight loss events beginning near 100 °C for polymers 8a–d (Figure S23), suggesting similar thermal reactivity patterns in the solid state and in solution. These studies reveal that at the temperatures required for appreciable thermal uncaging, decomposition of FLP 1 is inevitable.

Figure 4 (a) Thermal uncaging and (b) photochemical uncaging studies with model FLP-alkene adduct 9. % Conversion and % yields determined from 1H NMR spectra relative to an internal standard. aYield in parentheses is % yield based on recovered starting material (BRSM). See the SI for details.

Seeking to obviate thermal retro-hydroboration of uncaged vicinal FLPs, we considered photochemical activation of phosphonium47−52 borate53,549 as an alternative trigger for uncaging (Figure 4b). Photolysis of adduct 9 with a UV-C LED (λpeak = 280 nm) readily afforded NBE and FLP 1 as the major products (62% yield 1 BRSM), revealing uncaging mode A as the predominant pathway. Crucially, vinylphosphine 10 was not observed in all photochemical reactions (Figures S28–S30). FLP 1 persisted for >3 days after irradiation was stopped and showed negligible reformation of adduct 9 (Figure S30). Ethylene was also observed during UV-C photolysis in smaller quantities, implying formation of vicinal phosphine-borane 11 via uncaging mode B. As we were not able to isolate this putative vicinal FLP for definitive characterization, the assignment of 11 remains tentative at this stage. Based on the resonances attributed to 11 in NMR spectra, 11 is presumed to be a minor photoproduct relative to FLP 1 (1:11 = >10:1; see discussion in SI, Section 4c and Figures S27–S30). Adduct 12, a trans-diastereomer of 9, was also isolated as a minor byproduct in UV-C photoreactions (Figure 4b; XRD analysis in Figure S58). The stereochemical configuration of 12 suggests that it may be an “interrupted” product of a stepwise uncaging pathway (see Figure S43 and SI Section 4i for further mechanistic discussion).

Photouncaging of FLP 1 represents a valuable tool for modulating FLP reactivity akin to recent examples of photogenerated “Frustrated Radical Pairs” (FRPs).55 Unlike FLP 1 and other canonical closed-shell FLPs, FRPs are typically transient, open-shell radical ion pairs (LA•+/LB•–) that react with small molecules in a homolytic fashion.56,57 FRPs can be generated through photoexcitation of intermolecular charge transfer (CT) absorptions between a LA/LB pair,58 which was used to access polymeric FRPs in a recent example by Shaver.27 We considered that “vicinal FRPs” could be generated in secondary photoreactions of 1 after uncaging from 9, but we found that UV-C irradiation does not directly excite the intramolecular CT absorption band at 310–500 nm (Figure S53) of vicinal FLP 1 (Figure 5a). However, photoexcitation of the CT absorption with UV-A light yielded quantitative decomposition of FLP 1 to Mes2P-PMes2 as the major product (Figures S33 and S34), likely decomposing via an unstable vicinal FRP 13 (see Figure S35 for additional discussion). Likewise, UV-A irradiation of adduct 9 produced the same decomposition products (Figures S31 and S32). Thus, UV-C irradiation is key to avoiding secondary photodecomposition of uncaged FLP 1.

Figure 5 (a) UV-A photolysis (365 nm LED) of the CT absorption of 1 leads to decomposition. (b) UV-A photolysis of 9 results in similar decomposition products observed for direct irradiation of 1.

Next, we leveraged the FLP photouncaging reaction to achieve photochemically triggered hydrogenation of enamine 14 and imine 16, using adduct 9 as a latent source of catalytic FLP 1 (Figure 6a). Substrates 14 and 16 were selected to facilitate benchmarking with established literature precedent by Erker and co-workers,59 who reported using 1•H2 (the H2 adduct of FLP 1(31,32)) to achieve catalytic hydrogenation of 14 to 15 (88% yield, 20 h) and of 16 to 17 (87% yield, 45 min), under H2 atmosphere at room temperature. In our phototriggered experiments, excellent yields and reasonable catalytic efficiencies were observed for hydrogenation of 14 (84% yield 15, 5 d) and 16 (100% yield 17, 2 d). The slower reaction times compared to literature are ascribed to the gradual generation of FLP 1 in our photochemical system (Figure S30). Kinetic studies showed that hydrogenation continues after irradiation stops (Figures S44 and S45), confirming that continuous irradiation is unnecessary to maintain catalytic activity. Notably, we observed negligible hydrogenation of 14 in control reactions using FLP 1, which we attribute to formation of an unidentified, catalytically inactive species upon direct mixing of 1 with 14 (Figure S46). This observation clarifies why successful hydrogenation of 14 required the use of 1•H2 as a masked form of 1.59 While adduct 9 and 1•H2 are both latent sources of 1, the temporal control enabled by photouncaging 1 from 9 uniquely obviates the deleterious reaction of 1 with 14 before H2 is introduced.

Figure 6 (a) Phototriggered hydrogenation of enamine 14 and imine 16 using 9. % conversion and % yields determined from 1H NMR spectra using an internal standard. aTurnover number (TON) relative to uncaged FLP 1. (b) Photochemical uncaging of polymer 8c and cross-linked polymer network 18 and photochemical hydrogenation of imine 16 using 8c. See the SI for additional details.

We further confirmed that polymer 8c generates FLP 1 and ethylene under UV-C irradiation (Figure 6b), with roughly 20% of caged adducts in the starting polymer proceeding to 1 as the predominant molecular uncaging product after 2 days (Figures S48 and S49). The proposed structure of polymeric photoproduct 18 is consistent with the appearance of ethylene and broad alkene resonances in 1H NMR spectra. The presence of uncaged FLPs in 18 remains tentative pending further characterization, but the ratio (y:x) should be small and similar to the distribution seen in model studies with 9 (11:1 = <1:10). We also incorporated caged FLP units into a cross-linked polymer network, 8c-X, by conducting VA polymerization with 5 mol % of diene S1 as a cross-linker (Figure 6b). As a solvent-swollen gel, 8c-X releases FLP 1 and ethylene upon irradiation (Figure S50), confirming that the photouncaging reactivity is preserved in this heterogeneous system. As proof-of-concept that triggered FLP release from a polymer precursor can be utilized in catalytic applications, we achieved phototriggered hydrogenation of imine 16 to 17 using polymer 8c, with 100% yield of 17 observed after 2 days (Figure 6b, Figure S51). While this system is selective for “uncaging mode A”, ongoing efforts in our lab to optimize for “uncaging mode B” may afford bifunctional polymeric FLP materials that, among other potential applications, serve as reusable catalytic materials.

In conclusion, this work establishes the use of phosphine/borane FLP-alkene adducts as mutually protected FLPs, which enabled the incorporation of latent FLP functionality into polymers through vinyl addition polymerization. Our studies demonstrate that vicinal FLPs can be photochemically released from caged FLP polymers, uncaging persistent, closed-shell FLPs that are catalytically active. The synthetic advances in this work introduce new opportunities for stimuli-responsive delivery of FLP payloads from macromolecular scaffolds.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.4c09012.Synthetic procedures, characterization data, and supplementary figures (PDF)

Supplementary Material

ja4c09012_si_001.pdf

This material is based upon work supported by the Air Force Office of Scientific Research under award number FA9550-23-1-0079. The authors acknowledge the use of facilities and instrumentation supported by NSF through the UC San Diego Materials Research Science and Engineering Center (UCSD MRSEC), grant # DMR-2011924.

The authors declare no competing financial interest.

Acknowledgments

We are grateful to Prof. Brian Long (University of Tennessee, Knoxville) for insightful discussions on polymerization studies and to Promerus LLC for the generous donation of (η6-C6H5CH3)Ni(C6F5)2 used in this work. We acknowledge Dr. Anthony Mrse (UCSD) for assistance with NMR spectroscopy and analysis and Sean Dunphy (UCSD) for assistance with synthesis.
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References

Stephan D. W. ; Erker G. Frustrated Lewis Pairs: Metal-Free Hydrogen Activation and More. Angew. Chem., Int. Ed. 2010, 49 (1 ), 46–76. 10.1002/anie.200903708.
Stephan D. W. Frustrated Lewis Pairs: From Concept to Catalysis. Acc. Chem. Res. 2015, 48 (2 ), 306–316. 10.1021/ar500375j.25535796
Stephan D. W. Frustrated Lewis Pairs. J. Am. Chem. Soc. 2015, 137 (32 ), 10018–10032. 10.1021/jacs.5b06794.26214241
Stephan D. W. The Broadening Reach of Frustrated Lewis Pair Chemistry. Science 2016, 354 (6317 ), aaf7229 10.1126/science.aaf7229.27940818
Stephan D. W. ; Erker G. Frustrated Lewis Pair Chemistry: Development and Perspectives. Angew. Chem., Int. Ed. 2015, 54 (22 ), 6400–6441. 10.1002/anie.201409800.
Stephan D. W. ; Erker G. Frustrated Lewis Pair Mediated Hydrogenations. In Frustrated Lewis Pairs I; Erker G. , Stephan D. W. , Eds.; Topics in Current Chemistry; Springer: Berlin, Heidelberg, 2013; Vol. 332 , pp 85–110.10.1007/128_2012_392
Stephan D. W. Diverse Uses of the Reaction of Frustrated Lewis Pair (FLP) with Hydrogen. J. Am. Chem. Soc. 2021, 143 (48 ), 20002–20014. 10.1021/jacs.1c10845.34786935
Stephan D. W. Catalysis, FLPs, and Beyond. Chem. 2020, 6 (7 ), 1520–1526. 10.1016/j.chempr.2020.05.007.
Tan X. ; Wang H. Frustrated Lewis Pair Catalysis: It Takes Two to Make a Thing Go Right. Chin. J. Chem. 2021, 39 (5 ), 1344–1352. 10.1002/cjoc.202000570.
Chen E. Y.-X. Polymerization by Classical and Frustrated Lewis Pairs. In Frustrated Lewis Pairs II: Expanding the Scope; Erker G. , Stephan D. W. , Eds.; Topics in Current Chemistry; Springer: Berlin, Heidelberg, 2013; pp 239–260.10.1007/128_2012_372
Hong M. ; Chen J. ; Chen E. Y.-X. Polymerization of Polar Monomers Mediated by Main-Group Lewis Acid-Base Pairs. Chem. Rev. 2018, 118 (20 ), 10551–10616. 10.1021/acs.chemrev.8b00352.30350583
Vidal F. ; Jäkle F. Functional Polymeric Materials Based on Main-Group Elements. Angew. Chem., Int. Ed. 2019, 58 (18 ), 5846–5870. 10.1002/anie.201810611.
Yolsal U. ; Horton T. A. R. ; Wang M. ; Shaver M. P. Polymer-Supported Lewis Acids and Bases: Synthesis and Applications. Prog. Polym. Sci. 2020, 111 , 101313 10.1016/j.progpolymsci.2020.101313.
Trunk M. ; Teichert J. F. ; Thomas A. Room-Temperature Activation of Hydrogen by Semi-Immobilized Frustrated Lewis Pairs in Microporous Polymer Networks. J. Am. Chem. Soc. 2017, 139 (10 ), 3615–3618. 10.1021/jacs.6b13147.28247761
Willms A. ; Schumacher H. ; Tabassum T. ; Qi L. ; Scott S. L. ; Hausoul P. J. C. ; Rose M. Solid Molecular Frustrated Lewis Pairs in a Polyamine Organic Framework for the Catalytic Metal-free Hydrogenation of Alkenes. ChemCatChem. 2018, 10 (8 ), 1835–1843. 10.1002/cctc.201701783.
Vidal F. ; McQuade J. ; Lalancette R. ; Jäkle F. ROMP-Boranes as Moisture-Tolerant and Recyclable Lewis Acid Organocatalysts. J. Am. Chem. Soc. 2020, 142 , 14427–14431. 10.1021/jacs.0c05454.32787237
Horton T. A. R. ; Wang M. ; Shaver M. P. Polymeric Frustrated Lewis Pairs in CO2/Cyclic Ether Coupling Catalysis. Chem. Sci. 2022, 13 (13 ), 3845–3850. 10.1039/D2SC00894G.35432910
Huang Y.-B. ; Gu F. ; Hu B. ; Li K.-M. ; Xu Z.-M. ; Luo J. ; Lu Q. Flexible Borane-Nitrogen Frustrated Lewis Pair Organic Microsphere for Selective Alkyne Hydrogenation. Chem. Mater. 2023, 35 (15 ), 5752–5763. 10.1021/acs.chemmater.2c03507.
Wang M. ; Nudelman F. ; Matthes R. R. ; Shaver M. P. Frustrated Lewis Pair Polymers as Responsive Self-Healing Gels. J. Am. Chem. Soc. 2017, 139 , 14232–14236. 10.1021/jacs.7b07725.28915038
Yolsal U. ; Wang M. ; Royer J. R. ; Shaver M. P. Rheological Characterization of Polymeric Frustrated Lewis Pair Networks. Macromolecules 2019, 52 , 3417–4325. 10.1021/acs.macromol.9b00271.
Yolsal U. ; Horton T. A. R. ; Wang M. ; Shaver M. P. Cyclic Ether Triggers for Polymeric Frustrated Lewis Pair Gels. J. Am. Chem. Soc. 2021, 143 (33 ), 12980–12984. 10.1021/jacs.1c06408.34387464
Wang M. ; Holland J. ; Horton T. A. R. ; Yolsal U. ; Shaver M. P. A Novel Borinate Ester Copolymer for Poly(Frustrated Lewis Pair) Gels. Polymer 2022, 242 , 124576 10.1016/j.polymer.2022.124576.
Chen L. ; Liu R. ; Yan Q. Polymer Meets Frustrated Lewis Pair: Second-Generation CO2-Responsive Nanosystem for Sustainable CO2 Conversion. Angew. Chem., Int. Ed. 2018, 57 (30 ), 9336–9340. 10.1002/anie.201804034.
Liu R. ; Wang Y. ; Yan Q. CO2-Strengthened Double-Cross-Linked Polymer Gels from Frustrated Lewis Pair Networks. Macromol. Rapid Commun. 2021, 42 (6 ), 2000699 10.1002/marc.202000699.
Wang Y. ; Yan Q. CO2-Fueled Transient Breathing Nanogels That Couple Nonequilibrium Catalytic Polymerization. Angew. Chem., Int. Ed. 2023, 62 (1 ), e202217001 10.1002/anie.202217001.
Barbee M. H. ; Wright Z. M. ; Allen B. P. ; Taylor H. F. ; Patteson E. F. ; Knight A. S. Protein-Mimetic Self-Assembly with Synthetic Macromolecules. Macromolecules 2021, 54 (8 ), 3585–3612. 10.1021/acs.macromol.0c02826.
Wang M. ; Shanmugam M. ; McInnes E. J. L. ; Shaver M. P. Light-Induced Polymeric Frustrated Radical Pairs as Building Blocks for Materials and Photocatalysts. J. Am. Chem. Soc. 2023, 145 (44 ), 24294–24301. 10.1021/jacs.3c09075.37890166
Langer R. Polymeric Delivery Systems for Controlled Drug Release. Chem. Eng. Commun. 1980, 6 (1–3 ), 1–48. 10.1080/00986448008912519.
Shieh P. ; Hill M. R. ; Zhang W. ; Kristufek S. L. ; Johnson J. A. Clip Chemistry: Diverse (Bio)(Macro)Molecular and Material Function through Breaking Covalent Bonds. Chem. Rev. 2021, 121 (12 ), 7059–7121. 10.1021/acs.chemrev.0c01282.33823111
Bouchard N. ; Fontaine F.-G. Alkylammoniotrifluoroborate Functionalized Polystyrenes: Polymeric Pre-Catalysts for the Metal-Free Borylation of Heteroarenes. Dalton Trans. 2019, 48 (15 ), 4846–4856. 10.1039/C9DT00484J.30869102
Spies P. ; Erker G. ; Kehr G. ; Bergander K. ; Fröhlich R. ; Grimme S. ; Stephan D. W. Rapid Intramolecular Heterolytic Dihydrogen Activation by a Four-Membered Heterocyclic Phosphane-Borane Adduct. Chem. Commun. 2007, (47 ), 5072–5074. 10.1039/b710475h.
Spies P. ; Kehr G. ; Bergander K. ; Wibbeling B. ; Fröhlich R. ; Erker G. Metal-Free Dihydrogen Activation Chemistry: Structural and Dynamic Features of Intramolecular P/B Pairs. Dalton Trans. 2009, (9 ), 1534–1541. 10.1039/b815832k.19421595
Axenov K. V. ; Mömming C. M. ; Kehr G. ; Fröhlich R. ; Erker G. Structure and Dynamic Features of an Intramolecular Frustrated Lewis Pair. Chem. - Eur. J. 2010, 16 (47 ), 14069–14073. 10.1002/chem.201001814.20981668
Sajid M. ; Kehr G. ; Wiegand T. ; Eckert H. ; Schwickert C. ; Pöttgen R. ; Cardenas A. J. P. ; Warren T. H. ; Fröhlich R. ; Daniliuc C. G. ; Erker G. Noninteracting, Vicinal Frustrated P/B-Lewis Pair at the Norbornane Framework: Synthesis, Characterization, and Reactions. J. Am. Chem. Soc. 2013, 135 (24 ), 8882–8895. 10.1021/ja400338e.23627402
Bielawski C. W. ; Grubbs R. H. Living Ring-Opening Metathesis Polymerization. Prog. Polym. Sci. 2007, 32 (1 ), 1–29. 10.1016/j.progpolymsci.2006.08.006.
Janiak C. ; Lassahn P. G. Metal Catalysts for the Vinyl Polymerization of Norbornene. J. Mol. Catal. Chem. 2001, 166 (2 ), 193–209. 10.1016/S1381-1169(00)00475-1.
Blank F. ; Janiak C. Metal Catalysts for the Vinyl/Addition Polymerization of Norbornene. Coord. Chem. Rev. 2009, 253 (7 ), 827–861. 10.1016/j.ccr.2008.05.010.
Klabunde K. J. ; Anderson B. B. ; Bader M. ; Clark R. J. Bis(Pentafluorophenyl)(η6-Toluene) Nickel(II). In Inorganic Syntheses; Shriver D. F. , Ed.; Wiley: 1979; Vol. 19 , pp 72–74.10.1002/9780470132500.ch13.
Gastinger R. G. ; Anderson B. B. ; Klabunde K. J. π-Arene Complexes of Nickel(II). Synthesis (from Metal Atoms) of (π-Arene)Bis(Pentafluorophenyl)Nickel(II). Properties,π-Arene Lability, and Chemistry. J. Am. Chem. Soc. 1980, 102 (15 ), 4959–4966. 10.1021/ja00535a023.
Barnes D. A. ; Benedikt G. M. ; Goodall B. L. ; Huang S. S. ; Kalamarides H. A. ; Lenhard S. ; McIntosh L. H. ; Selvy K. T. ; Shick R. A. ; Rhodes L. F. Addition Polymerization of Norbornene-Type Monomers Using Neutral Nickel Complexes Containing Fluorinated Aryl Ligands. Macromolecules 2003, 36 (8 ), 2623–2632. 10.1021/ma030001m.
Potier J. ; Commarieu B. ; Soldera A. ; Claverie J. P. Thermodynamic Control in the Catalytic Insertion Polymerization of Norbornenes as Rationale for the Lack of Reactivity of Endo-Substituted Norbornenes. ACS Catal. 2018, 8 (7 ), 6047–6054. 10.1021/acscatal.8b00393.
Martinez-Arranz S. ; Sanchez-Perez E. ; Molina de la Torre J. A. ; Perez-Ortega I. ; Albeniz A. C. P-Bromoaryl- and ω-Bromoalkyl-VA-PNBs: Suitable Starting Materials for the Functionalization of Vinylic Addition Polynorbornenes via Palladium-Catalyzed Cross-Coupling Reactions. RSC Adv. 2016, 6 (107 ), 105878–105887. 10.1039/C6RA23123C.
Casares J. A. ; Espinet P. ; Martín-Alvarez J. M. ; Martínez-Ilarduya J. M. ; Salas G. Stable Nickel Catalysts for Fast Norbornene Polymerization: Tuning Reactivity. Eur. J. Inorg. Chem. 2005, 2005 (19 ), 3825–3831. 10.1002/ejic.200500121.
Martínez-Arranz S. ; Albéniz A. C. ; Espinet P. Versatile Route to Functionalized Vinylic Addition Polynorbornenes. Macromolecules 2010, 43 (18 ), 7482–7487. 10.1021/ma101137z.
Gmernicki K. R. ; Hong E. ; Maroon C. R. ; Mahurin S. M. ; Sokolov A. P. ; Saito T. ; Long B. K. Accessing Siloxane Functionalized Polynorbornenes via Vinyl-Addition Polymerization for CO2 Separation Membranes. ACS Macro Lett. 2016, 5 (7 ), 879–883. 10.1021/acsmacrolett.6b00435.35614763
Mömming C. M. ; Frömel S. ; Kehr G. ; Fröhlich R. ; Grimme S. ; Erker G. Reactions of an Intramolecular Frustrated Lewis Pair with Unsaturated Substrates: Evidence for a Concerted Olefin Addition Reaction. J. Am. Chem. Soc. 2009, 131 (34 ), 12280–12289. 10.1021/ja903511s.19658420
Ammer J. ; Sailer C. F. ; Riedle E. ; Mayr H. Photolytic Generation of Benzhydryl Cations and Radicals from Quaternary Phosphonium Salts: How Highly Reactive Carbocations Survive Their First Nanoseconds. J. Am. Chem. Soc. 2012, 134 (28 ), 11481–11494. 10.1021/ja3017522.22591218
Alonso E. O. ; Johnston L. J. ; Scaiano J. C. ; Toscano V. G. Laser Flash Photolysis Studies of the Formation and Reactivities of Phenyl(Naphthyl)Methyl Carbocations Generated from Phosphonium Salt Precursors. Can. J. Chem. 1992, 70 (6 ), 1784–1794. 10.1139/v92-223.
Imrie C. ; Modro T. A. ; Rohwer E. R. ; Wagener C. C. P. Photolysis of (Arylmethyl)Triphenylphosphonium Salts. Substituent, Counterion, and Solvent Effects on Reaction Products. J. Org. Chem. 1993, 58 (21 ), 5643–5649. 10.1021/jo00073a023.
Takata T. ; Takuma K. ; Endo T. Photoinitiated Cationic Polymerization of Epoxide with Phosphonium Salts as Novel Photolatent Initiators. Makromol. Chem. Rapid Commun. 1993, 14 (3 ), 203–206. 10.1002/marc.1993.030140309.
Nagao Y. ; Shima K. ; Sakurai H. The Photolysis of Ethoxycarbonylmethyltriphenylphosphonium Salts. Bull. Chem. Soc. Jpn. 1972, 45 (10 ), 3122–3126. 10.1246/bcsj.45.3122.
Griffin C. E. ; Kaufman M. L. Photolysis of Benzyltriphenylphosphonium and Tetraphenylphosphonium Chlorides. Tetrahedron Lett. 1965, 6 (12 ), 773–775. 10.1016/S0040-4039(01)83983-6.
Williams J. L. R. ; Doty J. C. ; Grisdale P. J. ; Searle R. ; Regan T. H. ; Happ G. P. ; Maier D. P. Boron Photochemistry. I. Irradiation of Sodium Tetraarylborates in Aqueous Solution. J. Am. Chem. Soc. 1967, 89 (20 ), 5153–5157. 10.1021/ja00996a013.
Wilkey J. D. ; Schuster G. B. Irradiation of Tetraphenylborate Does Not Generate a Borene Anion. J. Org. Chem. 1987, 52 (11 ), 2117–2122. 10.1021/jo00387a001.
van der Zee L. J. C. ; Pahar S. ; Richards E. ; Melen R. L. ; Slootweg J. C. Insights into Single-Electron-Transfer Processes in Frustrated Lewis Pair Chemistry and Related Donor-Acceptor Systems in Main Group Chemistry. Chem. Rev. 2023, 123 (15 ), 9653–9675. 10.1021/acs.chemrev.3c00217.37431868
Ju M. ; Lu Z. ; Novaes L. F. T. ; Martinez Alvarado J. I. ; Lin S. Frustrated Radical Pairs in Organic Synthesis. J. Am. Chem. Soc. 2023, 145 (36 ), 19478–19489. 10.1021/jacs.3c07070.37656899
Dasgupta A. ; Richards E. ; Melen R. L. Frustrated Radical Pairs: Insights from EPR Spectroscopy. Angew. Chem., Int. Ed. 2021, 60 (1 ), 53–65. 10.1002/anie.202010633.
Holtrop F. ; Jupp A. R. ; van Leest N. P. ; Paradiz Dominguez M. ; Williams R. M. ; Brouwer A. M. ; de Bruin B. ; Ehlers A. W. ; Slootweg J. C. Photoinduced and Thermal Single-Electron Transfer to Generate Radicals from Frustrated Lewis Pairs. Chem. - Eur. J. 2020, 26 (41 ), 9005–9011. 10.1002/chem.202001494.32259331
Spies P. ; Schwendemann S. ; Lange S. ; Kehr G. ; Fröhlich R. ; Erker G. Metal-Free Catalytic Hydrogenation of Enamines, Imines, and Conjugated Phosphinoalkenylboranes. Angew. Chem., Int. Ed. 2008, 47 (39 ), 7543–7546. 10.1002/anie.200801432.
