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. 2024 Aug 13;146(34):24042–24052. doi: 10.1021/jacs.4c07767

Synthesis and Suzuki–Miyaura Cross-Coupling of Alkyl Amine-Boranes. A Boryl Radical-Enabled Strategy

Cornelia S Buettner †, Chiara Stavagna †, Michael J Tilby ‡, Bartosz Górski ‡, James J Douglas §, Naoki Yasukawa ∥,*, Daniele Leonori †,*
PMCID: PMC11363021  PMID: 39137918

Abstract

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Alkyl organoborons are powerful materials for the construction of C(sp3)–C(sp2) bonds, predominantly via Suzuki–Miyaura cross-coupling. These species are generally assembled using 2-electron processes that harness the ability of boron reagents to act as both electrophiles and nucleophiles. Herein, we demonstrate an alternative borylation strategy based on the reactivity of amine-ligated boryl radicals. This process features the use of a carboxylic acid containing amine-ligated borane that acts as boryl radical precursor for photoredox oxidation and decarboxylation. The resulting amine-ligated boryl radical undergoes facile addition to styrenes and imines through radical-polar crossover manifolds. This delivers a new class of sp3-organoborons that are stable solids and do not undergo protodeboronation. These novel materials include unprotected α-amino derivatives that are generally unstable. Crucially, these aliphatic organoboron species can be directly engaged in Suzuki–Miyaura cross-couplings with structurally complex aryl halides. Preliminary studies suggest that they enable slow-release of the corresponding and often difficult to handle alkyl boronic acids.

Introduction

Boron-containing molecules are integral materials in organic synthesis.1 These species are the coupling partners in the Nobel Prize-winning Suzuki–Miyaura reaction2 and can be used in many other processes like Brown oxidation,3 Chan-Lam amination,4 and homologation.5 The value of borylated molecules to our society can be aptly realized considering that the Suzuki–Miyaura cross-coupling alone represents >20% of all processes run by the pharmaceutical sector.6

In general, the preparation of borylated molecules is dominated by 2-electron processes that harness the ability of B-reagents to act as electrophiles (path a) and nucleophiles (e.g., β-borylation,7 path b) and participate in transition metal-catalyzed processes (e.g., Miyaura borylation8 and/or C–H activation,9 path c) (Scheme 1A). Recently, methods based on the ability of diboron species (e.g., B2(cat)2, cat = catechol) to trap carbon radicals have further increased their synthetic capability (path d).10 A different and potentially orthogonal approach to the ones listed above would be to develop methods exploiting the reactivity of closed-shell molecules with boryl radicals (path e).11 These are interesting synthetic intermediates that, owing to the direct involvement of the open-shell B-atom in the key C–B bond-forming event, might provide alternative tactics for the assembly of borylated materials.

Scheme 1. (A) Overview of Synthetic Methods for C–B Assembly; (B) Key Mechanistic Differences in the Nature of NHC- vs Amine-Ligated Boryl Radicals and Their Application in Suzuki–Miyaura Cross-Couplings; (C) Our Previous Work Enabled the Minisci-Style Borylation of Azines for Following Suzuki–Miyaura Cross-Coupling; (D) This Work Focuses on the Radical Borylation of Styrenes and Imines to Give Saturated Alkyl Amine-Boranes That Can Be Used in Suzuki–Miyaura Cross-Couplings.

Scheme 1

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So far, the most used class of boryl radicals are the ones ligated to an N-heterocyclic carbene (NHC) (b) (Scheme 1B).12 These species are generated from the corresponding NHC–BH3 reagent (a) by H-atom transfer (HAT) and have been used in a broad spectrum of radical reactions spanning dehalogenation,13 Barton-McCombie-type deoxygenation,14 addition to electron-rich and electron-poor π-systems (e.g., olefins and imines),15 as well as radical polymerization.12 From a structural point of view, NHC-boryls have a π-configuration with the unpaired electron in conjugation with the NHC ligand.16 This decreases their nucleophilicity and impacts the types of radical processes that might be accessible. The starting NHC–BH3 reagent is also non commercially available, and more importantly, the resulting NHC–BH2-containing products (c) have a somewhat limited reactivity profile. For example, to date, they have not been directly used in Suzuki–Miyaura cross-couplings due to their reaction with the Pd catalysts to generate [Pd]–H species. This can be circumvented, but it requires their prior conversion into the corresponding difluorinated derivatives.17

Our group has recently started a research program aimed at evaluating the synthetic potential of amine-ligated boryl radicals (d).18 These species are available from the corresponding and inexpensive Me3N–BH31 (2.8 €/g) by HAT, as pioneered by Roberts.19 Interestingly, d features a tetrahedral B-atom with a σ-radical configuration and hence strong nucleophilic character.20 However, d’s ability to participate in radical borylation reactions as well as the possibility to engage the corresponding organyl amine boranes (e) in Suzuki–Miyaura cross-couplings has been overlooked by the synthetic community.

The mechanistic precedents from Roberts led us to initially develop a photoredox protocol where the highly nucleophilic boryl radical d was engaged in Minisci-type additions to a range of industrially relevant azines (f) (Scheme 1C).18 This reactivity enabled C(sp2)–H borylation at aromatic positions that are elusive to transition-metal-catalyzed C–H activation. Crucially, the resulting borylated azines (G) are stable, crystalline boronate complexes that do not undergo protodeboronation and can be successfully used in challenging “2-pyridyl”-type Suzuki–Miyaura C(sp2)–C(sp2) cross-couplings.21

We recently became interested in developing strategies to install the amine-borane functionality onto saturated fragments and then evaluate these materials as novel cross-coupling partners for the more challenging C(sp3)–C(sp2) bond formations. In this article, we discuss the realization of these two goals and showcase a novel type of boryl radical precursor (2, see Scheme 2) for application in reductive quenching photoredox catalysis (Scheme 1D). This reagent has enabled the easy assembly of alkyl amine borane building blocks via radical borylation of styrenes and imines. These novel sp3-rich organoborons are stable solid materials that do not undergo protodeboronation and can be readily engaged in Suzuki–Miyaura cross-couplings with complex and functionalized aryl halides.

Scheme 2. (A) Key Mechanistic Aspects Related to Boryl Radical Generation and Following Borylation of Azines vs Olefins; (B) Synthesis of a Boryl Radical Precursor for Application in Decarboxylative Photoredox Catalysis; (C) Cyclic Voltammetry and Computational Studies on the Redox Behavior of Carboxylates 2 and Isoelectronic 3.

Scheme 2

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Results and Discussion

Design Plan

Our previous approach toward boryl radical generation used 1 under an oxidative quenching photoredox manifold in the presence of a stoichiometric persulfate (e.g., (Bu4N)2S2O8) (Scheme 2A, path (i).18 These conditions generated an electrophilic oxygen radical h that underwent polarity-matched HAT with 1 to deliver the key boryl radical d. This highly nucleophilic species then participated in Minisci-style additions to protonated azines f–H+. The resulting aminium radicals i were finally deprotonated and oxidized to give the protonated products g–H+ in a redox neutral manner.

We initially decided to adopt related conditions to establish a radical-polar crossover22 reaction on styrenes j (Scheme 2A, path (ii). This approach would generate d, as discussed above, for the following addition to j. At this point, we envisaged the resulting benzylic radical k to be oxidized to the corresponding cation l for nucleophilic trapping (m). Unfortunately, despite extensive screening of all reaction parameters, we did not succeed in implementing this proposal and currently believe chemical incompatibility between the stoichiometric oxidant and styrene to be the main issue. Therefore, we become interested in the possibility of developing an “umpolung” manifold based on the opposite sequence of redox events (Scheme 2A, path (iii)). Thus, generation of d via reductive quenching photoredox catalysis (i.e., oxidation instead of reduction) would be followed by radical borylation (d + j → k) and reduction to the stabilized benzylic anion n for final protonation (o).

It is relevant to note that switching between photoredox cycles (i.e., reductive vs oxidative quenching) is a common practice in photoredox method development owing to the availability of several types of precursors that can generate the same radical by either oxidation (e.g., alkyl carboxylates, oxalates, tribluoroborates) or reduction (e.g., alkyl phtalimide esters, Katrizky’s salts, sulfonium salts).23 In our case, we were confronted by the lack of precursors for boryl radical generation as these species have, to the best of our knowledge, been only prepared by HAT from electrophilic radicals (e.g., t-BuO• or –O3SO•) that intrinsically require an initial reduction.20

To overcome this challenge, we first needed to develop a novel type of boryl radical precursor. Hence, we speculated that carboxylic acid-containing 2 might, upon deprotonation, undergo SET oxidation (p) and decarboxylation to d (Scheme 2B). This would introduce boryl radicals in the same mechanistic framework routinely exploited for the photoredox generation of alkyl radicals from broadly available carboxylates.24 However, this simple concept belies several questions since other heteroatom radicals, like nitrogen radicals or oxygen radicals, cannot be generated from the corresponding carbamic and carbonic acids.

Furthermore, while 2 might resemble an amino acid, it cannot benefit from the usual stereoelectronic stabilization provided by the N-lone pair as this is involved in a dative bond with the B-atom.25

Synthesis of a New Boryl Radical Precursor and Mechanistic Studies

In order to explore this mechanistic proposal, we prepared 2 in two steps from NaBH3CN by hydride → amine ligand metathesis (2a), followed by methylation and hydrolysis.26 While these processes required long reaction times, we routinely accessed >10 g of material per batch reaction. In analogy to other amine-ligated organoborons, 2 is a stable crystalline material that is safe to handle under standard synthetic conditions.27

With this compound in hand, we investigated the feasibility of our proposed redox steps through electrochemistry and computational chemistry (Scheme 2C). First, cyclic voltammetry analysis of 2 as the corresponding Cs salt resulted in an irreversible oxidation profile with a measured Eox = +0.38 V (vs SCE in CH3CN). While this clearly supports the feasibility of 2– to undergo decarboxylation upon SET oxidation (p → d), we were surprised by its rather low oxidation potential. Indeed, as a comparison, the isoelectronic Cs-salt of carboxylic acid 3 has a significantly higher oxidation potential (also irreversible) Eox = +1.03 V (vs SCE in CH3CN).

The contrast between isoelectronic “N–B” and “C–C” compounds has been a recurring academic interest and has presented itself when considering aromaticity in benzene (C6H6) vs borazine ((BN)3H6)28 or the fact that ethane (C2H6) is a gas while the isoelectronic ammonia-borane (NH3–BH3) is a stable solid.29 In order to provide insight into the difference in oxidation potential between 2–Cs and 3–Cs, we performed computational studies. Specifically, the computed dipole moments of the two carboxylates showed a large difference (Δμ = 4.9 D). Visual examination of the molecular electrostatic potentials (MEP) indicates that the highest region of negative charge is, as expected, at the carboxylate moieties. Additionally, quantifying the difference at an equivalent point in space reveals 2– to have a 0.56 eV lower electrostatic potential (V) than 3–. To provide a chemically intuitive framework for this observation, we conducted natural bond orbital (NBO) analysis, where one of the most prominent second-order perturbations (E2) for 3– was the interaction between the oxygen lone pair and its neighboring C–C σ* (E2 = 24.5 kcal mol–1). Crucially, the analogous interaction in 2– over B–C σ* is less pronounced (E2 = 11.8 kcal mol–1). Hence, we propose that the diminished hyperconjugation in 2– causes the O-lone pair to be more localized on the carboxylate functionality, which in turns contributes toward the increased susceptibility for SET oxidation.

Development of Radical Borylations

Given the positive indications discussed above and with gram amounts of 2 in hand, we focused our attention on the development of the radical “hydroboration” of j. Mechanistically, we proposed that, upon deprotonation, 2 could be oxidized (p) by a visible-light excited photocatalyst (*PC) (Scheme 3A). Following decarboxylation, d would trap j, and the resulting benzylic radical k would close the photoredox cycle by SET with the reduced photocatalyst (PC•–). The resulting benzylic anion n would then be easily protonated to yield the desired alkyl amine borane o. Pleasingly, this proposal was readily implemented using the electron poor styrene 4a [Ir(dtbppy)(ppy)2](PF6) as the photocatalyst and t-Bu-TMG as the organic base in DMF solvent under blue LED irradiation at room temperature (Scheme 3B). The addition of H2O (10 equiv) was crucial to the success of the reaction, and under these conditions, 5a was obtained in 91% yield. While all optimization efforts are detailed in the Supporting Information, a few experiments are interesting to highlight from a mechanistic perspective.27 First of all, no reaction proceeded in the absence of either the photocatalyst, the base, or continuous irradiation. Furthermore, while the use of d7-DMF led to no deuterium incorporation in 5a, the use of D2O delivered the product of benzylic deuteration (84% D-incorporation).27 Overall, these experiments confirm the intermediary of anion n along the reaction pathway.

Scheme 3. (A) Proposed Catalytic Cycle for Photoredox Decarboxylative Generation of Boryl Radicals and Their Reaction with Styrene; (B) Optimized Conditions for Radical Borylation of Styrenes and Imines; (C) Proposed Pathways for Radical Borylation of Imines.

Scheme 3

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We also speculated that a similar photoredox cycle could be used for the addition of d to imines (q). In this case, there could be two options for the key C(sp3)–B bond formation: (1) q could directly add to the imine π* system, thus leading to an aminyl radical for SET with PC•– and protonation; alternatively, (2) direct SET reduction of the imine to the corresponding radical anion (q•–) would enable a radical–radical coupling aided from the persistent radical effect (Scheme 3C).30

Pleasingly high reactivity was obtained for the reaction of 2 with 6a under almost identical reaction conditions to the ones previously discussed.27 In this case, Cs2CO3 gave better results as the base and H2O was not necessary albeit some examples in the scope (see Scheme 4) benefited from its presence as an additive (Scheme 3B). Importantly, 7a also proved to be a stable material that crucially does not undergo protodeboronation. This contrasts with the known thermal instability and high protodeboronation tendency of α-amino boronic acids.31 Indeed, these species require either N-protection, generally with a Piv group, or full protonation with a strong Bro̷nsted acid in order to enable their synthetic handle and further use.31

Scheme 4. Substrate Scope for the Radical Borylation of Styrenes and Imines.

Scheme 4

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The reaction was run with 10 equiv of H2O.

In terms of scalability, this reaction was successfully translated by AstraZeneca on >5 g scale (25 mmol), employing 0.5 mol % [Ir] photocatalyst and using a Lucent360 reactor fitted with a 700 mL batch reactor. Under these conditions, 7a was obtained in 72% in situ yield and 51% yield after purification by simple filtration.

Scope of the Process

With the optimized conditions in hand for the radical borylation of styrenes and imines, we evaluated the scope of both transformations (Scheme 4). In terms of olefins, we successfully applied the strategy to styrene (5b), as well as several para-substituted derivatives equipped with electron-withdrawing functionalities like nitrile (5c), ester (5d), amide (5e–h), sulfone (5i), free sulfonamide (5j), and chloride (5k). The chemistry was also extended to ortho-substituted substrates (5l and 5m) as well as styrenes featuring a para,meta (5n), di-meta (5o), and di-ortho (5p) substitution patterns. We were also able to borylate a 2-naphthyl derivative (5q) and 4-vinyl pyridine (5r). Interestingly, under these basic conditions, we completely suppressed the Minisci-type reactivity at C2 that requires pyridine protonation for radical borylation.18 Considering the generation of a benzylic anion as part of our reaction mechanism, it was not surprising that electron-rich p-OMe-styrene (5s) failed under these reaction conditions. 1,1-Diphenylethylene provided the corresponding product 5t in high efficiency, and finally, we used this chemistry for the late-stage functionalization of the antineoplastic agent bexarotene 8 that gave 5u in good yield.

The imine scope was evaluated by looking first at the N-substitution. Thus, derivatives featuring N-Ph (7b and 7c) as well as para-F (7d and 7e), -ester (7f), and -Ph (7g) and electron-rich Me (7a), t-Bu (7h) and OMe (7i and 7j) provided the corresponding products in good yields. The successful formation of 7i and 7j is useful as the PMP can be easily removed. The chemistry was also applied to substrates containing meta-t-Bu (7k) and ortho-para-disubstituted aromatics (7l and 7m). The imine aromatic group was evaluated next, and we successfully used both electron-poor (7n–q) and electron-rich (7r–u) derivatives as well as systems containing a 2-naphthyl (7v) and a 4-pyridyl unit (7w). The compatibility with CN, aryl-Cl, and aryl–CF3 groups is interesting as boryl radicals have been described to undergo either addition or halogen-atom transfer (XAT) with these functionalities.32 Our results imply that reaction with the imine is considerably easier. In terms of limitations, we could not engage N-Boc (7x) and N-Bus (7y) imines in the reactivity.33

Suzuki–Miyaura Cross-Couplings

The C(sp3)–C(sp2) Suzuki–Miyaura cross-coupling was initially attempted using amine-borane 5t and aryl bromide 9 in toluene–H2O with KOH as the base and Pd2(dba)3 as the catalyst (Scheme 5A). Due to the wealth of information on profiling monophosphines with databases such as LKB34 and Kraken,35 we were inspired to investigate our optimization campaign with a single-node decision tree36 to perform a classification task and identify reactivity cliffs. Pleasingly using a threshold of 10% yield, the data could be partitioned, where a larger P–X bond σ* occupancy classified the Suzuki–Miyaura results as true negatives. These results can provide useful initial insights into future synthetic planning. Additionally, we also noticed that within the data, the dialkylbiaryl phosphines spanned the full range of yields, and accordingly, we searched for a univariate correlation with this subset of ligands. Interestingly, using the difference in minimum and maximum sterimol L parameter between conformers (ΔL) provided a good correlation (R2 = 0.79), which shows optimum performance for RuPhos that gave product 10a in 86% yield. Such an analysis allows one to postulate that the ability of the ligand to modulate steric bulk may be pertinent for the success of this cross-coupling. Crucially, no protodeboronation was observed under these reaction conditions.

Scheme 5. (A) Optimization of the Suzuki–Miyaura Cross-Coupling between 5t and 9, P–X Bond σ* is Boltzmann Weighted from Various Conformations; (B) Hydrolysis Studies on 5b; (C) Comparison of Suzuki–Miyaura Cross-Couplings of Different Alkyl Organoborons; (D) Suzuki–Miyaura Cross-Coupling of α-Aminoalkyl Amino-borane 7a.

Scheme 5

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From the development of the C(sp3)–C(sp2) Suzuki–Miyaura cross-coupling reaction and initial insight into the role of the ligand, a key question concerned the exact nature of the organoboron species undergoing transmetalation at [Pd(II)].37 Since the presence of H2O was crucial to the reaction success, we decided to evaluate the hydrolytic behavior of 5b to generate the known alkyl boronic acid 11 (Scheme 5B). Kinetic studies by 11B NMR spectroscopy on the temperature-dependent conversion of 5b into 11 were conducted in CH3CN with 10 equiv of H2O. Under these conditions, 5b was hydrolytically stable for >4 h up to 65 °C, while full hydrolysis could be achieved in ∼1 h upon warming to 80 °C. Since we are in approximately pseudo-first-order conditions, this behavior equates to a unimolecular rate of hydrolysis = 1.2 × 10–3 s–1. These results suggest that alkyl amine-boranes might enable “slow release”38 of the corresponding alkyl boronic acids for following transmetalation and cross-coupling. Furthermore, it is interesting to note that the hydrolytic behavior of these reagents is fundamentally different to one of other organoboron species since H2 is generated upon protonation with H2O, which leads to the evolution of H2.27,39

Furthermore, we also evaluated the cross-coupling capability of 5t with other aryl electrophiles comparing it with other reagents such as the likely intermediate boronic acid 12a, as well as the corresponding boronic ester 12b and trifluoroborate salt 12c (Scheme 5C). This demonstrated that 5t can be successfully engaged with similar efficiency in reactions with aryl chlorides and triflates. Interestingly, under these reaction conditions, 5t significantly outperformed 12a–c in all coupling reactions despite all these species might generate the same transmetalating intermediates.40 We hope that this demonstrates the versatility these novel reagents can provide in the case of challenging C(sp3)–C(sp2) bond-forming processes.

Interestingly, the use of the same reaction conditions for the coupling of α-amino alkyl amine-borane reagent 7a with 9 resulted in no product formation and complete decomposition of the organoboron (Scheme 5D, entry 1). Pleasingly, using (t-Bu)3P•HBF4 as the ligand with a lower amount of H2O and at 80 °C, product 13a was obtained in 57% yield (entry 2). Furthermore, by adjusting the base to K2CO3, we obtained a high coupling efficiency (entry 3). In terms of scale-up potential, AstraZeneca was able to run this reaction at 2.5 mmol scale (0.75 g of 7a) also decreasing the amount of 9 to 1.5 equiv. Under these conditions, 13a was obtained in slightly diminished yield (entry 4). In this case, a direct comparison between the ability of 7a to under cross-coupling reactions and the one of other types of α-aminoalkyl organoborons (e.g., boronic acids) was not possible as these species are known to undergo facile protodeboronation.31

Scope of the Process

With the conditions in hand for the effective cross-coupling of 5t and 7a, we evaluated the scope of aryl bromides compatible with the process (Scheme 6). The alkyl organoboron 5t was successfully engaged in reaction with Ph–Br (10b) as well as different para- (10a, c–h), meta- (10i), and ortho-substituted (10j) aryl bromides in generally good yields (Scheme 6A). This demonstrated tolerance of many functionalities spanning different electronic properties. Furthermore, the coupling efficiency could be maintained using hindered mesitylene (10k) and indanone (10l) derivatives, as well as 1-napthalene (10m) and electron-rich dibenzothiophene (10n). Several electron-poor azines were screened next, and the reaction was demonstrated across C3–Br quinoline (10o), pyridines (10p–r), and pyrimidine (10s). Finally, we also demonstrated high coupling efficiency with styrenyl bromide (10t).

Scheme 6. (A) Scope for the Suzuki–Miyaura Cross-Coupling Using Alkyl Amine-Borane 5t; (B) Scope for the Suzuki–Miyaura Cross-Coupling Using the α-Aminoalkyl Amine-Borane 7a.

Scheme 6

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The Suzuki–Miyaura cross-coupling of 7a demonstrated a similar efficacy across a series of related aryl bromides. In this case, the stability of 7a toward protodeboronation means that no N-protection is required and the secondary amines can be directly obtained. This has enabled fast access to a broad range of diaryl benzylic amines 13a–t in generally good yield and broad functional group compatibility in terms of both functionalized benzenes as well as both electron-rich (e.g., indole) and electron-poor (e.g., pyrimidine, pyridine, and quinoline) heteroaromatics.

Application to Informer Library Cross-Coupling

The results presented above suggest that the Suzuki–Miyaura cross-coupling of these novel alkyl amine-borane reagents is sufficiently general in terms of compatible aryl bromides. Nonetheless, Pd-catalyzed reactions can still be challenging when applied to complex and densely functionalized derivatives.41 To further test its utility, especially in a MedChem settings, we benchmarked the reactivity of 7a across the Merck informer library comprising 16 highly complex aryl halides (141–16) (Scheme 7).42 This screening study was performed at AstraZeneca using microscale parallel experimentation settings, and the amount of Ar–Br was lowered to 1.2 equiv. While not all substrates were compatible under our reaction conditions, we were pleased to see that many informers gave satisfactory hits in the microscale reactions.

Scheme 7. Testing the Suzuki–Miyaura Cross-Coupling of 7a with the Merck’s Informer Aryl Halides 141–16.

Scheme 7

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Screening results shown are uHPLC area/area% of product at 220 nm identified by mass spectrometry analysis and supported by 19F NMR, see SI for further details.

With this information in hand, we successfully translated two experiments to a preparative scale with similar efficiency (153 and 1513). Overall, these examples indicate these novel organoboron building blocks can be compatible for application in MedChem settings, particularly in a library synthesis setting via a common organoboron intermediate.

Conclusions

Amine-ligated boryl radicals have been scarcely used for the generation of borylated materials. The results presented here demonstrate that their corresponding boryl radicals can be generated by decarboxylative photoredox catalysis for addition to styrenes and imines. The radical-to-polar crossover reactions provide access to stable sp3-rich organoborons that can be successfully engaged in Suzuki–Miyaura reactions. These reactions accommodate the use of complex and high-functionalized aryl halides and result in minimum protodeboronation.

Acknowledgments

D. L. thanks the European Research Council for a grant (101086901). C. S. B. thanks the Marie Curie Actions for a Fellowship (B-STRAIN 101102819). Ms Cornelia Vermeeren (RWTH) is kindly acknowledged for support with the purification of some of the reaction products. Dr Wolfgang Bettray (RWTH) is thanked for HRMS studies of hydrolysis products. Minakem is kindly acknowledged for running the stability studies on compound 2. Computational calculations were performed with computing resources granted by RWTH Aachen University under project RWTH1268.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.4c07767.

  • 1H and 13C NMR spectra for all new compounds; additional experimental and computational details; materials and methods, including photographs of experimental setups (PDF)

Author Contributions

# C.S.B. and C.S. contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.

Supplementary Material

ja4c07767_si_001.pdf (17.6MB, pdf)

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