Tuning the Electrochemical Redox Potentials of Catechol with Boronic

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Tuning the electrochemical redox potentials of catechol with boronic acid derivatives Zachary M. Robole, Kira L. Rahn, Bryan J. Lampkin, Robbyn K. Anand, and Brett VanVeller J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b03087 • Publication Date (Web): 25 Jan 2019 Downloaded from http://pubs.acs.org on January 27, 2019

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The Journal of Organic Chemistry

Tuning the Electrochemical Redox Potentials of Catechol with Boronic Acid Derivatives Zachary M. Robole,† Kira L. Rahn,† Bryan J. Lampkin, Robbyn K. Anand, Brett VanVeller* Department of Chemistry, Iowa State University, Ames, Iowa 50011, United States Supporting Information Placeholder

ABSTRACT: A strategy to control the oxidation potential of catechol using borinic acids is presented. Borinic acids reversibly bind catechol to form boron ‘ate’ complexes (BACs) that alter the electron density on the oxygen atoms of catechol and, in turn, the propensity of the catechol towards electrochemical oxidation. The effect of different substituents on the borinic acid are investigated to determine their efficacy in tuning the electron density within the BAC and the resulting oxidation potential.

studied previously,9, 10 we hypothesized that borinic acids (R2BBorinic acids are a class of compounds that have been receiving considerably more attention in recent years because of the OH) would provide a distinct advantage over boronic acids. In those previous studies, formation of the BAC required an addiunique reactivity they confer upon complexation with diols.1 This reactivity can be understood by considering an example tional ligand to boron in the form of a hydroxide or methoxide from the literature (Scheme 1).2 The mono-acylation of a diol is ion. Because borinic acids possess two carbon substituents, forslow in the absence of a catalyst. Alternatively, 5 mol% of mation of the BAC with catechol does not require an additional Ph2BOH returns near quantitative yield of the mono-acylated ligand. Thus, formation of the anionic BAC was predicted to be product. The selectivity for mono-acylation and rate acceleramuch more facile11 and not require super-stoichiometric quantion furnished by the borinic acid results from the dynamic cotities of borinic acid and co-ligand. valent bonding preference for 1,2-diols to give the boron ‘ate’ complex (BAC, 1, Figure Scheme 1). While the formal negative Scheme 1. Reaction properties of borinic acids for more charge in 1 is conventionally drawn on boron, computation refacile oxidation of catechol. veals the more electronegative oxygen atoms bear the increased O Ph2BOH (5 mol %) δ charge.2 As a result, the oxygen atom is more nucleophilic and OCOR OH RCOCl (1.2 eq.) O δ Ph undergoes acylation faster than the free diol. The borinic acid R Cl B O O Ph has a lower affinity for the mono-acylated product (2) and rapOH i-Pr2NEt (1.2 eq.) 2 δ CH3CN BPh2 1 idly transfers to unreacted diol, enabling catalyst turnover. This reactivity has been applied to the regioselective functionaliza4 O δ OH tion of carbohydrates with excellent yields and selectivities.2-8 O δ Ar Ar2BOH –2e BPh2 B We hypothesized that the increased electron density on oxy–H2O oxidation O Ar O low OH gen in 1 could be applied to the oxidation of catechol to o-quiδ affinity 3 catechol none, where BAC 3 should make oxidation of catechol to oquinone more facile (Scheme 1). Substituents on the aryl rings To test our hypothesis, we used cyclic voltammetry (CV) to of the borinic acid could therefore control the build-up of elecmonitor the reversible redox reactions of BACs related to 3. tron density on the oxygen atoms of catechol, allowing for tunBriefly, the potential of the electrode is swept between –1.5 and ing of the oxidative reactivity. We also considered that quanti1.5 V (versus Ag/AgNO3). Changes in the flow of current into fication of the explicit oxidation potential of a given BAC and out of the electrode—at a given potential—identify oxidawould allow correlation of the electron density within the BAC tion and reduction processes, respectively. Catechol can unto an experimentally determined parameter. dergo a two-electron oxidation to give o-quinone (Figure 1), Finally, while the effect of boronic acid (R-B(OH)2) comand the potential at which oxidation/reduction occurs is pounds on the electrochemical behavior of catechol has been ACS Paragon Plus Environment

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exquisitely sensitive to the electron density in catechol.12 Thus, changes in the oxidation potential of catechol upon binding of borinic acid will report on catechol electron density. We employed 3,5-ditertbutylcatechol (5, Figure 1) as an electrochemical standard to test if borinic acids can alter the oxidation potential of catechol through formation of BACs.13-15 The bulky tert-butyl substituents sterically bias against further reaction of o-quinone (6). Catechol substrates, once oxidized, can polymerize and deposit onto electrodes during electrochemical analysis.16, 17 Thus, 3,5-ditertbutylcatechol allowed us to probe the redox behavior in the absence of potential competing side reactions.

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Lewis acid complexation with the borinic acid would increase the reduction potential. The shift in the reduction potential between 6 and 9 is much smaller when compared to the shift between 7 and 8, suggesting a lower affinity of 6 for the borinic acid as expected. These results support the notion that Ph2BOH can bind catechol to form a BAC that effectively increases the electron density on oxygen relative to catechol 5. Thus, BAC formation leads to more facile oxidation relative to catechol 5. Accordingly, the electron density at oxygen is not increased to the same extent as the oxygen atoms in dianion 7, and oxidation potential remains at an intermediate value between 7 and 5. The two oxidation potentials of 7 and 5 therefore mark the limits of the range (~ 2 V) over which the oxidation potential might be effectively tuned by the borinic acid reagent. To investigate the effect that substituents on the aryl rings of the borinic acids might have in tuning the oxidation potential of BACs between the limits set by 7 and 5, we tested compounds 3a–c following the same CV procedure as for Ph2BOH above (Table 1). Synthesis of borinic acids is commonly achieved by quenching an aryl lithium or aryl Grignard with B(OMe)3. Thus, we were limited to reported derivatives20 that were compatible with these organometal reagents and electrophilic substituents (i.e., R = –COR, –NO2, –CN) were not investigated.

Figure 1. Individual CV scans of 3,5-ditertbutylcatechol (5, 2mM), 3,5-ditertbutyl-o-quinone (6, 2 mM) and Ph2BOH (2 mM). All voltammograms measured at a platinum disk electrode, in acetonitrile containing tetrabutylammonium hexafluorophosphate (100 mM) as supporting electrolyte, scan rate = 100 mV s-1, average of 3 CV scans, referenced to Ag/AgNO3 electrode.

Catechol oxidation is not electrochemically reversible in acetonitrile (Figure 1, I oxidation at ~ 0.9 V but no reduction peak on the return wave);18 however, initiating cyclic voltammetry with o-quinone 6 does lead to reversible CV curves (Figure 1, II and III). Thus, the reactivity of BACs was probed by initially reducing o-quinone 6 (III, peak at ~ –0.9 V) and observing subsequent catecholate (7) oxidation (II, peak at ~ –0.8 V). As expected, the oxidation peak of catechol 5 is at a higher potential than catecholate 7 because of the greater electronic charge on the oxygen atoms in 7 relative to the protonated oxygen atoms in 3 (Figure 1). Finally, Ph2BOH was not electrochemically active over the potential range (Figure 1), a consistent feature of the general class of boron acid compounds during CV analysis.9, 10, 19

We next sought to evaluate the effect that formation of BAC 8 would have on the oxidation potential of catechol. The cyclic voltammetry experiment was conducted with 6 plus an equimolar amount of Ph2BOH (Figure 2A). Accordingly, an oxidation peak for the putative BAC 8 was observed at a potential that was intermediate between 7 and 5 (Figure 2A). Our identification of the oxidation peak at ~0 V as 8 is supported by the data in Figure 2B, where the experiment with 6 was conducted in the presence of a 0.5 molar equivalent of Ph2BOH. The oxidation peak for 8 decreases and the peak for 7 returns, consistent with the proposed binding mode. Finally, the reduction wave shows a small shift relative to 6, which indicates that reduction was more facile. We ascribe this small shift to 9 (Figure 2A), where

Figure 2. Comparison of oxidation waves of (A) 5 (2 mM) and 7 (2 mM) against a mixture of 6 (2 mM) and Ph2BOH (2 mM) and (B) 6 (2 mM) and Ph2BOH (2 mM) against a mixture of 6 (2 mM) and Ph2BOH (1 mM). Same experimental conditions as Figure 1.

BACs derived from 3a–c were able to shift the oxidation potential relative to catechol 6 (Table 1). We correlated the measured oxidation potential of each BAC with the Hammett parameter for each substituent (σpara, Figure 3). Similarly, we calculated both the natural charge (Figure 3A) and the electrostatic potential (Figure 3B) on the boron and oxygen atoms for a wider series of functionalized BACs based on 3. Table 1 and Figure 3 revealed a surprising trend. Despite differences in the electron-

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donating and withdrawing ability of substituent R, as indicated by its Hammett parameter, very little change in the oxidation potential of 8 and 3a–c was observed for the CV experiments. Based on the variability in the calculated charge on the boron and oxygen atoms and the variability in the experimentally determined oxidation potential, it would appear that the identity of the R group might only be effective in altering the oxidation potential of the BAC on the order of several hundredths of volts. This result indicated that the electron-donating and withdrawing ability of the R group did not exert a strong enough influence to greatly alter the oxidation potential of the BAC. Table 1: Absolute peak potentials for oxidation. oxidation peak (V) 6, o-quinone

–0.79

8, R = H

–0.01

3a, R = OMe

–0.03

3b, R = t-Bu

–0.01

3c, R = F

+0.03

11

+0.12

12

+0.39

5, catechol

+0.91

R t-Bu δ O Bδ O δ

t-Bu

R

t-Bu δ δ O O Bδ O δ

t-Bu

t-Bu δ δ O O Bδ O δ

t-Bu

calculated natural charge (NBO, and )

A

OMe H t-Bu Me F

NMe2 1.06 1.04 1 0.96 0.92

O

CO2H

CN

11 12

NO2

0.5 natural charge on boron 0.25

3c 3a –0.70

0

3b 8

–0.25

–0.71 nat. charge on catechol oxygens

–0.72 –1

–0.5

0

0.5

1

oxidation peak potential (V, )

Figure 3. Comparison of computed charge (left axis) and experimentally determined oxidation peak potential (right axis). (A) Natural Bond Orbital natural charge calculated using B3LYP/6311+g(d,p). (B) Electrostatic potential, CHELPG calculated using HF/cc-PVDZ.

We speculate that the inability of the R substituent to dramatically influence the oxidation of BACs (8 and 3a–c) stems from the fact that the sp3 boron atom in the BACs prevents any resonance effects from the R substituent from reaching the oxygen atoms of the catechol. This effect is in contrast to the scale defined by the Hammett parameters—which were based on the acidity of benzoic acid—where para-substituents could directly interact with the carboxylic acid moiety via resonance with the carbonyl group. An important caveat to the discussion above is that the computed parameters in Figure 3 do not address how the affinity of the borinic acid changes with substitution. The Lewis acidity of the boron increases with more electron-withdrawing substitution. However, the CVs in Figure 2B show that an equimolar amount of Ph2BOH leads to a complete shift in the oxidation peak and a half equivalent of Ph2BOH shows the presence of only two distinct species in solution. We therefore conclude that there is tight binding between the borinic acid and 7. Additionally, based on the results of 8 and 3a–c, we assume that any changes in binding affinity of the borinic acid would only minimally affect the observed oxidation potential and result in a change on the order of several hundredths of volts. The results in Table 1 and Figure 3 drove us to investigate local inductive effects at the sp3 boron center by using benzoxaborole (to give BAC 11) or 2-carboxyphenylboronic acid (to give BAC 12). BACs 11 and 12 introduce a third oxygen substituent on the boron atom such that the formal negative charge is shared over three electronegative oxygen atoms instead of only two in 8 and 3a–c. Thus, the oxidation of 11 and 12 was anticipated to shift to a more positive potential relative to 8 and 3a–c. Indeed, the peak potential for the oxidation of BAC 11 was measured to be ~0.1 V less favorable than BAC 8 (Figure 4), signifying that less electron density was on the catechol system. Finally, the ester moiety in 12 was anticipated to withdraw even more electron density from the additional oxygen atom substituent. Accordingly, the oxidation potential of BAC 12 was measured to be ~0.4 V less favorable than BAC 8 (Table 1).

N/A

Hammett parameter (σpara)

calculated electrostatic potential (CHELPG charge, and )

B

OMe H t-Bu Me F

NMe2 1

CO2H

CN

11 12

NO2

0.5

CHELPG charge on boron

0.8 0.6

0.25

0.4

3c

0.2

3a

0 –0.2 –0.4

0

3b 8

–0.25

CHELPG charge on catechol oxygens

–0.6 –0.8 –1

–0.5

0

0.5

Hammett parameter (σpara)

1

N/A

oxidation peak potential (V, )

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The Journal of Organic Chemistry

Figure 4. Comparison of oxidation waves of 8 (2 mM) and 11 (2 mM). Same experimental conditions as Figure 1.

The conclusions of this study are that borinic acids can be used to increase the propensity towards oxidation of catechol through formation of boron ‘ate’ complexes. The boron atom in 8 and 3a–c appears to act as an insulator against significant alteration of the electron density on catechol via resonance effects

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(changes in the hundredths of volts). Instead, local inductive effects at the boron appear to be more effective in altering the electron density for oxidation (changes in the tenths of volts, 11 and 12). These observations are significant in the context of di-alkyl and di-aryl BACs in organic synthesis. Such complexes have received considerable attention as nucleophilic species at carbon, capable of C–C and C–heteroatom bond formation with electrophiles in the absence of transition metals.21 We propose that formulation of those di-alkyl and di-aryl BACs with catechol, and direct experimental measurement of the corresponding oxidation potential by CV, would provide a method to quantify the electron density in the complex. Presumably, this approach could create a universal measure to rank the nucleophilicity of BACs. Finally, catechol is an important feedstock chemical that is used as a synthetic intermediate towards numerous higher value products.22 Strategies to control the redox profile of o-quinone may aid in realizing new methods to exploit catechol. Alternatively, catechol derivatives (3,4-dihydroxyphenylalanine, DOPA) are common biological adhesives which have received increasing attention in recent years.23 The oxidation of catechol to o-quinone by catechol oxidase is a critical step in the curing process. This work demonstrates an alternative non-biological approach to inform the design of new adhesive systems. These examples highlight the potential for broad impact of tuning the redox chemistry of catechol through boron complexation.

EXPERIMENTAL SECTION General Information. The borinic acid precursors (Ar2BOH) to BACs 8 and 3a–d were synthesized following previously reported procedures.20 Structural identity was confirmed by comparison with previously reported NMR spectra. Silica gel (40 µm) was purchased from Grace Davison. All reagents and chemicals were purchased from commercial vendors and used without purification. Cyclic Voltammetry Experiments. Cyclic voltammetry was performed using an e-DAQ e-corder 410 potentiostat. Voltammograms were measured at a platinum disk electrode, in acetonitrile containing tetrabutylammonium hexafluorophosphate (100 mM) as supporting electrolyte at a scan rate = 100 mV s-1 unless noted otherwise. The average of 3 CV scans referenced to a Ag/AgNO3 standard electrode are shown in Figure S1–S4. Each component: catechol, o-quinone and Ar2B-OH was measured at a concentration of 2 mM unless noted otherwise. General Computational. All structures were optimized using the B3LYP24, 25 functional at the 6-311+g(d,p) basis set using the Gaussian g09 software.26 Solvent was modeled implicitly using the IEFPCM method27 with acetonitrile as the solvent. All optimized complexes verified to be minima with the lack of any imaginary frequencies found in the Hessian calculation. Energies reported are in hartrees/molecule without any zero-point correction enforced. The optimized structures were then subjected to NBO analysis28 of which natural charges were extracted from the boron and catechol oxygen and reported below. Electrostatic potential derived charges were also determined using the CHELPG method.29 See SI tabulated data.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website.

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CV and computational data (PDF).

AUTHOR INFORMATION Corresponding Author * [email protected].

Author Contributions † These authors contributed equally Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT Acknowledgement is made to the Donors of the American Chemical Society Petroleum Research Fund (57219DNI4) and Iowa State University of Science and Technology for support of this research.

DEDICATION This work is dedicated to Prof. Ronald T. Raines on the occasion of his 60th birthday.

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