Acid-Catalyzed Electron Transfer Processes in Naphthalene peri

Article pubs.acs.org/joc

Cite This: J. Org. Chem. 2018, 83, 11917−11925

Acid-Catalyzed Electron Transfer Processes in Naphthalene periDichalcogenides Tyler A. Tuck, David J. Press, Brandon LeBlanc, Todd C. Sutherland, and Thomas G. Back* Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada

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S Supporting Information *

ABSTRACT: 1,8-Naphthalene peri-dichalcogenides undergo protonation by Bronsted acids to produce electrophilic cations. Single electron transfer (SET) from the remaining unprotonated electron-rich peri-dichalcogenide to the cation then generates a radical cation and a radical. Thus, the formation of radical species results in severe peak broadening and coalescence of NMR signals when trifluoroacetic acid or other strong acids are added to the peri-dichalcogenide, and the process can be reversed by treatment with base. Further evidence for the formation of radicals stems from EPR, radical quencing with sodium dithionite, and computational experiments. The electron transfer is enhanced by the presence of 2,7-dialkoxy substituents that further increase the electron-donating ability of the dichalcogenides. This is an unusual example of a proton-coupled electron transfer process where an electron-rich molecule reacts with its own conjugate acid via a single electron transfer process.

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INTRODUCTION The naphthalene 1,8-peri-dichalcogenides 1a,1,2 1b2 and 1c2 (Chart 1) have been known for many years and are easily synthesized from 1,8-dibromonaphthalene via transmetalation with n-butyllithium,3 followed by treatment with elemental sulfur, selenium, or tellurium.2 These and closely related compounds have been studied extensively in several contexts, including their use as electron donors in charge transfer complexes and organic conductors.4 The ability of 1a and 1b and their derivatives to form complexes with various metals and metalloids via the corresponding dithiols and diselenols has been extensively studied by the groups of Woollins,5a−e Tilley,5f and Grainger.5g Woollins et al. have also reported the 77 Se solid-state NMR spectrum of 1b,6 as well as the X-ray crystal structures of 1a−c and related compounds.7 The oxidation of the 2,7-di-tert-butyl derivative of 1a to the corresponding cyclic thiosulfinate and α-disulfoxide, as well as rearrangements of the latter, have been investigated by Grainger et al.,8 while the trisulfide 2-oxide analogue of 1a and several derivatives were utilized for generating sulfur monoxide that was then trapped by dienes.9 Diselenide 1b and its diselenol analogue can also deiodinate thyroxine, as reported by Mugesh et al.10 In acyclic diaryl dichalcogenides, the preferred C−X−X−C (X = S, Se, or Te) dihedral angle is nearly orthogonal.11 Steric effects result in an increase in the dihedral angle, as in the case of diselenide 2 (Chart 1), where the angle was determined to be 112.1° and a bathochromic shift relative to less hindered acyclic dialkyl diselenides was observed in the UV−visible spectrum.12 It therefore appears that the HOMO was destabilized by unfavorable lone pair interactions and the HOMO−LUMO gap decreased as a result of the bulky substituents. The rigid, planar structures of the naphthalene © 2018 American Chemical Society

peri-dichalcogenides enforce departure from the preferred orthogonal geometry in the opposite sense by constraining their dihedral angles to essentially 0°, which again increases their HOMO energies and lowers their oxidation potentials.13 This behavior suggested that diselenide 1b could serve as a small-molecule mimetic of the antioxidant selenoenzyme glutathione peroxidase (GPx),14 which protects living organisms from the harmful effects of hydrogen peroxide and lipid hydroperoxides that are produced during the course of aerobic metabolism.15 It performs this function by catalyzing the reduction of such peroxides with the thiol glutathione, which is in turn converted into the corresponding disulfide. Because oxidation at selenium is the rate-determining step of the catalytic cycles of many types of GPx mimetics,16 the conformationally constrained diselenide 1b, with its low oxidation potential, was expected to undergo a faster overall catalytic cycle than that of acyclic diselenides. Indeed, catalysis with 1b proved to be ca. 13 times faster than that with diphenyl diselenide, while a further enhancement in the rate was obtained from the installation of electron-donating omethoxy substituents in 3b that further increase the HOMO energy through mesomeric effects.17 For similar reasons, the dichalcogenides 3a−c serve as effective electron donors in charge transfer complexes with acceptors such as TCNQ.13 The X-ray crystal structures of 3b and of its iron insertion complex with Fe3(CO)12 have also been reported.5g More recently, we extended these studies to the 2,7-di-n-pentyloxy analogues 4a−c, to observe how the longer alkyl chains would affect their optoelectronic properties. We noted that on several occasions the 1H NMR spectra of diselenides 3b and 4b in Received: July 16, 2018 Published: August 27, 2018 11917

DOI: 10.1021/acs.joc.8b01820 J. Org. Chem. 2018, 83, 11917−11925

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The Journal of Organic Chemistry Chart 1. Dichalcogenides with Constrained Dihedral Angles

Figure 1. (a) 1H NMR spectrum of recrystallized diselenide 4b in freshly opened CDCl3. (b) 1H NMR spectrum of diselenide 4b showing peak broadening.

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Figure 2. 1H NMR spectra of recrystallized 4b (0.015 mmol) in 0.6 mL of freshly opened CDCl3 in the absence and presence of varying amounts of TFA (1.0 μL = 0.013 mmol).

To confirm the hypothesis that the peak broadening was acid-catalyzed, we monitored the effects of varying amounts of added trifluoroacetic acid (TFA) upon the 1H NMR spectrum of 4b in CDCl3. Even the addition of just 1 μL of the acid caused immediate and severe broadening and collapse of the initially sharp aromatic and OCH2 methylene signals of 4b, as shown in Figure 2. Addition of more TFA eventually resulted in broadening of the other peaks in the pentyloxy side chains. Sulfuric acid produced similar behavior, while acetic acid required higher concentrations to have a comparable effect (see Supporting Information). The addition of pyridine-d5 to the sample that had been previously treated with TFA resulted in restoration of the original spectrum containing sharp signals with normal chemical shifts and coupling patterns (Figure 3). Furthermore, the addition of TFA to a solution of 4b in toluene-d8, where normally resolved peaks had been previously observed in the absence of the acid, caused immediate collapse of the signals (Supporting Information). Thus, the peakbroadening effect was clearly pH dependent, readily promoted by acids and inhibited by base. Similar experiments were conducted with the analogous disulfide 4a and ditelluride 4c, where the presence of TFA resulted in peak broadening similar to that observed with diselenide 4b. The 13C NMR spectrum of 4b in CDCl3 in the presence of TFA showed complete collapse and disappearance of the aromatic signals and of the downfield methylene peak from the side chains, while the 77Se NMR signal could not be observed at all. The unsubstituted diselenide 1b also displayed significant peak broadening but only at relatively high concentrations of TFA compared to the amount required for

CDCl3 displayed unexpected peak broadening and coalescence, while at other times the spectra showed normal sharp signals and couplings. To our knowledge, this NMR effect has not been previously reported for the naphthalene peridichalcogenides and we now describe our investigation into its cause.

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RESULTS AND DISCUSSION

We first considered that the peak-broadening effect could be due to aggregation phenomena. However, the effect was not observed in other solvents such as benzene-d6 or toluene-d8, and the NMR spectrum of diselenide 4b showed no significant concentration dependence, thus making aggregation an unlikely cause of the NMR behavior.18,19 Moreover, peak broadening was not abolished when the spectrum was recorded after exclusion of oxygen, thus ruling out the presence of paramagnetic radical species produced from single electron reduction of dioxygen by the diselenide as the cause of the NMR effect. Because chloroform that has been exposed to air for extended periods of time can contain traces of water and HCl,20a we reasoned that their adventitious presence in CDCl3 may have resulted in the occasionally observed NMR peakbroadening effect. On the other hand, samples of 4b purified by recrystallization20b and recorded in freshly opened CDCl3, or in CDCl3 pretreated with anhydrous potassium carbonate always gave normal (sharp) spectra (for typical normal and broadened spectra, see Figures 1, parts a and b, respectively). Similar effects were observed with the dimethoxy derivative 3b. 11919


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Figure 3. 1H NMR spectra of 4b (0.038 mmol) in 0.6 mL of CDCl3 after treatment with 1 μL of TFA (1.0 μL = 0.013 mmol) followed by increasing amounts of pyridine-d5 (1.0 μL = 0.015 mmol).

Scheme 1. Mechanism of Proton-Catalyzed Electron Transfer in Dichalcogenides 3 and 4

recalled that the methoxy-substituted dichalcogenides 3a−c have HOMOs with unusually high energies and low oxidation potentials (the oxidation potentials of 3a−c were determined to be 0.31, 0.18, and 0.060 V, respectively, vs Fc/Fc+).13 This is the result of conformational constraints imposed on the diselenide moiety by the rigidly planar naphthalene structure, as well as by the mesomeric effects of the electron-donating alkoxy groups. Thus, electron transfer from the electron-rich diselenide moieties of 3b and 4b to the electron-deficient

the di-n-pentyloxy diselenide 4b. Evidently, the absence of electron-donating alkoxy substituents lowers the basicity of the diselenide moiety in the required protonation step. These spectra are also provided in the Supporting Information. On the basis of the above results, we propose the mechanism shown in Scheme 1 for 3b and 4b, where the diselenide is first protonated by the acid catalyst to form cations 5b and 6b. This increases the electrophilic reactivity of the molecule toward electron transfer from a second molecule of 3b or 4b. It will be 11920


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intensity of the red color increased at higher TFA concentration. The UV−visible−near IR spectrum is shown in Figure 5, which reveals an increase in the intensity of the absorption at ca. 525 nm, along with a bathochromic shift to 544 nm at the highest TFA concentration. New absorptions at 469 and 822 nm, as well as a shoulder at 590 nm, appeared and intensified with increasing addition of TFA. To further validate the mechanism in Scheme 1, a series of computations were performed with the Gaussian 09 platform,26 using the DFT B3LYP geometry optimization method and the LANL2DZ basis set.27 The methoxy-substituted dichalcogenides 3a−c were chosen instead of 4a−c to simplify the computations. The differences in energies (ΔE) between the methoxy analogues of the products of electron transfer (radical cations 7a−c and radicals 9a−c) and the reactants (dichalcogenides 3a−c and cations 5a−c) were computed and are listed in Table 1. Table 2 provides computed X−X bond lengths and O···HX distances for 5 and 9, to indicate possible hydrogen bonding interactions with the methoxy group. Additional computational details are found in Supporting Information. These data reveal that the electron transfer from diselenide 3b to the corresponding cation 5b, as shown in Scheme 1, is only slightly endothermic, where ΔE was computed to be 0.3177 kcal mol−1 in the gas phase. This is comparable for the disulfide 3a in the gas state, with ΔE = 0.2288 kcal mol−1, but is more strongly endothermic in the case of ditelluride 3c, where ΔE = 9.6000 kcal mol−1. However, even in the less favorable equilibrium of the latter, it is likely that there is a sufficient concentration of paramagnetic species present to cause the observed peak broadening displayed in the analogous pentyloxy derivative 4c. The electron transfer process for diselenide 3b is slightly more endothermic in solution, especially in water, where ΔE = 1.5597 kcal mol−1, suggesting that solvation of cation 5b and, to a lesser extent, diselenide 3b, is more effective than solvation of radical cation 7b and radical 9b. The computed dichalcogen bonds (X−X) (Table 2) are longer in the naphthalene peri-dichalcogenides 3a−c than in typical acyclic derivatives.28 While the corresponding cations 5b and 5c and radical cations 7a−c have X−X bond lengths similar to those of the parent dichalcogenides 3b and 3c, the disulfide cation 5a and radicals 9a−c display particularly elongated X···X distances that are greater than or equal to 2.99 Å (Table 2). This indicates that these structures contain weakly coordinated X···X interactions rather than fully covalent bonds. Furthermore, the coplanar structures in 5a and 9a−c place the chalcogenol hydrogen atoms in relatively close proximity to the alkoxy oxygen atoms (between 1.99 Å in 5a to 2.61 Å in 9c), thereby permitting intramolecular hydrogen bonding. This contrasts with cations 5b,c, where the chalcogenol moiety adopts a roughly orthogonal orientation to the plane of the naphthalene rings and therefore is too far from the methoxy oxygen atom to support significant hydrogen bonding (Chart 2).

cations 5b and 6b is facile and generates the radical cations 7b and 8b, along with radicals 9b and 10b. These paramagnetic species then cause the observed broadening, coalescence, and eventual collapse of the NMR signals. A similar pathway can explain the results observed in the sulfur and tellurium analogues 4a and 4c, as well as for the diselenide 1b, although the absence of electron-donating alkoxy substituents in the latter renders the process less facile. The fate of the radical cations 8a−c and radicals 10a−c is not known with certainty, but it is most likely that they simply exist in equilibrium with 4a−c and 6a−c, as shown in Scheme 1. Alternatively, Seppelt et al.21 have reported that radical cations of acyclic diselenides and ditellurides can dimerize in the solid state, but this behavior was confined to alkyl derivatives and so is unlikely to occur in the present case. A third possible scenario is based on deprotonation of 10a−c to the corresponding radical anions, which could then undergo charge annihilation with radical cations 8a−c to regenerate two molecules of the original diselenides 4a−c. Further investigation would be required to test this possibility. Similar arguments apply to the dimethoxy-substituted species originating from 3a−c. The hypothesis of an electron transfer mechanism, as shown in Scheme 1, was confirmed by several additional experiments. The peak-broadening effect in 4b was reversed by the addition of sodium dithionite, which is known to reduce or scavenge radicals.22 Furthermore, an EPR spectrum of 4b was recorded in the presence of TFA, as shown in Figure 4. In related work,

Figure 4. EPR spectrum of diselenide 4b in CDCl3 upon treatment with TFA.

previous groups have reported EPR spectra derived from disulfide 1a in the course of their investigations of its chargetransfer complexes,23 while Bock, Meinwald et al.24 recorded EPR spectra of the radical cations produced when disulfide 1a and diselenide 1b underwent oxidation by the Lewis acid aluminum trichloride. The present process is also related to the broader category of proton-coupled electron transfer (PCET) reactions. However, in typical PCET processes both the proton and electron are usually transferred from the same molecule to a receptor molecule, although several mechanistic variations are known.25 Here protonation by a separate Bronsted acid is required to initiate the process. In addition to NMR peak broadening, a significant color change from dark purple to crimson was also observed upon protonation of diselenide 4b with TFA in chloroform. The

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SUMMARY AND CONCLUSIONS Unexpected peak broadening and coalescence phenomena were observed in the NMR spectra of naphthalene peridichalcogenides 1b, 3b, and 4a−c recorded in the presence of Bronsted acids. These effects were enhanced by the presence of electron-donating alkoxy substituents that interact mesomerically with the chalcogen atoms of these compounds, thereby lowering their oxidation potentials and allowing them 11921


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Figure 5. Effect of TFA addition upon the UV−visible−near IR spectrum of diselenide 4b. Spectra were recorded (bottom to top) after the addition of varying amounts of TFA to the cuvette. The inset shows a new absorption at ca. 822 nm in a vertical expansion of the spectrum. The sample contained the indicated amount of TFA (1.0 μL = 0.013 mmol) in a 1.74 mM solution of 4b in chloroform, made up to a total of 3.5 mL.

Table 1. Computed Energy Differences ΔE = [E(7) + E(9) − E(3) − E(5)] for Dichalcogenides 3a−c and Their Respective Cations 5a−c, Radical Cations 7a−c, and Radicals 9a−c dichalcogenide

ΔE (kcal mol−1)

3a (gas state) 3b (gas state) 3b (MeOH) 3b (H2O) 3b (CHCl3) 3c (gas state)

0.2288 0.3177 1.5286 1.5597 1.2312 9.6000

Chart 2. Possible Hydrogen Bonding in Protonated Dichalcogenides 5

conditions and from computational work that indicated the proposed single electron transfer process to be only slightly endothermic and therefore feasible under the conditions employed. While the naphthalene peri-dichalcogenides are well-known to produce charge transfer complexes with strong electron acceptors such as tetracyanoquinodimethane (TCNQ), the present phenomenon, where the same starting dichalcogenide serves as both the donor and acceptor (after protonation), has to our knowledge not yet been reported.

to serve as single electron reducing agents. The peakbroadening phenomenon requires the presence of an acid catalyst to protonate the dichalcogenide and render the resulting cation more electrophilic and susceptible to reduction by the remaining conjugate base. The rigorous exclusion of oxygen rules out its possible role as an electron acceptor in this process. Thus, the dichalcogenide serves as both the electron donor and, in its protonated form, as the acceptor. The effect can be reversed by the addition of a base such as pyridine-d5 or by the reducing agent sodium dithionite. The single electron transfer produces a radical cation from oxidation of the original dichalcogenide and a radical from reduction of its protonated form. The broadening and coalescence of NMR peaks is attributed to the presence of these paramagnetic species in the solution. Further corroboration of this mechanism was obtained from the EPR spectrum of diselenide 4b that confirmed the presence of radical species under acidic

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EXPERIMENTAL SECTION

The 1H, 13C, 77Se, and 125Te NMR spectra were recorded at 400, 101, 76, and 126 MHz, respectively. Diphenyl diselenide (463.0 ppm relative to dimethyl selenide)29 was employed as an external standard for 77Se NMR spectra, and diphenyl ditelluride (422 ppm relative to dimethyl telluride)30 was used similarly for 125Te spectra. In Figure 2, 1 H NMR spectra were recorded on ca. 0.6 mL of a 25 mM solution of 4b (0.015 mmol) to which were added the amounts of TFA (1.0 μL = 0.013 mmol) indicated in the figure. In Figure 3, a 63 mM solution of 4b (0.038 mmol) was used with amounts of TFA (1.0 μL = 0.013 mmol) and pyridine-d5 (1.0 μL = 0.015 mmol) as indicated in the

Table 2. Computed X−X Bond Lengths and O···HX Bond Distances in 3a−c and Corresponding Species 5, 7, and 9 compound 3a (gas state) 3b (gas state) 3b (MeOH) 3b (H2O) 3b (CHCl3) 3c (gas state)

X−X (Å) of 3 X−X (Å) of cation 5 2.33402 2.53640 2.53592 2.53600 2.53622 2.86877

2.98803 2.56071 2.54500 2.54437 2.55016 2.88980

O···HX (Å) of cation 5 X−X (Å) of radical cation 7 X−X (Å) of radical 9 O···HX (Å) of radical 9 1.99351 3.27533 3.33816 3.33817 3.33086 3.36523

2.29649 2.49298 2.48708 2.48679 2.49072 2.81582 11922

3.02297 3.02922 3.02294 3.02261 3.02519 3.26587

2.00654 2.48521 2.54750 2.54330 2.54269 2.61340 DOI: 10.1021/acs.joc.8b01820 J. Org. Chem. 2018, 83, 11917−11925

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The Journal of Organic Chemistry figure. Similar 1H NMR peak-broadening experiments induced by TFA were conducted on disulfide 4a and ditelluride 4b at concentrations of 31 mM and 48 mM, respectively. EPR spectra (9.4 GHz) were obtained at a temperature of 160 K with a 1.0 mM solution of 4b in CDCl3 containing sufficient TFA to cause peak broadening in its 1H NMR spectrum. The UV−visible−near IR spectra of 4b were recorded in a 5 mL cuvette containing a total of 3.5 mL of a 1.74 mM solution of 4b in chloroform to which had been added the amounts of TFA indicated in Figure 5. Computational details are provided in Supporting Information. Diselenide 1b was prepared according to the procedure of Meinwald et al.,2 and dichalcogenides 3a−c were prepared as reported in Supporting Information of our previous reports on naphthalene peri-dichalcogenides.13,17 Preparation of 2,7-Di-n-pentyloxynaphthalene (11).31 A mixture of potassium carbonate (3.427 g, 24.80 mmol), 2,7dihydroxynaphthalene (1.007 g, 6.287 mmol), 1-bromopentane (2.32 mL, 18.7 mmol), and tris(3,6-dioxaheptyl)amine (TDA-1) (0.20 mL, 0.63 mmol) in 20 mL of toluene was refluxed for 22 h. The mixture was washed with brine and dried over anhydrous MgSO4. The crude product was concentrated in vacuo and purified by flash chromatography (elution with hexanes) to afford 1.308 g (69%) of 2,7-di-n-pentyloxynaphthalene (11) as a white solid: mp 48−51 °C; IR (film) 2933, 2867, 1605, 1457, 1214, 1019, 838 cm−1; 1H NMR (400 MHz, CDCl3) δ 7.64 (d, J = 8.9 Hz, 2 H), 7.04 (d, J = 2.4 Hz, 2 H), 6.99 (dd, J = 8.9, 2.4 Hz, 2 H), 4.06 (t, J = 6.6 Hz, 4 H), 1.82 (crude pentet, J = 6.9 Hz, 4 H), 1.57−1.35 (m, 8 H), 0.96 (t, J = 7.2 Hz, 6 H); 13C NMR (101 MHz, CDCl3) δ 158.3, 136.6, 129.6, 124.7, 116.8, 106.6, 68.6, 29.6, 28.9, 23.1, 14.6; MS (EI-TOF) (m/z, %) 300 (M+, 85), 230 (20), 160 (100); HRMS (EI-TOF) calcd for C20H28O2 (M+): 300.2089; found: 300.2088. Preparation of 1,8-Dibromo-2,7-di(n-pentyloxy)naphthalene (12). N-Bromosuccinimide (9.41 g, 52.9 mmol) and pyridine (4.36 mL, 54.1 mmol) were dissolved in 250 mL of ethyl acetate and stirred for 1.5 h under an inert atmosphere. 2,7-Di-npentyloxynaphthalene (4.28 g, 14.2 mmol) was then added, and the reaction mixture was stirred at room temperature for 4 d. The mixture was washed with water and brine, dried over anhydrous MgSO4, and evaporated in vacuo. The crude product was purified by flash chromatography (elution with hexanes:ethyl acetate, 3:1 increasing to 3:2) to afford 4.10 g (62%) of 1,8-dibromo-2,7-di(n-pentyloxy)naphthalene (12) as a pale orange solid: mp 65−68 °C; IR (film) 2948, 2862, 1614, 1510, 1310, 1262, 1033, 810 cm−1; 1H NMR (400 MHz, CDCl3) δ 7.68 (d, J = 9.0 Hz, 2 H), 7.12 (d, J = 9.0 Hz, 2 H), 4.14 (t, J = 6.5 Hz, 4 H), 1.89 (crude pentet, J = 7.0 Hz, 4 H), 1.57− 1.50 (m, 4 H), 1.46−1.37 (m, 4 H), 0.95 (t, J = 7.3 Hz, 6 H); 13C NMR (101 MHz, CDCl3) δ 156.3, 132.1, 130.0, 127.5, 113.2, 107.0, 70.5, 29.3, 28.4, 22.6, 14.2; MS (EI-TOF) (m/z, %) 458 (M+, 70), 388 (10), 318 (100),; HRMS (EI-TOF) calcd for C20H26O279Br81Br (M+): 458.0279; found: 458.0287. Preparation of 2,7-Di-n-pentyloxynaphtho[1,8-cd]-1,2-dithiole (4a). n-Butyllithium (0.93 mL, 2.5 M in hexanes, 2.3 mmol) was added dropwise to 1,8-dibromo-2,7-di(n-pentyloxy)naphthalene (12) (380 mg, 0.829 mmol) in dry THF (15 mL) at −78 °C under nitrogen. After 1.5 h, the solution was warmed to room temperature. It was recooled to 0 °C, elemental sulfur (80 mg, 0.31 mmol based on S8) was added, and the mixture was stirred at room temperature for 6 h. The reaction was quenched with saturated ammonium chloride, washed with brine, dried over Na2SO4, and concentrated in vacuo. Recrystallization of the crude material from hexanes yielded the product as a red solid (165 mg, 55%), mp 73−74 °C; IR (film): 2942, 2910, 2857, 1610, 1495, 1252, 1210, 1029, 805 cm−1; 1H NMR (400 MHz, CDCl3); δ 7.36 (d, J = 8.8 Hz, 2 H), 6.98 (d, J = 8.8 Hz, 2 H), 4.13 (t, J = 6.6 Hz, 4 H), 1.80 (crude pentet, J = 7.0 Hz, 4 H), 1.51− 1.35 (m, 8 H), 0.95 (t, J = 7.1 Hz, 6 H); 13C NMR (101 MHz, CDCl3); δ 149.4, 137.5, 127.0, 126.1, 123.3, 114.0, 69.5, 29.1, 28.1, 22.4, 14.0; HRMS (EI-TOF) calcd for C20H26O2S2 (M+): 362.1370, found: 362.1374. Anal. calcd for C20H26O2S2: C 66.25, H, 7.23, found, C 66.51, H, 7.20.

Preparation of 2,7-Di-n-pentyloxynaphtho[1,8-cd]-1,2-diselenole (4b). 1,8-Dibromo-2,7-di(n-pentyloxy)naphthalene (12) (1.050 g, 2.291 mmol) was treated with n-butyllithium (2.54 mL, 2.5 M in hexanes, 6.4 mmol) as in the preceding procedure. The solution was cooled to 0 °C, and elemental selenium (380 mg, 4.81 mmol) was added, followed by stirring at room temperature overnight. The reaction was quenched with ammonium chloride, and air was bubbled through the mixture for 30 min. The solution was then extracted with ethyl acetate, and the organic extracts were combined, dried over anhydrous Na2SO4, and concentrated in vacuo. Recrystallization of the crude product from hexanes yielded 619.5 mg (59%) of diselenide 4b as purple crystals; mp 91−92 °C; IR (film) 2951, 2911, 2851, 1609, 1503, 1257, 1024, 805 cm−1; 1H NMR (400 MHz, CDCl3) δ 7.51 (d, J = 8.5 Hz, 2 H), 6.94 (d, J = 9.0 Hz, 2 H), 4.13 (t, J = 6.6 Hz, 4 H), 1.82 (crude pentet, J = 6.9 Hz, 4 H), 1.57− 1.34 (m, 8 H), 0.95 (t, J = 7.2 Hz, 6 H); 13C NMR (101 MHz, CDCl3) δ 152.6, 139.9, 127.8, 125.5, 123.5, 113.0, 69.4, 29.1, 28.2, 22.4, 14.0; 77Se NMR (76 MHz, CDCl3) δ 405.3; MS (EI-TOF) (m/ z, %) 458 (M+, 100), 387 (40), 315 (45); HRMS (EI-TOF) calcd for C20H26O280Se2: (M+), 458.0263; found: 458.0256. Anal. Calcd for C20H26O2Se2: C, 52.64; H, 5.74; found: C, 52.30; H, 5.59. Preparation of 2,7-Di-n-pentyloxynaphtho[1,8-cd]-1,2-ditellurole (4c). 1,8-Dibromo-2,7-di(n-pentyloxy)naphthalene (12) (557 mg, 1.22 mmol) was treated with n-butyllithium (1.36 mL, 2.5 M in hexanes, 3.4 mmol) as in the preceding procedure. Elemental tellurium (434 mg, 3.40 mmol) was then added at 0 °C, the solution was stirred at room temperature overnight, and the reaction was quenched and worked up as in the case of diselenide 4b. The crude solid was recrystallized from hexanes to afford 439 mg (65%) of ditelluride 4c as a green solid (439 mg, 65%); mp 93−94 °C (decomp); IR (film) 2952, 2905, 2857, 1614, 1495, 1267, 1038, 805 cm−1; 1H NMR (400 MHz, CDCl3); δ 7.67 (d, J = 8.7 Hz, 2 H), 6.89 (d, J = 8.7 Hz, 2 H), 4.15 (t, J = 6.5 Hz, 4 H), 1.83 (crude pentet, J = 6.9 Hz, 4 H), 1.57−1.36 (m, 8 H), 0.96 (t, J = 7.2 Hz, 6 H); 13C NMR (101 MHz, CDCl3); 157.3, 146.4, 130.0, 128.4, 111.5, 107.4, 69.4, 29.2, 28.3, 22.4, 14.1; 125Te (126 MHz, CDCl3); δ 237.8; HRMS (ESI); calcd for C20H26O2128Te2 (M+): 554.0028; found: 554.0036.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.8b01820.

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NMR spectra, procedures for sodium dithionite and oxygen exclusion experiments, computational details, and references (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Thomas G. Back: 0000-0002-3790-1422 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Natural Sciences and Engineering Research Council of Canada (NSERC) for financial support via grant RGPIN-06670. D.J.P. and B.L. thank NSERC for postgraduate scholarships, and D.J.P. also thanks Alberta Innovates − Technology Futures for scholarship support. We are grateful to Professor Tris Chivers for useful discussions and Mr. Wade White for assistance in recording the EPR spectrum in Figure 4. 11923


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length of 3b was similarly determined to be 2.3552(3) Å in the solid state (reference 5g), suggesting that the present computations are overestimating the Se−Se bond length. (29) Duddeck, H. Selenium-77 Nuclear Magnetic Resonance Spectroscopy. Prog. Nucl. Magn. Reson. Spectrosc. 1995, 27, 1−323. (30) Luthra, N. P.; Odom, J. D. In The Chemistry of Organic Selenium and Tellurium Compounds; Patai, S.; Rappoport, Z., Eds.; John Wiley & Sons: Chichester, 1986; Vol. 1, Chapter 6. (31) Hori, A.; Takeda, H.; Premkumar, J. R.; Sastry, G. N. 1:1 and 2:1 Cocrystallizations of Alkoxy-Substituted Naphthalene Derivatives with Octafluoronaphthalene Through Arene−Perfluoroarene Interactions. J. Fluorine Chem. 2014, 168, 193−197.

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Acid-Catalyzed Electron Transfer Processes in Naphthalene peri

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