INTRODUCTION

Stable nitrogen-centered radical cations can be smoothly electrogenerated in situ from triarylamines bearing substituents in the para position (1, 2), as early testified by the formation of trianisylamine radical cation, that was in turn employed as the redox catalyst in the oxidation of cyanide anions (3). Throughout the years, tris(2,4-dibromophenyl)amine radical cation (also known as Magic Green, MG, today commercially available) has been widely employed as an oxidizing reagent and tris(2,4-dibromophenyl)amine can be considered as the precursor of the corresponding radical cation.

Apart from the use in mechanistic studies (see below), some technological applications of MG have been recently developed, including, among the others, its use in measuring the overcharge protection in lithium-ion batteries (LIBs) (4). The species was also used as a co-reactant in producing an intense blue light emission of corannulene derivatives (5).

Recently, electrochemistry has emerged as sustainable approach to access chemical intermediates (including radical ions and radicals) under mild conditions and to perform reactions that it is not possible to be accomplished by other synthetic approaches (6, 7).

In this context, radical cations deriving from triarylamines such as tris(4-bromophenyl)amine and tris(2,4-dibromophenyl)amine have been employed as efficient redox mediators for the gem-difluorodesulfurization reaction of dithioacetals (8) for the monodesulfurization of phenylthio-β-lactams (9) and S-aryl thiobenzoates (10) and as redox mediator in the efficient conversion of substituted S-phenyl thiobenzoate into phenyl benzoates (11).

On the other hand, the electrochemical generation of tris(2,4-dibromophenyl)amine radical cation was exploited in the spectroscopic detection of short-lived aromatic intermediates deriving from anthracene derivatives and N-methyldiphenyl- and diphenylamine by means of electron transfer stopped-flow (ETSF) methods (12). Likewise, the same authors described in details the kinetic of the electron transfer process between a series of 9-substituted anthracene and tris(2,4-dibromophenyl)amine radical cation electrogenerated in situ and the reactivity of the anthracene derivative radical cations against nucleophiles like water and methanol applying electron transfer stopped-flow (ETSF) methodology (13).

This method was also extended to the evaluation of the reactivity of a series of mono- and dicationic states of meta-connected oligoarylamines and characterization of the species through their absorption spectra (14).

When stabilized as the hexachloroantimonate salt (15), MG is often employed oxygenation reactions in apolar solvents. As an example, the reaction of 4,4-dimethyladamantylidene with molecular oxygen in the presence of catalytic amounts of MG (5 mol%) in dichloromethane at −78°C provided 4,4-dimethylspiro[adamantane-2,3’-[1,2]-dioxetane] as the primary photoproduct that in turn is converted at room temperature to 2-methyladamantyl-methyl ketone (16). Likewise, the monoelectronic oxidation of hindered olefins by the aminium salts have been investigated at both low and room temperature in dichloromethane and a series of dioxetanes, epoxides, ketones or allylic derivatives were formed in good to excellent yields (17).

The cyclopropanation reaction of a series of m- and p-substituted trans-3-methylstyrenes with diazoacetate has been carried out with commercially available MG aminium salt. The thermal reaction provided the cyclopropyl styrenes in good yields as a mixture of anti:syn diastereomers (18, 19). Another synthetic example involves the use of tris(2,4-dibromophenyl)aminium hexachloroantimonate in the preparation of 1-(2-pyridyl)-1,2,4-triazoles from 2-pyridyl hydrazones (20).

An alternative approach to triarylaminium radical cations is the photoionization of triarylamines, and in this case, depending on the substitution pattern of the aryl moiety, cyclization to the corresponding carbazoles has been also observed (21-23). In this context, direct irradiation of tris(4-bromophenyl)amine led to the formation in situ of the corresponding radical cation known as the Magic Blue that was described to cyclize providing carbazole derivatives (23). Further, direct irradiation of structurally related di- and triaryl amines was reported to give smoothly the corresponding carbazole derivatives through an intramolecular cyclization reaction of the radical cation (21-23). Surprisingly, the photochemistry of tris(2,4-dibromophenyl)amine, which is the precursor of tris(2,4-dibromophenyl)aminium radical cation, has not been reported in the literature yet. Therefore, we embarked to study systematically the photochemical behavior of tris(2,4-dibromophenyl)amine in different polar and nonpolar solvents, under nitrogen and oxygen atmospheres, and in the presence of nucleophilic additives such as aryl and alkyl sulfides and triarylphosphines. Furthermore, the direct irradiation of tris(2,4-dibromophenyl)amine in the presence of the additives under oxygen-saturated solvents led to the preparation of the corresponding sulfoxides and phosphine oxides.

MATERIALS AND METHODS Materials

Tris(2,4-bromophenyl)amine (1), thioanisole, p-methoxythioanisole, diphenyl sulfide, triphenylphosphine, triphenylphosphine oxide, tris(2-methylphenyl)phosphine, diphenyl sulfoxide, methyl phenyl sulfoxide, p-methoxyphenyl methyl sulfoxide and benzaldehyde were commercially available. Sulfoxides and triarylphosphine oxides used as reference compounds were prepared by reported procedures (25, 26). The benzyl ethyl sulfide and the corresponding sulfoxide were prepared by reported procedures (27, 28).

Co-Oxidation reactions

The co-oxidation experiments were performed by using 1 (5.0 × 10−3 m) in the presence of 0.01 m solutions of the chosen sulfide or triarylphosphine in different solvents (2,2,2-trifluoroethanol, acetonitrile, acetonitrile/water mixture and dichloromethane). The solutions were contained in rubber-stoppered, 1-cm-diameter quartz tubes, and a stream of dry oxygen saturated with the appropriate solvent was passed into the solution through a needle for 10 min in the dark. The quartz tubes were exposed to 10 phosphor-coated 15 W lamps (Rayonet) emitting at 366 nm. The products were assessed by GC on the basis of calibration curves in the presence of cyclododecane as the internal standard.

Time-resolved laser flash spectroscopy

The laser pulse photolysis apparatus consisted of a Flash lamp-pumped Q-switched SpitLight-100 Nd:YAG laser from InnoLas used at the fourth harmonic of its fundamental wavelength. The LP920-K monitor system (supplied by Edinburgh Instruments), arranged in a cross-beam configuration, consisted of a high-intensity 450 W ozone free Xe arc lamp (operating in pulsed wave), a Czerny-Turner with triple grating turret monochromator and a five-stage dynode photomultiplier. The signals were captured by means of a Tektronix TDS 3012C digital phosphor oscilloscope, and the data were processed with the L900 software supplied by Edinburgh Instruments. The solutions to be analyzed were placed in a fluorescence cuvette (d = 10 mm).

Nuclear magnetic resonance spectroscopy

1H and 13C NMR spectra were recorded in CDCl3 on a 300 MHz spectrometer; chemical shifts (δ) are reported in parts per million (ppm), relative to the signal of tetramethylsilane, used as an internal standard. 2D NMR spectra (heteronuclear single quantum correlation (HSQC) and heteronuclear multiple quantum correlation (HMQC) sequences) were recorded in CDCl3 on 300 MHz and 500 MHz spectrometers. Coupling constant (J) values are given in hertz (Hz). The measurements were carried out using standard pulse sequences.

Ion chromatography analysis

Ion chromatography analyses have been performed by means of a Dionex GP40 instrument equipped with a conductimetric detector (Dionex 20 CD20) and an electrochemical suppressor (ASRS Ultra II, 4 mm) by using the following conditions: chromatographic column IONPAC AS23 (4 × 250 mm), guard column IONOPAC AG12 (4 × 50 mm), eluent NaHCO3 0.8 mm + Na2CO3 4.5 mm, flux: 1 mL min−1; current imposed at detector: 50 mA.

RESULTS Photochemistry of tris(2,4-dibromophenyl)amine in solution

The photolysis of 1 was carried out in both nitrogen- and oxygen-saturated acetonitrile; analysis of the photolyzed mixture pointed out the presence of bromocarbazole derivatives as main photoproducts, along with HBr (see Scheme 1 and Figures S1–S10). The formation of bromocarbazoles points out that a photoinduced cyclization reaction is involved and the main photoproduct is compound 1a when compound 1 is consumed in ca 50%. Furthermore, compound 1a was detected by GC-MS in the photolyzed reaction mixture.

Irradiation of 1 in acetonitrile solution and product distribution.

The course of the photoreaction of compound 1 in O2-saturated acetonitrile solution monitored by UV-visible spectroscopy is detailed in Fig. 1, where it is apparent that cyclization reaction to provide carbazole moiety (that is responsible for the absorption bands located in the 340–360 nm region) is the main process. The consumption profiles of 1 measured in N2- and O2-saturated acetonitrile solutions are overlapped, indicating that there is no effect of the atmosphere chosen for carrying out the photoreaction (see Fig. 1b). Furthermore, comparing the consumption profile of 1 in O2-saturated acetonitrile with the consumption profiles of tris(p-bromophenyl)amine and triphenylamine, respectively, measured in the same experimental conditions (Fig. 1b) led to conclude that the cyclization reaction of 1 under oxygen atmosphere is slower than that of the two other amines. This behavior can be ascribed to the quenching of the triplet excited state of compound 1 by molecular oxygen, which is the photoreactive excited state inhibiting in part the photoreaction due to singlet oxygen production (23). The population of the triplet excited state of 1 is favored because of the known heavy atom effect (HAE) of the bromine atoms attached on the phenyl moieties (29). A similar oxygen effect was observed for the photochemical reaction of tris(p-bromophenyl)amine and triphenylamine in different solvents. In the case of triphenylamine, the quenching of the triplet excited state by molecular oxygen produced singlet oxygen with a quantum yield (ϕΔ) value of 0.63, which is a competitive pathway of the electrocyclization reaction (24). Therefore, the photoreaction of triphenylamine is faster than that of tris(p-bromophenyl)amine and the photoreaction of this latter substrate is faster than that of 1 (see Fig. 1b). This is because the efficiency of singlet oxygen production is expected to increase as the number of bromine atoms attached to the phenyl moiety increases.

(a) Variation of the UV-visible spectra of 1 (λexc = 310 nm) in oxygen-saturated acetonitrile. (b) Kinetic profiles of consumption of: (○) 1 (N2); (▼) 1 (O2); (∆) tris(p-bromophenyl)amine (O2) and (□) triphenylamine (O2) in acetonitrile (λexc = 310 nm).

When the photochemical reaction of 1 was carried out in oxygen-saturated acetonitrile and dichloromethane solutions, bromocarbazole derivatives such as 1b and 1c were also formed and bromide ion was detected in the reaction mixture at long period of irradiation time (t > 60 min) as can be seen in Fig. 2. Bromocarbazole 1b was isolated from the photolyzed reaction mixture through column chromatography and fully characterized by NMR spectroscopy, whereas bromocarbazole 1c was detected by GC-MS in the photoreaction mixture. The formation of bromide ion in the reaction mixture was ascribed to secondary processes that are involved during the photoreaction of 1. Indeed, compound 1b and the regioisomers 1c are clearly formed because of a photoinduced C-Br fragmentation of bromocarbazole 1a and consequently bromide ion is released to the solution. A similar behavior was also observed under nitrogen atmosphere. Photoinduced homolytic fragmentation of the C(sp2)-Br bond in aromatic compounds is a known photochemical pathway that is also observed in heteroaryl bromide derivatives (30). Generally, in the case of chloroarenes (benzene, biphenyl, and naphthalene moieties) the excited singlet state possesses sufficient energy to produce C-Cl homolytic fragmentation, whereas the triplet state does not display sufficient energy to promote the homolytic fragmentation (31). Photoinduced heterolytic fragmentation of C(sp2)-X of polyhaloarenes can also occur from the triplet excited state in the presence of a one-electron donor, that is triethylamine, involving a photoinduced electron transfer process and producing the radical anion of the polyhaloarene (31).

Conversion of amine 1 (○) and yield of bromide ion (□) profiles measured under oxygen-saturated acetonitrile solution (λexc = 366 nm).

Time-resolved absorption spectroscopy of amine 1 in acetonitrile and dichloromethane, respectively, was carried out under N2 and O2 atmospheres. A broad band centered at 400 nm was detected in N2-saturated acetonitrile solution, whereas two absorption bands, the former centered at 410 nm and the latter as a shoulder starting at 700 nm, were detected in dichloromethane (see Fig. 3). These bands were assigned to the UV-visible absorption spectra of two transient species, viz. radical cation 1·+ and radical cation polybromosubstituted dihydrocarbazole (2·+) (see Scheme 2) (23, 24). A similar behavior was observed when the experiments were performed under O2 atmosphere.

Time-resolved absorption spectra recorded after a laser pulse of 355 nm of amine 1 (3.0 × 10−3 m) transients in: (a) acetonitrile and (b) dichloromethane in nitrogen atmosphere.

Formation of transients 1•+ and 2·+ in nitrogen-equilibrated media.

The decay traces and the grow-in trace were measured under N2- and O2-saturated acetonitrile and dichloromethane solutions of 1 at λabs = 400 nm showing mono-exponential decay and grow-in curves (see Fig. 4). Applying a nonlinear regression fitting, the lifetime (τ) values were easily obtained and are collected in Table 1. The lifetime values were assigned to the intramolecular cyclization process (τcyc) of the radical cation 1·+ leading to the radical cation bromodihydrocarbazole (2·+) in acetonitrile and dichloromethane, which show very long lifetime values (τ > 1000 μs, see Scheme 3). Another bromodihydrocarbazole radical cation intermediate (2’·+) can be formed involving a cyclization pathway since the radical cation 1·+ is an unsymmetrical intermediate (see Scheme 3). However, the sequential pathways followed by intermediate 2’·+ involve the elimination of bromonium cation (Br+), which is unlikely in comparison with the sequential pathways involved for intermediate 2·+.

Decay traces and grow-in trace of transient spectra arising from 1, recorded at λabs = 400 nm in (a) N2- and O2-saturated acetonitrile and (b) in N2-saturated dichloromethane after a laser pulse of 355 nm.

Table 1. Lifetime (τ) of radical cation 1•+, cyclization rate constants (kcyc) and concentration of radical cation 1•+ at zero time after the laser pulse. Solvent Atmosphere τ/μs kcyc × 104/s−1 [1·+]/m MeCN N2 22 ± 3 4.5 ± 0.2 1.8 × 10−7 O2 25 ± 4 4.0 ± 0.6 1.3 × 10−7 DCM N2 35 ± 5 2.9 ± 0.4 1.9 × 10−7 O2 41 ± 6 2.4 ± 0.4 1.5 × 10−7

Proposed electrocyclization of 1·+ and sequential pathways of intermediates 2·+ and 2’·+.

The cyclization rate constants kcyc of the radical cation 1·+ to the bromodihydrocarbazole intermediate (2·+) were easily calculated as the reciprocal of the lifetime values because the cyclization reaction is a unimolecular reaction. The cyclization rate constants kcyc are also collected in Table 1.

The kcyc rate constants were also measured under oxygen-saturated acetonitrile and dichloromethane solutions, and it was found that the values were similar to those obtained under nitrogen atmosphere. However, the rate constants measured in dichloromethane show values that are halved in comparison with those measured in acetonitrile (see Table 1). On the other hand, the concentration of the radical cation 1·+ at zero time after the laser pulse has been measured in both solvents (Table 1) showing a rather dependence on the atmosphere used. Indeed, the concentration of the radical cation measured under oxygen atmosphere is lower than in nitrogen, which was ascribed to the quenching of the triplet excited state of amine 1 by molecular oxygen.

Co-oxidation of sulfides and phosphines by tris(2,4-dibromophenyl)amine (1) in O2-saturated solution

The oxidative properties of transient 1·+ and 2·+ have been tested by irradiating oxygen-saturated solutions of 1 in the presence of sulfides and phosphines that can be easily oxidized by the photogenerated transients (see Scheme 4). In the first case, 1 is consumed during irradiation, while the sulfides were co-oxidized selectively to the corresponding sulfoxides (see Table 2) with rates ranging from 0.004 to 0.332 μmol min−1. Notably, diphenyl sulfide (Ph2S), that is inert toward singlet oxygen (32), was oxidized. When using benzyl ethyl sulfide, significant amounts of benzaldehyde were also detected in dichloromethane and acetonitrile.

Co-oxidation of sulfides and triarylphosphines in the presence of 1.

Table 2. Rate of co-oxidation of sulfides upon irradiation in the presence of 1. Substrate Solvent Rate of reaction (μmol.min−1) Sulfoxide 1 b Other products Ph2S MeCN 0.004 0.022 MeCN – H2O (9:1) 0.019 0.033 TFE 0.073 0.056 CH2Cl2 0.038 0.030 PhSMe MeCN 0.015 0.019 MeCN – H2O (9:1) 0.076 0.047 TFE 0.042 0.046 CH2Cl2 0.026 0.043 p-MeOC6H4SMe MeCN 0.025 0.012 MeCN – H2O (9:1) 0.179 0.039 TFE 0.332 0.075 CH2Cl2 0.034 0.020 PhCH2SEt MeCN 0.086 0.025 PhCHO; 0.032 MeCN – H2O (9:1) 0.164 0.035 TFE 0.101 0.101 CH2Cl2 0.109 0.093 PhCHO; 0.035

The results summarized in Table 3 describe the selective co-oxidation of triphenylphosphine and tris (o-tolyl)phosphine to the corresponding phosphine oxides (see Scheme 4) that occurs in up to 8.53 μmol min−1, while no consumption of 1 was detected in all the solvents studied. Irradiation of both substrates in the presence of oxygen but without 1 did not provide any sulfoxides or phosphine oxides. Likewise, irradiations under a nitrogen atmosphere but in the presence of 1 did not give any oxidation products.

Table 3. Rate of co-oxidation of phosphines upon irradiation in the presence of amine 1. Substrate Solvent Rate of reaction (μmol.min−1) Phosphine oxide 1 b Ph3P MeCN 1.34 0 MeCN – H2O (9:1) 1.12 0 TFE 3.30 0 CH2Cl2 3.25 0 (o-MeC6H4)3P MeCN 2.77 0 MeCN – H2O (9:1) 3.30 0 TFE 3.53 0 CH2Cl2 3.44 0 Time-resolved spectroscopy analysis of tris(2,4-dibromophenyl)amine (1) in the presence of sulfides and phosphines

Time-resolved spectroscopy of 1 in the presence of selected phosphines and sulfides was carried in N2- and O2-saturated acetonitrile and dichloromethane solutions, respectively. Thus, triphenylphosphine and tris(o-tolyl)phosphine were found to quench efficiently radical cation 1·+ and consequently inhibition of the formation of radical cation 2·+ was observed as shown in Fig. 5a. Furthermore, triethylamine (TEA) was also employed as the electron donor to quench the radical cation 1·+ (magenta line in Fig. 5a) and subsequently inhibiting the formation of intermediate 2·+. On the other hand, no quenching of the radical cation 1·+ was observed when sulfides such as diphenyl sulfide, thioanisole, p-methoxy thioanisole and benzyl ethyl sulfide were used as quenchers and no inhibition of the formation of transient 2·+ occurred for the case of p-methoxythioanisole (see Fig. 5b: compare black curve with blue curve in the figure). This spectroscopic behavior led to conclude that radical cation 1·+ is not able to oxidize the sulfides to the corresponding sulfoxides but cyclizes competitively to radical cation 2·+. However, there is another oxidizing intermediate formed by the reaction of radical cation 2·+ and molecular oxygen that displays a long lifetime undetectable by laser flash photolysis but is able to oxidize efficiently sulfides to the corresponding sulfoxides as it was observed under preparative conditions.

(a) Decay traces of radical cation 1·+ transient (1.0 × 10−3 m) recorded at λabs = 380 nm under O2-saturated dichloromethane after a laser pulse of 355 nm in the presence of PPh3 (1.2 × 10−3 m) (black line); (o-Tol)3P (1.2 × 10−3 m) (blue line); and triethylamine (3.4 × 10−3 m) (magenta line). (b) Decay traces of radical cation 1·+ transient (1.0 × 10−3 m) recorded at λabs = 380 nm under O2-saturated dichloromethane after a laser pulse of 355 nm in the absence of quencher (black line) and in the presence of p-MeOPhSMe (1.2 × 10−3 m) (blue line).

The quenching of radical cation 1·+ by triphenylphosphine is depicted in Fig. 6, showing that the observed first-order decay constant (kobs) of 1·+ depends linearly on the quencher concentration according to Eq. (1).

kobs=kcyc+kETQ(1)

Quenching of radical cation 1•+ with increasing concentrations of triphenylphosphine (○). Straight line is the best linear regression fit (r2 > 0.98).

Plotting kobs versus triphenylphosphine concentration resulted in a linear correlation (see Fig. 6), and the bimolecular rate constant kET of 1.4 × 105 m−1 s−1 was obtained from the slope of the linear regression. The kET values measured for triphenylphosphine, tris(o-tolyl)phosphine and triethylamine in different experimental conditions are collected in Table 4. As it is apparent from the table, the rate constants kET range from 104 to 106 m−1 s−1 including the data measured for triethylamine, and do not depend on the nature of the solvent. However, the rate constants measured under O2 atmosphere show values rather lower than those obtained under nitrogen flushed solutions.

Table 4. Quenching rate constants (kET) of radical cation 1·+ in different conditions. Quencher Solvent Atmosphere kET/m−1 s−1 Ph3P MeCN N2 2.3 × 106 Air 1.4 × 105 O2 2.8 × 106 Ph3P DCM N2 3.2 × 105 O2 2.4 × 105 (o-CH3C6H4)3P MeCN N2 4.9 × 105 (o-CH3C6H4)3P DCM N2 3.4 × 105 O2 3.3 × 105 Et3N DCM N2 7.5 × 104 DISCUSSION Photochemistry of tris(2,4-dibromophenyl)amine (1) under N2- and O2-saturated solutions

Direct irradiation of amine 1 with a laser pulse (355 nm) generates two key intermediates in both N2- and O2-saturated solutions, viz. radical cations 1·+ and 2·+, and it was found that radical cation 2·+ reacts efficiently with molecular oxygen as depicted in Scheme 5. Ionization of photo-excited triplet state of amine 1 under nitrogen atmosphere forms the radical cation (1·+) (path (a)). Two competitive pathways consumed 1·+. One involves the back-electron transfer process (path (c)) giving back the amine 1 with an estimated diffusional rate constant of 1010 m−1 s−1. The other pathway involves the intramolecular cyclization reaction (path (b)) giving brominated dihydrocarbazole radical cation 2·+ with a rate constant of 4.5 × 104 s−1 and 2.9 × 104 s−1 in acetonitrile and dichloromethane, respectively. Next, intermediate 2·+ loses two protons and oxidizes amine 1 to radical cation 1·+ (path (d)) to give polybromocarbazole 1a as the primary photoproduct. Finally, direct irradiation of compound 1a under preparative conditions produces debromination photoproducts (1b and 1c) because bromide ion was detected at long period of irradiation time (see Fig. 2). The photocyclization of di- and triarylamines is a known pathway to convert these substrates into carbazoles (23, 29, 33). The visible irradiation of triarylamines in the presence of a copper-based sensitizer provided a family of substituted carbazoles, and the proposed mechanism involved the formation of a triarylamine radical cation that in turn cyclized to the corresponding dihydrocarbazole. This intermediate then collapsed to bromo carbazole through oxidation and re-aromatization of the intermediate (34). Further photoinduced debromination of the primary photoproduct provides the bromocarbazole observed in the reaction mixture.