Two-phase oxidation of toluene derivatives by dioxygen using the 3-cyano-1-decylquinolinium ion as a photocatalyst

Abstract

The two-phase photocatalytic oxidation of toluene and p-xylene by dioxygen occurred efficiently using 3-cyano-1-decylquinolinium hexafluorophosphate (DeQuCN⁺PF₆⁻) as an organic photocatalyst. When toluene and p-xylene were used as the solvent with 2% H₂O, the oxygenated products were produced in the organic phase and hydrogen peroxide was produced in the aqueous phase at high turnover numbers.

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The photocatalytic oxidation of toluene and its derivatives by dioxygen with various organic photocatalysts under light irradiation has attracted increasing attention as an atom-economic and environmentally benign alternative to classical oxidation methods using inorganic oxidants.^1–4 Hydrogen peroxide (H₂O₂), which is the reduced product of dioxygen, often degrades organic photocatalysts under light irradiation, precluding a high turnover.^5,6 The stability of riboflavin tetraacetate as an organic photocatalyst has been shown to be enhanced by combination with a non-haem iron catalyst, which dissociates H₂O₂.⁷ H₂O₂ is an ideal alternative energy carrier to hydrogen because an aqueous solution of H₂O₂ can be used instead of gaseous hydrogen as an fuel in a one-compartment fuel cell to generate electricity.^8–10 If H₂O₂ can be separated from the reactant solution as soon as it is formed, the degradation of organic photocatalysts is avoided. However, such a separation of H₂O₂ during the photocatalytic oxygenation of substrates has yet to be achieved.

We report here the two-phase photocatalytic oxidation of toluene by dioxygen with 3-cyano-1-decylquinolinium hexafluorophosphate (DeQuCN⁺PF₆⁻) as an organic photocatalyst. The reaction was carried out in toluene with H₂O (2%); the oxygenated products were produced in the toluene and H₂O₂ was produced in the aqueous phase with high turnover numbers (Fig. 1).¹¹ Toluene could be replaced by p-xylene when higher concentrations of H₂O₂ were obtained in the aqueous phase. Such a two-phase system has been reported previously for the catalytic reduction of O₂ to produce H₂O₂.^12–14 However, the two-phase photocatalytic oxidation of substrates has not been reported for the production of H₂O₂ in an aqueous phase.

Fig. 1 Chemical structure of DeQuCN⁺PF₆⁻ and the two-phase photocatalytic oxidation of toluene by O₂ with DeQuCN⁺. The oxygenated products accumulate in the organic phase, whereas H₂O₂ is accumulated in the aqueous phase.

DeQuCN⁺PF₆⁻ was synthesized by the alkylation of quinoline with decyl triflate. The product was obtained by salt exchange with potassium hexafluorophosphate and characterized by ¹H NMR and high-resolution mass spectrometry (Fig. S1†). The detailed synthetic procedures are given in the ESI.†

The oxygenation of toluene and p-xylene by molecular oxygen (O₂) was achieved by using DeQuCN⁺ as a photocatalyst. Photo-irradiation of the absorption band (λ_max = 330 nm; Fig. S2†) of DeQuCN⁺ (50 μM) in an oxygen-saturated toluene/aqueous (36 mL/0.72 mL) bilayer solution with a high-pressure mercury lamp (1000 W) through a UV cutoff filter (λ < 310 nm) resulted in the formation of benzaldehyde and benzyl alcohol (Fig. 1). The products were identified by gas chromatography (Fig. S3†). After 96 h of irradiation, the amounts of benzaldehyde and benzyl alcohol produced in the toluene layer were 18 mM (0.66 mmol) and 6.0 mM (0.22 mmol), respectively (Fig. 2). Hydrogen peroxide accumulated in the aqueous layer and was quantified by spectroscopic titration with a [TiO(tpypH₄)]⁴⁺ complex (Ti-TPyP reagent; oxo[5,10,15,20-tetra(4-pyridyl)porphyrinato titanium(IV)] in acidic solution; see ESI†).¹⁵ The final concentration of H₂O₂ was 550 mM (0.40 mmol). The catalytic turnover numbers (TONs) were 490 for toluene and 510 for p-xylene based on the initial concentration of DeQuCN⁺.¹⁶ Most of the DeQuCN⁺ remained after photo-irradiation for 96 h. However, the reaction rate decreased at prolonged photo-irradiation times as a result of the photo-oxidation of the catalyst with H₂O₂. Because the H₂O₂ in H₂O is separated from the organic catalyst in acetonitrile (MeCN), photodegradation of the catalyst was avoided in this catalytic system.

Fig. 2 Time course of the two-phase photocatalytic oxidation of toluene by O₂ with DeQuCN⁺PF₆⁻ (50 μM) in toluene with H₂O (2%) under photo-irradiation with a high-pressure mercury lamp (1000 W) (λ > 310 nm).

When toluene was replaced by p-xylene, the formation of p-tolualdehyde and p-methylbenzyl alcohol was also observed (Fig. 3) (see Fig. S4† for gas chromatography data).¹⁷ The quantum yields of H₂O₂ were determined from the initial rate of the photocatalytic reaction as 5.6% in toluene and 12% in p-xylene (Fig. S5†). Table 1 summarizes the TONs and quantum yields of the photocatalytic oxidation of toluene and p-xylene by O₂ with DeQuCN⁺.

Fig. 3 Time course of the two-phase photocatalytic oxidation of p-xylene by O₂ with DeQuCN⁺PF₆⁻ (50 μM) in p-xylene with H₂O (10%) under photo-irradiation with a xenon lamp (500 W) (λ > 310 nm).

Table 1 TONs and quantum yields of photocatalytic oxidation of toluene and p-xylene by O₂ with DeQuCN⁺PF₆⁻ (50 μM)

Substrate	Product (TON)	Quantum yield (%)
Toluene	Benzyl alcohol (120)	5.6
	Benzaldehyde (370)
p-Xylene	p-Methylbenzyl alcohol (170)	12
	p-Tolualdehyde (340)

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Irradiation of the absorption band of DeQuCN⁺ (λ_max = 330 nm) resulted in a strong fluorescence at 432 nm in MeCN. However, the fluorescence was significantly quenched in toluene and p-xylene. The quenching rate constants in MeCN were determined from the slopes of the Stern–Volmer plots (Fig. 4) and the lifetimes of the singlet excited state of DeQuCN⁺ (τ = 34 ns, Fig. S6†). The k_q values obtained from eqn (1)

k_q = K_SV τ⁻¹ (1)

were 1.6 × 10¹⁰ M⁻¹ s⁻¹ for toluene and 1.9 × 10¹⁰ M⁻¹ s⁻¹ for p-xylene, which are close to the diffusion rate constant in MeCN (2.0 × 10¹⁰ M⁻¹ s⁻¹). The free energy change of the photoinduced electron transfer (ΔG_et in eV) from toluene to the singlet excited states of DeQuCN⁺ is given by eqn (2)

ΔG_et = e(E_ox − E^*_red) (2)

where e is the elementary charge, and E_ox and E^*_red are the one-electron oxidation potential of toluene (E_ox = 2.20 V vs. SCE)^5,18 and the one-electron reduction potential of the singlet excited state of DeQuCN⁺ (E^*_red = 2.72 V vs. SCE),¹⁹ respectively. The ΔG_et values were −0.52 eV for toluene and −0.79 eV for p-xylene (E_ox = 1.93 V vs. SCE),⁴ indicating that the photoinduced electron transfer reactions were energetically favourable.²⁰

Fig. 4 Upper panel: emission spectra of DeQuCN⁺PF₆⁻ (20 μM) in the presence of toluene (0–5.0 mM) in MeCN at 298 K. Lower panel: Stern–Volmer plots for the electron transfer quenching of DeQuCN⁺ by toluene and p-xylene in MeCN.

The occurrence of photoinduced electron transfer from the substrates to DeQuCN⁺ was confirmed by laser flash photolysis experiments (see ESI†). Laser excitation (λ = 355 nm from a Nd:YAG laser) of DeQuCN⁺ (3.0 × 10⁻⁴ M) in a deaerated toluene solution resulted in a transient absorption spectrum at 1 μs with new absorption bands at 540 and 950 nm (Fig. 5). Similar transient absorption spectra were also observed in p-xylene (Fig. S7†). The transient absorption band at λ_max = 540 nm was assigned to DeQuCN˙.¹⁹ The broad absorption band in the near-IR region (λ_max = 950 nm) was assigned to the toluene π-dimer radical cation because similar broad transient absorption bands in the long-wavelength region have been reported for the radical cations of toluene and other aromatic compounds.^19,21,22 The decay of the absorbance resulting from the radical ion pair obeyed second-order kinetics, indicating bimolecular back electron transfer between DeQuCN˙ and the toluene radical cation.

Fig. 5 Transient absorption spectra of DeQuCN⁺PF₆⁻ (100 μM) in deaerated toluene at 298 K taken at 1, 10 and 60 μs after nanosecond laser excitation at 355 nm.

The photocatalytic reaction was initiated by photoinduced electron transfer from toluene to the singlet excited state of DeQuCN⁺ (Scheme 1). The toluene radical cation, which is in equilibrium with the π-dimer toluene radical cation formed by photoinduced electron transfer, produces a benzyl radical via deprotonation. This is followed by the rapid addition of O₂ to give the peroxyl radical, leading to the final oxygenated products, benzaldehyde and benzyl alcohol.²³ However, the DeQuCN˙ produced by electron transfer reacts with O₂ to form O₂˙⁻. O₂˙⁻ disproportionates in the presence of protons to yield H₂O₂. The overall stoichiometry of the photocatalytic reaction is given by eqn (3):

2PhCH₃ + 3O₂ → 2PhCHO + 2H₂O₂ (3)

Scheme 1 Reaction mechanism of photocatalytic oxygenation of toluene derivatives and formation of H₂O₂ catalysed by DeQuCN⁺.

DeQuCN⁺ thus acts as an efficient organic photocatalyst for the two-electron reduction of O₂ to produce H₂O₂ as well as the oxygenation of organic substrates in a two-phase solution. The photochemical reaction is initiated by photoinduced electron transfer to produce a radical ion pair. The reaction intermediates were detected by time-resolved transient absorption measurements. This study provides a valuable method for the selective oxygenation of organic substrates with the simultaneous formation of high concentrations of H₂O₂.

Acknowledgements

This work was supported by Grants-in-Aid (no. 26620154 and 26288037 to K. O.) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), ALCA and SENTAN projects from JST, Japan (to S. F.).

Notes and references

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23. The yield of benzaldehyde is higher than that of benzyl alcohol (Fig. 2). Further oxidation of benzyl alcohol with photoexcited DeQuCN⁺ occurred to give benzaldehyde.

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Footnote

† Electronic supplementary information (ESI) available: Experimental details spectral data. See DOI: 10.1039/c6ra05993g

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