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Effect of redox potential on manganese-mediated benzylalcohol and sulfide oxidation

Tridentate 1,3-bis(2′-Ar-imino)isoindolinato manganese(II) complexes were found to efficiently catalyze the mild oxidation of organic sulfides to sulfoxides and benzyl alcohols to benzaldehydes with mCPBA and PhIO, but they proved almost ineffective by the use of tBuOOH and H2O2 at room temperature. The effect of electron-withdrawing and electron-donating substituents on the substrates (thioanisole and benzylalcohol), and the redox properties of the metal center by varying the aryl groups on the bis-iminoisoindoline were investigated, and showed a significant impact on the catalytic C–H oxidation and sulfoxidation reactions. Based on these results, including the linear correlations between the oxidation reactivity of the catalysts and MnIII/MnII redox potentials, the Hammett correlation with ρ =−0.27 for 4R-PhSMe and ρ =−0.27 for 4R-PhCH2OH, electrophilic oxomanganese(IV) intermediate has been suggested as key oxidant. Furthermore, the small negative slope (−0.5) from the logkrel versus Eo ox for 4R-PhSMe gives clear evidence for the direct oxygen atom transfer (OAT) mechanism instead of electron transfer (ET) mechanism between the MnIVO and PhSMe.


Introduction
High-valent oxoiron species play important roles in both heme and non-heme monooxygenase enzymes and their biomimetic reactions. These enzymes catalyze many oxidation reactions including oxygen atom transfer (hydroxylation, epoxidation) and dealkylation, but much less activation was observed when iron was replaced by manganese. However, in biomimetic systems oxomanganese complexes mediate most of the oxidations brought by the iron oxygenases leading to a wide range of manganese be used as hydrogen peroxide-based bleach catalysts [18]. In this ligand the introduction of various aryl groups on the bis-iminoisoindoline moiety provided a new method for tuning the reactivity and electronic structure of the catalyst. This work focuses on our recent efforts to tune the reactivity of our manganese system and investigate how the catalytic activity can be optimized compared to our iron system in the oxidation of organic substrates including thioanisols and benzyl alcohols. Organic sulfoxides and carbonyl compounds produced by the oxidation of alcohols are used as intermediates for the synthesis of various agrochemicals, plastic additives, pharmaceuticals and drugs [19][20][21][22][23][24]. Therefore, there has been considerable interest in the development of non-toxic, cheap and highly efficient oxidation catalysts for selective oxidation of organic sulfides and benzyl alcohols

Description of the catalytic oxidation reactions
For the oxidation of thioanisoles and benzylalcohols, the reactions were carried out under argon atmosphere at room temperature (25°C). In a typical experiment, 1 mL of H 2 O 2 (diluted from 38-40% solution), mCPBA (77%), or tBuOOH (diluted from 70% solution) in acetonitrile was delivered by syringe pump to a stirred solution (2 mL) of catalyst and p-substituted thioanisole or benzylalcohol derivatives. The final concentrations were 10 mM catalyst, 250 mM co-oxidants, and 500 mM substrate. The PhIO was added as a solid into the water (100 µL) containing CH 3 CN solution. The manganese complexes were removed by passing the internal standard containing reaction mixture through a silica column followed by elution with ethyl acetate. The products were identified by GC-MS based on authentic samples, and quantified by GC relative to bromobenzene as an internal standard.

Results and discussion
Iron and manganese containing porphyrin and phthalocyanine complexes as catalysts can be used for a wide range of oxidation reactions due to their distinguishing properties such as variable oxidation states and readily tunable Lewis acidic behavior and redox properties [54][55][56][57][58]. Based on the advantageous properties of the above systems the solubility, electronic structure and steric properties of the ligand as well as their complexes can be further developed by the synthesis of 1,3-bis(2 -Ar-imino)isoindolines, an open-chained phthalocyanine mimic, introducing various aryl groups on the bis-iminoisoindoline moiety [12]. In this study we have investigated the catalytic properties of manganese 1,3-bis(2 -Arimino)isoindolines for the oxidation of organic sulfides and benzyl alcohols with various co-oxidants compared to the recently published iron 1,3-bis(2 -Ar-imino)isoindoline system [59,60].   Table 1).

Catalytic oxidation of thioanisole
The catalytic activity of [Mn(HL 2 )Cl 2 ] (2) was studied in the oxidation of thioanisole and benzyl alcohol utilizing tert-butyl hydroperoxide (t BuOOH), metachloroperoxybenzoic acid (mCPBA), H 2 O 2 and PhIO   Figure 1 show that there is an increase of the yield when the co-oxidants employed are H 2 O 2 , t BuOOH, PhIO and mCPBA from 1 to 61% (entries 1, 6-8 in Table 1). The highest conversion is attained with mCPBA, where 15 turnovers were observed in the course of 20 min, after which time there was no further product formation (entries 1-5 in Table 1 [61]. In our case, unfortunately much worse or just slightly better results were obtained, depending on the amount of AcOH, which can presumably be explained by the formation of catalytically inactive complexes (Tables 1 and 2). Low yields were obtained for H 2 O 2 , probably because it competes with 2 for the catalase-like activity. The reactivities of para-substituted thioanisols relative to that of thioanisole were also investigated ( Figure 3). Hammett treatments of relative reactivities (k rel = log(X f /X i )/ log(Y f /Y i ), where X i and X f are the initial and final concentration of parasubstituted thioanisols, and Y i and Y f are the initial and final concentration of thioanisole) of various substituents against σ gave ρ value of −0.26  Table 1). ( Figure 4A), which suggests that the behavior of the oxidant generated from [Mn(HL 1 )Cl 2 ] with mCPBA is mildly electrophilic. Furthermore, this value is comparable to that of the closely related [Fe(HL 2 )] 2+ /H 2 O 2 system (ρ = −0.40) [59]. When the log k rel values were plotted against the E 0 ox potentials of para-substituted thioanisols, the plot gave a gradient of −0.50 ( Figure 4B), which is a little bit smaller than that obtained for [Fe(HL 2 )] 2+ /H 2 O 2 (−0.8). The magnitude of these values is consistent with a direct oxygen atom transfer mechanism (Scheme 2). Much higher values are typical for an electron transfer/oxygen rebound pathway [62].
in the formation of benzaldehyde in 30, 8.5, 11.4 and 2% yield, respectively (entries 2-5 in Table 2 and Figure 5). The highest reactivity with a 7.5 turnover was observed for a mCPBA co-oxidant in the course of 20 min, after which time there was no further product formation.
Based on our previous experience we expected that varying the aryl group on the bisiminoisoindoline moiety would directly tune electron density in the metal center and hence allow us to evaluate the influence of these electronic changes on reactivity. Cyclic voltammetry measurements on complexes 1-6 reveal that the Mn(III/II) redox potentials can vary by up to 560 mV between the most electron-withdrawing (948 mV for 1) and the most electron-donating (388 mV for 5) [18]. With a systematic series of [Mn II (HL 1−6 )Cl 2 ] (1-6) complexes, we investigated their oxidative reactivity for HAT (hydrogen atom transfer) and OAT (oxygen atom transfer) reactions with benzyl alcohol and thioanisole. Among these catalysts, we observed that the complex 5 and 6 containing the most electron-donating benzimidazole arms, show the smallest rates of oxidation for both OAT (Figure 7) and HAT (Figure 8) reactions. In other words, the electron-deficient, Lewis acidic catalyst (1 and 2) is much more active than the electron-rich ones (5 and 6). Similar trend was   Table 1).

Conclusion
We have investigated the effect of systematic changes on the 1,3-bis(2 -Ar-imino)isoindoline by introducing various heteroaromatic arms on the bis-iminoisoindoline moiety toward the reactivity of their manganese(II) complexes for HAT and OAT reactions. These transformations resulted in  Table 2). (A) Comparison of the product formation in the manganese-catalyzed (2) oxidation of substituted benzyl alcohols. (B) Plot of log k rel against the σ p of para-substituted benzyl alcohols (entries 2, 6-9 in Table 2).  Table 1).  Table 2). a predictable and significant 560 mV shift in the Mn III /Mn II redox potentials with concomitant changes in the reactivity toward thioanisols and benzyl alcohols. The effect is small in both cases, but based on these results, including the linear correlations between the oxidation reactivity of the catalysts and Mn III /Mn II redox potentials and the Hammett correlations (ρ = −0.27 for 4R-PhSMe and ρ = −0.27 for 4R-PhCH 2 OH) electrophilic oxomanganese(IV) intermediate has been suggested as key oxidant.

Conflicts of interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.