Metal-Catalyzed Hydrogen Peroxide Oxidation for Synthesis of Value-Added Chemicals

Journal of the Japan Petroleum Institute, 60, (4), 159-169 (2017) 159

[Review Paper] Metal-catalyzed Hydrogen Peroxide Oxidation for Synthesis of Value-added Chemicals

Yoshihiro KON＊

Interdisciplinary Research Center for Catalytic Chemistry, National Institute of Advanced Industrial Science and Technology (AIST), Central 5, Higashi 1-1-1, Tsukuba, Ibaraki 305-8565, JAPAN

(Received December 26, 2016)

Hydrogen peroxide is an ideal oxidant for industrial processes that produce useful chemicals such as propylene oxide, oxime and catechol because this environmentally benign oxidant has excellent oxygen atom efficiency and low cost. However, hydrogen peroxide is a weak oxidant with low selectivity, so application of hydrogen peroxide oxidation technology to the synthesis of complex organic compounds containing various functional groups remains challenging. We have found that various methods of hydrogen peroxide oxidation of olefins can be used to synthesize a wide range of fine chemicals. Here we report our recent investigations of a practical synthetic method that uses common metals and reusable catalysts to expand our previous catalytic system using tungsten- based catalysts using three components. For example, our new method employs high-speed styrene oxide syn- thesis using an iron picolinate catalyst, bulky sulfide oxidation employing a reusable titanosilicate zeolite catalyst, and high-conversion synthesis of nitroxide radical polymer. The desired compounds are formed in over 90 % selectivity despite their complex structures, under safe reaction conditions and with the high efficiency of hydrogen peroxide.

Keywords Oxidation catalyst, Hydrogen peroxide, Fine chemicals, Iron complex, Titanosilicate zeolite, Nitroxyl radical polymer

1. Introduction 1. 1. History of H2O2 Oxidation Hydrogen peroxide (H2O2) is one of the best candi- Oxidation of organic compounds is an important dates for generating high-value chemicals under envi- reaction for the production of value-added chemicals. ronmentally benign reaction conditions3). In particular, Generally, the selection of oxidants as well as that of the only byproduct of the reaction is water. Another 1) reactants is important to promote a specific reaction . important feature is that H2O2 is generally used as an For example, adipic acid, an important precursor to approximately 30-40 % aqueous solution (H2O2 aq.), in make 6,6-nylon, is synthesized by oxidation from contrast with oxygen gas. For example, H2O2 aq. can mixed cyclohexanol and cyclohexanone compounds be used for the generation of chemicals with high boil- using nitric acid as the oxidant, and epoxy resins, fre- ing points, by mixing with a liquid or dissolved solid quently used as insulating resins in electronic devices, precursor. are produced by oxidation of the corresponding olefin However, H2O2 has low potential, and identification using peracetic acid. However, these reactions also of suitable catalytic systems compatible with the reac- produce harmful waste chemicals such as nitrous oxide tivity of specific precursors has been very difficult. (N2O) and acetic acid. N2O is known as a greenhouse H2O2 oxidation using catalysts was first reported around gas, and the worldwide production of 2.2 million tons/ 1950, and catalytic reactions with high selectivity for year of adipic acid generates 400,000 tons/year of N2O. target compounds were frequently reported after 1980. Acetic acid causes water pollution, and much effort is For example, H2O2 oxidation using a tungsten complex required to separate it from the crude products. was reported in 1983, and H2O2 oxidation with tungsto- Therefore, future industrial systems will require less phosphoric acid and quaternary ammonium salt was environmentally damaging systems of oxidation1),2). reported in 19884)～6). The concept of sustainable chemistry was proposed worldwide in 1990, and halide- and solvent-free H2O2 oxidation using sodium tung- DOI: doi.org/10.1627/jpi.60.159 state, quaternary ammonium salt, and phosphonic acid ＊ E-mail: [email protected] was reported in 1996 as an example of an environmen-

J. Jpn. Petrol. Inst., Vol. 60, No. 4, 2017 160 tally benign reaction7). Our research group reported tungsten-based catalysts that can oxidize reactants to give target products, such as terpene oxides, in good to excellent yields8). Our research into H2O2 oxidation recognized the importance of practical H2O2 oxidation for the synthesis of value- added chemicals with high boiling points that are usually synthesized in organic solution. In addition, further development of basic techniques and concepts easily applicable to industrial synthesis is required to achieve Fig. 1● Concept of Hydrogen Peroxide Oxidation Catalyzed by Metal Compounds the goals of high-speed synthesis, use of recyclable cat- alysts, and applicability to large-scale synthesis while maintaining the excellent yield percentages achieved on 2. Result and Discussion a laboratory scale. Here we report three types of catalytic reactions that 2. 1. H2O2 Oxidation of Styrene Using Iron use H2O2 oxidation and meet the criteria of simplicity, Complex Catalyst ease of handling, and high selectivity to give useful Iron is one of the most abundant metals and is easy compounds. The reactions are selective production of to handle9). Iron is known to be an active catalyst for styrene oxide from styrene using an iron picolinate cat- H2O2 oxidation, as iron complexes are involved in cata- 10) alyst, oxidation of sulfides through tunable reactivity lytic H2O2 activation processes in biological systems . using an interlayer-modified titanosilicate zeolite cata- Since the beginning of this century, many efforts have lyst, which is easily separated from the products, and been made to construct artificial systems with bio- 9) synthesis of radical polymers on a kilogram scale by mimetic iron complexes for H2O2 oxidation . In addi- using tungsten-catalyzed peroxide species under low tion, many useful methods for high-yield synthesis have oxygen concentration conditions with appropriate reac- been reported. However, some critical problems must tion times. These three reactions achieve environmen- be solved to achieve practical synthesis, such as the use tally benign oxidation and low-cost synthesis and are of environmentally benign reagents that are chloride- good candidates for practical synthesis using H2O2. free, reduction of H2O2 as a side reaction, and develop- 1. 2. Concept for Making Metal Peroxide Active ment of an easy handling process. Species from Precatalyst and H2O2 We have demonstrated high yield, high rate syntheses Catalytic H2O2 oxidation can be achieved through the of styrene oxides using an iron complex coordinated by 11) formation of a metal peroxide species from H2O2 and two types of picolinates in the presence of H2O2 . metal catalyst, because the metal peroxide active spe- The developed iron catalyst was effective for the oxida- cies can oxidize the reactant with good selectivity. tion of various styrenes to give the corresponding epox- Selectivity for the desired product through H2O2 oxida- ides in over 90 % yields at 25 ℃ under chloride-free tion requires use of a suitable metal oxide precursor to conditions. The reaction must be easy to handle to form a metal_oxygen_oxygen (M_O_O_) bond with obtain the target styrene oxides. The iron complex superior characteristics to the simple decomposition of catalyst was prepared with simple mixing of 0.02 equiv- H2O2. The balance between the stability of the oxida- alent per substrate (eq.) of iron(II) acetate, 0.02 eq. of tion state and the redox potential of the precatalyst is 2-picolinic acid (picH), and 0.02 eq. of 6-methyl-2-pic- critical for targeted oxidation, as production of the olinic acid (Me-picH) in MeCN. Styrene (1.0 mmol) required metal peroxide in smooth and good yields was poured into the prepared iron catalyst solution at facilitates the specified reaction without the formation 25 ℃, followed by the addition of 1.40 eq. of 35 % of byproducts. Formation of a metal peroxide species H2O2 aq. for 10 min. After extra stirring for 5 min, the seems to be the rate-determining step, and the reaction styrene was fully converted to styrene oxide in 95 % proceeds smoothly if the active species is successfully yield (Table 1, entry 4). Benzaldehyde and 2-phenyl- formed. However, the effects of the solvent, the rela- 1-ethanol were produced as byproducts as 2 % of each tive thermal stabilities of the products and reactants, yield. We also carried out a 100 g-scale H2O2 oxida- and the size of the contact area between the active site tion of styrene using 1.25 eq. of 35 % H2O2 aq. with the and the reactant should all be considered in the condi- same iron catalyst solution used in entry 4 to produce tions of a highly selective reaction. The formed styrene oxide in 80 % isolated yield, as shown in M_OOH species can be converted to various configura- Scheme 1. tions such as an M_O_O triangle shape, M＝O double Oxidation of styrene was not observed in the absence bonded shape, and its dimer. The specific shape of iron metal (Table 1, entry 1). Using another formed depends on the characteristics of the metal and 0.02 eq. of picH instead of Me-picH also catalyzed the of the reaction environment (Fig. 1). epoxidation, but the reactivity was lower than that of

J. Jpn. Petrol. Inst., Vol. 60, No. 4, 2017 161

a),11) Table 1 Oxidation of Styrene by 35 % H2O2 aq. with Iron Catalysts

b) c) Entry Fe(OAc)2 [eq.] picH [eq.] Me-picH [eq.] Yield [%] Selec. [%] 1 0 0.02 0.02 0 0 2 0.02 0.04 0 42 74 3 0.02 0.03 0.01 78 85 4 0.02 0.02 0.02 95 95 5 0.02 0.01 0.03 52 81 6 0.02 0 0.04 37 71 7 0.01 0.01 0.01 69 93 8 0.04 0.04 0.04 92 92

a) Styrene : Fe(OAc)2 : ligand molar ratios were selected as shown here. MeCN solution, 25 ℃, dropwise addition of 1.4 eq. of 35 % H2O2 aq. for 10 min and further stirring for 5 min, unless otherwise stated. b) Determined by GC using biphenyl as an internal standard. c) Selectivity＝yield of styrene oxide/(100－persistence styrene)×100.

trans-epoxide in 80 % yield because the contact between the iron complex active site and the substrate occurred orthogonal to the stilbene plane, which con- sists of two phenyl groups, so these groups do not cause steric repulsion. Aliphatic olefins such as 1-octene were not effectively oxidized by this catalytic system. Scheme 1● Synthesis of Styrene Oxide on 100 g Scale by H2O2 Cyclooctene was oxidized relatively easily, giving Oxidation11) epoxycyclooctane in 77 % yield. The magnetic susceptibility of catalyst solutions pre- pared from Fe(OAc)2, picH, and Me-picH, as calculated the mixed-ligand system (Table 1, entry 2)12). The using the Evans NMR method, showed that the effec- yield was also decreased if picH was fully replaced tive magnetic moment was μeff＝5.00 μB, close to the with Me-picH (Table 1, entry 6). In the latter case, spin-only value of high-spin iron(III) species of 4.90 μB. II III generation of oxygen bubbles was observed at each ad- Fe of Fe(OAc)2 is thought to be oxidized to Fe spe- dition of H2O2, indicating that the iron complex with cies during preparation of the catalyst solution in air. Me-picH preferably catalyzes the disproportionation of The structure of the mixed picolinate-coordinated H2O2 into oxygen and water prior to the epoxidation. iron complex catalyst was observed by single-crystal Screening of picH : Me-picH ratios showed the opti- X-ray analysis. The molecular structure of the iron(III) III mum oxidation of styrene with picH : Me-picH＝1 : 1 complex [Fe (pic)(Me-pic)2] (1) is shown in Fig. 2. (Table 1, entries 2-6). In addition, 0.02 eq. of cata- The precatalyst complex 1 was a mononuclear iron(III) lyst was sufficient for the reaction (Table 1, entries 4, complex containing one pic moiety and two Me-pic 7, and 8). The reaction yield of styrene was lower moieties as ligands. None of the distances between using 1.0 eq. of H2O2 (73 % yield) because H2O2 was the three types of Fe and N, and Fe and O atoms consumed during its decomposition. showed any clear difference derived from the presence Application of the developed catalyst to the oxidation or absence of a methyl group at the 6-position of the of various styrenes and aromatic alcohols is shown in picolinate moiety. All distances were in the range of Table 2. The oxidations were efficiently carried out 1.950-2.166 Å (1 Å＝10–10 m), which is similar to those with p-substituted styrenes in good yields. Even reported for Fe(pic)3. The electrospray ionization tan- 4-phenyl styrene was smoothly oxidized to give the cor- dem mass spectrometry (ESI-MS) spectrum of 1 was responding epoxide in 85 % yield with 97 % selectivity observed in MeCN solution. The observed signal (m/z ＋ despite the disadvantage of its bulkiness. The catalytic ＝451[Fe(pic)(Me-pic)2＋H] ) showed partial preser- activities for α- and β-branched styrene derivatives vation of the mononuclear structure 1 even in MeCN were lower, probably due to the steric repulsion solution. Facile ligand exchange between Me-pic and between the substrates and the ligands located next to pic ligands of the iron complex was suggested by the ＋ the active site of the catalyst. Interestingly, trans-β- observation of signals of 465[Fe(Me-pic)3＋H] and ＋ styrene was selectively converted to trans-epoxide, and 437[Fe(pic)2(Me-pic)＋H] , and of three small signals cis-β-styrene to cis-epoxide. The opposite isomer of (m/z＝778, 764, and 750, which were estimated to be each compound was not obtained at all. The stereo- iron(III) dinuclear complexes including picolinate selectivities in the reactions of trans-β-styrene and cis- ligands (pic)3(Me-pic)2, (pic)2(Me-pic)3, and (pic)(Me- β-styrene were similar to those of epoxidation with the pic)4, respectively). tungsten catalyst. Oxidation of trans-stilbene gave Oxidation of styrene by a MeCN solution of catalyst

J. Jpn. Petrol. Inst., Vol. 60, No. 4, 2017 162

a),11) Table 2 Oxidation of Various Olefins by 35 % H2O2 aq. with Iron Catalysts

35 % H2O2 aq. Fe(OAc)2 R picH R 1 O 1 R4 Me-picH R4 R2 R2 Solvent R3 R3 25 ℃, 15 min O O O

F Cl y. 92 %b) (96 %c)) y. 96 %b) (99 %c)) y. 98 %b) (99 %c)) O O O

Br F3C Me y. 83 %b) (86 %c)) y. 69 %b) (99 %c)) y. 83 %b) (86 %c)) O O d) O d)

Ph y. 85 %b) (97 %c)) y. 78 %b) (82 %c)) y. 92 %b) (95 %c))

d) e) g) O O

b) c) y. 68 % (77 % ) y. 77 %b) (88 %c)) O

y. 80 %b) (99 %c)) O f)

y. 17 %b) (45 %c))

a) H2O2 : substrate : Fe(OAc)2 : picH : Me-picH＝125 : 100 : 2 : 2 : 2 molar ratio in MeCN solution, 25 ℃, dropwise addition of H2O2 for 10 min and further stirring for 5 min, unless otherwise stated. b) Determined by GC using biphenyl as an internal standard. c) Yield/ conversion×100. d) H2O2 : substrate : Fe(OAc)2 : picH : Me-picH＝150 : 100 : 5 : 5 : 5. e) H2O2 : substrate : Fe(OAc)2 : picH : Me-picH＝180 : 100 : 5 : 5 : 5. f) H2O2 : substrate : Fe(OAc)2 : picH : Me-picH＝200 : 100 : 8 : 8 : 8. g) H2O2 : substrate : Fe(OAc)2 : picH : Me-picH＝200 : 100 : 8 : 8 : 8 in MeCN : N,N-dimethylacetamide＝1 : 4 solution.

Fig. 2 Structure of Iron Complex 1 and Its Estimated Peroxo-active Species11)

1 (0.02 eq) with 35 % H2O2 (1.25 eq.) gave styrene epoxide. Competitive experiments using a 1 : 1 mix- oxide in 93 % yield with 95 % selectivity. This result ture of styrene and p-substituted styrene showed the clearly showed that the iron active species are made Hammett linear free-energy relationship σ＋ correlated from complex 1. Acetamide was not observed after well with ρ＝－1.1 (Fig. 3). The negative ρ value the oxidation, so MeCN did not act as an oxidant but as suggests the electrophilic nature of the iron active spe- a ligand and/or solvent. H2O2 epoxidation of styrene cies. using 0.02 eq. of 1 in the presence of 0.02 eq. of a radi- From these results, we tentatively postulated that cal trapping agent, such as N-t-butyl-a-phenylnitrone or [FeIII(pic)(Me-pic)(Me-picH)OOH] was generated from duroquinone, showed no reduction in the yields of 1 and H2O2, followed by abstraction of water to gener-

J. Jpn. Petrol. Inst., Vol. 60, No. 4, 2017 163

V _ 19) ate the [Fe (O)(pic)(Me-pic)(OOCC5H3N Me)] active systems . However, such reactions require large 14) V _ species . [Fe (O)(pic)(Me-pic)(OOCC5H3N Me)] is amounts of hazardous organic solvent and/or high tem- an iron(V) active species that is capable of oxygenating perature. In addition, a homogeneous catalyst is diffi- olefins. After the reaction, deoxygenated 1 is re- cult to separate and reuse after the reaction. The use V activated by H2O2 to regenerate [Fe (O)(pic)(Me-pic) of solid titanosilicate zeolite catalysts, such as TS-1 and _ (OOCC5H3N Me)]. MeCN is essential in stabilizing Ti-MWW, is very effective for achieving high-yield the iron(III) and iron(V) species during the reaction. syntheses of various sulfoxides and sulfones for indus- The 6-methyl pyridyl moiety of Me-pic has steric repul- trial processes, due to easy separation and reusability. sion between the iron ion and the methyl group, accel- Titanosilicate zeolite catalysts in the presence of H2O2 erating the formation of [FeV(O)(pic)(Me-pic) can oxidize sulfides to give sulfoxides and sulfones20). _ (OOCC5H3N Me)] and modifying the spin states of the However, the relationship between zeolite structure and iron center to afford iron(V) active species that can reactivity has not been investigated. Therefore, we enable efficient oxidation15). These results indicate investigated the effect of zeolite structure on catalytic that a suitable combination of picolinate derivatives can activity for the oxidation of sulfides and tried to develop control the reactivity of the iron(V) active species. In a working hypothesis based on our findings21). Three addition, the two types of picolinate ligands (pic and types of titanosilicate zeolite catalysts were prepared, Me-pic) can be easily purchased at low cost, and the TS-1, Ti-MWW, and Ti-IEZ-MWW, to check the effect unit chemical process of the simple mixing of the com- of titanosilicate zeolite pore size on the catalytic activity ponents is applicable to large-scale synthesis after of H2O2 oxidation. appropriate modification for the target reaction. TS-1, Ti-MWW, and Ti-IEZ-MWW were prepared in 2. 2. Recyclable Titanosilicate Zeolite Catalyst for collaboration with Prof. Yokoi, Dr. Yoshioka, and Prof. H2O2 Oxidation of Sulfides Tatsumi for the experiments. TS-1 was chosen as a Sulfoxides and sulfones are useful intermediates for standard titanosilicate zeolite catalyst with an MFI-type various pharmaceuticals such as antifungal medica- structure containing three-dimensional 10-membered tions16). Numerous methods are available for the oxi- ring (3D 10-MR) pores22). Ti-MWW was prepared as an 17) dation of sulfides using aqueous H2O2 as an oxidant . MWW-type structure containing two types of ring: 12- Sulfides are easily oxidized by H2O2 to give sulfoxides MR pores with a system of independent 10-MR chan- and sulfones at high temperature and/or with long reac- nels23). Ti-IEZ-MWW was prepared as an interlayer- tion time without a metal catalyst18). However, selec- expanded MWW zeolite by silylation of the layered tive oxidation of sulfides under mild conditions can be precursor of the MWW-type titanosilicate (Ti-MWW(P)) achieved with optimized catalysts and/or reaction with diethoxydimethylsilane (DEDMS) under acidic conditions followed by calcination24). The atomic ratios of Si/Ti in the titanosilicate zeolite catalysts were estimated by inductively coupled plasma (ICP) analysis as 72, 57, and 86 for TS-1, Ti-MWW, and Ti-IEZ- MWW, respectively (Table 4). The BET (Brunauer- Emmett-Teller) surface area of each titanosilicate zeo- lite catalyst was calculated from the amount of absorbed N2 on the surface as 471, 499, and 521 for TS-1, Ti- MWW, and Ti-IEZ-MWW, respectively (Table 4). Thioanisole was selected as a screening substrate to estimate the catalytic activities of titanosilicate zeolite catalysts. The reaction was conducted with 35 % H2O2 (1.2 eq. to thioanisole) and titanosilicate zeolite Fig. 3 Hammett Plot for p-Substituted Styrenes11) catalyst (10 mg) in open air at 25 ℃ for 2 h with vigor-

Table 3 Competitive Oxidation of p-Substituted Styrenesa),11)

Entry p-Substituents (X) Conv. of p-X [%]b) Conv. of p-H [%]b) 1 Me 7.7 2.5 2 Cl 2.5 3.5 3 OMe 11.4 1.0

4 NO2 1.6 7.2 a) Reaction was run using a 1 : 1 mixture of styrene and p-substituted styrenes (2.0 mmol), 35 % H2O2, and 1 in a 100 : 10 : 0.5 molar ratio at 25 ℃ for 3 min. b) Determined by GC analysis.

J. Jpn. Petrol. Inst., Vol. 60, No. 4, 2017 164

a),21) Table 4 Oxidation of Thioanisole with TS-1, Ti-MWW, and Ti-IEZ-MWW Catalysts Using 35 % H2O2 aq. O 35 % H2O2 aq. (1.2 eq.) S Ti-zeolite (10 mg) S 25 ℃, 2 h Yield [%]d) Entry Zeolite catalyst Si/Ti ratiob) BET surface areac) Selectivity [%]e) sulfoxide sulfone 1 None - - 18 0 95 2 TS-1 72 471 53 8 82 3 Ti-MWW 57 499 89 10 89 4 Ti-IEZ-MWW 86 521 94 6 94

a) Reaction conditions: thioanisole (1.0 mmol), 35 % H2O2 (1.2 mmol), titanosilicate catalyst (10 mg), 25 ℃, 1000 rpm, 2 h. b) Atomic ratio of Si/Ti in Ti zeolite was estimated by ICP analysis.

c) BET surface area in Ti zeolite was calculated from the amount of adsorbed N2 on the surface. d) Yield on the basis of thioanisole, determined by GC analysis with biphenyl as an internal standard. e) (Yield of product)/(100－remaining sulfide)×100. ous stirring. The results are summarized in Table 4. p-OMe＝5.46 (2.73). The calculated least-squares lin- Reaction at lower temperature (25 ℃) without organic ear function of σ＋versus log(kp-substituted/kH) showed a solvent reaction conditions for 2 h resulted in lower slope of －0.41. This negative ρ value suggests that levels of sulfoxidation (18 % yield, Table 1, entry 1). the Ti active species generated in Ti-IEZ-MWW have TS-1 is a common titanosilicate zeolite catalyst with electrophilic nature. This tendency is consistent with 26) good activity for H2O2 oxidation of sulfides in an opti- the previously reported sulfide oxidation with H2O2 . mized organic solvent. However, TS-1 did not show The application of Ti-IEZ-MWW-catalyzed H2O2 high activity under organic solvent-free conditions, but oxidation to various sulfides is shown in Table 5. gave methyl phenyl sulfoxide in 53 % yield and methyl Methyl p-substituted phenyl sulfoxides were formed phenyl sulfone in 8 % yield (Table 4, entry 2). Ti- from the corresponding sulfides via H2O2 oxidation in MWW showed high activity even under organic solvent- high yields (75-92 %, Table 5). Reaction of 2-methyl free conditions to catalyze the sulfoxidation of thio- sulfanyl pyridine, which includes a nitrogen atom as anisole to give the corresponding sulfoxide in high well as a sulfur atom as reactive sites, gave the corre- yield (89 %, Table 4, entry 3). Ti-MWW, with 10- sponding sulfoxide in moderate to good yields (79 % MR interlayer pores, is much more active than TS-1 in yield). In these reactions, the N-oxide of 2-methyl sul- 25) the epoxidation of linear alkenes with H2O2 . Ti- fanyl pyridine was not observed. The chemoselectivity IEZ-MWW showed high activity, giving methyl phenyl of Ti-IEZ-MWW was also checked using the reactions sulfoxide in 94 % yield with 94 % selectivity (Table 4, of allyl phenyl sulfide and 2-phenylthioethanol. Allyl entry 4). The optimized amount of H2O2 was 1.2 eq. phenyl sulfide and 2-phenylthioethanol were oxidized of 35 % H2O2. In addition, calcination of Ti-IEZ- to give the corresponding sulfoxides in 72 % and 95 % MWW at 550 ℃ for 5 h just before the reaction was yields, respectively (Table 5). No epoxide or carbonyl required to achieve optimize the catalytic oxidation of compounds were observed. thioanisole. Although Ti-IEZ-MWW seems to show Ti-IEZ-MWW was especially effective for the oxida- high activity derived from the 12-MR interlayer pores tion of sulfides with bulky substituents, such as diphe- in addition to the MWW-type structure, it also tends to nyl sulfide. Oxidation of diphenyl sulfide using the adsorb water and/or organic compounds which reduce Ti-IEZ-MWW catalyst gave the corresponding sulfox- the activity. The calculated turnover frequencies ide in good yield (50 % yield, Table 5). In contrast, (TOFs) in Ti-IEZ-MWW during the oxidation of thio- oxidation of diphenyl sulfide using the TS-1 or Ti- anisole were ca. 2000 h–1. MWW catalyst gave the corresponding sulfoxide in low Competitive experiments using a 1 : 1 mixture of yields (8 % and 29 % yields for TS-1 and Ti-MWW, thioanisole and p-substituted thioanisole were per- respectively). The calculated TOFs were 15 for Ti- formed to determine the Hammett linear free-energy MWW and 41 for Ti-IEZ-MWW. Ti-IEZ-MWW relationship. The experiment was carried out using a showed higher catalytic activity for oxidation of diphe- 1 : 1 mixture of thioanisole and p-substituted thio- nyl sulfide than Ti-MWW. The BET surface area of anisole (1.0 mmol), 35 % H2O2 aq. (1.4 mmol), and Ti- the titanosilicate zeolite catalysts was almost within the IEZ-MWW (10 mg) at 25 ℃ for 5 min. The conver- same range, so the reactivity of the catalysts correlated sions of p-substituted compounds (conversions of directly with the relative ease with which sulfides con- competitive thioanisole are shown in parentheses) were tacted the Ti active sites of the zeolites, similar to the p-Me (p-H)＝7.60 (6.03), p-Cl＝7.29 (8.60), and case of Ti-beta. Potential advantages of MWW-type

J. Jpn. Petrol. Inst., Vol. 60, No. 4, 2017 165

Table 5 Oxidation of Various Sulfides to Sulfoxides Using Ti-IEZ-MWWa),b),21)

35 % H2O2 aq. (1.2 eq.) O S Ti-IEZ-MWW (10 mg) S R R’ R R’ 25 ℃, 2 h O O O S S S

Cl y. 94 % (83 %c)) y. 92 % (84 %d)) y. 87 % (82 %d)) O O O S N S S

Br y. 75 % (77 %d)) y. 79 % (84 %e)) y. 72 % (84 %e)) O O f) g) O S S OH S 3 3 y. 82 % (79 %d)) e) y. 95 % (84 % ) y. 50 % (47 %d))

a) Reaction conditions: sulfide (1.0 mmol), 35 % H2O2 (1.2 mmol), Ti-IEZ-MWW (10 mg), 25 ℃, 1000 rpm, 2 h, unless otherwise stated. b) Yield on the basis of sulfide, determined by GC analysis. c) Isolated yield (10 mmol scale). d) Isolated yield (3 mmol scale). e) Mixture of sulfoxide and sulfone was obtained, the yield was calculated by 1H NMR (3 mmol scale). f) Diphenyl sulfide was oxidized at 40 ℃, 1000 rpm, 18 h. g) Dibutyl sulfide was oxidized at 25 ℃, 1000 rpm, 4 h.

Table 6● Oxidation of Thioanisole to Methyl Phenyl Sulfoxide with Ti-IEZ-MWW Catalyst Using a),21) 35 % H2O2 aq., and Recycling of Catalyst O 35 % H2O2 aq. (1.2 eq.) S Ti-IEZ-MWW S 25 ℃, 2 h Number of reuses 1 2 3 4 5 Yield of methyl phenyl sulfoxide [%]b) 85 90 91 89 94

a) Substrate: catalyst ratio (wt%); thioanisole : Ti-IEZ-MWW＝1 : 0.08, 35 % H2O2 (1.2 eq.) was used. 25 ℃, 1000 rpm, 2 h. b) Yield on the basis of thioanisole, determined by GC analysis.

materials are expected from the presence of supercages These results clearly showed that the sulfoxidation was and side pockets, but access to the supercages is seri- catalyzed at the Ti active sites on solid Ti-IEZ-MWW. ously restricted by the openings of the elliptical 10-MR The Ti-IEZ-MWW catalyst could easily be reused in pores. In contrast, Ti-IEZ-MWW has 12-MR interlayer the next reaction. Sulfoxidation of thioanisole gave pores, so bulkier aryl sulfides can access the Ti active the corresponding sulfoxide in 85-94 % yields, with no sites of Ti-IEZ-MWW. TS-1 and Ti-MWW do not decrease in yields after five cycles (Table 6). The Ti- have 12-MR interlayer pores, so bulkier aryl sulfides IEZ-MWW catalyst was removed from the product cannot easily access the Ti active sites of TS-1 and Ti- after the reaction, washed with water and acetone, and MWW. Di-n-butyl sulfide was effectively oxidized by then calcined at 550 ℃ for 5 h for use in the next reac- the Ti-IEZ-MWW catalyst to give di-n-butyl sulfoxide tion. in 82 % yield. Oxidation of various sulfides to the corresponding Sulfoxidation of diphenyl sulfide using Ti-IEZ- sulfones was successfully carried out using 2.5 eq. of MWW catalyst was stopped by removal of the catalyst 35 % H2O2 aq. with Ti-IEZ-MWW catalyst at 50 ℃ for in the middle of the reaction. ICP analysis did not 3 h (Table 7). This catalyst gave methyl phenyl detect Ti in the filtrate of diphenyl sulfide oxidation. sulfone quantitatively, and oxidized thioanisole deriva-

J. Jpn. Petrol. Inst., Vol. 60, No. 4, 2017 166

a),21) Table 7 Oxidation of Various Sulfides to Sulfones Using Ti-IEZ-MWW Catalyst with 35 % H2O2 aq. 35 % H O aq. (2.5 eq.) 2 2 O O S Ti-IEZ-MWW (10 mg) S R R’ R R’ 50 ℃, 3 h O O O O O O S S S

Cl y. 99 % (99 %c)) y. 87 % (83 %c)) y. 97 % (84 %c)) O O O O O O S N S S

Br y. 94 % (92 %c)) y. 78 % (85 %c)) y. 61 % (61 %c))

d) e) O O O O S OH S O O S 3 3 c) y. 88 % (88 %c)) y. 55 % (50 % ) y. 38 % (23 %c))

a) Reaction conditions: sulfide (1.0 mmol), 35 % H2O2 (2.5 mmol), Ti-IEZ-MWW catalyst (10 mg), 50 ℃, 1000 rpm, 3 h, unless otherwise stated. b) Yield and conversion on the basis of sulfide, determined by GC analysis. c) Isolated yield (3 mmol scale). d) Diphenyl sulfide was oxidized under toluene solution for 18 h. e) Dibutyl sulfide was oxidized for 4 h. tives, such as 4-methyl, 4-chloro, and 4-bromo thio- investigated20). Reaction of titanosilicate zeolite cata- _ anisole, to give the corresponding sulfones in high lyst with H2O2 forms ≡Ti OOH active species accom- yields (87 %, 97 %, and 94 %, respectively). In con- panied by the desorption of silanol. In the presence of _ trast, TS-1 and Ti-MWW catalyzed oxidation of p- water, coordination of H2O molecules to the≡Ti OOH substituted thioanisoles to the corresponding sulfones in active species forms cyclic structures. Previous 28-35 % and 76-94 % yields, respectively. Nitrogen- reports20) and Hammett plots indicate the reaction containing sulfide was effectively oxidized with Ti-IEZ- mechanism occurs as follows (Fig. 4). Nucleophilic MWW catalyst to give methyl pyridyl sulfone in 78 % attack of the sulfur atom to an oxygen atom of the ≡Ti_ yield. Allyl phenyl sulfide was oxidized with Ti-IEZ- OOH active site leads to one oxygen transfer to form MWW catalyst to give allyl phenyl sulfone in 61 % sulfoxide. Deoxygenated ≡Ti_OH is stabilized by yields. Oxidation of 2-phenylthioethanol using Ti- coordination with water and reoxidized by the next _ IEZ-MWW gave 2-phenylsulfonylethanol in 55 % H2O2 to reproduce the ≡Ti OOH active species. The yield. Diphenyl sulfone was not observed (0 % yield) formation of sulfone is also explained by this mecha- from oxidation using 2.5 eq. of 35 % H2O2 aq. with Ti- nism; that is, nucleophilic attack of the sulfur atom of IEZ-MWW catalyst at 50 ℃ for 18 h under organic sulfoxide is a key step. Sulfoxides are generated with solvent-free conditions, and diphenyl sulfoxide was the good selectivity at 25 ℃ because nucleophilic attack by only product; presumably because the generated sulfox- sulfide is far superior to that by sulfoxide. The reac- ide is solid and does not make sufficient contact with tivity can be linked to the topology of the titanosilicate H2O2 aq. and the titanosilicate zeolite catalyst. In con- zeolite catalysts due to the relative ease of accessibility trast, diphenyl sulfide was oxidized with Ti-IEZ-MWW of the nucleophilic S atom on the sulfide to the O atom catalyst to the corresponding sulfone in 38 % yield with on the ≡Ti_OOH active site. 93 % selectivity in toluene solution. However, the 2. 3. Production of Poly(2,2,6,6,-tetramethyl- TS-1 and Ti-MWW catalysts showed no oxidation pyperidinoxyl-4-yl-methacrylate) with 20 kg 27) activity for diphenyl sulfide. The larger interlayer Scale Synthesis by Catalyzed H2O2 Oxidation pore size of Ti-IEZ-MWW compared to those of TS-1 Innovative radical polymers, especially poly(2,2,6,6,- and Ti-MWW is effective for enabling the oxidation of tetramethylpyperidinoxyl-4-yl-methacrylate) (PTMA), bulky sulfides. Oxidation of alkyl sulfides such as di- are extremely interesting for various value-added chem- n-butyl sulfide with Ti-IEZ-MWW catalyst also formed icals because of the unique electronic property of the corresponding sulfone in good yield (88 % yield). PTMA derived from its unique structure combining sta- The reaction mechanisms underlying the oxidation of ble N_O radicals28) and ease of handling as a polymer. sulfides catalyzed by titanosilicate zeolite have been Two main oxidation methods are known to give PTMA:

J. Jpn. Petrol. Inst., Vol. 60, No. 4, 2017 167

21) Fig. 4 Possible Reaction Mechanism of Ti-IEZ-MWW-catalyzed H2O2 Oxidation

Table 8 Comparison between Conventional and New Processes27)

n n

O O reaction conditions O O

N N H O

Solvent Oxidant [eq.] Catalyst Time [h] Conv. [mol%]a) CH Cl m-CPBA 1.2 - 16 95 Conventional process 2 2 TBA H2O2 5 H2WO4 6 93

H2WO4 New process DMAC H2O2 2 2 95 PhP(O)(OH)2 a) Calculated from ESR data.

oxidation of the precursor neutral polymer by m- 29) CPBA and by H2O2 aq. with alcohol solvent. However, the m-CPBA method requires chloroform or dichloromethane solvent, and forms m-chlorobenzylic acid as a deoxidized compound of m-CPBA, whereas the H2O2-alcohol method consumes four equivalents of H2O2 during the generation of explosive O2 gas. The present method of catalysis can inexpensively, safely, and easily produce PTMA by oxidation of an imino-containing compound using only 2 eq. of H2O2 27) in nonhalogenated solvents to give the corresponding Generated PTMA with 20 kg yield (right) . nitroxide radical in over 90 % yield. This particular Fig. 5● Comparison of Generated O2 Gas and Reaction Time work is a collaboration with Mr. Hashimoto, Mr. between the Conventional and New Systems (left) Kinpara, and Mr. Fujimoto of Sumitomo Seika Chemicals Co., Ltd. This new catalytic system was achieved by applying two key technologies: the combi- system. The optimized catalytic reaction was success- nation of tungstic acid catalyst with phenyl phosphonic fully scaled up to give 20 kg of the desired PTMA acid to activate the oxidation reaction, and the applica- under safe reaction conditions (Fig. 5). This synthe- tion of N,N-dimethylacetamide (DMAC) solvent to sis is a good example of catalyst-derived production of inhibit the unnecessary production of O2. Table 8 fine chemicals, and is applicable to environmentally summarizes the advantages of our developed catalytic sustainable industrial processes.

J. Jpn. Petrol. Inst., Vol. 60, No. 4, 2017 168

3. Conclusion 6) Ishii, Y., Yamawaki, K., Ura, T., Yamada, H., Yoshida, T., Ogawa, M., J. Org. Chem., 53, (15), 3587 (1988). 7) Sato, K., Aoki, M., Ogawa, M., Hashimoto, T., Noyori, R., J. A practical method of synthesis based on H2O2 oxi- Org. Chem., 61, (23), 8310 (1996). dation was proposed, focusing iron metal complex 8) Kon, Y., Chishiro, T., Uchida, H., Sato, K., J. Jpn. Petrol. Inst., catalyst, titanosilicate zeolite catalyst, and PTMA pro- 55, (5), 277 (2012). duction. This method has the advantages of high 9) Mayer, A. C., Bolm, C., “Iron Catalysis in Organic Chemistry,” speed, easy recycling of the catalyst, and applicability ed. by Plietker, B., Wiley-VCH, Weinheim (2008), p.73. 10) Shikama, K., Chem. Rev., 98, 1357 (1998). to large-scale synthesis with low O2 emissions. A 11) Chishiro, T., Kon, Y., Nakashima, T., Goto, M., Sato, K., Adv. working hypothesis for generating peroxide species Synth. Catal., 356, 623 (2014). _ such as the metal OOH shape is required to start the 12) Tung, H.-C., Kang, C., Sawyer, D. T., J. Am. Chem. Soc., 114, construction of a method for H2O2 catalytic oxida- 3445 (1992). tion30), and continuous development will be required to 13) Kon, Y., Hachiya, H., Ono, Y., Matsumoto, T., Sato, K., Synthesis, 1092 (2011). achieve practical synthesis. 14a) Chen, K., Costas, M., Que Jr., L., J. Chem. Soc., Dalton Trans., 672 (2002). Acknowledgment 14b) Katona, G., Carpentier, P., Nivière, V., Amara, P., Adam, V., This work was supported in part by the New Energy Ohana, J., Tsanov, N., Bourgeois, D., Science, 316, 449 (2007). and Industrial Technology Development Organization 15) Kodera, M., Kano, K., Bull. Chem. Soc. Jpn., 80, 662 (2007). 16) Kon, Y., Yokoi, T., Yoshioka, M., Tanaka, S., Uesaka, Y., (NEDO), Japan, and by the Cooperative Research Mochizuki, T., Sato, K., Tatsumi, T., Tetrahedron, 70, 7584 Program ‘Network Joint Research Center for Materials (2014). and Devices’ (No. 2014256), Japan. I express my 17) Trost, B. M., Bull. Chem. Soc. Jpn., 61, 107 (1988). thanks to Mr. Shun Hashimoto, Mr. Yuji Kinpara, Mr. 18) Smith, M. B., March, J., “March’s Advanced Organic Nobutaka Fujimoto, and Mr. Eiichi Araki (all of Chemistry, 6th ed.,” Wiley, New York (2007), p. 1780. 19) Jereb, M., Green Chem., 14, 3047 (2012). Sumitomo Seika Co., Ltd.); as well as to Dr. Toshiyuki 20) Kholdeeva, O. A., Top. Catal., 40, 229 (2006). Yokoi, Dr. Masato Yoshioka, and Prof. Takashi Tatsumi 21) Notari, B., Adv. Catal., 41, 253 (1996). (all of Tokyo Institute of Technology) for their research 22) Fan, W., Duan, R.-G., Yokoi, T., Wu, P., Kubota, Y., Tatsumi, T., collaboration. I also express thanks to Dr. Shinji J. Am. Chem. Soc., 130, 10150 (2008). Tanaka, Dr. Takefumi Chishiro, Mr. Takuya Nakashima, 23) Wu, P., Tatsumi, T., Komatsu, T., Yashima, T., Chem. Lett., 774 (2000). Ms. Yumiko Uesaka, Dr. Kazuhiko Sato, Dr. Midori 24) Wu, P., Ruan, J., Wang, L., Wu, L., Wang, Y., Liu, Y., Fan, W., Goto, and Dr. Hiromichi Shimada for their useful dis- He, M., Terasaki, O., Tatsumi, T., J. Am. Chem. Soc., 130, 8178 cussions. (2008). 25) Wu, P., Tatsumi, T., J. Phys. Chem. B, 106, 748 (2002). References 26) Kamata, K., Hirano, T., Ishimoto, R., Mizuno, N., Dalton Trans., 39, 5509 (2010). 27) Hishimoto, S., Kinpara, Y., Fjimoto, N., Kon, Y., Sato, K., 1) Sheldon, R. A., Chem. Ind., 903 (1992). Catalysts and Catalysis, 57, (5), 262 (2015). 2) Ishii, Y., Sakaguchi, S., “Modern Oxidation Methods,” ed. by 28) Nishide, H., Suga, T., Electrochem. Soc. Interface, 14, (4), 32 Bäckvall, J. E., Wiley-VCH, Weinheim (2004), p. 119. (2005). 3) Jones, C. W., “Applications of Hydrogen Peroxide and 29) Kurosaki, T., Lee, K. W., Okawara, M., J. Polym. Sci. A-1 Derivatives,” Royal Society of Chemistry, Cambridge (1999), Polym. Chem., 10, (11), 3295 (1972). p. 37. 30) Kon, Y., Tanaka, S., Sato, K., Synthesiology-English edition, 8, 4) Payne, G. B., Williams, P. H., J. Org. Chem., 24, (1), 54 (1959). (1), 16 (2015). 5) Venturello, C., Alneri, E., Ricci, M., J. Org. Chem., 48, (21), 3831 (1983).

J. Jpn. Petrol. Inst., Vol. 60, No. 4, 2017 169

要 旨

金属触媒による過酸化水素酸化反応を利用した高付加価値化学品の合成

今 喜裕

（国研）産業技術総合研究所 触媒化学融合研究センター，305-8565 茨城県つくば市東1-1-1つくば中央第5

過酸化水素はプロピレンオキシド，オキシムやカテコールな ホン酸からなる触媒系の基盤をさらに拡充させて，他の汎用金 ど有用化学品の工業プロセスに用いられる理想的な酸化剤とし 属の適用可能性や回収再使用といった利便性の向上を目指した て知られている。過酸化水素は他の酸化剤に比べ有効酸素の比 基盤技術に挑戦したので報告する。具体的には，鉄触媒による 率が高く安価だがそれ自体の酸化力は低いため，標的とする反 高速スチレンオキシドの合成，チタノシリケートゼオライト触 応に最適な触媒の発見と触媒反応の設計が鍵となる。我々はこ 媒による回収再使用可能触媒の反応，安定ラジカルポリマーの れまでに独自の活性触媒発見指針に基づく触媒反応の設計を行 キログラムスケール合成の事例を示す。いずれの反応において い，種々の機能性化学品製造に有用な過酸化水素酸化技術を開 も，ターゲットとする機能性化学品を選択率 90 % 以上で合成 発し，基盤技術として蓄積してきた。今回，これまでに開発し することに成功した。 てきたタングステン酸ナトリウム－四級アンモニウム塩－ホス

J. Jpn. Petrol. Inst., Vol. 60, No. 4, 2017