1． Introduction

From the viewpoint of the diversity of organic reactions and synthetic methodologies, the development of new organic synthesis methods using various reaction devices and energy sources such as continuous �ow reaction and microwave heat-ing method is actively being conducted. Among them, mecha-nochemistry is a methodology for achieving chemical conver-sion reactions through the addition of mechanical energy. In the past, mechanochemistry applied to organic synthesis reac-tions did not receive much attention except for limited �elds, but it has recently been recognized as a tool for ef�ciently advancing organic reactions. 1 In particular, because an organic synthetic reaction can be carried out under solvent─ free condi-tions using the rotation, friction, and vibration energy of the balls, as well as the reaction vessels in mechanochemical reac-tions using a ball mill, it is bene�cial from the viewpoint of green chemistry. In a mechanochemical reaction using a ball mill, the reaction proceeds smoothly with a substrate having low solubility or low stability in a solution, and the substrate, reagent, and catalyst can avoid dilution or solvation through a solvent. 2

We previously reported a palladium carbon (Pd/C)─ cata-lyzed solvent─ free hydrogenation (solid─ solid─ gas phase hydrogenation) in which a solid reducible substrate and hydro-gen gas directly react at room temperature. 3 Applying this reaction, we also developed a solid─ solid─ solid─ solid phase ligand─ free Suzuki─ Miyaura coupling reaction simply by mix-ing solid aromatic halide, aromatic boronic acid, sodium car-bonate, and Pd/C at 100 ℃. 3

Based on this background, we became interested in mecha-nochemical reactions that proceed without a solvent, and began research on solvent─ free reactions using a stainless steel (SUS 304) planetary ball mill. As a result, ef�cient hydrogen generation methods from water and organic solvents mediated by the mechano─ energy and constituent metals of SUS304

have been developed, and hydrogen produced in the ball mill container can be directly �xed to other organic compounds (hydrogenation). In this paper, the hydrogen generation and subsequent hydrogenation reactions under SUS 304 ball─ milling conditions are outlined.

2． Hydrogen Generation from H 2O under Planetary Ball─ Milling Conditions using Stainless Steel Balls 4

2.1　 Discovery OriginA ball mill machine is a device intrinsically aimed at a par-

ticle size reduction of a solid or powder material through pul-verization. Ball milling technology has recently been utilized for solvent─ free reactions using solid reagents, which can be effectively contacted through a ball collision, 1,5 although organic synthesis using a ball─ milling device has been regarded as a lesser─ known enabling technology. 5 We also investigated a solid─ phase reaction (unpublished data) using a planetary ball mill machine, in which the main board and containers (vessels) are both rotated in reverse directions to create intense mecha-nochemical energy. In the course of such research, there have been occasions in which the reaction sludge �rmly attached to the wall could not be cleaned using ordinary brushing; how-ever, such sludge could be cleaned using planetary ball milling with an excess amount of H 2O for a 1 min period. Owing to the ef�ciency of an impacting ball, this method was also found to slightly increase the internal pressure of the vessel (Figure 1). An analysis revealed that the only gas generated from the water is pure hydrogen (H 2), which was the starting point for the studies summarized in this paper.2.2　 Planetary Ball Mill Machine

Ball mills are classi�ed into drum ball mills, jet─ mills, bead─ mills, vibration ball mills, planetary ball mills, horizontal rotary ball mills, and mixer mills, among others. The hydrogen generation reaction described herein can be achieved using a planetary ball mill, which creates high mechanochemical energy, whereas the reaction cannot proceed at all with a vibra-

Stainless Steel Ball Milling for Hydrogen Generation and its Application for Reduction

Yoshinari Sawama ＊, Miki Niikawa, and Hironao Sajiki ＊

＊Laboratory of Organic Chemistry, Gifu Pharmaceutical University 1─ 25─ 4, Daigaku─ nishi, Gifu 501─ 1196, Japan

(Received July 11, 2019; E─ mail: sawama@gifu─ pu.ac.jp, sajiki@gifu─ pu.ac.jp)

Abstract: Hydrogen (H 2) is an environmentally friendly energy source that produces only water through com-bustion and is a useful reducing agent in organic chemistry. However, according to the law, it needs to be strictly stored, transported, and managed in pressure vessels. Although H 2 is mainly produced industrially by the steam reforming of methane (CH 4), this is not environmentally friendly owing to the emissions of carbon dioxide (CO 2) as well as the energy input from the endothermic reaction. Herein, hydrogen generation is achieved from water (H 2O and D 2O), alkane, and diethyl ether (Et 2O) without CO 2 emissions through the mechanochemical collision of stainless─ steel balls using a planetary ball mill machine. Furthermore, in situ generated H 2 can be directly utilized as a reductant for various reducible functionalities, such as alkynes, alkenes, ketones, nitro groups, and aromatic halides. In addition, D 2 derived from D 2O promotes a reductive deuteration to provide the corresponding deuterated products. When using Et 2O as a hydrogen source, a reduc-tion of arene can be achieved. In this paper, the hydrogen generation from water, alkanes, and Et 2O, subsequent hydrogenation, and the role of stainless─ steel balls are summarized.

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tion ball mill or other device owing to the insuf�cient gene-ration of energy.

Hydrogen generation studies have primarily been con-ducted using a Fritsch Pulverisette Premium Line 7 Ball Mill (PLP─ 7; Figure 2, right), the vessel of which can be tightly sealed and enables the accessible collection of internal gases through a valve attached to the lid. In addition, the internal gas pressure and temperature are directly monitored by a dedi-cated system equipped with a wireless manometer terminal to observe the real─ time internal pressure and temperature. Meanwhile, the hydrogenation of the reducible substrates was conducted using Fritsch Pulverisette 7 Classic Line Ball Mill (P─ 7; Figure 2, left), which is a simpler device without a gas valve. For each reaction described herein, balls of approxi-mately 5 mm in diameter were used.

2.3　 Time Dependence of the Internal Gas and TemperatureFirst, 15 mmol (270 μL) of H 2O was degraded to H 2 with

stainless steel (SUS304: ca. 69% Fe, 8─ 10% Ni, and 18─ 20% Cr) balls with a diameter of approximately 5 mm in a 80 mL SUS304 vessel at a rotational speed of 800 rpm (Figure 3). The internal pressure and temperature increased in a time─ depen-dent manner and reached a plateau within 60 min at approxi-mately 6,800 hPa and 60 ℃. After cooling to 27 ℃, the internal pressure was shown to be 5,600 hPa, from which it was calcu-lated that 18.0 mmol of gases was accumulated in an 80 mL SUS304 vessel (Figure 4). Because 3.25 mmol of the original gases existed in the vessel (80 mL) before the reaction, 14.75 (=18.0─ 3.25) mmol of gas was generated. An analysis of the internal gas implied that H 2 was only generated without the emission of molecular oxygen (O 2) derived from the H 2O. Namely, no detonating gas composed of H 2 and O 2 was gene-rated, and H 2 could be obtained almost exclusively quantita-tively without a separation process.

The H 2 generation ef�ciencies are dramatically dependent on the rotation speed of the ball mill machine (Figure 5), and

the internal pressure rapidly increased at 800─ 1,100 rpm. The H 2 generation also proceeded ef�ciently at a rotational speed of 600 rpm or more, and the quantitative conversion of H 2O into H 2 could be achieved at 600 to 1,100 rpm within 30 to 360 min. After cooling to 27 ℃, the internal pressure of the reaction vessel at each rotation speed was approximately 5,600 hPa, which corresponded to a nearly quantitative amount of H 2 generated from H 2O in the vessel (Figure 4). The internal pressure of each reaction vessel at only the reac-tion endpoint of each rotational speed was signi�cantly differ-ent owing to the difference in the internal temperature, and depended on the rotational energy, as shown in Figure 5 (a) and (b). The H 2 generation was incomplete at 400 rpm even after 360 min, and was never observed at 200 rpm.

Although iron powder was reported to react with H 2O to form H 2 under ball─ milling conditions, 6 and the use of SUJ─ 2 and carbon steel balls, both composed mostly of iron metal (Table 1), instead of SUS304 balls, were inactive for H 2 gene-ration from H 2O under the same ball─ milling conditions (Figure 6). By contrast, SUS316 (ca. 65% Fe, 10─ 14% Ni, and 16─ 18% Cr) consisting of similar main components as SUS304 effectively facilitated the H 2 generation.2.4　 Direct Hydrogenation using in situ Generated Hydrogen

from H 2O using Stainless Steel Balls 7

A mixture of diphenylacetylene (1, 0.5 mmol) and H 2O (15 mmol) was milled with SUS304 balls (50 pieces) in a 12 mL SUS304 vessel at 800 rpm for 3 h to provide a mixture of cis ─ and trans ─ stilbenes (2 and 3) together with 1,2─ diphenylethane (4) as the hydrogenated products (Table 2, entry 1). Namely,

Figure 2.　 Fritsch Pulverisette 7 Classic Line Ball Mill (P─ 7, left) and Fritsch Pulverisette Premium Line 7 Ball Mill (PLP─ 7, right).

Figure 1.　Hydrogen generation from water through ball milling.

Figure 3.　 Monitoring of internal pressure and temperature (a SUS304 ball with a diameter of approximately 5 mm was used).

Figure 4.　 Theoretical calculation of H 2 generation in 80 mL SUS304 vessel after a ball─ milling reaction at 800 rpm for 60 min and subsequent cooling to 27 ℃.

Vol.77 No.11 2019 （ 19 ） 1071

H 2 was generated from H 2O mediated by the collision of SUS304 balls and immobilized to an alkyne moiety of 1. Although the reaction using SUS316 balls instead of SUS304 balls also provided the hydrogenated products (entry 2), the use of SUJ─ 2 balls, which were inactive for the H 2 generation, as well as the use of brass (Cu/Zn), alumina (Al 2O 3), and zirco-nia (ZrO 2) balls failed to facilitate the hydrogenation of 1 (entries 3─ 6).

The use of H 2O, the number of SUS304 balls applied, and rotation speed strongly in�uenced the hydrogenation ef�cien-cies (Table 3). As a result of elongation, after a rotation time of 12 h under the conditions shown in entry 1 of Table 2, the hydrogenation of 1 was completed to provide 4 at a 98% iso-lated yield. A reduction in the rotation speed (to 650, 500, or 250 rpm) resulted in a low conversion or no reaction. A small number of balls (entry 1 versus entries 5 and 6) and H 2O (entry 1 versus entries 7 and 8) also caused less effective hydro-genation.

The H 2O─ mediated hydrogenation in a SUS─ Fritsch Pul-verisette Premium Line 7 Ball Mill (PLP─ 7) using stainless

Figure 5.　 Rotation speed─ dependent increase in H 2 generation ef�ciency.

Figure 6.　H 2 generation ef�ciency of several iron alloy balls.

Table 1.　Component composition of various materials.

Table 2.　Hydrogenation using various balls.

Table 3.　 Optimization of hydrogenation using H 2O and SUS304 balls.

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steel was applicable to the hydrogenation of a wide variety of substrates bearing various functional groups (Table 4). The multiple bonds of 4─ methoxyphenyl acetylene, 1,1─ diphenyl-ethylene, and cinnamyl alcohol were effectively hydrogenated (entries 1─ 3). While aromatic halides underwent hydrodebro-mination or hydrodeiodination to provide dehalogenated ben-zene derivatives (entries 4─ 6), the debenzylation of benzyl (Bn) ether or the hydrolysis of ester failed to proceed. Hydrogena-tion reactions of a nitro group into an amine group and a ketone group into a hydroxy group were also conducted to provide the corresponding hydrogenated products (entries 7─ 12). In addition, azide was ef�ciently reduced into an amine product (entry 13).

2.5　 Deuterium Generation from D 2O and its Application to Reductive Deuteration 7

In addition, D 2O (15 mmol) was effectively degraded into D 2 under SUS304 ball─ induced milling conditions as with the case of H 2O degradation (Figure 7). Although the C─ D bond is fundamentally more stable than the C─ H bond owing to the isotopic effect, the generation rate of D 2 was similar to that of H 2 (Figure 5 and 7, respectively).

Encouraged by these results, the reductive deuteration

(deuterogenation) was examined. Deuterium─ labeled com-pounds are utilized in a wide variety of scienti�c �elds, such as a mechanistic investigation of organic reactions, tracers used in microanalyses, elucidation of the drug metabolism, heavy drugs, and quantitative mass spectrometry analysis. 8 Therefore, various synthetic methods have been extensively investi-gated. 9,10 Although gaseous D 2 is a useful and commercially available deuterium source, the use of expensive D 2 is still problematic owing to its cost and �ammability. After the rota-tion of a SUS304 vessel including the substrate (0.5 mmol), D 2O (15 mmol, 30 equiv), and SUS304 balls (diameter of approximately 5 mm, 50 balls) at 800 rpm, the deuterated products were obtained through deuterogenation using in situ generated D 2 in a SUS304 vessel (Table 5). The alkyne moiety was transformed into a 1,1,2,2─ tetra─ deuterated ethane func-tion (entries 1─ 3), whereas the 1,2─ dideutero ethane function was converted from the corresponding ole�n moiety (entries 4 and 5). Mono─ deuterated benzene products were obtained through the reductive deuteration of aromatic halides (entries 6 and 7). The ketone was converted into a secondary alcohol, the α ─ position of which was selectively deuterated (entries 8 and 9).2.6　 Effect of Metal Components of SUS304 4,7

As shown in Table 1, SUS304 is composed of Fe (ca. 69%), Ni (8─ 10%), and Cr (18─ 20%) as the main components. The generation of H 2 was not observed using ZrO 2 balls in a ZrO 2 vessel (Table 6, entry 1). Although a remarkable generation of H 2 was not observed through the addition of Fe or Ni powder (entries 2 and 4) under the ball milling conditions using ZrO 2 balls and a vessel, H 2 gas was effectively produced with the coexistence of additional Cr powder (entry 3).

Similarly, the hydrogenation of diphenylacetylene (1) under atmospheric hydrogen does not proceed using ZrO 2 balls in a ZrO 2 vessel (Table 7, entry 1) and the addition of Fe or Cr was also ineffective for the hydrogenation (entries 2 and 3). However, the hydrogenation of 1 proceeded to provide a mix-ture of hydrogenated products (2─ 4) using Ni powder as an additive (entry 4) .

After the generation of H 2 from H 2O using SUS304 balls and a vessel, a black powder was obtained, as shown in Figure 8. On the surface (sputtering time of 0 min) of the fresh SUS304 powder, oxidized Cr, Fe, and Ni ions exist, whereas

Table 4.　Scope of substrates used in hydrogenation. a

Figure 7.　SUS304─ induced D 2 generation from D 2O.

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zerovalent Cr, Fe, and Ni metals were observed in the internal parts that had sputtered for a suitable time (ca. 15 min or more) based on the X─ ray photoelectron spectroscopy (XPS) analysis and its consecutive depth pro�ling (Ar + sputter rate = 20.0 nm/min@SiO 2, total time ≈ 30 min) (Figure 8, lower

part). Meanwhile, only an oxidized Cr ion peak was observed on the surface, and the internal areas of the black power obtained and the abundance ratio of the zerovalent Fe metal relatively decreased, although the Ni 0 metal remained unchanged.

These results (shown in Tables 6 and 7 and Figure 8) imply that H 2 is �rst generated through the oxidation of Cr 0 into Cr ions (Figure 9). The single─ electron transfer from excess Fe 0 likely oxidized into Cr ions, inducing the regeneration of Cr 0 and the generation of oxidized Fe ions. Ni 0 acts as a hydroge-nation catalyst in the presence of H 2 derived from H 2O. Although such reactions involving independent Fe 0, Cr 0, or Ni 0 metal do not generally proceed, the mechanochemical activation through ball─ milling using SUS304 balls enables the present H 2 generation and the subsequent hydrogenation in collaboration with the intervening (possibly catalytic) effect of the constituent metals of SUS304.

Figure 8.　 XPS analysis of black powder obtained after the generation of H 2.

Figure 9.　 Cr─ mediated H 2 generation and Ni─ catalyzed hydrogenation on SUS304.

Table 5　Application to deuterogenation with D 2O.

Table 6.　Essential metals in H 2 generation.

Table 7.　Essential metals in hydrogenation.

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3． Hydrogen Generation from Alkanes and Diethyl Ether 11

3.1　 SUS304─ mediated Degradation of AlkanesThe powerful mechanochemical energy achieved using the

planetary ball mill device also realized a degradation of the alkanes to produce H 2 (Table 8). Surprisingly, n ─ pentane (C 5H 12), n ─ hexane (C 6H 14), and n ─ heptane (C 7H 16) (15 mmol each) bearing a comparatively short carbon chain were degraded under SUS304 ball─ milling conditions at 800 rpm for 1 h to produce H 2 (0.9─ 2.0 mmol) and CH 4 (0.6─ 2.3 mol, entries 1─ 3), whereas n ─ decane (C 10H 22) was somehow inactive (entry 4). Because the gaseous CH 4 does not undergo a degra-dation, H 2 should be generated directly from the correspond-ing liquid alkanes. The detailed reaction mechanism of this remains unclear. Furthermore, diethyl ether (Et 2O) can also be a hydrogen source to provide 3.01 mmol of H 2 (entry 5). Simi-larly, di─ n ─ propyl ether (n ─ Pr 2O) bearing one more carbon in each side chain does not transform into H 2. The length effect of the aliphatic chain is also under consideration.

3.2　 Hydrogenation using Alkane and Diethyl EtherAlthough the amount of H 2 generation from alkanes and

Et 2O were not quantitative, in situ generated H 2 was directly utilized for hydrogenation (Scheme 1). Here, 1,1─ diphenyleth-ene (5) was partially hydrogenated under SUS304 ball─ milling conditions at 800 rpm for 6 h into 1,1─ diphenylethane (6) in only a 24% yield together with a 60% recovery of 5 (Scheme 1, upper part). Unexpectedly, the reduction using Et 2O as a hydrogen source effectively proceeded into 1,1─ dicyclohexyle-thane (7) through the hydrogenation of the ole�n moiety and both arene nuclei. Arene reduction is a useful method for the preparation of cyclohexane derivatives, which are utilized in the �eld of next─ generation energy development as liquid organic hydrogen carriers such as methylcyclohexane (MCH) 12 and others. 13 However, the reduction (hydrogenation) of reso-nance─ stabilized arenes generally requires the use of excess

�ammable H 2 gas. We developed a H 2─ free arene reduction method using 2─ PrOH as a hydrogen source (an apparent reducing agent) in the presence of Pt/C. 14 Considering these backgrounds, an incredible Et 2O─ induced reduction in arene is valuable as an H 2─ free arene reduction method.

Various arene moieties of heptylbenzene, biphenyl, and diphenylmethane were effectively hydrogenated using in situ generated H 2 derived from Et 2O under SUS304 ball─ milling conditions to provide the corresponding cyclohexane products (Table 9, entries 1─ 3). Multiple bonds such as alkyne and alkene were also reduced simultaneously (precedingly) with the arene reduction (entries 4 and 5). Naphthalene was trans-formed into a mixture of cis ─ and trans ─ decaline (entry 6). Benzophenone was transformed into dicyclohexylmethane through the Et 2O─ mediated hydrogenation of both aromatic rings and ketone moieties and the subsequent hydrogenolysis of the intermediary benzyl alcohol (entry 7). Aliphatic ketone and alkyne derivatives without aromatic rings within the mole-cule were also hydrogenated to provide the corresponding alcohol and alkane derivatives (entries 8─ 10).

The generation of H 2 from Et 2O was not observed under the ball─ milling conditions using ZrO 2 balls and a vessel (Table 10, entry 1). In similarity to the mechanochemical deg-radation of H 2O into H 2, the addition of Cr metal facilitated the H 2 generation from Et 2O along with the formation of CH 4 (entry 3). By contrast, Fe and Ni metals were inactive for the gas generation from Et 2O (entries 2 and 4).

Ni powder operates as a catalyst for the arene hydrogena-

Table 8.　H 2 generation from alkanes and ether.

Table 9.　Arene reduction using Et 2O as a hydrogen source.

Scheme 1.　 Hydrogenation and arene nuclei reduction using in situ generated H 2 from alkane and Et 2O.

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tion of n ─ heptylbenzene (8) under atmospheric H 2 conditions using ZrO 2 balls and vessels (Table 11, entry 4), whereas the reaction without an additive or with Fe or Cr powder as an additive resulted in no reaction (entries 1─ 3). These are similar to the reactions using H 2O for H 2 generation and the subse-quent hydrogenation (see Tables 6 and 7).

The present mechanochemical degradation reaction accompanied by the generation of hydrogen gas from H 2O, alkanes, and Et 2O is summarized below.(1) Cr metal, which is a component of SUS304, is essential in generating H 2 from H 2O, alkanes, and Et 2O.(2) Ni metal as another component of SUS304 acts as a cata-lyst for the hydrogenation of the coexisting reducible functiona-lities using in situ generated H 2.(3) It is likely that Fe metal partially suppresses the oxidation of Cr metal and regenerates Cr metal through the reduction of Cr ions formed after H 2 generation.(4) Fe metal is a pivotal component used to increase the strength of the balls, which creates an intense mechanochemi-cal energy assisting in the degradation of H 2O, alkanes, and Et 2O into H 2.

4． Conclusion

The authors developed a unique hydrogen generation method without the emissions of CO 2 under mechanochemical (ball─ milling) conditions using SUS304 stainless steel balls and a vessel. With this method, H 2O, alkanes, and Et 2O can be used as a hydrogen source, and the hydrogen transfer (hydroge-nation) reaction is also possible using in situ generated H 2. It is noteworthy that Et 2O is applicable as an apparent reductant for an arene reduction, which has required the use of an exces-sive amount of H 2 in previous methods. An investigation into other target materials for H 2 generation and its subsequent reduction (hydrogenation) using the presented methodology is ongoing in our laboratory. The cooperative activation by three metals (Fe, Cr, and Ni) in SUS304 is exciting and expected to contribute to the further development of various chemical �elds.

AcknowledgementsThis work was partially supported by a Grant─ in─ Aid for

Scienti�c Research (B) (26670005) 16H05075 and a Canon Foundation for H.S., and a Grant─ in─ Aid for Challenging Exploratory Research (26670005) and a Koshiyama Research Grant for Y.S. We sincerely thank Shiono Chemical Co., Ltd., for the partial �nancial support. We are also grateful for the kind assistance provided by Fritsch Japan Co, Ltd., relevant to the Fritsch Pulverisette Premium Line 7 Ball Mill (PLP─ 7). We would like to thank Editage (www.editage.com) for English language editing.

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Table 11.　Metal effect under atmospheric H 2. a

Figure 10.　Role of each metal in SUS304.

Table 10.　The metal effect for H 2 generation from Et 2O.

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PROFILE

Yoshinari Sawama has been an Associate Professor at Gifu Pharmaceutical University since 2017. He received his Ph.D. in 2006 in the laboratory of Prof. Dr. Yasuyuki Kita at Osaka University. After working as the post-doctoral fellows at Osaka University, Dort-mund Technical University, and Ritsumeikan University, he was appointed as an Assistant Professor at Gifu Pharmaceutical University in 2010. His research interests are in mecha-nochemical reaction, Lewis acidic chemistry, green sustainable chemistry, isotopic labeling, and total synthesis of natural products.

Miki Niikawa has been a researcher of the Laboratory of Organic Chemistry at Gifu Pharmaceutical University since 2013 and a visiting Associate Professor at Gifu Pharma-ceutical University since 2016. She received her Ph.D. in 2007 from Gifu Pharmaceutical University under Professor Hisamitsu Na-gase.Her research interests are in environmental chemistry and the development of next─ generation energy.

Hironao Sajiki has been a Professor at Gifu Pharmaceutical University since 2006. He re-ceived his Ph.D. from Gifu Pharmaceutical University in 1989 under the direction of Prof. Yoshifumi Maki. After serving as a Postdoctoral Fellow at the State University of New York at Albany and Massachusetts Institute of Technology, he joined to Epix Pharmaceuticals, MA, USA as a group lead-er. In 1995, he moved to Gifu Pharmaceutical University as an Assistant Professor. He has also been the president of the Japanese Soci-ety of Process Chemistry since 2017. His re-search interests include the development of heterogeneous transition metal catalysts, post─ synthetic deuteration, catalytic hydro-gen production, and practical synthetic methodologies.

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