Supported Metal Hydroxides As Efficient Heterogeneous Catalysts for Green Functional Group Transformations

Journal of the Japan Petroleum Institute, 57, (6), 251-260 (2014) 251

[Review Paper] Supported Metal Hydroxides as Efficient Heterogeneous Catalysts for Green Functional Group Transformations

Kazuya YAMAGUCHI and Noritaka MIZUNO＊

Dept. of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, JAPAN

(Received August 23, 2014)

This review article mainly describes our research on the development of highly active heterogeneous catalysts based on the properties of metal hydroxides and efﬁcient green functional group transformations using these catalysts. Metal hydroxide species have both Lewis acid (metal center) and Brønsted base (hydroxy group) sites on the same metal sites. Combined action of the Lewis acid and Brønsted base paired sites can activate various types of organic substrates; for example, alcohols→alkoxides, amines→ amides, nitriles→η2-amidates, and terminal alkynes→acetylides. In the presence of supported ruthenium hydroxide catalyst (Ru(OH)x/Al2O3), oxidative dehydrogenation of alcohols and amines efficiently proceeds, forming the corresponding carbonyl compounds (aldehydes and ketones) and nitriles, respectively. Ru(OH)x /Al2O3 can also catalyze hydration of nitriles to primary amides, ammoxidation of primary alcohols to nitriles, and formal α-oxygenation of primary amines to primary amides. In addition, oxidative alkyne homocoupling and 1,3-dipolar cycloaddition efﬁciently proceed in the presence of supported copper hydroxide catalysts (Cu(OH)x /OMS-2 for homocoupling, Cu(OH)x/TiO2 and Cu(OH)x/Al2O3 for cycloaddition; where OMS-2 is manganese oxide-based octahedral molecular sieve, KMn8O16). The catalytic processes for these transformations are heterogeneous, and the catalysts can be reused several times with maintained high catalytic activity.

Keywords Supported metal hydroxide, Heterogeneous catalysis, Concerted activation, Lewis acid, Brønsted base, Green chemistry

1. Introduction lysts will be very signiﬁcant. Our strategy to design highly active heterogeneous The development of efﬁcient heterogeneous catalysts catalysts is to create monomerically (or at least highly) is of particular research interest because of the potential dispersed metal hydroxide species on appropriate sup- to realize green functional group transformations1)～7). ports. Such monomerically (or highly) dispersed The immobilization of catalytically active species onto metal hydroxide species would possess both Lewis acid (inert) solid supports and the attachment of active and Brønsted base paired sites on the same metal sites, homogeneous catalysts through ionic or covalent bonds so that various types of substrates, including alcohols, onto surface modified supports have been extensively amines, nitriles, and terminal alkynes, would be activated investigated1)～7). However, various drawbacks are fre- by the concerted action of the Lewis acid and Brønsted quently observed; for example, (i) the intrinsic catalytic base paired sites, as shown in Fig. 18). performance of the parent active homogeneous catalysts Coordinated activation of substrates by the Lewis is often decreased by immobilization or attachment, acid and Brønsted base paired sites has already been and/or (ii) leaching of the active species is sometimes demonstrated in nitrile hydration by a monomeric cobalt observed. Therefore, the development of truly stable hydroxo complex9). The hydration is initiated by heterogeneous catalysts with at least comparable or coordination of a nitrile to the cobalt center (Lewis acid preferably better catalytic performance than the corre- site). As the nitrile is coordinated to the cobalt center, sponding homogeneous catalysts is very important. In the p orbital on the nitrile carbon can overlap with the addition, the development of effective new transforma- sp3 orbital on the neighboring hydroxo species. tions possible only by using such heterogeneous cata- Consequently, intramolecular nucleophilic attack of the hydroxo species (Brønsted base site) on the proximal DOI: dx.doi.org/10.1627/jpi.57.251 nitrile carbon is activated by the Lewis acid site and 2 ＊ To whom correspondence should be addressed. readily proceeds to form the η -amidate species, result- ＊ E-mail: [email protected] ing in promotion of hydration (Fig. 2)9). This coordi-

J. Jpn. Petrol. Inst., Vol. 57, No. 6, 2014 252

tion of NaOH (the detailed procedures for preparation of supported metal hydroxide catalysts are published elsewhere10)～19)). As we expected, various types of substrates could be activated by the coordinated action of the Lewis acid and Brønsted base sites (Fig. 1); for example, alcohols→alkoxides, amines→amides, nitriles →η2-amidates, and terminal alkynes→acetylides. Therefore, we successfully achieved various green functional group transformations using these supported metal hydroxide catalysts. The present review article summarizes our achieve- ments with heterogeneously catalyzed green functional group transformations using supported metal hydroxide catalysts as follows: (i) oxidative dehydrogenation of 10)～13) alcohols and amines by Ru(OH) x/Al2O3 , (ii) 14) hydration of nitriles by Ru(OH)x/Al2O3 , (iii) ammoxi- 15),16) dation of primary alcohols by Ru(OH)x/Al2O3 , (iv) formal α-oxygenation of primary amines by Ru(OH)x / 17) Al2O3 , (v) oxidative homocoupling of terminal 18) alkynes by Cu(OH)x /OMS-2 , and (vi) 1,3-dipolar Fig. 1● Activation of Various Substrates (alcohols→alkoxides, 2 cycloaddition of azides to terminal alkynes by Cu(OH)x / amines→amides, nitriles→η -amidates, and terminal 19) alkynes →acetylides) by Supported Metal Hydroxide TiO2 . Catalysts (M＝Ru or Cu) 2. Oxidative Dehydrogenation of Alcohols and Amines by Ru(OH)x /Al2O3

Oxidative dehydrogenation of alcohols and amines are very important reactions because of the versatile use of the oxidation products, i.e., aldehydes, ketones, nitriles, and imines, as starting materials for important pharmaceuticals, agrochemicals, fragrances, and fine chemicals20)～23). Numerous heterogeneous catalysts have been developed for oxidative dehydrogenation of alcohols and amines using O2 as the terminal oxi- dant22),23). The performance, applicabilities, and limi- tations of recently reported heterogeneous catalysts for aerobic oxidative dehydrogenation of alcohols have been comprehensively summarized22),23). Our10)～13) and other research groups24)～26) have focused on ruthenium hydroxide Ru(OH)x (hydrate oxide RuO2・nH2O) for the aerobic oxidative dehydrogenation of alcohols. Easily prepared Ru(OH)x /Al2O3 has the potential to 10 Fig. 2● (a) A 10 -Fold Rate Acceleration of Acetonitrile Hydration act as an efﬁcient heterogeneous catalyst for oxidative by the Concerted Activation and (b) the Formation of η2- Amidate Species9) dehydrogenation of various structurally diverse alcohols and primary amines using O2 or air (1 atm) as the terminal oxidant (Fig. 3)10)～13). The reaction hardly pro- nated activation of a nitrile by the Lewis acid and ceeds in the absence of catalysts or in the presence of Brønsted base paired sites on the same metal site pro- only supports, unsupported ruthenium hydroxide spe- 10 vided about a 10 -fold rate acceleration compared with cies Ru(OH)x, or anhydrous RuO2. A simple physical – 9) the common OH -catalyzed nitrile hydration (Fig. 2) . mixture of Ru(OH)x and Al2O3 had no catalytic activity. We have successfully prepared various types of sup- We also prepared a supported ruthenium chloride cata- ported metal hydroxide catalysts (M(OH)x /support; with lyst (RuClx /Al2O3). Oxidative dehydrogenation of M＝Ru or Cu, support＝Al2O3, TiO2, or OMS-2) by the benzyl alcohol hardly proceeded with RuClx /Al2O3 reaction of metal salts (RuCl3 or CuCl2) and appropriate under the conditions described in Fig. 3. RuClx /Al2O3 supports in aqueous solutions followed by base treat- supported catalyst was then treated with an aqueous ment using a 1.0 M (1 M＝1 mol dm–3) aqueous solu- solution of NaOH (0.1 M) at room temperature.

J. Jpn. Petrol. Inst., Vol. 57, No. 6, 2014 253

Fig. 4● Possible Reaction Mechanism for the Ru(OH)x /Al2O3- catalyzed Oxidative Dehydrogenation of Alcohols10)～13)

Reaction conditions: Ru(OH)x /Al2O3 (2.5 mol% for alcohol; 2.8 mol% for amine), alcohol or amine (1 mmol), PhCF3 (1.5 mL for alcohol; 5 mL for amine), 83 ℃ for alcohol; 100 ℃ for amine, O2 that the state of the ruthenium species in Ru(OH)x / (1 atm). a) Ru(OH)x /Al2O3 (5 mol%). b) Hydroquinone (1 equiv. Al2O3, e.g., the degree of polymerization and oxidation to Ru) was used. c) p-Xylene (5 mL), 130 ℃. state, hardly changed after these reactions. Moreover, Ru(OH)x/Al2O3 could be reused several times with Fig. 3● Ru(OH)x /Al2O3-catalyzed Oxidative Dehydrogenation of Alcohols and Amines10)～13) maintained high catalytic performance for these reactions. The addition of radical scavengers such as 2,6-di-t- Oxidative dehydrogenation of benzyl alcohol with the butyl-4-methylphenol and hydroquinone did not affect treated catalyst under the same conditions showed high the reaction rates and product selectivities, suggesting catalytic activity of the base-treated RuClx /Al2O3 and that radical intermediates were not formed during these resulted in a quantitative yield of benzaldehyde. These oxidative dehydrogenation reactions. Competitive results suggest that highly dispersed ruthenium hydrox- oxidative dehydrogenation of benzyl alcohol and ide species are important for the oxidative dehydroge- 1-phenylethanol with Ru(OH)x /Al2O3 showed that the nation of alcohols. dehydrogenation of benzyl alcohol proceeded ca. 10 Aerobic oxidative dehydrogenation of various types times faster than that of 1-phenylethanol. This faster of structurally diverse alcohols efﬁciently proceeded in oxidation of benzyl alcohol even in the presence of the presence of Ru(OH)x /Al2O3, forming the corre- more reactive 1-phenylethanol suggests the formation sponding aldehydes and ketones (Fig. 3). Oxidative of alkoxide species27)～29). The competitive dehydro- dehydrogenation of benzylic alcohols with electron- genation of p-substituted benzyl alcohols had a donating or electron-withdrawing substituents at each Hammett ρ value of －0.46. This negative Hammett ρ position of the benzene rings also efﬁciently proceeded value can be interpreted in terms of the formation of a to form the corresponding benzaldehydes. Allylic carbenium ion-type transition state through hydride alcohols were converted to the corresponding α,β- abstraction, in which electron-donating substituents sta- unsaturated carbonyl compounds without hydrogena- bilize the carbenium ion-type transition state30). On tion of the double bonds. This catalytic system could the basis of these results, a possible reaction mechanism also be applied to the conversion of aliphatic and hetero- for Ru(OH)x /Al2O3-catalyzed oxidative dehydrogena- aromatic alcohols. Furthermore, various types of tion of alcohols is proposed (Fig. 4). The oxidative structurally diverse primary amines including benzylic, dehydrogenation of primary and secondary amines also allylic, and aliphatic amines could be converted into the proceeds through the same pathway as that for alcohols; corresponding nitriles (Fig. 3). In addition, second- that is, amide formation followed by β-elimination. ary amines such as N-phenylaniline and dibenzylamine This experimentally proposed reaction mechanism could be converted into the corresponding imines was further theoretically investigated by density func- (Fig. 3). These oxidative dehydrogenation reactions tional theory (DFT) calculations using model catalysts completely stopped after removing the Ru(OH)x/Al2O3 “Ru(OH)3(OH2)3” (Fig. 5) and “RuCl3(OH2)3” for 31) catalyst by hot filtration. X-ray diffraction (XRD), Ru(OH)x /Al2O3 and RuClx /Al2O3, respectively . X-ray photoelectron spectroscopy (XPS), and X-ray Using 2-butanol as the model substrate, the calculated absorption fine structure (XAFS) analyses confirmed activation energy for alkoxide formation (step 1 in

J. Jpn. Petrol. Inst., Vol. 57, No. 6, 2014 254

Reaction conditions: Ru(OHx /Al2O3 (1 mol%), alcohol (1 mmol), toluene (3 mL), 80 ℃, Ar (1 atm), 24 h.

Fig. 6● Ru(OH)x /Al2O3-catalyzed Racemization of Chiral Secondary Alcohols32),33) ] 1 – E [ k J m o l

Fig. 7 Classical Synthetic Procedure for Primary Amides

bond, resulting in lower activation energy for the O2 insertion step. Then, the hydroxide (or alkoxide) spe- Blue, red, and white spheres indicate the Ru, O, and H atoms, respec- cies is regenerated with the formation of H2O2. tively.

3. Hydration of Nitriles by Ru(OH)x /Al2O3 Fig. 5● ( a ) O p t i m i z e d S t r u c t u r e o f t h e M o d e l C a t a l y s t “Ru(OH)3(OH2)3” and Energy Diagrams of (b) Alkoxide Formation and (c) β-Elimination Steps31) Amides are one of the most important compounds in modern chemistry as well as biology, and are widely utilized as raw materials for engineering plastics, fra- –1 Fig. 4) was 27.7 kJ mol with Ru(OH)3(OH2)3 and grance intensiﬁers, anti-block reagents, color pigments –1 123.2 kJ mol with RuCl3(OH2)3. Therefore, alkoxide for inks, detergents, lubricants, and starting materials 34),35) formation with Ru(OH)x /Al2O3 would proceed much for peptide and protein synthesis . Aminolysis of more easily than with RuClx /Al2O3. This calculation activated carboxylic acid derivatives, e.g., acid chlo- result is consistent with the above-mentioned experi- rides and acid anhydrides, with amines is still widely 36),37) mental results using Ru(OH)x /Al2O3 and RuClx /Al2O3. utilized for synthesis of amides . However, ami- The activation energy for β-elimination (step 2 in nolysis requires stoichiometric reagents, e.g., SOCl2, Fig. 4) was calculated to be 77.2 kJ mol–1 (Fig. 5). for the pre-activation of carboxylic acids. In addition, Therefore, step 2 is included in the rate-determining at least equimolar amounts of byproducts are formed step for the present oxidative dehydrogenation. The not only during the aminolysis but also the pre-activation kinetic isotope effects also support this conclusion10)～13). step; a typical procedure for the synthesis of nicotin- The activation energies for the reverse reactions of amide is shown in Fig. 736),37). Thus, the development β-elimination (hydride readdition) and alkoxide forma- of green catalytic procedures for the synthesis of tion were calculated to be 38.5 kJ mol–1 and 19.8 kJ amides from readily available and inexpensive starting mol–1, respectively (Fig. 5). Therefore, racemization materials is a very important subject38). Currently, of 2-butanol can readily proceed in the absence of O2. benzonitrile or its substitutes and acrylonitrile are in- Indeed, racemization of various optically active second- dustrially produced by ammoxidation using O2 as the ary alcohols efficiently proceeded in the presence of oxidant and NH3 as the nitrogen source. Thus, nitrile Ru(OH) x /Al2O3 under “anaerobic” conditions hydration is a reliable procedure from both environmen- (Fig. 6)32),33). tal and economical viewpoints for the synthesis of ben- For the reaction of the hydride species with O2 (step zamide derivatives and acrylamide. 3 in Fig. 4), the coordination of the electron-donating As mentioned in Section 1., nitrile hydration can be ligands (in particular, an alcohol molecule and H2O) to promoted by the coordinated activation of nitriles by form the six-coordinated ruthenium monohydride (Ru_ the Lewis acid and Brønsted base paired sites (Fig. 2)9). H) species is the key to promote O2 insertion into the Inspired by this pioneering work, we prepared various hydride species31). Electron donation from these types of supported metal hydroxide catalysts for nitrile ligands to the hydride species can weaken the Ru_H hydration. Fortunately, we found that nitrile hydration

J. Jpn. Petrol. Inst., Vol. 57, No. 6, 2014 255

Reaction conditions: Ru(OHx/Al2O3 (10 mol%), primary alcohol

Reaction conditions: Ru(OHx /Al2O3 (4 mol%), nitrile (1 mmol), (0.5 mmol), THF solution of NH3 (0.45 M, 2 mL), 120 ℃, air (6 atm). water (3 mL), 140 ℃. a) 130 ℃ . Fig. 10● Ru(OH)x /Al2O3-catalyzed Ammoxidation of Primary 14) 15),16) Fig. 8 Ru(OH)x /Al2O3-catalyzed Hydration of Nitriles Alcohols

304 m2 g–1) using only 2 equiv. (or less) of water with 39) respect to the nitrile . Such MnO2-catalyzed nitrile hydration efficiently proceeded even without any sol- vents39).

4. Ammoxidation of Primary Alcohols by Ru(OH)x / Al2O3 Fig. 9● (a) Reaction Mixture for the Ru(OH)x /Al2O3-catalyzed Hydration of Benzonitrile, (b) Filtrate after Removal of Nitriles are an important class of chemicals that are Ru(OH)x /Al2O3 by Hot Filtration, and (c) Crystals of Benzamide Appeared after Cooling the Filtrate at 0 ℃14) widely used as starting materials for various important pharmaceuticals, agrochemicals, and fine chemicals40)～43). The SN2 substitution of alkylhalides, the Sandmeyer efficiently proceeded in water in the presence of several reaction, and the Wittig reaction are the most commonly metal hydroxide catalysts; in particular, Ru(OH)x /Al2O3 utilized procedures for laboratory scale nitrile synthe- showed high catalytic activity and selectivity14). The sis40)～43). Oxidative dehydrogenation of primary potential of the Ru(OH)x /Al2O3-catalyst system for the amines is a good candidate, as mentioned in Section 2. synthesis of various types of structurally diverse nitriles Ammoxidation is an attractive procedure for green is summarized in Fig. 8. Benzonitriles were hydrated nitrile synthesis. to the corresponding benzamides with little change in Ru(OH)x/Al2O3 showed high catalytic performance the reaction rates caused by the electronic variation due for ammoxidation of primary alcohols to nitriles using to substituents on the benzene rings. In addition, a THF (or 1,4-dioxane) solution of NH3 in the presence 15),16) Ru(OH)x /Al2O3 successively catalyzed the hydration of of air as the oxidant (Fig. 10) . Ammoxidation of heteroaromatic nitriles. Less reactive aliphatic nitriles benzylic alcohols, which contain electron-donating as could also be selectively hydrated to the corresponding well as electron-withdrawing substituents, efficiently aliphatic primary amides without formation of carbox- proceeded, giving the corresponding substituted benzo- ylic acids. nitriles. Aromatic allylic alcohols were converted to After the hydration of benzonitrile was completed, the corresponding unsaturated nitriles without hydroge- Ru(OH)x /Al2O3 could readily be separated from the re- nation, isomerization, or hydration of the double bonds. action mixture by simple hot filtration, and no ruthenium Heteroaromatic alcohols were also converted into the was detected in the filtrate (Fig. 9). After separation corresponding heteroaromatic nitriles. Although the of the catalyst, the filtrate was cooled to 0 ℃ and ana- present system was applicable to the ammoxidation of lytically pure white crystals of benzamide were deposited benzylic, aromatic allylic, and heteroaromatic alcohols, (Fig. 9). The crystals could easily be retrieved from the reaction of simple aliphatic alcohols was unfortu- the water solvent by simple decantation. Thus, the nately unsuccessful; for example, 1-octanol gave only present hydration process was totally free of organic 4 % yield of n-octanenitrile under the conditions de- solvent even in the workup process. The observed scribed in Fig. 10. This is a limitation of the present catalysis reaction was truly heterogeneous in nature. Ru(OH)x /Al2O3-catalyzed ammoxidation method. In addition, the retrieved catalyst could be reused at The reaction profiles for the ammoxidation of benzyl least twice for the hydration of benzonitrile. alcohol showed that benzaldehyde was initially pro- Recently, we found that hydration of various types of duced followed by the formation of benzonitrile. nitriles efficiently proceeded in the presence of amor- Moreover, Ru(OH)x /Al2O3 showed high catalytic per- phous MnO2 (with relatively large BET surface area, formance for the transformation of various types of

J. Jpn. Petrol. Inst., Vol. 57, No. 6, 2014 256

Boc＝t-butoxycarbonyl.

Fig. 11 Classical α-Oxygenation of Primary Amines44) Reaction conditions: Ru(OHx /Al2O3 (5 mol%), primary amine (0.25 mmol for benzylic amines; 0.5 mmol for aliphatic amines), aldehydes to the corresponding nitriles under the condi- water (1 mL for benzylic amines; 2 mL for aliphatic amines), 140 ℃, tions described in Fig. 1015),16). In particular, aliphatic air (5 atm). a) 160 ℃. b) 130 ℃. aldehydes could be converted into the corresponding Fig. 12● Ru(OH)x /Al2O3-catalyzed Formal α-Oxygenation of 15),16) aliphatic nitriles . Primary Amines17) The present ammoxidation reaction possibly proceeds through three sequential reactions of (i) aerobic oxidative dehydrogenation of the alcohol to aldehyde, (ii) dehydrative condensation of the aldehyde and NH3 to aldimine, and (iii) aerobic oxidative dehydrogenation of the aldimine to form the corresponding nitrile as the ﬁnal product. The observed catalysis was intrinsically heterogeneous, and the catalyst could be reused several times.

5. Formal α-Oxygenation of Primary Amines by Ru(OH)x /Al2O3

As mentioned in Section 3., nitrile hydration is one of the most reliable procedures for the synthesis of benzamide derivatives and acrylamide. On the other hand, other nitriles have generally been synthesized by expensive non-green procedures40)～43), so that nitrile hydration is not the best choice. α-Oxygenation of primary amines would be one of the most useful synthetic procedures for primary amides. However, in general, oxygenation of the α-position of primary Fig. 13● (a) Structure of OMS-2 and (b) Possible Reaction Path for amines is very difficult, although in-situ generation of the OMS-2-catalyzed Oxidative Amidation48)～51) strong oxidant RuO4 from RuO2・nH2O and NaIO4 has been utilized with protection of the amino groups, for example (Fig. 11)44). As mentioned in Sections 2. and amine. In addition, non-activated linear, branched, 3., Ru(OH)x /Al2O3 acts as an efficient heterogeneous and cyclic aliphatic amines could be converted into the catalyst for oxidative de hydrogenation of primary corresponding aliphatic primary amides. Catalysis of amines to nitriles as well as hydration of nitriles to pri- this transformation was heterogeneous, and Ru(OH)x/ mary amides. Therefore, sequential one-pot perfor- Al2O3 could be reused without significant loss of cata- mance of these two reactions would represent “formal” lytic performance. α-oxygenation of primary amines to primary amides. Quite recently, we found that manganese oxide-based Fortunately, in the presence of Ru(OH)x/Al2O3, vari- octahedral molecular sieve (OMS-2, KMn8O16, ous primary amines are efficiently converted into the Fig. 13)45)～47) could act as an efficient reusable hetero- corresponding primary amides in water using air as the geneous catalyst for formal α-oxygenation of primary 17) 48) only oxidant . The potential of the Ru(OH)x/Al2O3- amines to primary amides (Fig. 13) . In addition, catalyzed formal α-oxygenation of primary amines to amidation of primary alcohols49),50) and alkylarenes51) primary amides is summarized in Fig. 12. The using NH3 as the nitrogen source also efficiently pro- α-oxygenation of benzylamine derivatives efficiently ceeded in the presence of OMS-2. proceeded to form the corresponding benzamide derivatives. Nicotinamide could be obtained from 3-picolyl-

J. Jpn. Petrol. Inst., Vol. 57, No. 6, 2014 257

Fig. 14● Pictures of Reaction Solutions (a) before and (b) after the Homocoupling of Phenylacetylene53)

Reaction conditions: Cu(OH)x /OMS-2 (2 mol%), terminal alkyne (0.5 mmol), toluene (2 mL), 100 ℃, O2 (1 atm), 10 min. a) 5 mol%. b) 10 mol%. c) 3 mol%.

Fig. 16● Cu(OH)x /OMS-2-catalyzed Oxidative Homocoupling of Terminal Alkynes18)

Fig. 15● Strategy for the Cu(OH)x /OMS-2-catalyzed Oxidative performance for the homocoupling of terminal alkynes 18) 53) Homocoupling of Terminal Alkynes18) (Fig. 16) . In sharp contrast to Cu(OH)x /TiO2 , little deactivation of Cu(OH)x /OMS-2 was observed, and Cu(OH)x /OMS-2 could be reused for the homocoupling 6. Oxidative Homocoupling of Terminal Alkynes of phenylacetylene at least thirteen times without appre- 18) by Cu(OH)x/OMS-2 ciable loss of its high catalytic activity . To investigate the mechanisms of OMS-2 in detail, Diyne derivatives are key structural elements for nat- control experiments were performed with the following ural product synthesis, polymer chemistry, and supra- findings. Treatment of Cu(OH)x /TiO2 (Cu: 10 μmol) molecular chemistry52). Oxidative homocoupling of with phenylacetylene under an Ar atmosphere converted terminal alkynes is one of the most widely utilized pro- an almost equimolar amount of phenylacetylene with cedures for the synthesis of symmetrical diynes52). We respect to the copper species into the corresponding found that supported copper hydroxide species on TiO2 diyne. On the other hand, similar treatment of (Cu(OH)x /TiO2, with mean Cu oxidation state of ＋2), Cu(OH)x /OMS-2 (Cu: 10 μmol) ca. 20 times larger could catalyze the aerobic oxidative homocoupling of amount of phenylacetylene than the copper species in 53) terminal alkynes . Various types of terminal alkynes Cu(OH)x /OMS-2, clearly showing that reoxidation of could be converted into the corresponding diynes. the reduced Cu(I) species occurs even under Ar atmo- However, the Cu(OH)x /TiO2-catalyzed system had the sphere in the presence of OMS-2. In addition, XPS serious problem of formation of inactive (polymeric) analysis confirmed that the oxidation state of the copper copper(I) acetylide species (Fig. 14, right) during the species in Cu(OH)x /OMS-2 was still ＋2 even after the homocoupling, probably due to the slow reoxidation of 13th reuse. Therefore, the reduced manganese species Cu(I) species with O2. As a result, severe deactivation in OMS-2 is possibly reoxidized by O2 under catalytic 45)～47) of the Cu(OH)x /TiO2 catalyst was observed. turnover conditions . To overcome such deactivation, the reoxidation rate Various types of terminal alkynes could be converted of the reduced copper species should be increased. A into the corresponding diynes using Cu(OH)x /OMS-2 “biomimetic oxidation strategy” has been developed (Fig. 16). Phenylacetylenes with electron-donating as using electron-transfer mediators (ETMs) to lower the well as electron-withdrawing substituents at various activation energies between reduced catalysts and oxi- positions were efficiently converted to the correspond- dants54). Based on this biomimetic oxidation strategy, ing 1,4-diphenyl-1,3-butadiyne derivatives. Hetero- we utilized OMS-2 as the support as well as the ETM aromatic, enyne, and silylacetylene derivatives were for the copper hydroxide species for the following rea- converted into the corresponding diynes. Homo- sons (Fig. 15): OMS-2 has (i) thermodynamically suit- coupling efficiently proceeded even for less reactive ali- able redox potential for reoxidation of Cu(I) using O2, phatic alkynes. Homocoupling of propargylic alcohol (ii) high electron-conduction property, (iii) oxygen and amine was also successful. reduction ability, and (iv) relatively large (external) surface area45)～47). As we expected, Cu(OH)x /OMS-2 had high catalytic

J. Jpn. Petrol. Inst., Vol. 57, No. 6, 2014 258

8. Conclusion

This review article summarized our research into efficient heterogeneously catalyzed green functional group transformations using supported metal hydroxide catalysts. Besides the above-mentioned transformations, we have successfully developed several functional group transformations using hydroxide-based heterogeneous catalysts; for example, oxygenation and aromatization of alkylarenes58), oxidative homocoupling of 2-naphthols and substituted phenols59), N-alkylation

Reaction conditions: Cu(OH)x /TiO2 (2.5 mol%), azide (0.5 mmol), of amines and NH3 using alcohols as the alkylation terminal alkyne (0.5 mmol), toluene (1.5 mL), 60 ℃, Ar (1 atm). reagents60),61), self-condensation of primary amines to secondary amines62), rearrangement of aldoximes to pri- Fig. 17● Cu(OH)x /TiO2-catalyzed 1,3-Dipolar Cycloaddition of 63),64) Azides to Terminal Alkynes19) mary amides , and dehydration of aldoximes to nitriles65). Our hydroxide-based strategy demonstrated here has high generality and applications especially in the fields of catalytic chemistry and synthetic organic chemistry.

Acknowledgment This work was accomplished through tremendous efforts of co-workers in our laboratory. This work was supported in part by Grants-in-Aid for Scientific Reaction conditions: Cu(OH)x /Al2O3 (1.5 mol%), azomethine Researches from Ministry of Education, Culture, (0.5 mmol), terminal alkyne (0.55 mmol), toluene (2 mL), 40 ℃, Ar (1 atm). Sports, Science and Technology.

Fig. 18● Cu(OH)x /Al2O3-catalyzed 1,3-Dipolar Cycloaddition of References Azomethine Imines to Terminal Alkynes57) 1) Sheldon, R. A., van Bekkum, H., “Fine Chemicals through Heterogeneous Catalysis,” Wiley, Weinheim (2001). 2) Mizuno, N., “Modern Heterogeneous Oxidation Catalysis 7. 1,3-Dipolar Cycloaddition of Azides to Terminal ─Design, Reactions and Characterization,” Wiley-VCH, Alkynes by Cu(OH)x /TiO2 Weinheim (2009). 3) Leadbeater, N. E., Marco, M., Chem. Rev., 102, 3217 (2002). As mentioned in Section 6., the copper species in 4) Lu, Z., Linder, E., Mayer, H. A., Chem. Rev., 102, 3543 (2002). 5) Wight, A. P., Davis, M. E., Chem. Rev., 102, 3589 (2002). Cu(OH)x /TiO were readily reduced to Cu(I) in the 2 6) De Vos, D. E., Dams, M., Sels, B. F., Jacobs, P. A., Chem. Rev., presence of terminal alkynes. This is a serious disad- 102, 3615 (2002). vantage for oxidation reactions but an advantage for 7) Corma, A., García, H., Chem. Rev., 102, 3837 (2002). 1,3-dipolar cycloaddition of azides to terminal alkynes 8) Yamaguchi, K., Mizuno, N., Synlett, 2365 (2010). because the active Cu(I) species55),56) are formed in-situ 9) Kim, J. H., Britten, J., Chin, J., J. Am. Chem. Soc., 115, 3618 without additional reducing agents. As we expected, (1993). 10) Yamaguchi, K., Mizuno, N., Angew. Chem., Int. Ed., 41, 4538 the 1,3-dipolar cycloaddition of various combinations (2002). of organic azides (including aromatic and aliphatic 11) Yamaguchi, K., Mizuno, N., Angew. Chem., Int. Ed., 42, 1479 ones) and terminal alkynes (including aromatic, aliphatic, (2003). and double bond-containing ones) exclusively proceeded 12) Yamaguchi, K., Mizuno, N., Chem. Eur. J., 9, 4353 (2003). 13) Yamaguchi, K., Kim, J. W., He, J. L., Mizuno, N., J. Catal., in the presence of Cu(OH)x /TiO to form the corre- 2 268, 343 (2009). sponding 1,4-disubstituted-1,2,3-triazole derivatives in 14) Yamaguchi, K., Matsushita, M., Mizuno, N., Angew. Chem., a completely regioselective manner, as shown in Int. Ed., 43, 1576 (2004). 19) Fig. 17 . Cu(OH)x /OMS-2 did not catalyze the 15) Oishi, T., Yamaguchi, K., Mizuno, N., Angew. Chem., Int. Ed., 1,3-dipolar cycloaddition. However, 1,3-dipolar 48, 6286 (2009). cycloaddition of azomethine imines to terminal alkynes 16) Oishi, T., Yamaguchi, K., Mizuno, N., Top. Catal., 53, 479 (2010). efficiently proceeded to form the corresponding N,N- 17) Kim, J. W., Yamaguchi, K., Mizuno, N., Angew. Chem., Int. bicyclic pyrazolidinone derivatives in the presence of Ed., 47, 9249 (2008). 57) Cu(OH)x/Al2O3 (Fig. 18) . 18) Oishi, T., Yamaguchi, K., Mizuno, N., ACS Catal., 1, 1351 (2011). 19) Yamaguchi, K., Oishi, T., Katayama, T., Mizuno, N., Chem.

J. Jpn. Petrol. Inst., Vol. 57, No. 6, 2014 259

Eur. J., 15, 10464 (2009). 42) Magnus, P., Scott, D. A., Fielding, M. R., Tetrahedron Lett., 20) Arends, I. W. C. E., Sheldon, R. A., ed. by Bäckvall, J.-E., 42, 4127 (2001). “Modern Oxidation Methods,” Wiley-VCH, Weinheim (2004), 43) Fatiadi, A. J., eds. by Patai, S., Rappaport, Z., “Preparation and pp. 83-118. synthetic applications of cyano compounds, in Triple-Bonded 21) Murahashi, S.-I., Komiya, N., ed. by Bäckvall, J.-E., “Modern Functional Groups, Vol. 2,” John Wiley & Sons, Ltd., Oxidation Methods,” Wiley-VCH, Weinheim (2004), pp. 165- Chichester, UK (1983). 191. 44) Tanaka, K., Yoshifuji, S., Nitta, Y., Chem. Pharm. Bull., 36, 22) Mallat, T., Baiker, A., Chem. Rev., 104, 3037 (2004). 3125 (1988). 23) Matsumoto, T., Ueno, M., Wang, N., Kobayashi, S., Chem. 45) DeGuzman, R. N., Shen, Y. F., Neth, E. J., Suib, S. L., O’Young, Asian J., 3, 196 (2008). C. L., Levine, S., Newsam, J. M., Chem. Mater., 6, 815 (1994). 24) Matsumoto, M., Watanabe, N., J. Org. Chem., 49, 3435 (1984). 46) Suib, S. L., J. Mater. Chem., 18, 1623 (2008). 25) Zhan, B.-Z., White, M. A., Sham, T.-K., Pincock, J. A., Doucet, 47) Suib, S. L., Acc. Chem. Res., 41, 479 (2008). R. J., Rao, K. V. R., Robertson, K. N., Cameron, T. S., J. Am. 48) Wang, Y., Kobayashi, H., Yamaguchi, K., Mizuno, N., Chem. Chem. Soc., 125, 2195 (2003). Commun., 48, 2642 (2012). 26) Yu, H., Fu, X., Zhou, C., Peng, F., Wang, H., Yang, J., Chem. 49) Yamaguchi, K., Kobayashi, H., Oishi, T., Mizuno, N., Angew. Commun., 2408 (2009). Chem., Int. Ed., 51, 544 (2012). 27) Sharpless, K. B., Akashi, K., Oshima, K., Tetrahedron Lett., 50) Yamaguchi, K., Kobayashi, H., Wang, Y., Oishi, T., Ogasawara, 17, 2503 (1976). Y., Mizuno, N., Catal. Sci. Technol., 3, 318 (2013). 28) Kanemoto, S., Matsubara, S., Takai, K., Oshima, K., Utimoto, 51) Wang, Y., Yamaguchi, K., Mizuno, N., Angew. Chem., Int. Ed., K., Nozaki, H., Bull. Chem. Soc. Jpn., 61, 3607 (1988). 51, 7250 (2012). 29) Markó, I. E., Giles, P. R., Tsukazaki, M., Chellé-Regnaut, I., 52) Siemsen, P., Livingston, R. C., Diederich, F., Angew. Chem., Urch, C. J., Brown, S. M., J. Am. Chem. Soc., 119, 12661 Int. Ed., 39, 2632 (2000). (1997). 53) Oishi, T., Katayama, T., Yamaguchi, K., Mizuno, N., Chem. 30) Connors, K. A., “Chemical Kinetics, The Study of Reaction Eur. J., 15, 7539 (2009). Rates in Solution,” VCH Publishers, Inc., New York (1990). 54) Piera, J., Bäckvall, J.-E., Angew. Chem., Int. Ed., 47, 3506 31) Nikaidou, F., Ushiyama, H., Yamaguchi, K., Yamashita, K., (2008). Mizuno, N., J. Phys. Chem. C, 114, 10873 (2010). 55) Rostovtsev, V. V., Green, L. G., Fokin, V. V., Sharpless, K. B., 32) Yamaguchi, K., Koike, T., Kotani, M., Matsushita, M., Angew. Chem., Int. Ed., 41, 2596 (2002). Shinachi, S., Mizuno, N., Chem. Eur. J., 11, 6574 (2005). 56) Tornøe, C. W., Christensen, C., Meldal, M., J. Org. Chem., 67, 33) Yamaguchi, K., Koike, T., Kim, J. W., Ogasawara, Y., Mizuno, 3057 (2002). N., Chem. Eur. J., 14, 11480 (2008). 57) Yoshimura, K., Oishi, T., Yamaguchi, K., Mizuno, N., Chem. 34) Greenberg, A., Breneman, C. M., Liebman, J. F., “The Amide Eur. J., 17, 3827 (2011). Linkage: Structural Significance in Chemistry, Biochemistry 58) Kamata, K., Kasai, J., Yamaguchi, K., Mizuno, N., Org. Lett., 6, and Material Science,” Wiley, New York (2000). 3577 (2004). 35) Carey, J. S., Laffan, D., Thomson, C., Williams, M. T., Org. 59) Matsushita, M., Kamata, K., Yamaguchi, K., Mizuno, N., J. Biomol. Chem., 4, 2337 (2006). Am. Chem. Soc., 127, 6632 (2005). 36) Constable, D. J. C., Dunn, P. J., Hayler, J. D., Humphrey, G. R., 60) He, J. L., Kim, J. W., Yamaguchi, K., Mizuno, N., Angew. Leazer Jr., J. L., Linderman, R. J., Lorenz, K., Manley, J., Chem., Int. Ed., 48, 9888 (2009). Pearlman, B. A., Wells, A., Zaks, A., Zhang, T. Y., Green 61) Yamaguchi, K., He, J. L., Oishi, T., Mizuno, N., Chem. Eur. J., Chem., 9, 411 (2007). 16, 7199 (2010). 37) Valeur, E., Bradley, M., Chem. Soc. Rev., 38, 606 (2009). 62) Kim, I., Itagaki, S., Jin, X., Yamaguchi, K., Mizuno, N., Catal. 38) Allen, C. L., Williams, J. M. J., Chem. Soc. Rev., 40, 3405 Sci. Technol., 3, 2397 (2013). (2011). 63) Fujiwara, H., Ogasawara, Y., Yamaguchi, K., Mizuno, N., 39) Yamaguchi, K., Wang, Y., Kobayashi, H., Mizuno, N., Chem. Angew. Chem., Int. Ed., 46, 5202 (2007). Lett., 41, 574 (2012). 64) Fujiwara, H., Ogasawara, Y., Kotani, M., Yamaguchi, K., 40) Smith, M. B., March, J., “March’s Advanced Organic Mizuno, N., Chem. Asian J., 3, 1715 (2008). Chemistry: Reactions, Mechanisms, and Structure, Sixth 65) Yamaguchi, K., Fujiwara, H., Ogasawara, Y., Kotani, M., Edition,” John Wiley & Sons, Inc., New Jersey (2007). Mizuno, N., Angew. Chem., Int. Ed., 46, 3922 (2007). 41) Miller, J. S., Manson, J. L., Acc. Chem. Res., 34, 563 (2001).

J. Jpn. Petrol. Inst., Vol. 57, No. 6, 2014 260

要旨

担持金属水酸化物を基盤とした環境調和型官能基変換反応のための高機能固体触媒開発

山口和也，水野哲孝

東京大学大学院工学系研究科応用化学専攻，113-8656 東京都文京区本郷7-3-1

我々は，金属水酸化物が同一金属上に，Lewis 酸と Brønsted Ru(OH)x /Al2O3 を用いた第1級アルコールのアンモ酸化，第1級塩基を併せもつという性質に着目し，金属水酸化物を基盤としアミンの形式的 α-酸素添加反応などの酸化的官能基変換の開た固体触媒開発を行った。担持金属水酸化物触媒は，アルコー発にも成功した。さらに我々は，本触媒設計概念がルテニウムル，アミン，ニトリル，末端アルキンなどの様々な基質を以外の種々の遷移金属種にも適用可能で一般性が高いことを示 Lewis 酸と Brønsted 塩基の協奏作用により効率よく活性化できし，これらの同一金属上に存在する金属由来の Lewis 酸と水酸

ることを明らかにした。担持水酸化ルテニウム触媒（Ru(OH)x / 基由来の Brønsted 塩基の協奏的効果を利用した種々の高効率

Al2O3）は，分子状酸素を酸化剤としたアルコールやアミンの官能基変換反応，たとえば担持水酸化銅触媒を用いたアルキン酸化的脱水素反応に極めて高い活性を示すことを明らかにしの酸化カップリング反応やアジドと末端アルキンの1,3-双極子

た。本 Ru(OH)x /Al2O3 は酸化反応だけでなく，水素移行型還元付加環化などの開発にも成功した。反応やニトリルの水和反応に対しても高活性を示した。また，

J. Jpn. Petrol. Inst., Vol. 57, No. 6, 2014