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 cata- lysts. 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 pri- mary amides, ammoxidation of primary alcohols to nitriles, and formal α-oxygenation of primary amines to pri- mary 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) 24)～26) and other research groups have focused on ruthe- nium 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 ter- minal 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 reac- tions. 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