Synergistic Catalysis by Multifunctionalized Solid Surfaces for Nucleophilic Addition Reactions

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1 Journal of the Japan Petroleum Institute, 57, (3), (2014) 95 [Review Paper] Synergistic Catalysis by Multifunctionalized Solid Surfaces for Nucleophilic Addition Reactions Ken MTKURA Dept. of Environmental Chemistry and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama , JAPAN (Received February 6, 2014) This review paper treats with acid-base and Pd-complex-base bifunctional catalyst surfaces used to catalyze nucleophilic addition reactions. A silica _ alumina-supported tertiary amine was found to be a highly active hetero geneous catalyst for Michael additions and cyano-ethoxycarbonylation. These reactions barely proceeded with either a homogeneous amine or silica _ alumina support, indicating that synergistic catalysis occurred between the silica _ alumina surface and basic amine. Silica _ alumina, which contains both tertiary and primary amines, was an efficient catalyst for one-pot synthesis of 1,3-dinitroalkanes from aldehydes and nitromethane. The acidbase synergistic catalysis was applied to metal complex-base synergistic catalysis. Both the Pd-complex and tertiary amine-immobilized silica was an excellent heterogeneous catalyst for the Tsuji-Trost reaction. Spectroscopic analyses, such as solid-state MAS NMR, revealed the structures of prepared catalysts, immobilization mechanism of amine groups on the silica _ alumina surface, and reaction intermediates and catalytic reaction mechanism. Substrate scope for the cyanation, Michael reaction, 1,3-dinitroalkane synthesis, and the Tsuji-Trost reaction are also described. Keywords Synergistic catalysis, Heterogeneous catalyst, Acid-base catalyst, Nucleophilic addition reaction, Palladium 1. Introduction Immobilization of active species can allow the coexistence of incompatible catalytic species, such as an acid and a base, on the same solid surface. For nucleophilic addition reactions, acid-base bifunctional catalysts can activate both nucleophile and electrophile substrates to promote new bond formation. In general, two types of activation modes exist for catalytic nucleophilic addition reactions: (1) activation of nucleophile precursors by basic catalysts to abstract their acidic portions, such as α-hydrogen atoms, and (2) lowering the LUM levels of electrophiles by interaction with Brønsted or Lewis acidic catalysts. Thus, the ideal pathway for nucleophilic reactions is dual activation of both electrophiles and nucleophiles by acidic and basic catalysts, respectively (Scheme 1) 1),2). However, the existence of both strongly acidic and basic species in solution induces neutralization resulting in inactive salts. Immobilization of both the acidic and basic species on a solid surface can prevent this mutual neutralization. Immobilization of these acidic and basic species on the same solid surface can produce a bifunctional catalytic DI: dx.doi.org/ /jpi motokura@chemenv.titech.ac.jp surface possessing acidic and basic species which able to participate in a single reaction step to significantly accelerate catalytic reactions 3)5). These heterogeneous bifunctional catalysts can be categorized into three types: (1) catalysts possessing both an immobilized Brønsted acid site and a basic site derived from organic groups, (2) immobilized basic organic groups and Brønsted acid sites derived from the support surface, and (3) immobilized metal complexes or metal nanoparticles and basic organic groups. For a Nu: nucleophile, E: electrophile. Scheme 1 Acid-base Synergistic Catalysis on a Solid Surface

2 96 Values in parentheses represent 13 C NMR chemical shifts of the terminal carbon atom of the tertiary amine. Scheme 2 Immobilization of a Tertiary Amine on a Si 2 _ Al 2 3 Surface via Silane-coupling Reaction type (1) catalyst, in 2005, Lin and co-workers demonstrated a urea _ amine bifunctionalized silica surface for carbon _ carbon bond-forming reactions, such as an aldol reaction 6). The first report on the immobilization of two incompatible groups, such as an organic acid (carboxylic or sulfonic acid), and a base, on the same catalyst particle surface was reported by Davis and co-workers in ). Later reports described, several bifunctional catalysts with both an organic acid and base used for carbon _ carbon bond-forming reactions 8)11). For type (2) catalysts, the silanol group on the silica surface was reported as a weak acid site for the promotion of amineor NH2 group-catalyzed reactions 12),13). In 2003, Kubota and co-workers reported that silanol groups on Si2 significantly enhanced amine-catalyzed aldol reactions 14). A detailed investigation of the role of silanol groups was reported by Katz et al. 15) and Asefa et al. 16). The bifunctional catalysis of basic organic molecules and silanol goups was highlighted by cyclic carbonate synthesis from epoxides and carbon dioxide 17),18). For type (3) catalysts, Pd complexes and Pt nanoparticles have been reported as counterparts of organic bases for synergistic catalysis 19)21). This review describes our recent work on synergistic catalysis of type (2) and type (3) catalysts. The application of bifunctional catalysts for one-pot syntheses is mentioned. This paper also focuses on the synergistic catalysis compared with homogeneous analogues and synthetic utility of the heterogeneous catalysts. 2. Silica Alumina-supported Amine Catalyst Due to the weak acidity of silanol groups, organic amines can exist together with silanols. However, their restricted application to organic reactions is one of the problems of the amine _ silanol pairing system. To solve this problem, a silica _ alumina as a more acidic support was used instead of silica for the preparation of acid-base bifunctional catalysts 22)27). A silica _ alumina-supported tertiary amine (Si2 _ Al23/NEt2) was prepared by the silane-coupling reaction between the silica _ alumina (Si2 _ Al23) surface and 3-(diethylamino)propyltrimethoxysilane in toluene under reflux (Scheme 2). The immobilization pathway of the silane-coupling reagents containing amine functional groups to Si2 _ Al23 surfaces was characterized by solid-state MAS NMR (magic-angle spinning nuclear magnetic resonance) analysis. The Si2 _ Al23 was treated with 3-(diethylamino)propyltrimethoxysilane in toluene for 5 min at room temperature, followed by filtration. The solid obtained was subjected to solid-state 13 C and 29 Si NMR analyses. The 13 C NMR signal of the terminal carbon of the ethyl amino group shifts upfield due to interaction between an acid (H ) and the nitrogen atom. A 13 C NMR peak assignable to the terminal carbon of the adsorbed amine was observed around 8.0 ppm after room temperature treatment for 5 min. In contrast, the chemical shift for Si2 _ Al23/NEt2, completely immobilized after reflux for 24 h, was 9.4 ppm. These results indicate that the acid-base interaction in the material treated at room temperature was stronger than that in the Si2 _ Al23/ NEt2. The 29 Si MAS NMR analysis indicated an increase in the T 2 sites and a decrease in the T 1 sites after reflux. Thus, the mechanism for amine immobilization on the Si2 _ Al23 surface was proposed as follows (Scheme 2): (i) before immobilization, the nitrogen atom of the tertiary amine interacts with the surface strong Brønsted acid site (Si _ (H ) _ Al) on Si2 _ Al23, (ii) one Si _ Me group of the silane-coupling reagent reacts with a surface Si _ H group near the strong acid site to form a covalent Si Si (surface) bond, and (iii) another Si _ Me group reacts with a neighboring Si _ H upon heating, resulting in a decrease in the acid-base interaction between the strong acid site and the nitrogen atom. The 13 C CP (cross polarization)/mas NMR signals from mixtures of Si2 _ Al23 and triethylamine appeared more upfield than those for Si2 _ Al23/NEt2. For example, a peak assigned to triethylamine adsorbed on the Si2 _ Al23 surface showed the largest upfield shift [from 11.8 (triethylamine in CDCl3 solution) to 7.5 ppm]. These results indicate that the acid-base interaction between the tertiary amine and the acid site on Si2 _ Al23 weakens upon the silane-coupling immobilization. Michael reaction of a nitrile with ethyl acrylate and cyano-ethoxycarbonylation of benzaldehyde were

3 97 Table 1 Michael Reaction and Cyano-ethoxycarbonylation with Various Catalysts a) NC C 2 Et C 2 Et catalyst NC C 2 Et Et NC Et catalyst CN Et Entry Catalyst Yield (Michael reaction) b) [%] Yield (cyanation) b) [%] 1 Si _ 2 Al 2 3 /NEt Si 2 /NEt Triethylamine c) Si _ 2 Al c) TriethylamineSi _ 2 Al a) Michael reaction: catalyst (amine: mmol), nitrile (1.0 mmol), ethyl acrylate (3 mmol), toluene (1 ml), 90, 24 h, under N 2 atmosphere. Cyanation: catalyst (amine: mmol), benzaldehyde (0.5 mmol), ethyl cyanoformate (1.0 mmol), toluene (5 ml), room temp., 1 h, under N 2 atmosphere. b) Determined by GC and 1 H NMR. Based on nitrile and benzaldehyde used. c) Si 2 _ Al 2 3 ( g for Michael reaction, g for cyanation). Table 2 Michael Reaction of α-substituted Ethyl Cyanoacetates with Electron-deficient Alkenes a) NC R Et EWG Si 2 _ Al 2 3 /NEt 2 Entry R EWG Yield b) [%] 1 c) Me C 2 Et 94 2 Ph C 2 Me 99 3 Ph C 2 Me 99 d) 4 Ph C 2 Me 94 e) 5 f) Me CN 95 NC R Et EWG a) Catalyst (amine: mmol), nitrile (1.0 mmol), alkene (3.0 mmol), toluene (1 ml), 60, 3 h, under N 2 atmosphere. b) Determined by GC and 1 H NMR. Based on nitrile. c) Catalyst (amine: mmol), 90, 24 h. d) 2nd reuse experiment. e) 4th reuse experiment. f) 24 h. examined using the Si2 _ Al23/NEt2 catalyst. The Si2 _ Al23/NEt2 showed the greatest catalytic activity for both the Michael reaction and cyanation to afford the corresponding addition products in 94 % and 95 % yields, respectively (Table 1, entry 1). Si2/NEt2 was much less active under the reaction conditions (Table 1, entry 2). Neither triethylamine nor Si2 _ Al23 promoted the desired addition reaction (Table 1, entries 3 and 4). Notably, product yields were lower using the mixture of triethylamine and Si2 _ Al23 (Table 1, entry 5) than the use of the supported catalyst (Table 1, entry 1). High catalytic performance was achieved only for the immobilized tertiary amine on the Si2 _ Al23 surface. The high performance of the Si2 _ Al23/NEt2 was applied to the Michael reaction (Table 2) and to cyano-ethoxycarbonylation of various substrates (Table 3). For the Michael reaction, several α-substituted ethylcyanoacetate and unsaturated esters and nitriles were good donors and acceptors, respectively. The heterogeneous Si2 _ Al23/NEt2 catalyst was easily separated from the reaction mixture, and reusable at least four times for the Michael reaction (Table 2, entry 4). Various aldehydes can be used for the cyanoethoxycarbonylation to afford the corresponding cyanation products in excellent yield. Due to weak interactions between the H site and immobilized tertiary amine group, both the acid site and amine group can act as catalytically active species. In contrast, a nonimmobilized tertiary amine, such as triethylamine, strongly adsorbed on the acid site, causing deactivation. Therefore, Si2 _ Al23/NEt2 possessed much greater activity for Michael reactions and cyanoethoxycarbonylation. These reactions did not progress with only Si2 _ Al23 or tertiary amine. The catalytic

4 98 Table 3 Cyano-ethoxycarbonylation of Carbonyl Compounds Using Si 2 _ Al 2 3 /NEt 2 a) R 2 Si _ 2 Al 2 3 /NEt 2 NC Et toluene, rt R1 R 2 R 1 CN Et R Carbonyl compound Time [h] Conversion b) [%] Yield b) [%] RH RCl RMe RMe S a) Carbonyl compounds (0.5 mmol), ethyl cyanoformate (0.6 mmol), toluene (1 ml), Si 2 _ Al 2 3 /NEt 2 ( g, N: mmol), room temp., under N 2 atmosphere. b) Based on carbonyl compounds, determined by GC and 1 H NMR. reaction pathway including acid-base cooperative activation is shown in Scheme 3. Similar to Si2 _ Al23, Al-MCM-41 has also been reported as a support, which can enhance amine-catalyzed nitro-aldol reactions 28). 3. Control of Acid-base Interactions of Silica Alumina-supported Amine Catalyst Scheme 3 Reaction Mechanisms of a Si 2 _ Al 2 3 /NEt 2 -catalyzed Michael Reaction and Cyano-ethoxycarbonlylation Reaction To more precisely control acid-base interactions of the Si2 _ Al23-supported tertiary amine catalyst, the density of surface Si _ H groups was investigated 29). According to the proposed immobilization mechanism, acid-base interactions should decrease with the Si _ H density because the amine-immobilization positions are distant from the strong acid sites, resulting in fewer acid-base interactions and greater catalytic performance. Si2 _ Al23 was pretreated at under vacuum before use as a support for amines. Treated samples are referred to as Si2 _ Al23(treatment temperature T). Tertiary amines were immobilized on Si2 _ Al23(T) by treatment it with a toluene solution of 3-(diethylamino)propyltrimethoxysilane under reflux for 24 h. Scheme 4 represents the preparation path-

5 99 Values in parentheses represent 13 C NMR chemical shifts of the terminal carbon atom of the tertiary amine. Scheme 4 Preparation of Si 2 _ Al 2 3 (500)/NEt 2 way of Si2 _ 13 Al23(500)/NEt2. C MAS NMR analysis was performed to determine the acid-base interactions between the strong acid site and immobilized amine group. After immobilization of the tertiary amine groups onto Si2 _ Al23(120), the 13 C NMR signal of the terminal carbon (9.4 ppm) shifted upfield compared to that of the amine precursor (11.9 ppm) (Scheme 2). In contrast, the same signal from Si2 _ Al23(500)/NEt2 was observed at 11.0 ppm (Scheme 4), which indicated only a very small shift from the precursor. Variable-contact-time 13 C CP/MAS NMR is a technique to determine the molecular motion of solid materials 30)32). Additionally, contact-time array 13 C CP/MAS NMR measurements of Si2 _ Al23(500)/ NEt2 and Si2 _ Al23(120)/NEt2 were conducted to determine the molecular mobility of the immobilized amines. Variable-contact-time 13 C CP/MAS NMR measurement for Si2 _ Al23(500)/NEt2 showed the highest intensity of the terminal carbon (5.0 ms contact time). In contrast, the highest intensity for Si2 _ Al23(120)/NEt2 was for 1.0 ms contact time. Therefore, the cross-polarization time constant values of Si2 _ Al23(500)/NEt2 were much longer than those of Si2 _ Al23(120)/NEt2, indicating much greater mobility of the tertiary amine group in Si2 _ Al23(500)/ NEt2. The strong acid-base interaction between the amine group and acid site on Si2 _ Al23 in Si2 _ Al23(120)/NEt2 suppressed tertiary amine mobility compared with Si2 _ Al23(500)/NEt2. The calcination of Si2 _ Al23 at 500 reduced the number of surface silanol groups, resulting in high dispersion of silanols on the Si2 _ Al23 surface, and immobilizing the amines on positions distant from a strong acid site (Scheme 4). In contrast, a Si2 _ Al23(120) support with a higher concentration of Si _ H groups reduces the distance between the amine group and acid site compared to Si2 _ Al23(500) (Scheme 2), implying strong acid-base interaction. The 1,4-addition reaction of nitroethane to methyl vinyl ketone was examined using Si2 _ Al23(T)/NEt2. Results are summarized in Table 4. The catalyst Si2 _ Al23(500)/NEt2 was remarkably active and afforded the doubly alkylated product in 93 % yield (entry 1). Decreasing the pretreatment temperature of Si2 _ Al23 reduced the catalytic activity of Si2 _ Al23(T)/NEt2 (entries 1-4). Note that this reaction minimally proceeded with either Si2 _ Al23(500) or homogeneous amines (entries 6 and 7). A mixture of Si2 _ Al23(500) and triethylamine showed catalytic activity, but the yield of the product was very low (entry 8). This indicates that both the surface acid site and amine group in Si2 _ Al23(500)/NEt2 without acidbase neutralization can act as a catalytically active species. The reaction pathway may be similar to that of 1,4-addition of nitriles (Scheme 3). Substrate scope for Michael reaction of nitroalkanes to unsaturated carbonyl compounds using Si2 _ Al23(500)/NEt2 are summarized in Table 5. Various acyclic, cyclic, linear, and branched nitroalkanes acted as good substrates for Michael reaction with methyl vinyl ketone (entries 1-6). Unsaturated ketones and an aldehyde were used for Michael acceptors (entries 7-10). The catalyst could be reused without appreciable loss of its catalytic activity (entry 1). For a strong base, such as sodium ethoxide, as a homogeneous catalyst, selectivity was as low as 1-7 % over the entire reaction due to further conversion of the product to an undesired cyclization product 33),34). The cooperative activation by the surface acid and amine group, a weaker base compared to sodium ethoxide, realizes selective catalysis compared to the use of only a strong base. 4. Application of Supported Amine Catalysts toward ne-pot Synthesis A heterogeneous catalyst allows multi-catalytic functions on its solid surface. Such multifunctional catalyst

6 100 Table 4 1,4-Addition of Nitroethane to Methyl Vinyl Ketone with Various Catalysts a) N 2 2 catalyst N 2 catalyst Entry Catalyst Conversion of nitroethane b) [%] Yield of dialkylated product b) [%] Initial rate b),c) [mmol/h] 1 d) Si _ 2 Al 2 3 (500)/NEt Si _ 2 Al 2 3 (500)/NEt Si _ 2 Al 2 3 (400)/NEt Si _ 2 Al 2 3 (200)/NEt Si _ 2 Al 2 3 (120)/NEt e) Si _ 2 Al 2 3 (500) Triethylamine e) Si _ 2 Al 2 3 (500)Triethylamine None a) Reaction conditions: nitroethane (1.0 mmol), methyl vinyl ketone (3.0 mmol), catalyst (amine: mmol), toluene (1 ml), 50, 20 h, under N 2 atmosphere. b) Determined by 1 H NMR and GC using internal standard technique. Based on amount of nitroethane used. No other byproducts were detected except for a mono-alkylated product in the case of supported catalysts. c) Initial formation rate of the addition products. d) 24 h. e) 0.08 g of Si 2 _ Al 2 3 was used. N 2 promotes not only a single reaction step by synergistic catalysis, but also allows multiple reactions in a single reactor (one-pot synthesis) 35)37). Tertiary amines can act as Lewis and Brønsted bases for activation of nucleophiles, whereas supported primary amines efficiently catalyze condensation reactions of carbonyl compounds such as nitro-aldol reactions 38)40), where the primary amines activate the carbonyl compounds through formation of imine intermediates 15),38)40). These observations suggest that immobilization of both tertiary and primary amines onto the same solid surface can produce an efficient heterogeneous catalyst for carbon _ carbon bond-forming reactions via activation of both nucleophiles and electrophiles. Both tertiary and primary amine-immobilizing Si2 _ Al23 catalyst (Si2 _ Al23/NH2/NEt2) demonstrated tremendous potential for successful one-pot synthesis of 1,3-dinitroalkanes 41). Reaction between benzaldehyde and nitromethane was examined using Si2 _ Al23-supported amine catalysts, as shown in Table 6. Remarkably, 93 % dinitroalkane product selectivity at 100 % conversion of benzaldehyde was achieved after 8 h in the presence of the Si2 _ Al23/NH2/NEt2. A Si2 _ Al23/NH2 sample possessing a full loading of tertiary amine group (Si2 _ Al23/NH2-f: 1.11 mmol/g) had better catalytic activity (entry 2) than did Si2 _ Al23/NH2 with a similar primary amine loading to that of Si2 _ Al23/NH2/NEt2 (entry 1); however, the yield of dinitroalkane was 39 %. The Si2 _ Al23/NEt2-f and Si2 _ Al23/NEt2 samples also were less active and less selective (entries 4 and 5) for the synthesis of dinitroalkane. These results indicate that both primary and tertiary amine groups promote dinitroalkane synthesis. The generality of the Si2 _ Al23/NH2/NEt2- catalyzed 1,3-dinitroalkane synthesis was examined using various aldehydes (Table 7). Electron-donating groups at the para-position of benzaldehyde enhanced reactivity, and the corresponding 1,3-dinitroalkanes were formed in excellent yields (entries 2-4, %). This reaction also proceeded successfully with piperonal, 3,4-dimethoxybenzaldeyde, and the heteroaromatic aldehyde 2-thiophenecarboxyaldehyde (entries 7, 8, and 10). Benzaldehyde containing a carboxyl group also reacted with nitromethane in the presence of Si2 _ Al23/NH2/NEt2 catalyst to afford 65 % yield of the corresponding 1,3-dinitroalkane product (Scheme 5). This reaction minimally proceeded with sodium methoxide as a strong base due to neutralization of the base by the carboxylic acid. A reaction mechanism for the synthesis of 1,3-dinitroalkane on Si2 _ Al23/NH2/NEt2, including the effect of the acid support, is illustrated in Scheme 6. During the synthesis, (i) the aldehyde is activated by the surface acid site and reacts with a NH2 group to form an imine intermediate, (ii) an α-proton of nitromethane is abstracted by a tertiary amine group, accompanied by nucleophilic attack of the deprotonated nitromethane to the imine resulting in nitrostyrene formation, and (iii) another nitromethane is activated by the tertiary amine

7 101 Table 5 Michael Reaction of Nitroalkanes to Unsaturated Carbonyl Compounds Using Si2_Al23(500)/NEt2a) R2 R1 Entry N2 R4 N2 R3 Nitroalkane R4 Si2_Al2 500 /NEt2 3 R3 Alkene R2 R1 Yieldb) [%] Product 1 N2 N2 85, 83c) N2 87 N N2 3 N2 4 d) H N2 N2 62 H 5e) N2 N2 96 N2 6 N2 78 7f) N2 N2 8g) N2 9g),h) N2 87 N2 83 N2 10 i) N2 93 H H N2 92 a) Reaction conditions: nitroalkane (3.0 mmol), alkene (1.0 mmol), Si2_ Al23(500)/NEt2 (amine: mmol), toluene (1 ml), 50, 24 h, under N2 atmosphere. b) Determined by 1H NMR and GC. Based on alkene. c) Reuse experiment. d) 70 h. e) 12 h. f) 80. g) 100. h) 50 h. i) room temp., 1 h. group and Michael reaction with nitrostyrene occurs to give the 1,3-dinitroalkane. 5. Silica-supported Palladium Complex and Tertiary Amine Catalyst The Tsuji-Trost reaction (allylation of nucleophiles) catalyzed by a Pd complex is an efficient method for J. Jpn. Petrol. Inst., Vol. 57, No. 3, 2014

8 102 Table 6 Reaction between Benzaldehyde and Nitromethane Using Amine-immobilized Catalysts a) H N 2 CH 3 N 2 catalyst N 2 N 2 N 2 Entry Catalyst Yield b) [%] nitrostyrene nitroalcohol dinitroalkane 1 Si _ 2 Al 2 3 /NH 2 /NEt Si _ 2 Al 2 3 /NH 2 -f Si _ 2 Al 2 3 /NH Si _ 2 Al 2 3 /NEt 2 -f Si _ 2 Al 2 3 /NEt a) Reaction conditions: benzaldehyde (1.0 mmol), nitromethane (2.0 ml), _ NH 2 ( mmol), _ NEt 2 ( mmol), 100, 8 h, under N 2 atmosphere. Amine loading: Si 2 _ Al 2 3 /NH 2 /NEt 2 : NH 2 : 0.44 mmol/g, NEt 2 : 0.36 mmol/g; Si 2 _ Al 2 3 /NEt 2 -f: 0.90 mmol/g; Si 2 _ Al 2 3 /NEt 2 : 0.40 mmol/g; Si 2 _ Al 2 3 /NH 2 -f: 1.11 mmol/g; Si 2 _ Al 2 3 /NH 2 : 0.39 mmol/g. b) Determined by 1 H NMR. Based on benzaldehyde. R Table 7 1,3-Dinitroalkane Synthesis from Various Aldehyde a) Si 2 _ Al 2 3 /NH 2 /NEt 2 2 CH 3 N N 2 R Entry Aldehyde Time [h] Conversion of aldehyde b) [%] Yield b) [%] X 1 XH XMe XMe XH XCl c) 6 XN c) Me Me N S a) Reaction conditions: aldehyde (1.0 mmol), nitromethane (2 ml), Si 2 _ Al 2 3 /NH 2 /NEt 2 (0.10 g, NH 2 : mmol, NEt 2 : mmol), 100, under N 2 atmosphere. b) Determined by 1 H NMR. Based on aldehyde. c) 8 % yield of nitroalcohol was formed. d) 36 % yield of nitroalcohol was formed. carbon _ carbon and carbon _ oxygen bond formation 42)44). Not only homogeneous catalysts but also heterogeneous Pd catalysts have been used for the Tsuji-Trost reaction 45)52). The reaction proceeds in the presence of a Pd species and a base 53)55). The role of the base is electron-donating activation of the nucleophile (R _ H) to enhance addition to the η 3 -allylpalladium species (Scheme 7). Synergistic catalysis of a silica-supported Pd complex and tertiary amine (Si2/diamine/Pd/NEt2) was used to accelerate the Tsuji-Trost reaction 56),57). Scheme 8 represents preparation of Si2/diamine/ Pd/NEt2. 3-(2-Aminoethylamino)propyltrimethoxysilane and 3-diethylaminopropyltrimethoxysilane were immobilized on the Si2 surface using a silanecoupling reaction to give silica-supported amines (Si2/ diamine/net2). Si2/diamine/Pd/NEt2 was prepared from Si2/diamine/NEt2 by treatment with a PdCl2(PhCN)2 solution in 2-propanol. The Si2/di-

9 103 Scheme 5 Reaction of Nitromethane with Benzaldehyde Containing a Carboxyl Group Scheme 6 Proposed Reaction Mechanism for 1,3-Dinitroalkane Synthesis from an Aldehyde and Nitromethane Scheme 7 Palladium-base Synergistic Catalysis for the Tsuji-Trost Reaction Scheme 8 Preparation Route of Si 2 /diamine/pd/net 2

10 104 Table 8 The Tsuji-Trost Reaction of Ethyl 3-xobutanate with Allylmethylcarbonate Using Pd Catalysts a) Pd catalyst Entry Catalyst Conversion of ketoester b) [%] Yield (mono-allylation) b) [%] Yield (di-allylation) b) [%] 1 Si 2 /diamine/pd/net Si 2 /diamine/pd c) Si 2 /diamine/net Si 2 /diamine/pddiethylbutylamine d) Si 2 /diamine/pdsi 2 /NEt a) Reaction conditions: ethyl 3-oxobutanate (1.0 mmol), allylmethylcarbonate (2.5 mmol), K 2 C 3 (3.0 mmol), catalyst (Pd: mmol), THF (4.0 ml), 70, 5 h, under Ar atmosphere. b) Determined by 1 H NMR using 1,3,5-triisopropylbenzene as the internal standard. c) g of Si 2 /diamine/net 2 was used. d) mmol of diethylbutylamine was added. amine/pd was prepared by a similar procedure. The surface structure and amount of Pd and amine loading were determined by solid-state NMR (nuclear magnetic resonance), XPS (X-ray photoelectron spectroscopy), and elemental analysis 56). The Tsuji-Trost allylation of ethyl 3-oxobutanate with allylmethylcarbonate using the Pd catalysts was examined. Results are shown in Table 8. The Si2/ diamine/pd/net2 produced 99 % yield of total allylated products (entry 1), whereas the yield of allylated products was 26 % for Si2/diamine/Pd (entry 2). To examine the effect of immobilization of a tertiary amine on catalytic activity, reaction using Si2/ diamine/pd with a free tertiary amine (diethylbutylamine) was conducted (entry 4). The amount of free tertiary amine used was similar to that in Si2/diamine/ Pd/NEt2. As a result, the Si2/diamine/Pd with a free amine showed less activity (6 % yield) than did Si2/ diamine/pd (entry 4 versus 2). Additionally, increasing the amount of free tertiary amine decreased product yield, indicating deactivation of the catalytic Pd site by the free tertiary amine. Product yield was not increased using a physical mixture of Si2/NEt2 and Si2/diamine/Pd (entry 5 versus 2). nly a tertiary amine immobilized on the same Si2 surface can enhance the Pd-catalyzed Tsuji-Trost reaction. Substrate scope for the Si2/diamine/Pd/NEt2 catalyst in the Tusji-Trost reaction is shown in Table 9. Reaction using bulky ketoesters having methyl, cyclic, and phenyl groups proceeded with Si2/diamine/Pd/ NEt2, to give % yields of products (entries 2-4). The slightly lower product yields compared with ethyl 3-oxobutanate indicate difficulty in substrate access to the catalyst surface due to steric bulk. The Si2/ diamine/pd/net2 catalyst showed high activity for reaction of a diester (entry 5). The reaction also was compatible with aliphatic (entry 6), aromatic (entry 7), and cyclic diketones (entries 8-10). To extend the scope of the Si2/diamine/Pd/NEt2- catalyzed Tsuji-Trost reaction, synthesis involving phenols and carboxylic acids were examined. Results are summarized in Table 10. Phenol reacted with allylmethylcarbonate to form allyl phenyl ether in 86 % yield using Si2/diamine/Pd/NEt2 (entry 1). Electronwithdrawing groups, such as nitro and chloro groups, at the para position of the phenyl ring resulted in higher product yields (entries 2 and 3) compared to phenol (entry 1) with a shorter reaction time. In contrast, electron-donating groups, such as methyl and methoxy groups, led to reduced product yields (entries 4 and 5). The Tsuji-Trost reaction of carboxylic acids also proceeded with Si2/diamine/Pd/NEt2 catalyst (entries 6 and 7). The Si2/diamine/Pd/NEt2 catalyst was reusable at least three times for the reaction of p-chlorophenol (entry 3). Formation of η 3 -allylpalladium species on the Si2/ diamine/pd/net2 catalyst was confirmed by solid-state 13 C CP/MAS NMR analysis. Proton abstraction from 1,3-dicarbonyls by the tertiary amine group on the Si2 surface was confirmed by results of the Michael reaction of ethyl 3-oxobutanate with methyl vinyl ketone using Si2/NEt2, which demonstrated the synergistic catalysis of surface Pd species and tertiary amine group, as shown in Scheme Conclusion This review summarizes synergistic catalysis of silica _ alumina-supported amines as well as a silica-supported palladium complex and tertiary amine. Multi-catalytic functions on the same solid surface can participate in a single reaction step for acceleration of various nucleophilic addition reactions, such as Michael reactions. Additionally, a multifunctional catalyst was successfully applied to the one-pot synthesis of 1,3-dinitroalkanes by successive promotion of Henry and Michael reactions. A Brønsted acid as well as palladium complex can act as a counterpart of an organic base for synergis-

11 105 Table 9 The Tsuji-Trost Reaction of Various 1,3-Dicarbonyl Compounds Using Si 2 /diamine/pd/net 2 Catalyst a) R 1 R 3 R 2 Si 2 /diamine/pd/net 2 R 1 R 3 R 2 Entry 1,3-Dicarbonyl compound Time [h] Yield b) [%] Mono- : Di- b) : Ph : : : 54 7 c) Ph d) 97 : : a) Reaction conditions: 1,3-dicarbonyl (1.0 mmol), allylmethylcarbonate (2.5 mmol), K 2 C 3 (3.0 mmol), catalyst (Pd: mmol), THF (4.0 ml), 70, under Ar atmosphere. b) Determined by 1 H NMR using 1,3,5-triisopropylbenzene as the internal standard. c) Reaction conditions: 1,3-dicarbonyl (3.0 mmol), allylmethylcarbonate (7.5 mmol), K 2 C 3 (9.0 mmol), catalyst (Pd: mmol), THF (12.0 ml), 70. d) Isolated yield. tic catalysis. Immobilization of functional groups on a solid surface shows great potential for significantly accelerating organic reactions through synergistic catalysis. Acknowledgment The author thanks to the following people who have contributed to the research described in this review. Prof. Yasuhiro Iwasawa, Prof. Mizuki Tada, Dr. Satoka Tanaka, Mr. Mitsuru Tomita (The University of Tokyo), Prof. Toshihide Baba, Dr. Akimitsu Miyaji, and Mr. Hiroto Noda (Tokyo Institute of Technology). References 1) Gröger, H., Chem. Eur. J., 7, 5246 (2001). 2) Kanai, M., Kato, N., Ichikawa, E., Shibasaki, M., Pure Appl. Chem., 77, 2047 (2005). 3) Margelefsky, E. L., Zeidan, R. K., Davis, M. E., Chem. Soc. Rev., 37, 1118 (2008). 4) Motokura, K., Tada, M., Iwasawa, Y., Chem. Asia J., 3, 1230 (2008). 5) Shylesh, S., Thiel, W. R., ChemCatChem, 3, 278 (2011). 6) Huh, S., Chen, H.-T., Wiench, J. W., Pruski, M., Lin, V. S.-Y., Angew. Chem., Int. Ed., 44, 1826 (2005).

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14 G Michael 3 NMR 1 3 Henry Michael 1,3- one-pot Pd 3 Tsuji-Trost