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Catalytic activities of nickel (II) complexes of a salen analog, [Ni(babp)], in epoxidation of olefins
395
Journal Of hfOk?CUf!UF catalysis, 87 (1994) 195-202 Elsevier Science B.V., Amsterdam M311
Catalytic activities of nickel (II) complexes of a salen analog, [ Ni (babp ) 1, in epoxidation of olefins Masaki Yamada+, Shinji Ochi, Hatsuhiko Suzuki, Akihiro Hisazumi, Shigeyasu Kuroda and Ichiro Shimao Department of Materials Science and Engineering, 3190 Gofuku, Toyama 930 (Japan)
Faculty of Engineering,
Toyama University,
Koji Araki’ Institute
of Industrial Science, University
of
Tokyo, 7-22-l Roppongi, Minato-ku,
Tokyo 106
(Jap4 (Received April 30,1993; accepted September 17,1993)
Abstract A salen analog [Ni(babp) ] (babp=6,6’-bis(benzoylaminato)-2,2’-bipyridine (2 - ) ) and its t-butyl derivative catalyzed epoxidation of olefins in the presence of sodium hypochlorite as the oxidant. These complexes showed higher catalytic activity than [ Ni (salen) ] in epoxidation of an electron-deficient olefin, ally1 chloride, yielding epichlorohydrin in 43% yield. Key words: ally1 chloride; epoxidation; nickel catalysis: salen analog, square-planar complex
Introduction We reported syntheses, structures, and properties of a new type of salen (NJ -di (salicylidene)ethylenediamine) analog [M (babp) ] (M = Cu, Ni, Co, and Zn), where babp represents a deprotonated form of 6,6’ bis (benzoylamino)-2,2’-bipyridine [l-3]. Various salen analogs have been developed by structural modification of the diamine and/or phenoxy units of salen in order to improve the notoriously low durability of [Co (salen) ] as an oxygenation catalyst [ 41. In the [M (babp) ] complexes, however, the diamine and phenoxy units of salen were replaced by entirely different units and the square-planar NzOz coordination sites were reconstructed by using more stable building blocks, i.e., 2,2’-bipyridine and deprotonated amide. A cobalt (II) complex [Co (babp) ] was shown to be an excellent catalyst for oxygenation of 2,6disubstitutedphenols. Selectivity and durability of [Co (babp) ] were much higher than those of [Co (salen) 1, which was suggested to be due, at least in *Corresponding author. ‘Deceased.
SSDZ 0304-5102(93)E0245-C
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part, to the higher stability of the ligand moieties under oxidative conditions [ 31. Recently several groups have reported epoxidation of olefins, mostly electron-rich olefins like styrene or &methylstyrene, by nickel (II) complexes including [ Ni (aalen) ] [ 5-91, and a higher oxidation state of the complex [ 5-71, which is a highly active oxidant [lo], was proposed as being the active species of the catalyst. Therefore, the present study is suitable for testing the scope and limitations of the new type of salen analog prepared by using entirely different units. Here we report that [ Ni (babp ) ] derivatives exhibited catalytic activities in the epoxidation of olefins, especially for electron-deficient ally1 chloride, in the presence of sodium hypochlorite as the terminal oxidant.
x
Terminal Oxidant, Ni(II) Complex
-X
0
[Ni(salen)]
R=H; [Ni(babp)] R=r-Bu; [Ni(Bu-babp)] [Ni(saloph)]
Results and discussion Properties of the complexes The complex [Ni (babp) ] is sparingly soluble in most of the common organic solvents, and solubility of [ Ni (babp ) ] in dichloromethane, which is often used as a solvent for catalytic oxidation [5-E!], is lower than 10m4mol dmm3. Therefore, we prepared a t-butyl-substituted derivative, [ Ni (Bu-babp) ] (Bubabp = 6,6’ -his (4-t-butylbenzoylaminato) -2,2’ -bipyridine (2 - ) ), which was sufficiently soluble in dichloromethane (more than 10m2mol dmm3at r.t.). A TG study of [ Ni (Bu-babp ) ] showed that heating to its melting point at 320 ’ C under aerobic conditions caused no weight loss, showing that introduction of the t-butyl groups did not affect the high thermal stability of [ Ni (babp ) ] [ 21. We studied the spectroscopic and redox properties of [ Ni (babp ) ] and [ Ni (Bu-babp ) ] prior to use as an epoxidation catalyst. Characteristic IR bands of [ Ni (babp ) ] assigned to the deprotonated amide moieties (1558 and 1366 cm-’ ) [ 111 were also observed for [ Ni (Bu-babp ) ] [ 21, confirming that introduction of the t-butyl groups did not affect the square-planar structure of the
M. Yamada et al. /J. Mol. Catal. 87 (1994) 195-202
300
400
500
197
600
wavelength/nm Fig. 1. Electronic spectra of nickel complexes (6.67 x lo-’ mol dm-’ ) in dichloromethane at 20” C. (-): [Ni(babp)], (- - - -):[Ni(Bu-babp)], (- - -): [Ni(salen)], and (- - - -): [Ni(saloph)l (concentration of this complex is 3.33 x 10m6mol dme3).
complex. The electronic spectra of [ Ni (babp) ] and [ Ni (Bu-babp) ] in dichloromethane are shown in Fig. 1 along with that of [ Ni (salen) ] and a com[Ni(saloph)] (saloph=oanother Schiff-base ligand plex of bis (salicylideniminato )-benzene (2 - ) ) . The spectra of [ Ni (babp) ] and were quite similar to that of [Ni(salen) ] [ 121 showing the [Ni(Bu-babp)] characteristic band at around 420 nm [ 21, whereas a large red-shift of the corresponding band was observed in the spectrum of [Ni (saloph) 1. Unlike [ Ni (salen) 1, [ Ni (saloph) ] has a non-planar structure, and, therefore, the results support that [Ni(babp) ] and [ Ni (Bu-babp) ] have a square-planar structure quite similar to that of [ Ni (salen) 1. The redox property of [Ni(babp) ] was slightly different from that of [Ni(salen) 1. A cyclic voltamogram of [Ni(babp) ] in dimethyl sulfoxide (DMSO ) showed a single, quasi-reversible reduction peak presumably due to a Ni”/Ni’ redox couple at E1,2= - 1.33 V vs. SCE, which was slightly more positive than that of [Ni(salen) ] (El,z= - 1.61 V vs. SCE in DMSO) [ 131. Introduction of the t-butyl groups showed only a small effect, and the reduction potential of [ Ni (Bu-babp ) ] was - 1.45 V vs. SCE. However, no oxidation peak was observed for [ Ni (babp ) ] and [ Ni (Bu-babp ) ] within the sweep range (up to + 2 V), although [ Ni (salen) ] exhibited a quasi-reversible oxidation peak due to a Nin/Ni”’ redox couple at Elj2 = +0.71 V vs. SCE in DMSO [ 131. Use of glassy carbon instead of platinum wire as the working electrode did not affect the results. These results indicate that generation of the Ni”’ species in [ Ni (babp ) ] and [ Ni (Bu-babp) ] might be difficult. Epoxidation of olefins Epoxidation of olefins by sodium hypochlorite was carried out in a two phase system under phase-transfer conditions [ 61, where an organic phase ( 10
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cm3) was a dichloromethane solution of olefin (4 mmol), complex (0.04 mmol, 1 mol% of the substrate), and benzyltri-n-butylammonium chloride (phase transfer reagent, 0.15 mmol) and an aqueous phase (20 cm3) was a sodium hypochlorite solution (0.7 mol dme3, pH 12.2). The reaction, catalyzed by [ Ni (babp) ] or [ Ni (Bu-babp) 1, proceeded in the presence of sodium hypochlorite as the oxidant (Table 2). Olefins used in the study were styrene, cyclohexene (Table 1), and ally1 chloride (Table 2). After an induction period, a black precipitate appeared in the organic phase and then vigorous bubbling took place in the aqueous phase. When this stage was over, the black precipitate was completely bleached, and the olefin in the organic phase was conTABLE 1 Epoxidation of styrene and cyclohexene with sodium hypochlorite at 25 ’ C? Entry
7 8
Olefin
Catalyst
Time/h
Conversion/%
Epoxide yield/%
styrene
[Ni (babp ) 1 [Ni(Bu-babp)] [Ni(Bu-babp)]” [Ni(salen)] [Ni(Bu-babp)ld [Ni(salen)ld
b
24 2 2 24 24
100 100 100 100 37 65
20 28 21 42 0 0
[Ni(Bu-babp)] [Ni(salen)]
24 2
100 100
24 23
cyclohexene
“Conversions and yields were determined by GC. bIrreguIarinduction period was observed, see text. “Aqueous sodium hypochlorite was adjusted to pH 9.3 by addition of borate buffer. dOxidation with iodosylbenxene, see text. TABLE 2 Epoxidation of ally1chloride with sodium hypochlorite for 2 h at 25 ’ C Entry
1 2s 3 4 5 6 7
Catalyst
[Ni(Bu-babp)] [Ni (babp ) 1 [Ni(salen)] [Ni(saloph)] ]Co(babp) 1 [Co(salen)]
Conversion/%
98 18 98 64 73 3 75
Yield/% Epichlorohydrin
Trichloropropane
43 17 39 19 19 0 11
32 22 27 22 22 0 10
*Conversions and yields were determined by GC. bSecond use of the catalyst, see text. Conversion and yields are those of the second reaction.
M. Yarmdaetd./J.Mol.CataL87(1994)195-202
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sumed completely when styrene or cyclohexene was used as the substrate. Iodine titration of the aqueous phase indicated that the oxidant sodium hypochlorite was also consumed completely. This process was quite similar to that catalyzed by [ Ni (salen) ] [ 6,7]. In the case of styrene or cyclohexene, yields of epoxides were no better than those catalyzed by [ Ni (salen) ] (Table 1), yielding a variety of degradation and chlorination products. As reported by Yoon and Burrows [ 61, iodosylbenzene was ineffective as an oxidant. However, the induction period in the [ Ni(babp) ] catalyzed reaction was much longer (approximately 8 h) than that for the [Ni(salen) ] one (a few minutes), and was not reproducible requiring up to several days in some cases. As [Ni(babp) ] did not dissolve in dichloromethane completely under the initial conditions employed, this could be the reason for the long and irregular induction periods. Epoxidation catalyzed by the more organic-soluble analog, [Ni(Bu-babp) 1, started within 8 h in all cases, although the induction period was still long compared to that of [Ni (salen) 1. Addition of pyridine (10 v/v% ) as an additional donor ligand [5] to the organic phase slightly diminished the long induction period to approximately 4 h. Decrease of the pH of the aqueous phase to 9.3 by using borate buffer effectively shortened the long induction time, and epoxidation started within a few minutes [ 71. However the yield of the epoxide was not improved. On the other hand, [Ni(Bu-babp) ] showed superior catalytic activity in the epoxidation of electron-deficient ally1 chloride (Table 2). Since electrondeficient olefins are less reactive toward electrophilic addition, relatively few have been studied such as in the direct epoxidation of ally1 chloride. Epichlorohydrin, one of the key reagents for organic synthesis, was obtained in 43% yield by direct epoxidation of ally1 chloride with sodium hypochlorite; this is twice as high as the reaction catalyzed by [ Ni (salen) ] or [ Ni (saloph) 1. The apparent changes in the solution during the reaction were similar to those for styrene or cyclohexene. However, the long induction time necessary for the reaction of styrene or cyclohexene was drastically lowered and epoxidation of ally1 chloride started within a few minutes even without lowering the pH of the aqueous phase. After the reaction, sodium hypochlorite was completely consumed in all cases. However, considerable amounts of ally1 chloride remained unreacted when [ Ni (salen) ] was used as a catalyst. The major by-product of the reaction was found to be 1,2,3trichloropropane, the chlorination product of ally1chloride in the reaction [ 7,141. Epoxide selectivity is higher for [ Ni (Bubabp) ] (epoxide/trichloropropane molar ratio = 1.3~)than [ Ni (salen) ] (0.9). The complex [ Ni (babp) ] showed similar catalytic activity to that of [ Ni (Bubabp ) 1, and the reaction started within a few minutes in most cases although occasionally a long induction period was still observed. Neither iodosylbenzene nor hydrogen peroxide was effective as oxidant in the epoxidation of ally1 chloride in the presence of the Ni” complexes. Cobalt complexes, some of which were known to catalyze epoxidation with sodium hypochlorite [ 9,151, were also ineffective in this epoxidation.
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Since considerable amounts of ally1 chloride remained unreacted when (salen) ] was used as the catalyst, higher yield of epichlorohydrin catalyzed by [Ni(Bu-babp) ] is due, at least in part, to the higher stability of the ligand moiety. In the [ Ni (salen) ] -catalyzed reaction, addition of sodium hypochlorite solution after completion of the first reaction did not improve the conversion, indicating the deterioration of the catalyst. Higher durability of the catalytic activity of [Ni(Bu-babp) ] was clearly shown in the further use of the catalyst. By addition of another feed of ally1 chloride and fresh sodium hypochlorite to the reaction mixture after completion of the first reaction, vigorous bubbling occurred again and substantial amounts of the added ally1 chloride were consumed giving epichlorohydrin in 17% yield (entry 2). The results demonstrated that [Ni(Bu-babp) ] retained its catalytic activity to some extent even after completion of the first reaction [ 31. Thus, it is shown that the Ni” complex of the salen analog catalyzes the direct epoxidation of electron-deficient ally1 chloride with sodium hypochlorite under phase-transfer conditions, yielding epichlorohydrin in 43% yield. Among the metal-catalyzed epoxidation of olefins, only a limited number of reports have appeared as to the nickel catalysis [91, and most of them are limited to the epoxidation of reactive electron-rich olefins. In the case of epoxidation catalyzed by [ Ni (salen) 1, highly active oxidant, oxo-Ni”’ or oxo-NP’ complex were suggested as being the active species [ 5,7]. In the present case, [ Ni (babp) ] and [ Ni (Bu-babp) ] showed no oxidation wave in the cyclic voltamogram, and their reduction potentials were slightly higher than that of [Ni(salen) 1. Therefore, formation of the higher oxidation state of [ Ni (babp ) ] and [ Ni (Bu-babp ) ] might be difficult, requiring longer induction periods. A lower yield in the epoxidation of styrene might be due to the higher reactivity of the active species leading to unidentified degradation products. [ Ni
Conclusion
Most of the salen analogs are limited to modifications of the phenoxy and/ or diamine units [ 41. This and previous studies confirm that the Con and Ci” complexes of the salen analog, [M (babp) ] have similar catalytic activities to those of [M (salen) 1, although the building blocks of the analog, babpHz are entirely different from those of salen, and only the Nz02 square-planar coordination sites are preserved. Replacement of their structure by more stable units increased the stability of the complex under severe reaction conditions, contributing to their superior performance as oxygenation and epoxidation catalysts.
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Experimental Electronic spectra were taken on a Shimadzu UV-265FS spectrophotometer at 20°C. Cyclic voltammetry was carried out with a Yanaco Polarographic Analyzer Model P-1100 at room temperature at a scan rate of 50 mV s-l under nitrogen atmosphere. A platinum wire, a glassy carbon stick, and a saturated calomel electrode were used as working, counter, and reference electrodes, respectively. Complexes Preparations of 6,6’ -his (benzoylaminato ) -2,2’ -bipyridine (2 - ) nickel (II) and 6,6’ -his (benzoyl-aminato) -2,2’ -bipyridine (2 - ) ([Ni(babp)l) were reported elsewhere [ 21. The Schiff base comcobalt(B) ([Co(babp)]) plexes [ Ni (salen) ] and o-bis (salicylideniminato) benzene (2 - )nickel (II) ( tNi(saloph) I) were prepared by published procedures [16]. The complex 6,6’ -his (4- t-butylbenzoyl-aminato ) -2,2’ -bipyridine (2 - ) nickel (II) ( [ Ni (Bubabp ) ] ) was prepared by a procedure similar to that of [ Ni (babp ) ] using 4-tbutylbenzoyl chloride instead of benzoyl chloride. Orange flakes. M.p. (from ligroin, b.p. 70-125°C) 320°C. Elemental analysis, Found: C, 68.39; H, 5.86; N, 9.91%. Calcd. for C3zH3zN402Ni: C, 68.23; H, 5.73; N, 9.95%. Epoxidations Epoxidation was performed in a round bottom glass ampoule (4 cm/a x 12 cm) at 25°C. To avoid loss of volatile materials during the reactions, a condenser cooled with ice water was attached on the ampoule. For oxidation with sodium hypochlorite; a dichloromethane solution (10 cm3) containing nickel catalyst (0.04 mmol), olefin (4 mmol), and phase transfer reagent (benzyltrin-butylammonium chloride, 0.15 mmol) and an aqueous NaOCl solution (0.7 M, pH 12.2, 20 cm3) in the ampoule were stirred vigorously. In contrast, oxidation with iodosylbenzene was carried out in 4 mmol of iodosylbenzene/dichloromethane solution without using phase transfer reagent. The amounts of substrate and products were monitored by GLC. Column: Shimadzu silicone OV-17/Shimalite W (1 m x3 mm0), silicone OV-210/Shimalite W (3 m x 3 mm@), and polyethylene glycol20M, 20%/Shimalite W (2 m x 3 mm@ ) for styrene, cyclohexene, and ally1 chloride, respectively.
References 1 2 3
M. Yamada, K. Araki and S. Shiraishi, Bull. Chem. Sot. Jpn., 60 (1987) 3149. M. Yamada, K. Araki and S. Shiraishi, Bull. Chem. Sot. Jpn., 61 (1988) 2787. M. Yamada, K. Araki and S. Shiraishi, J. Chem. Sot., Chem. Commun., 1988,530; M. Yamada, K. Araki and S. Shiraishi, J. Chem. Sot., Perkin Trans., 1 (1990) 2687.
202 4
5 6 7 8 9 10 11 12 13 14 15 16
M. Yamada et al. /J. Mol. Catal. 87 (1994) 195-202 A. Nishinaga and H. Tomita, J. Mol. Catai., 7 (1980) 179; M. Fro&in-Rio, D. Pujol, C. BiedCharreton, M. Peree-Fauvet and A. Gaudemer, J. Chem. Sot., Perkin Trans., 1, (1984) 1971; B.B. Corden, R.S. Drago and R.P. Perito, J. Am. Chem. Sot., 107 (1985) 2903. J.D. KooIa and J.K. Kochi, Inorg. Chem., 26 (1987) 908. H. Yoon and C.J. Burrows, J. Am. Chem. Sot., 110 (1988) 4087. H. Yoon, T.R. Wagler, K.J. O’Connor and C.J. Burrows, J. Am. Chem. Sot., 112 (1990) 4568. T. Yamada, T. Takai, 0. Rhode and T. Mukaiyama, BuII. Chem. Sot. Jpn., 64 (1991) 2109. K.A. Jorgensen, Chem. Rev., 89 (1989) 431. R.H. Hohn, Chem. Rev., 87 (1987) 1401. M. Nonoyama, S. Tomita and K. Yamasaki, Inorg. Chim. Acta, 12 (1975) 33. D.H. Brown, J.H. Morris, G. Oates and W.E. Smith, Spectrochim. Acta, 38A (1982) 333. P. Zanello and A. Cinquantini, Transition Met. Chem., 10 (1985) 370. K. O’Connor and C.J. Burrows, J. Org. Chem., 56 (1991) 1344. A. Nishinaga, K. Maruyama, M. Kakutani, T. Mashino andT. Umeda, Stud. Surf. Sci. CataI., 66 (1991) 675. P. Pfeiffer, E. Breith, E. Lubbe and T. Tsumaki, Annalen, 503 (1933) 84.
Journal Of hfOk?CUf!UF catalysis, 87 (1994) 195-202 Elsevier Science B.V., Amsterdam M311
Catalytic activities of nickel (II) complexes of a salen analog, [ Ni (babp ) 1, in epoxidation of olefins Masaki Yamada+, Shinji Ochi, Hatsuhiko Suzuki, Akihiro Hisazumi, Shigeyasu Kuroda and Ichiro Shimao Department of Materials Science and Engineering, 3190 Gofuku, Toyama 930 (Japan)
Faculty of Engineering,
Toyama University,
Koji Araki’ Institute
of Industrial Science, University
of
Tokyo, 7-22-l Roppongi, Minato-ku,
Tokyo 106
(Jap4 (Received April 30,1993; accepted September 17,1993)
Abstract A salen analog [Ni(babp) ] (babp=6,6’-bis(benzoylaminato)-2,2’-bipyridine (2 - ) ) and its t-butyl derivative catalyzed epoxidation of olefins in the presence of sodium hypochlorite as the oxidant. These complexes showed higher catalytic activity than [ Ni (salen) ] in epoxidation of an electron-deficient olefin, ally1 chloride, yielding epichlorohydrin in 43% yield. Key words: ally1 chloride; epoxidation; nickel catalysis: salen analog, square-planar complex
Introduction We reported syntheses, structures, and properties of a new type of salen (NJ -di (salicylidene)ethylenediamine) analog [M (babp) ] (M = Cu, Ni, Co, and Zn), where babp represents a deprotonated form of 6,6’ bis (benzoylamino)-2,2’-bipyridine [l-3]. Various salen analogs have been developed by structural modification of the diamine and/or phenoxy units of salen in order to improve the notoriously low durability of [Co (salen) ] as an oxygenation catalyst [ 41. In the [M (babp) ] complexes, however, the diamine and phenoxy units of salen were replaced by entirely different units and the square-planar NzOz coordination sites were reconstructed by using more stable building blocks, i.e., 2,2’-bipyridine and deprotonated amide. A cobalt (II) complex [Co (babp) ] was shown to be an excellent catalyst for oxygenation of 2,6disubstitutedphenols. Selectivity and durability of [Co (babp) ] were much higher than those of [Co (salen) 1, which was suggested to be due, at least in *Corresponding author. ‘Deceased.
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part, to the higher stability of the ligand moieties under oxidative conditions [ 31. Recently several groups have reported epoxidation of olefins, mostly electron-rich olefins like styrene or &methylstyrene, by nickel (II) complexes including [ Ni (aalen) ] [ 5-91, and a higher oxidation state of the complex [ 5-71, which is a highly active oxidant [lo], was proposed as being the active species of the catalyst. Therefore, the present study is suitable for testing the scope and limitations of the new type of salen analog prepared by using entirely different units. Here we report that [ Ni (babp ) ] derivatives exhibited catalytic activities in the epoxidation of olefins, especially for electron-deficient ally1 chloride, in the presence of sodium hypochlorite as the terminal oxidant.
x
Terminal Oxidant, Ni(II) Complex
-X
0
[Ni(salen)]
R=H; [Ni(babp)] R=r-Bu; [Ni(Bu-babp)] [Ni(saloph)]
Results and discussion Properties of the complexes The complex [Ni (babp) ] is sparingly soluble in most of the common organic solvents, and solubility of [ Ni (babp ) ] in dichloromethane, which is often used as a solvent for catalytic oxidation [5-E!], is lower than 10m4mol dmm3. Therefore, we prepared a t-butyl-substituted derivative, [ Ni (Bu-babp) ] (Bubabp = 6,6’ -his (4-t-butylbenzoylaminato) -2,2’ -bipyridine (2 - ) ), which was sufficiently soluble in dichloromethane (more than 10m2mol dmm3at r.t.). A TG study of [ Ni (Bu-babp ) ] showed that heating to its melting point at 320 ’ C under aerobic conditions caused no weight loss, showing that introduction of the t-butyl groups did not affect the high thermal stability of [ Ni (babp ) ] [ 21. We studied the spectroscopic and redox properties of [ Ni (babp ) ] and [ Ni (Bu-babp ) ] prior to use as an epoxidation catalyst. Characteristic IR bands of [ Ni (babp ) ] assigned to the deprotonated amide moieties (1558 and 1366 cm-’ ) [ 111 were also observed for [ Ni (Bu-babp ) ] [ 21, confirming that introduction of the t-butyl groups did not affect the square-planar structure of the
M. Yamada et al. /J. Mol. Catal. 87 (1994) 195-202
300
400
500
197
600
wavelength/nm Fig. 1. Electronic spectra of nickel complexes (6.67 x lo-’ mol dm-’ ) in dichloromethane at 20” C. (-): [Ni(babp)], (- - - -):[Ni(Bu-babp)], (- - -): [Ni(salen)], and (- - - -): [Ni(saloph)l (concentration of this complex is 3.33 x 10m6mol dme3).
complex. The electronic spectra of [ Ni (babp) ] and [ Ni (Bu-babp) ] in dichloromethane are shown in Fig. 1 along with that of [ Ni (salen) ] and a com[Ni(saloph)] (saloph=oanother Schiff-base ligand plex of bis (salicylideniminato )-benzene (2 - ) ) . The spectra of [ Ni (babp) ] and were quite similar to that of [Ni(salen) ] [ 121 showing the [Ni(Bu-babp)] characteristic band at around 420 nm [ 21, whereas a large red-shift of the corresponding band was observed in the spectrum of [Ni (saloph) 1. Unlike [ Ni (salen) 1, [ Ni (saloph) ] has a non-planar structure, and, therefore, the results support that [Ni(babp) ] and [ Ni (Bu-babp) ] have a square-planar structure quite similar to that of [ Ni (salen) 1. The redox property of [Ni(babp) ] was slightly different from that of [Ni(salen) 1. A cyclic voltamogram of [Ni(babp) ] in dimethyl sulfoxide (DMSO ) showed a single, quasi-reversible reduction peak presumably due to a Ni”/Ni’ redox couple at E1,2= - 1.33 V vs. SCE, which was slightly more positive than that of [Ni(salen) ] (El,z= - 1.61 V vs. SCE in DMSO) [ 131. Introduction of the t-butyl groups showed only a small effect, and the reduction potential of [ Ni (Bu-babp ) ] was - 1.45 V vs. SCE. However, no oxidation peak was observed for [ Ni (babp ) ] and [ Ni (Bu-babp ) ] within the sweep range (up to + 2 V), although [ Ni (salen) ] exhibited a quasi-reversible oxidation peak due to a Nin/Ni”’ redox couple at Elj2 = +0.71 V vs. SCE in DMSO [ 131. Use of glassy carbon instead of platinum wire as the working electrode did not affect the results. These results indicate that generation of the Ni”’ species in [ Ni (babp ) ] and [ Ni (Bu-babp) ] might be difficult. Epoxidation of olefins Epoxidation of olefins by sodium hypochlorite was carried out in a two phase system under phase-transfer conditions [ 61, where an organic phase ( 10
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M. Yamada et al. /J. Mol. Catal. 87 (1994) 195-202
cm3) was a dichloromethane solution of olefin (4 mmol), complex (0.04 mmol, 1 mol% of the substrate), and benzyltri-n-butylammonium chloride (phase transfer reagent, 0.15 mmol) and an aqueous phase (20 cm3) was a sodium hypochlorite solution (0.7 mol dme3, pH 12.2). The reaction, catalyzed by [ Ni (babp) ] or [ Ni (Bu-babp) 1, proceeded in the presence of sodium hypochlorite as the oxidant (Table 2). Olefins used in the study were styrene, cyclohexene (Table 1), and ally1 chloride (Table 2). After an induction period, a black precipitate appeared in the organic phase and then vigorous bubbling took place in the aqueous phase. When this stage was over, the black precipitate was completely bleached, and the olefin in the organic phase was conTABLE 1 Epoxidation of styrene and cyclohexene with sodium hypochlorite at 25 ’ C? Entry
7 8
Olefin
Catalyst
Time/h
Conversion/%
Epoxide yield/%
styrene
[Ni (babp ) 1 [Ni(Bu-babp)] [Ni(Bu-babp)]” [Ni(salen)] [Ni(Bu-babp)ld [Ni(salen)ld
b
24 2 2 24 24
100 100 100 100 37 65
20 28 21 42 0 0
[Ni(Bu-babp)] [Ni(salen)]
24 2
100 100
24 23
cyclohexene
“Conversions and yields were determined by GC. bIrreguIarinduction period was observed, see text. “Aqueous sodium hypochlorite was adjusted to pH 9.3 by addition of borate buffer. dOxidation with iodosylbenxene, see text. TABLE 2 Epoxidation of ally1chloride with sodium hypochlorite for 2 h at 25 ’ C Entry
1 2s 3 4 5 6 7
Catalyst
[Ni(Bu-babp)] [Ni (babp ) 1 [Ni(salen)] [Ni(saloph)] ]Co(babp) 1 [Co(salen)]
Conversion/%
98 18 98 64 73 3 75
Yield/% Epichlorohydrin
Trichloropropane
43 17 39 19 19 0 11
32 22 27 22 22 0 10
*Conversions and yields were determined by GC. bSecond use of the catalyst, see text. Conversion and yields are those of the second reaction.
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sumed completely when styrene or cyclohexene was used as the substrate. Iodine titration of the aqueous phase indicated that the oxidant sodium hypochlorite was also consumed completely. This process was quite similar to that catalyzed by [ Ni (salen) ] [ 6,7]. In the case of styrene or cyclohexene, yields of epoxides were no better than those catalyzed by [ Ni (salen) ] (Table 1), yielding a variety of degradation and chlorination products. As reported by Yoon and Burrows [ 61, iodosylbenzene was ineffective as an oxidant. However, the induction period in the [ Ni(babp) ] catalyzed reaction was much longer (approximately 8 h) than that for the [Ni(salen) ] one (a few minutes), and was not reproducible requiring up to several days in some cases. As [Ni(babp) ] did not dissolve in dichloromethane completely under the initial conditions employed, this could be the reason for the long and irregular induction periods. Epoxidation catalyzed by the more organic-soluble analog, [Ni(Bu-babp) 1, started within 8 h in all cases, although the induction period was still long compared to that of [Ni (salen) 1. Addition of pyridine (10 v/v% ) as an additional donor ligand [5] to the organic phase slightly diminished the long induction period to approximately 4 h. Decrease of the pH of the aqueous phase to 9.3 by using borate buffer effectively shortened the long induction time, and epoxidation started within a few minutes [ 71. However the yield of the epoxide was not improved. On the other hand, [Ni(Bu-babp) ] showed superior catalytic activity in the epoxidation of electron-deficient ally1 chloride (Table 2). Since electrondeficient olefins are less reactive toward electrophilic addition, relatively few have been studied such as in the direct epoxidation of ally1 chloride. Epichlorohydrin, one of the key reagents for organic synthesis, was obtained in 43% yield by direct epoxidation of ally1 chloride with sodium hypochlorite; this is twice as high as the reaction catalyzed by [ Ni (salen) ] or [ Ni (saloph) 1. The apparent changes in the solution during the reaction were similar to those for styrene or cyclohexene. However, the long induction time necessary for the reaction of styrene or cyclohexene was drastically lowered and epoxidation of ally1 chloride started within a few minutes even without lowering the pH of the aqueous phase. After the reaction, sodium hypochlorite was completely consumed in all cases. However, considerable amounts of ally1 chloride remained unreacted when [ Ni (salen) ] was used as a catalyst. The major by-product of the reaction was found to be 1,2,3trichloropropane, the chlorination product of ally1chloride in the reaction [ 7,141. Epoxide selectivity is higher for [ Ni (Bubabp) ] (epoxide/trichloropropane molar ratio = 1.3~)than [ Ni (salen) ] (0.9). The complex [ Ni (babp) ] showed similar catalytic activity to that of [ Ni (Bubabp ) 1, and the reaction started within a few minutes in most cases although occasionally a long induction period was still observed. Neither iodosylbenzene nor hydrogen peroxide was effective as oxidant in the epoxidation of ally1 chloride in the presence of the Ni” complexes. Cobalt complexes, some of which were known to catalyze epoxidation with sodium hypochlorite [ 9,151, were also ineffective in this epoxidation.
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Since considerable amounts of ally1 chloride remained unreacted when (salen) ] was used as the catalyst, higher yield of epichlorohydrin catalyzed by [Ni(Bu-babp) ] is due, at least in part, to the higher stability of the ligand moiety. In the [ Ni (salen) ] -catalyzed reaction, addition of sodium hypochlorite solution after completion of the first reaction did not improve the conversion, indicating the deterioration of the catalyst. Higher durability of the catalytic activity of [Ni(Bu-babp) ] was clearly shown in the further use of the catalyst. By addition of another feed of ally1 chloride and fresh sodium hypochlorite to the reaction mixture after completion of the first reaction, vigorous bubbling occurred again and substantial amounts of the added ally1 chloride were consumed giving epichlorohydrin in 17% yield (entry 2). The results demonstrated that [Ni(Bu-babp) ] retained its catalytic activity to some extent even after completion of the first reaction [ 31. Thus, it is shown that the Ni” complex of the salen analog catalyzes the direct epoxidation of electron-deficient ally1 chloride with sodium hypochlorite under phase-transfer conditions, yielding epichlorohydrin in 43% yield. Among the metal-catalyzed epoxidation of olefins, only a limited number of reports have appeared as to the nickel catalysis [91, and most of them are limited to the epoxidation of reactive electron-rich olefins. In the case of epoxidation catalyzed by [ Ni (salen) 1, highly active oxidant, oxo-Ni”’ or oxo-NP’ complex were suggested as being the active species [ 5,7]. In the present case, [ Ni (babp) ] and [ Ni (Bu-babp) ] showed no oxidation wave in the cyclic voltamogram, and their reduction potentials were slightly higher than that of [Ni(salen) 1. Therefore, formation of the higher oxidation state of [ Ni (babp ) ] and [ Ni (Bu-babp ) ] might be difficult, requiring longer induction periods. A lower yield in the epoxidation of styrene might be due to the higher reactivity of the active species leading to unidentified degradation products. [ Ni
Conclusion
Most of the salen analogs are limited to modifications of the phenoxy and/ or diamine units [ 41. This and previous studies confirm that the Con and Ci” complexes of the salen analog, [M (babp) ] have similar catalytic activities to those of [M (salen) 1, although the building blocks of the analog, babpHz are entirely different from those of salen, and only the Nz02 square-planar coordination sites are preserved. Replacement of their structure by more stable units increased the stability of the complex under severe reaction conditions, contributing to their superior performance as oxygenation and epoxidation catalysts.
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Experimental Electronic spectra were taken on a Shimadzu UV-265FS spectrophotometer at 20°C. Cyclic voltammetry was carried out with a Yanaco Polarographic Analyzer Model P-1100 at room temperature at a scan rate of 50 mV s-l under nitrogen atmosphere. A platinum wire, a glassy carbon stick, and a saturated calomel electrode were used as working, counter, and reference electrodes, respectively. Complexes Preparations of 6,6’ -his (benzoylaminato ) -2,2’ -bipyridine (2 - ) nickel (II) and 6,6’ -his (benzoyl-aminato) -2,2’ -bipyridine (2 - ) ([Ni(babp)l) were reported elsewhere [ 21. The Schiff base comcobalt(B) ([Co(babp)]) plexes [ Ni (salen) ] and o-bis (salicylideniminato) benzene (2 - )nickel (II) ( tNi(saloph) I) were prepared by published procedures [16]. The complex 6,6’ -his (4- t-butylbenzoyl-aminato ) -2,2’ -bipyridine (2 - ) nickel (II) ( [ Ni (Bubabp ) ] ) was prepared by a procedure similar to that of [ Ni (babp ) ] using 4-tbutylbenzoyl chloride instead of benzoyl chloride. Orange flakes. M.p. (from ligroin, b.p. 70-125°C) 320°C. Elemental analysis, Found: C, 68.39; H, 5.86; N, 9.91%. Calcd. for C3zH3zN402Ni: C, 68.23; H, 5.73; N, 9.95%. Epoxidations Epoxidation was performed in a round bottom glass ampoule (4 cm/a x 12 cm) at 25°C. To avoid loss of volatile materials during the reactions, a condenser cooled with ice water was attached on the ampoule. For oxidation with sodium hypochlorite; a dichloromethane solution (10 cm3) containing nickel catalyst (0.04 mmol), olefin (4 mmol), and phase transfer reagent (benzyltrin-butylammonium chloride, 0.15 mmol) and an aqueous NaOCl solution (0.7 M, pH 12.2, 20 cm3) in the ampoule were stirred vigorously. In contrast, oxidation with iodosylbenzene was carried out in 4 mmol of iodosylbenzene/dichloromethane solution without using phase transfer reagent. The amounts of substrate and products were monitored by GLC. Column: Shimadzu silicone OV-17/Shimalite W (1 m x3 mm0), silicone OV-210/Shimalite W (3 m x 3 mm@), and polyethylene glycol20M, 20%/Shimalite W (2 m x 3 mm@ ) for styrene, cyclohexene, and ally1 chloride, respectively.
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