Catal Lett (2009) 131:440–443 DOI 10.1007/s10562-009-9886-1
Allylic Oxidation of Cyclohexene with Molecular Oxygen Using Cobalt Resinate as Catalyst Caixia Yin Æ Zehui Yang Æ Bin Li Æ Fengmei Zhang Æ Jiaqiang Wang Æ Encai Ou
Received: 26 January 2008 / Accepted: 15 April 2008 / Published online: 24 February 2009 Ó Springer Science+Business Media, LLC 2009
Abstract Allylic oxidation of cychohexene under atmospheric pressure of molecular oxygen was carried out over cobalt resinate in the absence of solvent. It was shown that cobalt resinate exhibited promising catalytic activity for the oxidation of cyclohexene to 2-cyclohexen-1-ol and 2-cychohexen-1-one under mild condition for the first time. Keywords Allylic oxidation Cyclohexene Cobalt resinate catalyst 2-cyclohexene-1-ol 2-cyclohexene-1-one
1 Introduction The allylic oxidation of olefin into a, b-unsaturated alcohols and ketone is an important transformation in natural product synthesis [1]. In particular, the oxidation products of cyclohexene (CHE) and their derivatives, viz. 2-cyclohexene-1-one (2-CHON), 1-metylcyclohex-1-en-3-one, etc., are important in organic synthesis owing to the presence of a highly reactive carbonyl group, which is utilized in cycloaddition reaction [2]. Moreover, the products of allylic oxidation of CHE are important intermediates for the synthesis of medication, pesticide, and insect pheromone [3]. Whitomore et al. synthesized 2-CHON by the oxidation of CHE using chromic anhydride as oxidation in acetic acid solution [4]. However, the separation of reactants and products from the reaction mixture was difficult and the yields were low by using this method. The process
may also cause severe environmental pollution. Several homogenous and heterogeneous contained manganese, copper, iridium, iron and cobalt complexes catalysts were employed in the allylic oxidation of CHE [5–11]. But most of the cited work suffered from the drawback more or less. For example, the conversion was low and some reactions need organic solvents and special oxidants, such as, tert-butylhydroperoxide, and chloroperoxybenzoic. Some reactions were carried out using molecular oxygen as oxidant at high pressure. Particularly, some homogeneous medium has several disadvantages owing to the typical problems of separation of products from the catalyst. Sakthivel et al. overcame the typical drawback of homogeneous catalysis by using chromium containing mesoporous molecular sieves as catalyst for the oxidation of CHE to 2-CHON [12]. However, the conversion of CHE was also low and peroxide was used as oxidant. Interestingly, Sridhar and co-workers studied the effect of various kinetic parameters on the uncatalyzed oxidation of cyclohexene with molecular oxygen [13]. A few resinates, such as calcium, zinc, and manganese resinate, were synthesized and found these kind resinates wide application as dryers for paints and varnishes and as constituents of printing inks. But the catalytic properties of these materials have not been explored so far. Therefore, in this work cobalt resinate was synthesized and used as an efficient catalyst for the oxidation of CHE for the first time.
2 Experimental C. Yin Z. Yang B. Li F. Zhang J. Wang (&) E. Ou Department of Applied Chemistry, Key Laboratory of Medicinal Chemistry for Natural Resource, Ministry of Education, Yunnan University, 650091 Kunming, People’s Republic of China e-mail:
[email protected]
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2.1 Cobalt Resinate Preparation The preparation route of cobalt resinate is depicted in Scheme 1 [14].
Allylic Oxidation of Cyclohexene with Molecular Oxygen
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Scheme 1
O O
Co(NO 3 ) 2
NaOH
O O Co
Cobalt resinate was prepared involving the following processes: (a)
Abietic acid (6 g, Technical Grade), alcohol (AR, 12.5 mL), hydrochloric acid (AR, 1 mL) were used as received without further purification and added successively into a temperature-controlled, round bottom flask having a reflux condenser. Saturated sodium hydroxide was not added into dropwise until the reaction mixture was heated for 15 min and then stopped adding when the solution became orange. Reaction mixture was cooled and a light yellow crystalline sodium resinate was obtained. And then sodium resinate was washed with ethanol under reduced pressure and dried. (b) Sodium resinate (3.3 g) was mixed with saturated cobalt nitrate (2.9 g, AR) solution. The solution was extracted by xylene (AR) to remove impurity and residual abietic acid. Then the residue water in the solution was removed by distillation and then amaranth cobalt resinate could be obtained and used as catalyst for the oxidation of CHE after dry.
2.2 The Preparation CHE A mixture of cyclohexanol (AR, 15 g) and vitriol oil (AR, 1 mL) were added into a 50 mL round-bottom flask equipped a distillation condenser. The mixture was heated to the set temperature and an immersed cold water acceptor was used to accept the distillate under 363 K/5.3 kPa. Sodium chloride was used to saturate the distillate, and the organic layer was separated and then washed with water and 5% sodium carbonate. And then organic layer was separated again from the water and sodium carbonate. Anhydrous CaCl2 (AR) was used to remove more water from the separated organic phase. The dried organic material was distilled in a water bath and the products were collected between 348 and 350 K/ 5.3 kPa. The refractive index (n20 D ) of the obtained product (8 g) was 1.4460 which is accordance with refractive index of the reference (n20 D = 1.4465). The obtained products were analyzed by IR (BIO-RADFTS40) and GC–MS (Fı´nnigan GC800 TP/MS Voyager) using DB-5MS capillary column. The results of IR and MS are also accordance with the reference.
COONa
2.3 Oxidation of CHE The oxidation reactions were carried out as follows: The 20 g CHE was added into a temperature-controlled, round bottom, 3-necked-flask having a reflux condenser. The molecular oxygen (1,000–1,200 mL/min) was passed though the reaction solution under atmospheric pressure and cobalt resinate was added after the reaction mixture was heated to the set temperature. Reaction mixture was filtered under reduced pressure after the set time. The obtained crude products were analyzed by GC–MS (Fı´nnigan GC800 TP/MS Voyager) using DB-5MS capillary column. Reference substances were used for the identification of the products.
Table 1 Comparison of catalytic activities of cobalt resinate and Co-MCM-41 for the oxidation of CHE Catalyst
Conversion of CHE (wt%)
Selectivity of 2-CHOL (wt%)
Selectivity of 2-CHON (wt%)
Co-MCM-41
*100
20.1
15.5
Cobalt resinate
94.5
40.2
44.4
Reaction condition: CHE: cobalt resinate (wt/wt) = 10:1; CHE: Co-MCM-41 (wt/wt) = 40: 1; temperature = 343 K; reaction time = 7 h; airflow rate = 1,000–1,200 mL/min
100
Conversion/selectivity (wt%)
COOH
Conversion of CHE
80 60
selectivtiy of 2-CHON
40 20 Sselectivity of 2-CHOL 0 310
320 330 340 Reaction temperature (K)
Fig. 1 Effect of reaction temperature on the conversion and selectivity (reaction conditions: CHE = 20 g; airflow = 1,000–1,200 mL/min; reaction time = 7 h; cobalt resinate = 2 g)
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C. Yin et al.
Even though the conversion of CHE was almost 100% by using Co-MCM-41 as catalyst, the selectivity of 2-CHOL and 2-CHON was only 20.1 and 15.5%, respectively. By contrast, a rather higher conversion of CHE (94%) was also obtained by using cobalt resinate as catalyst under the same condition and the selectivity of 2-CHOL and 2-CHON arrived 41.9 and 44.4%, respectively. It is obvious that cobalt resinate has better performance on the oxidation of CHE. On the other hand, we have also studied the oxidation of CHE in blank experiments without catalyst under the oxidation conditions we used at 343 K for 7 h. The oxidation products were too low to be observed. The result of Sridhar and co-workers [13] indicated that the conversion rate of CHE would be less than 5% in stainless-steel reactor at 343 K for 5 h when oxygen pressure = 4 atm. Since in our study a glass reactor was used and the reaction pressure (atmospheric pressure) was lower than 4 atm, it would be expected that the conversion rate of CHE would be much less than 5%. Thus, we would say our blank result agrees with the result of Sridhar and co-workers [13] to some extent. Therefore, in the following we concentrate on the study of the influence of various parameters on the CHE conversion and selectivity of 2-CHOL and 2-CHON over cobalt resinate. The higher activity of cobalt resinate on the oxidation of CHE may be explained that cobalt resinate has a lipophilic
Conversion of CHE
Conversion/selectivity (wt%)
100 80 60
Selectivity of 2-CHON
40 20 Selectivity of 2-CHOL 0 2
3
4 5 Reaction time (h)
6
7
Fig. 2 Effect of reaction time on the conversion and selectivity (reaction conditions: CHE = 20 g; airflow = 1,000–1,200 mL/min; reaction temperature = 343 K; cobalt resinate = 2 g)
3 Results and Discussion The results of CHE oxidation with O2 over cobalt resinate are depicted in Table 1. For comparisons, the oxidation reaction of CHE with O2 over Co-MCM-41 [15] is also listed in Table 1. The catalysts are compared under similar conditions and found that 2-cyclohexene-1-ol (2-CHOL) and 2-CHON were detected as the major products. From Table 1 it is seen that both cobalt resinate and Co-MCM-41 exhibited rather high activity for the oxidation of CHE.
Scheme 2 The classic Haber– Weiss radical-chain sequence mechanism proposed for the oxidation of CHE over cobalt resinate [11, 13, 16]. (Cobalt resinate was presented as Co2?)
Initiation : OOH
H
O
H
Co 2+
O2
+ H
O
H
OH
OH
-
+
Co 3+
.
+
+
Propagation : .
OO .
H
H
O2
H
OOH
.
+ H
OOH
H
Co 3+
OO .
+
Co 2+
Termination : H
2
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OO .
H
OO - OO
H
OH
O
+
+
O2
Allylic Oxidation of Cyclohexene with Molecular Oxygen
structure, which increased the mutual solubility between CHE and cobalt resinate catalyst. So the oxidation of CHE could happen without any solvent. The influence of reaction temperature on CHE reaction over cobalt resinate at 7 h was investigated and shown in Fig. 1. It is seen that the conversion of CHE increased almost linearly with reaction temperature up to 333 K. The selectivity of 2-CHON also increased with increase in reaction temperature up to 333 K. A further increase in the reaction temperature resulted in the decrease in the conversion rate of CHE and the selectivity of 2-CHON, probably owing to the polymerization of CHE or 2-CHON at higher temperature. Whereas the selectivity of 2-CHOL decreased with the increase of reaction temperature up to 333 K, which is attributed to the further reaction of 2CHOL to 2-CHON. Beyond 333 K, the selectivity of 2CHOL increased with increase in reaction temperature. Considering the effect of temperature on both conversion of CHE and the selectivity of 2-CHON, 333 K was chosen as the suitable temperature for the oxidation of CHE. The effect of reaction time on CHE reaction over cobalt resinate was also investigated (Fig. 2) and found that the rate of CHE conversion increases with increase in reaction time up to 5 h while the selectivity of 2-CHON increased a little during this reaction time. The reaction time is longer than 5 h, the conversion of CHE nearly leveled off whereas selectivity of 2-CHON decreased a little which suggests the further reaction of 2-CHON. By contrast, the selectivity of 2-CHOL decreased with increase in reaction time from 2 to 5 h. The selectivity of 2-CHOL increased after 5 h. Thus, 5 h is suitable reaction time for the production of 2-CHON. A classic Haber–Weiss radical-chain sequence mechanism which is most plausible mechanism to form the two major products is depicted in Scheme 2; [11, 13, 16]. In this mechanism, cobalt resinate catalyst acted as radical initiator.
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4 Conclusions In summary, cobalt resinate was an efficient catalyst for the synthesis of 2-CHOL and 2-CHON under mild reaction conditions without adding any initiator and solvent. A high conversion of CHE (94%) can be obtained at 333 K. Acknowledgments The authors thank the National Natural Science Foundation of China (Project 20463003) and Natural Science Foundation of Yunnan Province (Project 2004E0003Z and 2003E0007R).
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