098038ne

http://www.chemistrymag.org/cji/2007/098038ne.htm	Aug.28, 2007 Vol.9 No.8 P.38 Copyright

Photopromoted carbonylation of alkyl bromides catalyzed by copper powder under ambient conditions

Zou Shunying, Jia Yingping, Yin Jingmei, Gao Dabin, Zhou Guangyun
(Liaoning Key Laboratory of Bio-Organic Chemistry, Dalian University, Dalian 116622)

Abstract Photopromoted carbonylation of alkyl bromides with carbon monoxide catalyzed by copper powder can be carried out under ambient conditions and the corresponding ester was produced. The yield and the selectivity of ester can be improved by addition of CH₃COONa. And after the reaction, the catalyst can be separated easily and reused.
Keywords photopromoted carbonylation, copper powder, alkyl bromides, carbon monoxide, recycle used.

The synthesis of esters by carbonylation is one of the most important reactions, which usually requires high temperatures (150-200ºC) and high pressure (10-20 MPa) or precious metal catalysts (Ru, Rh, Ir)^[1]. There are many advantages, however, for photopromoted carbonylations such as ambient conditions and non-precious metal catalyst^{[2, 3]}.
In the previous paper^[4], we have reported that the photopromoted carbonylation of alkyl bromides with CO could be carried out under the catalysis of copper salts. Notice that the copper salts are soluble in methanol, these homogeneous catalysts are difficult to be separated after reaction and reused then. Thus, the use of heterogeneous catalysts is worthy to be considered in the photopromoted carbonylations.
We report herein that copper powder has been used as catalyst for the photopromoted carbonylation of alkyl bromides with CO under ambient conditions and the effect of CH₃COONa as additive on the reaction. The catalyst could be recycle used for several times. To our best knowledge, there are no reports about copper powder as heterogeneous catalyst in the photopromoted carbonylations of alkyl bromides.

1 EXPERIMENTAL
Carbon monoxide (99.9%, Dalian Guangming Institute); methanol (G.R., Beijing Chemical Engineering Co.); bromocyclohexane, 1-bromopentane, 1-bromohexane, 1-bromoheptane, 1-bromooctane, 1-bromodedecane, n-decane, copper powder, CH₃COONa (A.P., Beijing Phentex Chemicals Co., Ltd.).
A typical reaction procedure was as follows: First, a quartz reactor was equipped with a 400 W high pressure mercury lamp and cooled with circulating cold water to maintain room temperature. Then a quartz test tube, which contained the methanol solution of alkyl bromides (100 mmol/L), copper powder (0.1 mmol) and additive, was placed closely against the outer wall of the quartz reactor. The reactions were carried out with irradiation for 6 hours at room temperature under 0.1 MPa of CO pressure. The reaction products were characterized by GC retention time and GC-MS. During the procedure, n-decane was used as internal standard.

2 RESULTS AND DISCUSSION
The photopromoted carbonylation of bromocyclohexane with carbon monoxide and methanol catalyzed by copper powder could be carried out under ambient conditions and cyclo-C₆H₁₁COOCH₃ was produced (Scheme 1). The results indicated that the yield of 22% and the selectivity of 31% for cyclo-C₆H₁₁COOCH₃could be obtained with copper powder as catalyst and the major byproduct was cyclohexene.

Scheme 1

Furthermore, the effect of different additives on carbonylation of bromocyclohexane with CO was investigated. The results are shown in Table 1.

Table 1 Effect of different additives on photopromoted carbonylation of bromocyclohexane

Additives^a	Conversion (%)	Yield (%)	Selectivity (%)
None	71	22	31
HCOONa	73	30	41
CH₃COONa	66	32	48
NaHSO₃	60	4	7
p-toluenesulfonic acid	72	4	6

a: additives 30mmol/L

    It can be seen that the catalytic activity of copper powder is improved by addition of the basic additives, e.g. the yield of the ester can be increased from 22% to 32% and the selectivity improved from 31% to 48% with CH₃COONa as additive. However, acidic additives such as NaHSO₃ and p-toluenesulfonic acid were disadvantageous only with cyclo-C₆H₁₁COOCH₃in 4% yield. The role of the basic additives could be assumed to neutralize hydrogen bromide formed in the reaction^[3-5]. And in the experiment of control when copper powder was absent, the solution of methanol and bromocyclohexane under CO was irradiated with no additive or CH₃COONa as additive, respectively. No ester was produced. The result indicated that the photopromoted carbonylation did not proceed without catalyst^[3].
   In order to improve the reactivity, the effect of CH₃COONa concentration on the reaction was investigated. The results are shown in Figure 1.
    From Figure 1, it can be seen that the yield and selectivity of the ester depended on the amount of CH₃COONa when the concentration of CH₃COONa was lower than 40mmol/L. For the concentration of CH₃COONa was over 40mmol/L, the change of the yield was not obvious, but the selectivity was decreased with the increasing of the byproduct. The best concentration of CH₃COONa was 40mmol/L, the yield and the selectivity was 36% and 53%, respectively.

Figure 1 Effect of CH₃COONa concentration on photopromoted carbonylation of bromocyclohexane

In addition, the recycle use of copper powder as catalyst for the carbonylation of bromocyclohexane was studied. The results are shown in Figure 2.

Figure 2 Effect of recycle times of copper powder on the photopromoted carbonylation of bromocyclohexane with CO

It can be seen that the yield of cyclo-C₆H₁₁COOCH₃ decreased monotonically from 36% to 32% in the eight recycles. The observations demonstrate that the catalytic activity of copper powder is not change obviously in the cycle and the catalysts can be used repeatedly.
Under the same conditions, the photopromoted carbonylation of linear alkyl bromides has also been investigated with copper powder (Scheme 2). The results are shown in Table 2.

Scheme 2

Table 2 Effect of copper powder on photopromoted carbonylation of different substrates

Substrates	Conversion (%)	Yield (%)	Selectivity (%)
1-bromopentane	96	15	16
1-bromohexane	92	20	22
1-bromoheptane	96	26	27
1-bromooctane	90	20	22
1-bromodedecane	92	16	17

It can be seen that copper powder can also catalyze photopromoted carbonylation of linear alkyl bromides with CH₃COONa as additive, and the corresponding ester was obtained. Further research on mechanism is underway.

Acknowledgment The authors are indebted to the National Natural Science Foundation of China (No.20372012) for the generous financial support.

REFERENCES
[1] Tao Y T. Chow T J. Lin J T et al.. J. Chem. Soc. Perkin Trans. I, 1989, 1: 2509-2511.
[2] Tao Y T. Yeh W L. Chow Y L et al.. J. Mol. Catal., 1991, 67: 105-115.
[3] Sun Y. Yin J M. Gao D B et al.. Chin .Chem. Lett., 2003, 14(6): 575-578.
[4] Wen T. Jia. Y P. Gao D B et al.. Chin .Chem. Lett., 2005,16(11): 1455-1458
[5] Cash D. Combs A. Dragojlovic V. Tetrahedron Lett., 2004, 45: 1143-1145.

光促进温和条件下铜粉催化溴代烷烃的羰基化反应
邹顺瑛，贾颖萍，高大彬，尹静梅，周广运
（大连大学生物有机省重点实验室,大连 116622）
摘要在温和条件下，以铜粉为催化剂，实现了溴代烷烃与一氧化碳的光促进羰基化反应，得到相应的羧酸甲酯。添加物醋酸钠对反应有促进作用，反应后，催化剂铜粉易于分离，可重复使用。
关键词 光促进羰基化，铜粉，溴代烷烃，一氧化碳，重复使用

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