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

AbstractPhotopromoted carbonylation ofalkyl bromideswith carbon monoxide catalyzed by copper powder can be carried outunder ambient conditions and the corresponding ester was produced. The yield and the selectivity of ester can be improved by addition of CHCOONa. And after the reaction, the catalyst can be separated easily and reused.

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 CHCOONa 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, -decane, copper powder, CHCOONa (A.P., Beijing Phentex Chemicals Co., Ltd.).
A typical reaction procedure was as follows: First, a quartz reactor was equipped with a 400 Whigh 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, -decane was used as internal standard.

RESULTS AND DISCUSSION
The photopromoted carbonylation of bromocyclohexane with carbon monoxideand methanol catalyzed by copper powder could be carried out under ambient conditions and cyclo-C₁₁COOCH was produced (Scheme 1). The results indicated that the yield of 22% and the selectivity of 31% for cyclo-C₁₁COOCH could 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 1Effect of different additives on photopromoted carbonylation of bromocyclohexane

Additives	Conversion (%)	Yield (%)	Selectivity (%)
None	71	22	31
HCOONa	73	30	41
CHCOONa	66	32	48
NaHSO	60
-toluenesulfonic acid	72

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 CHCOONa as additive. However, acidic additives such as NaHSO and -toluenesulfonic acid were disadvantageous only with cyclo-C₁₁COOCH in 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 CHCOONa 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 CHCOONa 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 CHCOONa when the concentration of CHCOONa was lower than 40mmol/L. For the concentration of CHCOONa 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 CHCOONa was 40mmol/L, the yield and the selectivity was 36% and 53%, respectively.

Figure 1 Effect of CHCOONa 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-C₁₁COOCH 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 CHCOONa 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.