균일계 Rh 촉매에 의한 □,□-Unsaturated Ester의 Hydrofomylation 반응

Theories and Applications of Chem. Eng., 2002, Vol. 8, No. 1

이 철우, 이 원묵, H. Alper*

화학공학의 이론과 응용 제8권 제1호 2002년

한밭대학교 응용화학공학부/RRC, *Ottawa대학교 화학과

Hydrofomylation of □,□-Unsaturated Esters Catalyzed by Rhodium Complexes

C. W. Lee, W. M. Lee, H. Alper*

Dept. of Chemical Engineering/RRC, Hanbat National University

*Department of Chemistry, University of Ottawa

Introduction

The hydroformylation of □,□-unsaturated esters is an attractive synthetic route for the preparation of 1,3-and 1,4-difunctional compounds, which can subsequently be converted to malonic acid derivatives and □-butyrolactones. Numerous neutral and cationic rhodium compounds have been investigated as catalysts, and recently the zwitterionic rhodium compound, Rh(cod)(□6-PhBPh3), has been shown to be an excellent precatalyst for the reaction. The regiochemistry of the reaction strongly depends on the catalyst system and the substrates employed. For example, Prokai-Tatrai et al. investigated the hydroformylation of methyl methacrylate and found that the ratio of □-formyl to □-formyl product is 94/6 in the case of Rh4(CO)12 as the catalyst, but it changes to 9/91 for the Rh4(CO)12-PBu3-NEt3 catalyst system. It has been known that the presence of  1,4-bis(diphenylphosphino)butane (dppb) as an ligand in metal-catalyzed carbonylation reactions enables reactions to occur which do not otherwise proceed and can alter the regiochemistry of a reaction in some cases.

We now describe how the use of dppb as an added ligand influences the zwitterionic Rh(Ⅰ)-catalyzed hydroformylation of □,□-unsaturated esters and the effect of the bidentate phosphine ligand when cationic or neutral rhodium complexes are employed as catalysts.

Experimental

Dichloromethane and THF were dried and distilled under nitrogen before use. The zwitterionic rhodium complex and [Rh(1,5-hd)(phen)]+Cl- were prepared according to literature methods. The other rhodium complexes, substrates and ligands were purchased from Aldrich Chemical Co., and were used as received. Proton and carbon NMR spectra were recorded on a Varian Gemini 200 spectrometer using CDCl3 as the solvent. A 45ml stainless steel autoclave (Parr Instrument) was used as a batch reactor. In a typical experiment, a mixture of the easter (4.0 mmol), Rh complex (0.040 mmol), and dppb (0.808mmol) in CH2Cl2 (10mL) was charged into the reactor. The gas phase in the reactor was flushed with carbon monoxide three times and pressurized to 300 psi, and then hydrogen was introduced up to the total pressure of 600psi. The reactor was then placed in an oil bath at 80 oC and reacted for 18 h. The reactor was then cooled to room temperature, the reaction mixture was filtered through neutral alumina, and the solvent was removed by rotary evaporation. Quantitative analysis was carried out by a GLC (varian 3400) equipped with a 1.5% OV-17 and 1.95% OV-210 packed column, with biphenyl as an internal standard. The ratio of isomers was determined by NMR spectroscopy. Pure product was isolated by silica gel column chromatography.

Results and Discussion

The hydroformylation reaction of the □,□-unsaturated esters, methyl acrylate, methyl methacrylate, and □-methylene-□butyrolactone were carried out at 50 - 130 oC under 600 psi of synthesis gas (CO/H2 = 1/1) using zwitterionic, cationic, and neutral rhodium(Ⅰ) complexes as catalyst precursors, in the presence or absence of dppb. Rh(cod)(□6-PhBPh3)(1), [Rh(cod)(dppb)]+BF4-(2), and [Rh(cod)Cl]2(3) were used as the zwitterionic, cationic, and neutral rhodium complexes, respectively.

1. Hydroformylation of Methyl Acrylate(4)

The rhodium-catalyzed hydroformylation of 4 gave both methyl □-formylpropionate (5) and methyl □-formylpropionate (6). A small amount (＠5%) of methyl propionate was formed as a byproduct. The result are summerized in Table 1. The NMR spectra of 5 shows that it is in equilibrium with its enol form [CH2 = C(CHOH)CO2Me]. The ratio of the enol to the aldehyde in CDCl3 at room temperature is 0.75/1.0.

In the absence of dppb, the catalytic activities of rhodium complexes were low to moderate, and the regioselectivity for 5 was 75 - 79%. The order of the catalytic activity was 1Ｂ2Ｂ3 in the absence of dppb. However, addition  of dppb ligand significantly increased both the catalytic activity and degree of regioselectivity. Almost the same results were observed regardless of the nature of the rhodium complexes. The most dramatic example was observed when 3 was used as the catalyst. In the absence of dppb,  the conversion, yield, and □-selectivity were 16, 5, and 75%, respectively. However, the conversion, yield, and □-selectivity increased to 100, 94, and 98%, respectively, when 2 eq of dppb were present in the reaction system. Triphenylphosphite, P(OPh)3, showed excellent □-selectivity with Rh(acac)(CO)2 in the hydroformylation of ethyl acrylate. On the contrary, the phosphite gave excellent □-selectivity using the zwitterionic rhodium complexes (entry3, Table 1)

2. Hydroformylation of methyl Methacrylate (7)

The hydroformylation of 7 was carried out at 100 oC under 600 psi of synthesis gas in dichloromethane for 18h and the results are presented in Table 2. When 1 was used as the catalyst in the absence of dppb, the reaction proceeded rapidly and the ratio of □-isomer to □-isomer was 20:80. A trace amount (＠2%) of the hydrogenation product was formed in the case. The catalytic activity of the zwitterionic compound 1 was superior to that of the cationic (2) or the neutral (3) complex. Interestingly, the hydrogenation product was formed in 47% yield using 3 as the catalyst in the absence of dppb. The addition of dppb to the zwitterionic (1) reaction system increased the □-selectivity remarkably to 91% (2:1 ratio of dppb:1) from 20%. However, in marked contrast to the hydroformylation of 4, the conversion and yield decreased by the addition of dppb to catalyst 1. Also note that when 2 and 3 were used in the presence of 2 mol equiv of dppb, the catalyst system was totally inactive.

3. Hydroformylation of □-Methylene-□-butyrolactone (10).

In the absence of dppb, the reaction of 10 with synthesis gas did not afford any aldehyde, although the conversion was very high (Ｂ90%). Instead, the isomerized lactone was the predominant product, and some hydrogenation occurred. Therefore, it seems that CO insertion does not take place effectively in the absence of dppb. However, in the presence of dppb, the branched aldehyde containing a quaternary carbon atom, to which was attached the formyl group, was obtained in excellent yield. The hydroformylation process is regiospecific. When the zwitterionic complex 1, the cationic complex 2, and neutral rhodium complex 3 were used as catalysts in the absence of dppb at 100 oC under 600 psi, the isomerization product was the major one with the yield being 86 - 96%. When dppb was added as aligand (2: 1 ratio of dppb/Rh), the branched aldehyde was formed as the main product in 81 - 82% yield (isolated yield of 67 - 70%).

Conclusions

Excellent □-selectivity occurs when dppb is added as a ligand in the hydroformylation of □,□-unsaturated esters catalyzed by rhodium(Ⅰ) complexes. When methyl acrylate and □-methylene-□-butyrolactone are used as substrates, dppb significantly promotes the hydroformylation. However, dppb inhibits the reaction when methyl methacrylate is used as the substrate. In the absence of dppb, the hydromylation of methyl methacrylate catalyzed by 1 shows increasing □-selectivity with increasing temperature.

References

1. Botteghi, C., Ganzerla, R., Lenarda, M. and Moretti, G., J. Mol. Catal. 40, 129 (1987).

2. Pruett, R. L., Adv. Organomet. Chem., 17, 1 (1979).

3. Yamashita, H., Roan, B. L., Sakakura, T., Tanaka, M., J. Mol. Cat., 81, 255 (1993).

4. Amer, I. and Alper, H., J. Am. Chem. Soc., 112, 3674 (1990).

5. Alper, H. and Zhou, J.-Q., J. Org. Chem., 57, 3729 (1992).

6. Prokai-Tatrai, K., Toros, S. and Heil, B., J. Organomet. Chem., 332, 331 (1987).

7. El Ali, B. and Alper, H., J.-Q., J. Org. Chem., 56, 5357 (1991).

8. Lin, I. J. B. and Alper, H., J. Chem. Soc., Chem. Commun., 248 (1987)

Table 1. Rhodium-Catalyzed Hydroformylation of Methyl Acrylate

화학공학의 이론과 응용 제8권 제1호 2002년

with Synthesis Gas (CO/H2=1/1)

entry	[Rh]	L (L/Rh)	conv(%)	yield(%)	□-selectivity
1	1	-	71	51	76
2	1	dppb (2)	100	93	97
3	1	P(OPh3) (4)	96	90	98
4	2	-	34	25	79
5	2	dppb (2)	100	89	99
6	2	-	16	5	75
7	3	dppb (1)	33	25	91
8	3	dppb (2)	100	94	98
9	3	dppb (3)	100	94	99

Table 2. Rhodium-Catalyzed Hydroformylation of Methyl Mthacrylate

with Synthesis Gas (CO/H2=1/1)

entry	[Rh]	L (L/Rh)	conv(%)	yield(%)	□-selectivity(%)
1	1	-	98	96	20
2	1	dppb (2)	72	72	91
3	2	-	29	24	16
4	2	dppb (2)	nr	-
5	3	-	100	53	16
6	3	dppb (2)	nr	-