3 MgO as solid base catalysts in Michael addition

3.1 Introduction

↓50

Alkaline earth oxides are generally used as solid base catalysts. Among them, MgO is the most widely used. MgO has been extensively studied as a promising catalyst [13, 1, 2, 3], modifier or promoter [4,5, 6] and catalyst support [7,8,9] in heterogeneous catalysis. MgO exhibits high activities in numerous base-catalyzed organic reactions, such as self-Michael addition to form methyl diesters [163], the Tishchenko reactions [164] , the Meerwein-Ponndorf-Verley reactions [165] , dimerisation of ethanol to butanol [10], and self-condensation of propanol [11].

The basicity of MgO has been studied by FTIR, CO2-TPD. Normally, MgO acts as strong base catalyst. Coluccia and Tench [ 12] proposed a surface model for MgO (Fig. 3.1). There are several types of Mg–O ion pairs with different coordination numbers. Ion pairs of low coordination numbers exist at corners, edges, or high Miller index surfaces of the (100) plane [13]. Different basic sites generated by increasing the pre-treatment temperature appear to correspond to the ion pairs of different coordination numbers. However, the correspondence between the catalytically active sites for different reaction types and the coordination number of the ion pairs is not definite yet [13].

↓51

Fig. 3.1 Ions in low coordination on the surface of MgO [174]

MgO can be prepared by simple thermal decomposition of magnesium compounds such as Mg(OH)2, MgCO3, Mg(NO3)2 under controlled conditions. However, the pretreatment time, calcination temperature, gas environment can influent the properties of the resulting MgO catalysts, and therefore decide the catalytic behaviour. Choudhary et al. studied the influence of precursors used in preparation of MgO and its surface properties and catalytic activity in oxidative coupling of methane and found MgO obtained from magnesium carbonate and magnesium acetate were comparable and were much better than MgO obtained from the other precursors [14]. However, Aramendia et al. found in Meerwein-Ponndorf-Verley reaction of cyclohexanone with isopropyl alcohol, the most active catalyst was the solid prepared by rehydration and subsequent calcination of a magnesium oxide that was previously obtained from commercially available magnesium hydroxide [165]. Nanoscale and high surface area MgO can also be prepared from Mg(OCH3)2 by use of autoclave hypercritical drying (aerogel) procedure [ 15]. Meanwhile, aerogel prepared MgO showed higher reactivity than commercial and conventionally prepared MgO in Wadsworth–Emmons reactions [16]. However, the same MgO was poorer catalyst in toluene benzylation by benzyl chloride [17]. Therefore, for different reactions, various MgO catalysts may have quite different catalytic behaviors.

MgO has been proved as a good catalyst in a self-Michael addition of methyl crotonate [152], but not in the Michael additions of nitromethane to ,-unsaturated carbonyl compounds [140]. The factors to be considered MgO as an efficient catalyst for Michael addition depend on the strength of the basic sites, the acidity of reactant, and the charge on the carbon atom at -position to carbonyl group. Because of the importance of MgO as a solid base catalyst, in this chapter, various MgO prepared from different precursors were first tested in the liquid-phase Michael addition of 2-methylcyclohexane-1,3-dione to methyl vinyl ketone to get the preliminary result.

3.2 Preparation and characterization of MgO prepared by different methods

↓52

Table 3.1 shows the preparation andcharacteristics of the MgO catalysts prepared from different precursors. In most cases, simple thermal decomposition of various precursors at different temperatures was used to prepare MgO. All the calcination time is 4 h. As shown in Table 3.1, MgO-1 was prepared by calcination of Mg(NO3)2⋅6H2O at 600 °C in air, and very low surface area, 1.8 m2/g, was obtained. MgO-2 and MgO-3 were obtained by thermal decomposition of MgCO3 at 600 and 500 °C in air, respectively. MgO-3 had a higher surface area (106.6 m2/g) than MgO-2 (76.6 m2/g) because a lower calcination temperature was used. MgO-4 was obtained by calcination at 600 °C of dried Mg(OH)2, which was prepared by precipitation of MgCl2 using KOH and dried at 110 °C. MgO-4 had a surface area of 76.7 m2/g. MgO-5 to MgO-7 were prepared by calcination of dried Mg(OH)2 (the hydroxide was precipitated from Mg(NO3)2 solution with KOH and dried at 110 °C) at 600, 500, 400 °C, respectively. The surface area of these three MgO increased with decreasing calcination temperature. This is due to an increase in the crystal size caused by sintering of MgO at the higher temperatures.

Table 3.1 Preparation and characteristics of MgO catalysts

Sample

Precursor

Preparation

SBET

(m2/g)

VP a

(cm3/g)

dp b

(Å)

MgO-1

Mg(NO3)2

calcination at 600 °C in air

1.8

0.017

381

MgO-2

MgCO3

calcination at 600 °C in air

76.6

0.337

175

MgO-3

MgCO3

calcination at 500 °C in air

106.4

0.321

120

MgO-4

MgCl2

precipitation c and calcination at 600 °C in air

76.7

0.971

508

MgO-5

Mg(NO3)2

precipitation c and calcination at 600 °C in air

75.0

0.960

258

MgO-6

Mg(NO3)2

precipitation c and calcination at 500 °C in air

116.5

1.095

375

MgO-7

Mg(NO3)2

precipitation c and calcination at 400 °C in air

170.2

1.074

252

Mg(OH)2

Mg(NO3)2

precipitation and dried at 110 °C in air

-

-

-

a: BJH desorption cumulative pore volume of pores between 17.0 and 3000.0 Å diameter
b: Average pore diameter by BET
c: Mg(OH)2 was dried at 110 °C in air before calcination

Fig. 3.2 XRD patterns of MgO. MgO-1 (a), MgO-2 (b), MgO-3 (c), MgO-4 (d), MgO-5 (e), MgO-6 (f), MgO-7 (g), Mg(OH)2 (h)

↓53

Fig. 3.3 N2 adsorption/desorption isotherms of MgO catalysts

XRD patterns of MgO catalysts are shown in Fig. 3.2. All the MgO catalysts are pure phases of periclase MgO (PDF No.45-946) that exhibits the three characteristic peaks at 2θvalues of 37.0 (1 1 1), 43.0 (2 0 0) and 62.4 (2 2 0) degrees. The hydroxide (Fig. 3.1h) precipitated from Mg(NO3)2 exhibited a brucite-like Mg(OH)2 structure (PDF No.44-1482). If the strongest peak at around 43.0 degree for periclase is used as a reference for crystallinity, then MgO-1 with the smallest surface area is the most crystalline sample. MgO-3 is the least crystalline. From MgO-5 to MgO-7, with the decrease of the calcination temperature, the crystallinity decreases.

The N2 adsorption/desorption isotherms of MgO catalysts are shown in Fig. 3.3. Except of the MgO calcined from the precursor of Mg(NO3)2⋅6H2O, all other MgO exhibited similar isotherms (type IV). According to IUPAC classification, they are typical of mesoporous materials [18]. The shape of isotherms and the hysteresis loop can give the information about the pore structure. The desorption isotherms of MgO-2 and MgO-3 followed different paths than the adsorption isotherm down to a P/P0 value of 0.5 and 0.4, respectively. The hysteresis loop of MgO-3–MgO-7 closed at P/P0 values of ca.0.8. These results indicated that MgO-2 and MgO-3 had smaller mesoporous than MgO-3–MgO-7. Based on the shape of hysteresis loops, all MgO have ‘ink-bottle’ pores, spheroidal cavities or volids between close-packed spherical-like particles.

3.3 Catalytic behavior of MgO catalysts

↓54

Scheme 3.1 Possible reaction procedure in the reaction of 2-methylcyclohexane-1,3-dione (1) with methyl vinyl ketone (2) using MgO as catalyst

When MgO was used as catalyst in Michael addition of 2-methylcyclohexane-1,3-dione to methyl vinyl ketone, in the 1H NMR spectra of the reactionproducts, besides the signals of expected Michael adduct 2-(#SYMBOL#-oxobutyl)-2-methylcyclohexane-1,3-dione (single peaks at 1.11 and 2.04 ppm, DMSO-d 6), additional main CH3 signals at 0.97 and 1.23 ppm, were found. Based on the results of NMR and GC-MS, another main product 6-hydroxy-1,6-dimethyl-2,9-dioxobicyclo[3.3.1]nonane was confirmed, which was formed by the aldol cyclization of Michael adduct 2-methyl-2-(3-oxo-butyl)-cyclohexane-1,3-dione. In case of MgO as catalyst, the possible reaction procedure is shown in Scheme 3.1. Besides the expected Michael addition, a consecutive aldol cyclization took place to give a bridged ketol (product 4, in Scheme 3.1). Negligible amount of product 5 and 6 were also detected by GC-MS.

Table 3.2 The results of the Michael addition using different MgO as catalysts

Catalyst

Yield of Michael adduct and (bridged ketol) %

0.5 h

1 h

2 h

4 h

6 h

8 h

24 h

MgO-1

-

-

6 (0)

21 (0)

43 (4)

67 (9)

57 (32)

MgO-2

-

61 (30)

56 (32)

48 (39)

26 (55)

21 (56)

18 (51)

MgO-3

65 (14)

64 (28)

55 (33)

30 (53)

24 (59)

18 (55)

13 (45)

MgO-4

51 (18)

59 (31)

52 (34)

34 (47)

14 (62)

14 (60)

4 (43)

MgO-5

43 (28)

65 (26)

59 (28)

42 (40)

17 (60)

15 (54)

6 (47)

MgO-6

70 (22)

66 (26)

59 (30)

39 (47)

27 (54)

19 (57)

10 (42)

MgO-7

68 (17)

65 (24)

58 (30)

43 (30)

30 (49)

21 (54)

13 (42)

Mg(OH)2

-

23 (0)

51 (0)

75 (18)

60 (32)

52 (38)

42 (43)

↓55

Table 3.2 shows the reaction results of the different MgO catalysts. For the catalyst MgO-1, prepared by the decomposition of Mg(NO3)2, at the beginning of the reaction (before the reaction time 4 h), no bridged ketol was formed, only 21% yield of Michael adduct was obtained in 4 h. At 8 h, the yield of bridged ketol was 9% and the yield of Michael adduct arrived maximum 67%. Compared with the other MgO catalysts, MgO-1 showed quite different behavior probably due to its unusual low surface area. For other MgO catalysts (MgO-2 to MgO-7), both Michael adduct and bridged ketol appeared at the beginning of the reaction (in 0.5–1 h). Within the reaction time of 8 h, with the decrease of the yields of Michael adduct from the maxima of about 60–70% (after 0.5 h for MgO-3, MgO-6 and MgO-7; 1 h for MgO-4 and MgO-5) to 15–21% (8 h), the yields of bridged ketol increased from 25–30% (1 h) to 55–65% (8 h). However, in the literature, 60% isolated yield of bridged ketol was obtained after 38 h at 20 °C under high pressure using the same reactants and the homogeneous catalyst Et3N [ 19]. For all cases, at 24 h, low yields of some possible known compounds (product 5 and 6 in Scheme 3.1) were obtained, which could be the reason for the decrease of both Michael adduct and bridged ketol. In case of Mg(OH)2 as catalyst, within the reaction time of 2 h, 51% yield of Michael adduct with 100% selectivity was obtained. The Michael adduct yield went through a maximum of over 75% yield after 4 h but did not decrease as drastically as in the case of MgO (final yield : 42%, not less than 20%). The different behavior of Mg(OH)2 and MgO may ascribed to the different type of base sites on the surface. For the different MgO catalysts, the catalytic performances have slight difference. Many parameters, like the nature of the base sites on the MgO surface, the surface area of MgO, the crystallite size of MgO, can affect the reaction results. Since MgO did not catalyze the Michael addition selectively, the discussion here will not go further to point out what is the exact reason for the different catalytic behavior from one MgO catalyst to another. However, the behavior of MgO will be compared and discussed with other catalysts systems in the following chapter based on the characterization of its basic properties.

Here, the Michael adduct is an important precursor of Wieland-Miescher keton [20], which is particularly useful for the synthesis of biologically active compounds [21,22]. However, although bridged ketols are not normally the desired product of the reaction, they are proven to be versatile synthetic intermediates since they can be readily dehydrated to the bicyclic enones or converted to the fused Robinson products under mild acid conditions. Moreover, the bridgehead or angular carbomethoxy of the bicyclic or fused-ring compound, respectively, expands the possibilities for further synthetic transformations at this position [181]. In our case, by careful selection of the reaction time and catalyst, the reaction can, to an extent, be controlled to give high yields of the Michael adduct 2-(γ-oxobutyl)-2-methyl-cyclohexan-1,3-dione or the bridged ketol 6-hydroxy-1,6-dimethyl-2,9-dioxobicyclo [3.3.1]nonane, respectively.

In summary, MgO can not catalyze the Michael addition of 2-methylcyclohexane-1,3-dione to methyl vinyl ketone selectively. However, either Michael adduct or bicyclic ketol can be selected to be the dominant product by choosing the MgO catalyst and reaction time. Moreover, two-step reaction can be performed in one-pot system.

3.4 Conclusions

↓56

MgO prepared by different methods or conditions have various textural properties. MgO with surface area from 1.8 to 170.2 m2/g and pore volume from 0.01 to 1.0 cm3/g can be prepared. MgO-1 prepared by calcination of Mg(NO3)2⋅6H2O at 600 °C in air has very low surface area of 1.8 m2/g; however, MgO-7 prepared by calcination of dried Mg(OH)2 at 400 °C has a high surface area of 170.2 m2/g.

All the MgO catalysts can not catalyze the Michael addition of 2-methylcyclohexane-1,3-dione to methyl vinyl ketone selectively because a consecutive aldol cyclitzation occurred. Michael adduct or bicyclic ketol can be selected to be the dominant product by choosing the MgO catalyst and the reaction time. The catalytic behavior of MgO in Michael addition is ascribed to its strong basicity.


Footnotes and Endnotes

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19 [] W. G. Dauben, R.A. Bunce, J. Org. Chem. 48 (1983) 4642–4648.

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