Reduction of heterocyclic carboxaldehydes via Meerwein–Ponndorf–Verley reaction

Applied Catalysis A: General 303 (2006) 23–28 www.elsevier.com/locate/apcata

Reduction of heterocyclic carboxaldehydes via Meerwein–Ponndorf–Verley reaction Ce´sar Jime´nez-Sanchidria´n **, Julia Marı´a Hidalgo, Jose´ Rafael Ruiz * Departamento de Quı´mica Orga´nica,Universidad de Co´rdoba, Campus de Rabanales, Edificio Marie Curie, Carretera Nacional IV-A, km. 396, 14014 Co´rdoba, Spain Received 4 November 2005; received in revised form 12 January 2006; accepted 16 January 2006

Abstract A series of Mg/Al and Mg/Al/Zr layered double hydroxides (LDHs) was prepared for use as catalysts in the Meerwein–Ponndorf–Verley (MPV) reaction of heterocyclic carboxaldehydes. Analysis of the solids revealed that zirconium is in fact incorporated into the LDH structure; as a result, their calcination produces Mg/Al/Zr oxides that are effective catalysts for the MPV reduction of heterocyclic compounds. In fact, the catalysts thus obtained exhibited higher activity than a catalyst obtained from an LDH containing Mg and Al only. The reaction was conducted under very mild conditions (viz. atmospheric pressure and a temperature of 82 8C) and provided conversions close to 90 and 100% selectivity in all instances. # 2006 Elsevier B.V. All rights reserved. Keywords: Hydrotalcite; LDH; Meerwein–Ponndorf–Verley reaction; Hydrogen transfer; Heterocyclic carboxaldehydes

1. Introduction The synthesis of alcohols by reduction of a carbonyl bond is one of the processes most widely used in Organic Chemistry. The reduction is facilitated by the presence of a metal catalyst and gaseous hydrogen [1–6]. However, many sulphur-containing carbonyl compounds cannot be reduced in this way as the sulphur poisons the catalyst and stops the reaction. In this work, we developed a new method for preparing heterocyclic alcohols based on the Meerwein–Ponndorf–Verley reaction in the liquid phase [7–11]. This reaction provides an interesting pathway for the reduction of aldehydes and ketones to alcohols. Essentially, it involves the aluminium isopropoxide-catalysed hydrogen transfer from a secondary alcohol (usually 2-propanol) to the carbonyl compound and yields the corresponding alcohol and acetone (see Scheme 1). This way of reducing C O bonds is highly attractive on account of its high selectivity towards carbonyl groups. In fact, other reducible functions such as C C and C–halogen bonds are left untouched. However, obtaining acceptable yields in the MPV reaction entails using a large excess

* Corresponding author. Tel.: +34 957 218638; fax: +34 957 212066. ** Corresponding author. E-mail address: [email protected] (J.R. Ruiz). 0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2006.01.035

of isopropoxide. Also, destroying the residual alkoxide remaining after the reaction can be cumbersome and regenerating the catalyst is usually labour-intensive and time-consuming if not impossible. A number of acid and basic heterogenous catalysts have been developed in recent years that allow the MPV reaction to be conducted in the liquid phase and avoid the need to isolate the catalyst from the reaction mass. Specially efficient among such catalysts are layered double hydroxides (LDHs). Our research group and the group of Professor Figueras, of the Lyon CNRS (France), have pioneered the use of these solids as catalysts for MPV reactions involving substrates of diverse nature such as naturally occurring aldehydes [12,13], cyclohexanone and its derivatives [14,15] and aromatic aldehydes [16]. This hydrogen transfer process has also been conducted in the presence of other types of catalysts including zeolites [17– 19] and metal oxides [16,20–27]. Layered double hydroxides are anionic clays where the same divalent cations in the brucite layers are replaced by trivalent cations. As a result, the layers acquire a charge deficiency that is offset by anions present in the spacing between pairs or brucite layers (see Fig. 1). A wide variety of LDHs containing diverse divalent and trivalent ions in combination with various anions have been reported [28–30]. Their calcination at 400–500 8C gives a mixture of oxides that can be used as catalysts or

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C. Jime´nez-Sanchidria´n et al. / Applied Catalysis A: General 303 (2006) 23–28

Scheme 1. General mechanism of the Meerwein–Ponndorf–Verley reaction.

catalyst supports by virtue of their high specific surface area, purity and structural stability, in addition to their favourable basic properties. This paper reports the results obtained in the liquid-phase MPV reduction of five-membered heterocyclic carboxaldehydes using 2-propanol as hydrogen donor and a catalyst obtained from an LDH containing magnesium, aluminium and zirconium. The results are compared with those provided by a catalyst containing magnesium and aluminium only. The proposed zirconium catalyst has for the first time been used in the MPV process, where other types of Zr-based catalysts had previously proved highly efficient [31–33]. The catalyst composition was optimized by examining the influence of its Zr content on the yield of the MPV reduction of benzaldehyde. 2. Experimental 2.1. General Mg(NO3)2
6H2O, Al(NO3)3
9H2O, NaOH and Na2CO3 were purchased from Panreac. Zr(IV) oxychloride, carbonyl compounds and 2-propanol were purchased from Aldrich and used without purification. All of the reaction products were identified by mass spectrometry.

at pH 10 at 60 8C under vigorous stirring. The pH was kept constant by adding appropriate volumes of 1 M NaOH during precipitation. The suspension thus obtained was kept at 80 8C for 24 h, after which the solid was filtered and washed with 2 L of deionized water. The magnesium, aluminium and zirconium LDHs with Mg(II)/[Al(III) + Zr(IV)] = 2 was obtained by following the same procedure, using appropriate amounts of the Mg(II) and Al(III) nitrates and Zr(IV) oxychloride. The LDHs thus prepared were ion-exchanged with carbonate to remove ions intercalated between layers. The procedure involved suspending the solids in a solution containing 0.345 g of Na2CO3 in 50 mL of bidistilled, deionized water per gram of LDH at 100 8C for 2 h. Then, each solid was filtered off in vacuo and washed with 200 mL of bidistilled, de-ionized water. The LDHs thus obtained were subjected to a second ion-exchange operation under the same conditions. The exchanged Mg/Al solid was named LDH-Al and its Mg/Al/Zr counterparts LDH-Zr1 and LDH-Zr2 (the former being that with the lower Zr content) (see Table 1). These solids were calcined at 500 8C in the air for 8 h, using a temperature gradient of 1 8C/min. The resulting products, named LDH-Al-500, LDH-Zr1-500 and LDH-Zr2-500, were used as catalysts in the MPV reactions. 2.3. Experimental techniques

2.2. Preparation of LDHs The LDH containing only magnesium and aluminium was prepared from solutions of Mg(NO3)2
6H2O and Al(NO3)3
9H2O in Mg(II)/Al(III) ratio = 2, using a coprecipitation method described elsewhere [30]. In a typical synthetic run, a solution containing 0.3 mol of Mg(NO3)2
6H2O and 0.15 mol of Al(NO3)3
9H2O in 250 mL of de-ionized water was used. This solution was slowly dropped over 500 mL of an Na2CO3 solution

The LDHs used and the catalysts obtained from them were characterized by using various instrumental techniques. Thus, the metal contents of the LDHs were determined by inductively coupled plasma spectroscopy on a Perkin-Elmer 1000 ICP spectrophotometer under standard conditions. X-ray diffraction patterns were recorded on a Siemens D-5000 diffractometer using Cu Ka radiation. Scans were performed over the 2u range from 2 to 808.

Fig. 1. Structure of an Mg/Al layered double hydroxide.

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Table 1 Chemical formulae and lattice parameters of the LDHs and catalysts Catalyst

Chemical formulae

Mg/Al/Zra

˚) a (A

˚) c (A

LDH-Al LDH-Zr1 LDH-Zr2 LDH-Al-500 LDH-Zr1-500 LDH-Zr2-500

Mg0.681Al0.319(OH)2(CO3)0.165
0.65H2O Mg0.684Al0.280Zr0.036(OH)2(CO3)0.160
0.80H2O Mg0.710Al0.217Zr0.073(OH)2(CO3)0.145
0.79H2O Mg(Al)Ox Mg(AlZr)Oy Mg(AlZr)Oz

2/0.84/0 2/0.81/0.11 2/0.61/0.21 – – –

3.033 3.532 3.562 4.155 4.178 4.214

21.97 22.97 23.92 – – –

a

Actual metal ratio.

2.4. Reaction conditions Catalytic hydrogen transfer runs were performed at 82 8C in a two-mouthed flask containing 0.003 mol of carbonyl compound, 0.06 mol of 2-propanol and 1 g of freshly calcined catalyst. One of the flask mouths was fitted with a reflux condenser and the other was used for sampling at regular intervals. The system was shaken throughout the process. Products were identified from their retention times as measured by GC–MS analysis on an HP 5890 GC instrument furnished with a Supelcowax 30 m  0.32 mm column and an HP 5971 MSD instrument. The experiments were repeated for two times and the results obtained in both were similar. 3. Results and discussion 3.1. Characterization of catalysts Table 1 shows the formulae and crystal phases of the three LDHs studied as determined using the XRD technique. The corresponding XRD patterns (Fig. 2) exhibit the typical diffraction peaks for LDH phases (JCPDS file No. 22-700). The incorporation of Zr(IV) into the solid layers is signalled by a change in the lattice parameters a and c (see Fig. 1), which are listed in Table 1. As can be seen, both are strongly influenced by the replacement of a trivalent metal such as Al(III) (ionic radius ˚ ) with a tetravalent one such as Zr(IV) (ionic radius 0.79 A ˚ ). 0.54 A It has been reported by several authors [28,34,35] that in the case of Mg/Al LDHs, thermal decomposition at 400–500 8C yields an ill-crystallized MgO phase whose lattice constant a is less than that of pure MgO and has been attributed to the dissolution of a small amount of Al3+ in the MgO lattice to form solid solutions. This lattice parameter a of the resulting MgO-like cubic phase (MgO solid solution) was evaluated from both d(2 0 0) and d(2 2 0) peaks, using the relation a = d  (h2 + k2 + l2)1/2 and the values for the catalysts are shown in Table 1. The calculated a parameter for LDH-Al-500 is ˚ , which is less than that of pure MgO (4.211 A ˚ ), indicating 4.155 A that a small amount of Al3+ is dissolved in the MgO lattice. As the Zr content in the LDH increases, the a parameter of the resulting phase increases and attains a value similar that of MgO. Since the ˚ ) is similar to that of Mg2+ (0.72 A ˚) ionic radius of Zr4+ (0.79 A [36], the a parameter of the resulting phase reaches the a value of MgO even at very low Zr4+ contents in the sample. From these results, it can be concluded that a part of Zr4+, in addition to Al3+, dissolves in the MgO lattice to form Mg–Al–Zr–O solid solution.

Calcining Mg/Al LDHs at 500 8C is known to produce a solid solution of an Mg(Al)Ox oxide with a disordered structure the XRD patterns for which exhibit the typical reflections of periclase MgO (see Fig. 3a) [30]. Zirconium-containing LDHs become Mg(AlZr)Ox oxides the XRD patterns for which (Fig. 3b and c) also exhibit a periclase-type structure. 3.2. Catalytic activity As noted in the previous section, we examined the activity of our catalysts in the MPV reduction of benzaldehyde prior to addressing that of heterocyclic carboxyaldehydes. The three catalysts studied provided benzyl alcohol with 100% selectivity. Also, the natural logarithm of the aldehyde concentration was found to be proportional to the reaction time,   c0 ln ¼ kt c where c0 is the initial aldehyde concentration, c that at a given time t and k is the rate constant. This relationship indicates that

Fig. 2. X-ray diffraction patterns for the studied LDHs.

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Table 3 Rate constants and conversions obtained in the MPV reduction of heterocyclic carboxyaldehydes with 2-propanol using the studied catalysts k (103 min1)a

Conversion (%)b

LDHA1-500

LDHZr1-500

LDHA1-500

1

1.20

2.00

69

90

2

2.00

2.68

97

87

3

3.22

3.75

99

95

4

3.57

4.56

91

98

5

0.40

0.73

89

100

Entry

a b

Fig. 3. X-ray diffraction pattern for the calcined LDHs.

the reaction is first order in the aldehyde concentration and allowed us to calculate the rate constant from the slope of a plot of ln(c0/c) as a function of time. As can be seen from Table 2, the Mg/Al LDH was that with the best rate constant. However, the conversion to benzyl alcohol at 2 h with this catalyst was similar to that obtained with the catalyst containing the lower proportion of zirconium (viz. LDH-Zr1-500). Also, the rate constant and the conversion decreased dramatically with increasing Zr content in the catalyst. We thus chose to use the solid containing the smaller amount of zirconium for subsequent tests. The reduction of heterocyclic carboxyaldehydes is of a high synthetic interest. Thus, the reduction of thiophene gives thiophenomethanols, which are major intermediates in the synthesis of some herbicides [37] and pharmacologically useful compounds [38,39]. The selectivity was 100% in all reduction runs. Also, as with benzaldehyde, the natural logarithm of the aldehyde concentration was proportional to the reaction time, so the reactions were first order in the aldehyde concentration. Table 3 shows the substrates used and their reduction products, Table 2 Rate constants and conversions obtained in the MPV reduction of benzaldehyde with 2-propanol using the studied catalysts Catalyst

k (103 min1)a

Conversion (%)b

LDH-Al-500 LDH-Zr1-500 LDH-Zr2-500

22.47 6.60 3.53

98 96 48

a b

Rate constant. Conversion to benzyl alcohol at 2 h of reaction.

Substrate

Product

LDHZr1-500

Rate constant. Conversion to appropriate alcohol at 24 h of reaction.

as well as the rate constants and conversions obtained in the MPV reduction with 2-propanol. As can be seen, the catalyst containing zirconium was more active than that containing magnesium and aluminium only. However, the conversion levels at 24 h were quite similar and exceeded 90% in most cases. Also, catalytic activity was higher for the sulphur-containing substrate (entry 4 in Table 3) than for the furan derivative (entry 3), which in turn outperformed the pyrrole derivative (entry 5), where the carbonyl group was at position 2 of the ring. Also, the activity was higher for the furan ring (entry 2) than for the tetrahydrofuran ring (entry 1). Finally, the conversion and rate constant were slightly higher for the furan derivatives with the carbonyl group at position 2 (entry 3) than for those having the group at position 3 (entry 2). 3.3. Mechanism of the process In previous work [13,14], our group developed a mechanism for LDH-catalysed MPV reductions. Scheme 2 depicts such a mechanism for the reduction of a generic aldehyde on an MgAlOx catalyst. As can be seen, the hydrogen transfer from the donor to the acceptor takes place via a six-membered cyclic intermediate with the alcohol and carbonyl compound adsorbed at a surface acid–base pair. As found in previous work, the ratedetermining step of the process is related to the interaction of the alcohol with the acid–base site, which causes its dissociation to the corresponding alkoxide. Carbonyl compounds are known to interact with acid and basic sites in solids to give condensation reactions in much the same way as alcohols are dehydrated to olefins. In the proposed mechanism, the alkoxide formed at a surface acid–base pair from the alcohol (step 1 in Scheme 2) transfers a hydride ion (step 3) that attacks the previously adsorbed carbonyl compound (step 2) at an acid Al3+ ion next to the acid–base pair where the alcohol is

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Scheme 2. Proposed mechanism for the MPV reduction with calcined Mg/Al LDH catalysts.

adsorbed. The concerted hydride transfer (step 3) leads to the formation of a new alcohol and a new carbonyl compound (acetone). A similar mechanism was been described by others authors in MPV reaction in the gas phase of carbonyl compounds with MgO [25,40–42] and Mg3(PO4)2 [43] as catalysts. This mechanism is consistent with the results obtained in the MPV reduction of heterocyclic carboxyaldehydes. Thus, the fact that the reaction rate decreases in the heteroatom sequence S > O > N can be ascribed to the heteroatom participating in the adsorbed complex. As can be seen in Scheme 3, the electron pairs in the sulphur and oxygen atoms take part differently from that in the nitrogen atom in the adsorbed complex; thus, the adsorbed complexes of the sulphur and oxygen are more stable and the reaction favoured as a result.

Fig. 4. Hydrogen bond formed in 2-carboxaldehydepyrrole.

The acidity of 2-carboxaldehydepyrrole allows it to form a hydrogen bond (Fig. 4) that interferes with the adsorption of the carbonyl group on the surface. In practice, this results in a low conversion and reaction rate relative to its furan and thiophene counterparts, which can form no such hydrogen bond. Finally, the formation of the complex shown in Scheme 3 may also account for the fact that the reduction of the carbonyl group at position 2 is faster than that of the group at 3 as, in the latter, the formation of the adsorbed complex only provides an anchoring point. 4. Conclusion

Scheme 3. Adsorbed complex of heterocyclic carboxaldehydes.

The method used in this work to prepare zirconiumcontaining layered double hydroxides allows the metal to be incorporated into the structure, as confirmed by the XRD patterns for the solids. Thermal treatment of such LDHs at 500 8C for several hours destroys their layered structure and produces mixed oxides of the constituent metals.

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The results obtained in this work show that conversion in the Meerwein–Ponndorf–Verley reduction of five-membered heterocyclic carboxyaldehydes with 2-propanol in the presence of thermally treated Mg/Al and Mg/Al/Zr catalysts is higher when the solid contains a small amount of zirconium. In any case, conversions are close to or even higher than 90% at 24 h with any of these catalysts. Regarding the influence of the heteroatom in the carboxyaldehyde, the reaction rate decreases in the following sequence: thiophene > furan > pyrrole. Also, the reaction is faster with the carbonyl substituent at position 2 than at 3. These results are all consistent with the supposed mechanism involving the unpaired electrons in the heteroatom. Acknowledgments The authors gratefully acknowledge funding by Spain’s Ministerio de Educacio´n y Ciencia, Feder Funds and to the Consejerı´a de Innovacio´n, Ciencia y Empresa de la Junta de Andalucı´a. References [1] M. Bartok, Stereochemistry of Heterogeneous Metal Catalysis, John Wiley & Sons, Chichester, 1985. [2] G.V. Smith, F. Notheisz, Heterogeneous Catalysis in Organic Chemistry, Academic Press, New York, 1999, pp. 29. [3] S. Nishimura, Handbook of Heterogeneous Catalytic Hydrogenation for Organic Synthesis, John Wiley & Sons, New York, 2001, pp. 170. [4] R.A. Sheldon, H. Van Bekkum, Fine Chemicals Through Heterogeneous Catalysis, Wiley-VCH, Weinheim, 2001, pp. 363. [5] M. Studer, H.U. Blaser, C. Exner, Adv. Synth. Catal. 345 (2003) 45. [6] M.V. Rajashekharan, I. Bergault, P. Fouilloux, D. Schewich, H. Delmas, R.V. Chaudhari, Catal. Today 48 (1999) 83. [7] H. Meerwein, R. Schmidt, Justus Liebigs Ann. Chem. 444 (1925) 221. [8] A. Verley, Bull. Soc. Chim. Fr. 37 (1926) 537. [9] W. Ponndorf, Angew. Chem. 39 (1926) 138. [10] K. Nishide, M. Node, Chirality 14 (2002) 759. [11] O. Pamies, J.E. Backvall, Chem. Eur. J. 7 (2001) 5052. [12] P.S. Kumbhar, J. Sa´nchez-Valente, J. Lo´pez, F. Figueras, J. Chem. Soc., Chem. Commun. (1998) 535.

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