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Meerwein-Ponndorf-Verley and Oppenauer reactions catalysed by heterogeneous catalysts
Heterogeneous Catalysis and Fine Chemicals IV H.U. Blaser, A. Baiker and R. Prins (editors) © 1997 Elsevier Science B.V. All richts reserved.
Meerwein-Ponndorf-Verley heterogeneous catalysts
and
531
Oppenauer
reactions
catalysed
by
E.J. Creyghton, J. Huskens, J.C. van der Waal and H. van Bekkum Laboratory of Organic Chemistry and Catalysis, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands
Summary Meerwein-Ponndorf-Verley and Oppenauer reactions (MPVO) are catalysed by metal oxides which possess surface basicity or Lewis acidity. Recent developments include the application of basic alkali or alkaline earth exchanged X-type zeolites and the Lewis-acid zeolites BE A and [Ti]-BEA. The BEA catalysts show high stereoselectivity, as a result of restricted transition state selectivity, in the MPV reduction of substituted alkylcyclohexanones with z-PrOH. 1. Introduction The Meerwein-Ponndorf-Verley reduction of aldehydes and ketones and the Oppenauer oxidation of alcohols are reactions that can be performed under mild conditions. Furthermore, the MPVO reactions can be applied without the risk of reducing or oxidising other functional groups. The hydrogen donors are easily oxidisable secondary alcohols (e.g. isopropanol or 2butanol) while the oxidants are simple ketones (e.g. acetone or cyclohexanone). The MPVO reactions are usually catalysed by metal alkoxides such as Al(0z-Pr)3. The activity of these catalysts is related to their Lewis acidic character in combination with ligand exchangeability. The reaction mechanism of the homogeneous MPVO reactions proceeds via a cyclic sixmembered transition state in which both the reductant and the oxidant are coordinated to the metal centre of a metal alkoxide catalyst (Scheme 1). The alcohol reactant is coordinated as alkoxide. Activation of the carbonyl by coordination to Al(III) initiates the hydride transfer reaction from the alcoholate to the carbonyl. The formed alkoxide may leave the catalyst via an alcoholysis reaction with the bulk alcohol [1]. Industrial applications of the MPVO reactions are found in the fragrance and pharmaceutical industries. ;AI
:AI^
o ! ro
o'
^AI.
o
o
o
A H Rf R2
R3
Ri
R3
Ri
R4 R3
Scheme 1. Reaction mechanism for the MPVO reaction.
532 A major advantage of heterogeneous over homogeneously catalysed MPVO reactions is that the catalysts can easily be separated from the liquid reaction mixture. So far, several examples of heterogeneously catalysed MPVO reactions have been reported. The catalysts comprise (modified) metal oxides which exhibit either Lewis acid or basic properties. The reaction mechanisms involved have in common that the first step consists in the formation of an alkoxide-like species, while the reactions proceed via cyclic six-membered transition states, comparable to those in homogeneous systems. This paper presents a comprehensive overview of heterogeneously catalysed MPVO reactions. It includes the recent application of zeolites as new recycleable solid catalysts for the MPVO reaction. The activity of these catalysts is related to their Lewis acid and/or basic properties. Some remarkable examples of shape-selective conversions resulting in high stereoselectivities have recently been found by our group.
2. MPVO reactions catalysed by metal oxides Horner and Kaps have used chlorinated Y-AI2O3 in combination with a small amount of Al(0/-Pr)3 as the catalyst in the MPV reduction of benzaldehyde, cyclohexanone, and acetophenone by z-PrOH [2]. In the absence of Al(0z-Pr)3, no reaction occurred. A large reaction rate enhancement was found by the addition of a strong base, e.g. diisopropylamine. Analogous phenomena have been observed in the Oppenauer oxidation of several secondary alcohols. Strong bases presumably assist the deprotonation of alumina-surface coordinated /PrOH, thereby forming the required isopropoxide surface species. The modified alumina, which contained about 85 mmol chloride / 100 g alumina, was obtained by heating dry alumina in thionyl chloride during 24 h. The chloride at the surface increases the Lewis acidity of the aluminium ions and the addition of the base facilitates the deprotonation of /-PrOH. Posner et al have applied Y-AI2O3 in the MPV reduction of unsaturated carbonyl compounds by /-PrOH [3]. Dehydrated alumina was able to deprotonate /-PrOH, by which means an aluminium isopropoxide catalyst was formed in situ. However, high temperatures were needed (up to 300°C) and only low yields of alcohol were obtained due to the occurrence of several side-reactions. Wismeijer et al studied the liquid phase transfer hydrogenation of 4-tertbutylcyclohexanone by 2-propanol at 83°C over activated y-Al203 as the catalyst [4]. The activity of the catalyst was found to increase with increasing activation temperature. Selective poisoning experiments indicated that coordinatively unsaturated Al surface ions (Lewis acid sites), formed upon dehydroxylation, were essential for catalytic activity. During reaction the catalyst was found to become conditioned by irreversible alcoholysis of the initial active sites, producing less-active sites. The reaction mechanism, however, remained essentially the same as indicated by the constant ratio of c/5//rfl«^-4-^er/-butylcyclohexanol (9/91). Gargano et al. investigated the pretreatment of the alumina catalyst with hydrogen at 270°C [5]. This allowed much lower reaction temperatures, in this way increasing the selectivity towards the alcohols aimed at. Several other metal oxides have been tested and La203 proved to be the best catalyst, both with regard to conversion and to selectivity. Kuno et al. have used Zr02 as solid catalyst in the oxidation of both secondary and primary alcohols [6]. The catalyst was prepared via precipitation of an aqueous solution of
533 ZrCl20.8H20 with sodium hydroxide at room temperature. The resuhing hydroxide was calcined at 300°C. Batch-oxidations were performed at 80°C in benzene or toluene with 60 equivalents of acetone as the oxidant. These authors have also tested alumina, aluminium silicate, and zeolite NaA. These catalysts were shown to give lower yields in the oxidation of 2-octanol, whereas Si02, aqueous Ti02, and aqueous Sn203 did not show any activity at all. A continuous liquid phase system with a fixed catalyst bed in a tube reactor was applied, and the reaction temperature was optimised for several secondary alcohol substrates. Primary alcohols were oxidised by/?-benzoquinone or benzophenone with Zr02 as the catalyst at 140°C in xylene as the solvent. Kaspar et al demonstrated the reduction of a,P-unsaturated ketones to ally lie alcohols with /-PrOH in the gas phase over MgO as fixed bed catalyst at 250°C [7]. The MgO was formed in situ by heating Mg(0H)2 at 350°C in an air current during 4 hours. Regeneration of the catalyst was done in the same way. In a subsequent paper the chemoselective reduction of the carbonyl group of 4-hexen-3-one over various solid catalysts was reported [8]. MgO was found to show the highest chemoselectivity. However, as a result of its high basicity several side reactions were also observed. Doping of the MgO catalysts with HCl afforded solid catalysts with improved selectivity. Kijenski et al. studied hydrogen transfer reactions of various reactants having different functional groups [9]. The reactions were performed in the gas phase over MgO in a fixed bed, activated at either 550 or 750°C, at reaction temperatures in the range of 350-450°C. Donor molecules included alcohols and also hydrocarbons. All the (de)hydrogenation reactions studied were important for the preparation of perfumery flavours and fragrances. They included the reduction of carbonyl groups and the dehydrogenation of long chain aliphatic alcohols. Other related reactions that were screened included the hydrogenation of epoxy compounds, the hydrogenation of styrene and the reduction of aromatic nitro-compounds. Moreover, the successful hydrogenation of nitrobenzene with ethanol to aniline initiated further research in this field, leading to a separate communication [10]. Kijenski et al extended their research in the field of hydrogen transfer reactions to a series of main group and transition metal oxides, supported on silica [11]. The reductions of ethyl methyl ketone, methyl isopropyl ketone and 4-methylacetophenone with 2-propanol were chosen as model reactions. Most of the tested metal oxides exhibited catalytic activity. Catalytic titration, using poisons suppressing various types of surface sites, was applied for the identification of the active centres of the catalysts. The results indicated the participation of basic and/or one electron donor sites of the MgO surface. Acidic centres were mainly responsible for undesired consecutive dehydration. Ravasio et al. investigated the hydrogen transfer from different secondary alcohols to a steroidic conjugated enone and a saturated ketone over a CU/AI2O3 catalyst at 90°C [12]. The stereoselectivity of the transfer reaction was found to depend upon the secondary alcohol applied as hydrogen donor. Selectivities to the 5p isomer ranging from 48 and 85% were observed. In the reduction of the 3-keto group also a strong effect of the donor alcohol on the stereoselectivity was found. However, an excess of the equatorial alcohol was obtained in all cases. Ivanov et al. studied the MPV reaction between ethanol and acetone over various metal oxides having different acid-base properties [13]. Reaction was found to occur over both Lewis acid (AI2O3-CI) and base (MgO, Zr02) catalysts. Based on FT-IR and specific poisoning experiments, two mechanistic variants were proposed differing in the mode of formation of
534 the surface alkoxide species, while both mechanisms proceeded via cyclic six-membered transition states, comparable to those in homogeneous systems. The coordination of the carbonyl, however, was assumed to occur by hydrogen bonding to an acid hydroxy group instead of to the metal. For amphoteric catalysts (AI2O3) both mechanisms were possible.
3. MPVO reactions catalysed by zeolites Shabtai et al studied the potential of alkali and alkaline earth exchanged X-type zeolites in the gas-phase (100-180°C) MPV reduction of various saturated and unsaturated aldehydes and ketones, using isopropanol as reducing agent [14]. In the reduction of linear aldehydes over NaX a gradual decrease in the reduction rate was observed with increasing chain length, which was attributed to increasing diffusional limitations in the micropores. Selectivities to the corresponding 1-alcohols were generally high (> 95%). Application of Lewis-acidic CaX gave acetalisation of the aldehydes as an important side-reaction. This could be prevented, however, by applying higher reaction temperatures. Unfortunately, the X-type zeolite/isopropanol system was not capable of reducing a,P-unsaturated aldehydes. Shapeselectivity was found in the selective conversion of citronellal under MPV conditions. In NaX there was enough space for the substrate to undergo an intramolecular ring closure to isopulegol wheras over CsX reduction to the linear citronellol was observed (Scheme 2). In the reduction of methylcyclohexanone isomers at 100°C it was observed that the 4-isomer reacted relatively fast and gave a thermodynamically determined product distribution (cis:trans = 24:76). The 2- and 3-methylcyclohexanone reacted more slowly and gave a kinetically determined product distribution (cis:trans = 62.5:37.5 and 23.5:76.5 for the 2- and 3-isomer, respectively). The mechanism was proposed to involve the formation of a surface isopropoxide group attached to a cationic site (basic mechanism). It could not be excluded, however, that incompletely coordinated Si- or Al-sites contributed to the catalytic activity (Lewis-acid mechanism).
NaX sel.86% conv. 87 %
isopulegol
CsX |1 JL
O
citronellal
sel.92% conv. 77 %
citronellol
Scheme 2. Shape-selective conversions of citronellal to isopulegol or citronellol under MPV conditions, after reference 14.
535 The reaction of cyclopentanol in the presence of cyclohexanone at 350°C over amorphous metal oxides and zeolites was studied by Berkani et al (scheme 3) [15]. MgO was found to be the most active catalyst for the hydrogen transfer reaction, followed by potassium impregnated gamma alumina (Y-AI2O3-K), Y-AI2O3 and CsNaX zeolites. For the zeolites, the MPVO activity decreased with decreasing cesium content. The reverse trend was observed for the acid catalysed dehydration activity. Addition of CO2 poisoned only the hydrogen transfer reaction while the amount of cyclopentene remained constant. It was therefore concluded that hydrogen transfer occurred only on the basic sites and dehydration only on the acid sites of the catalysts.
OH MPVO
+ H2O
H2O +
Scheme 3. Reaction of the cyclopentanol/cyclohexanone mixture at 350°C over various metal oxide catalysts, from reference 15. Recently, Creyghton et al reported the application of zeolite beta (BEA) in the stereoselective (> 95%) reduction of 4-/gr/-butylcyclohexanone to cz\s'-4-rer^butyl-cyclohexanol in the liquid phase [16,17]. This zeolite-based catalyst proved to be fully regenerable without loss in activity or stereoselectivity. This is of industrial relevance, as the c/^-isomer is a fragrance-chemical intermediate. Other active solid catalysts, including zeolites, invariably gave the thermodynamically more stable trans-isomQx. The activity of the BEA catalyst was found to increase upon increasing activation temperature. Furthermore, deep-bed calcination conditions gave a higher catalytic activity than a shallow-bed procedure, indicating a relation between the catalytic activity and the extent of framework dealumination since the former method results in a greater degree of auto-steaming. However, -^^Al-NMR spectra did not show any increase in octahedral aluminium. FT-IR results indicated a relation between the catalytic activity and the amount of aluminium which is only partially bonded to the framework (Lewisacid sites). The MPV mechanism was therefore proposed to involve a six-membered transition state which is formed upon chemisorption of a secondary alcohol on a Lewis-acid aluminium site and coordination of the ketone to the same site. A base mechanism was ruled out because of the low aluminium content (Si/Al=12), the absence of alkali or alkaline earth cations in the active H-BEA catalysts and the very similar activity of the Li-, Na-, K-, Rb- and Csexchanged catalysts. Furthermore, the catalyst could be poisoned by the base piperidine.
536 The transition states which lead to the cis- or trans-3[coho\ differ substantially in spatial requirements (Figure 1). That for the c/^'-isomer is more or less linear in form and aligned with the BE A channel while the formation of the trans-2i\co\v6[ requires an axially oriented (bulkier) transition state. Although the latter might still fit in the intersections of BEA it is questionable whether there is an active site available at the required position. More coordination possibilities are available for the c/5-transition state, which can easily be accommodated within the straight channels of BEA. The observed kinetically determined product distribution is thus satisfactorily explained by true transition state selectivity.
,H.
-Al-
r
.CH3 .CH3
,0
7-7~rr77^^i^
r
.CH3 CHs
-Al
.0
zeolite
Figure 1. Transition states for the formation of c/5-4-r^r/-butylcyclohexanol (top) and trans-A^err-butylcyclohexanol (bottom). In addition to the stereoselective MPV reaction presented above, van der Waal et al. reported the catalytic activity of aluminium-free titanium beta ([Ti]-BEA) zeolite in the same MPV reaction [18]. Again, a very high selectivity of 98% to the cz^y-isomer was found which was also explained by a restricted transition state, here around a Lewis acid titanium site. The Lewis acid properties of tetrahedrally incorporated titanium in zeolite [Ti]-BEA had already become clear during catalytic studies on the epoxidation of olefins with hydrogen peroxide in alcoholic solvents. The oxophilic Lewis acidity of the titanium site was confirmed by UV-VIS which showed an increased coordination number for the originally 4-coordinated titanium atom upon adsorption of alcohols and water. Kinetically determined product distributions were also obtained in the MPV reduction of 2-, 3- and 4-methylcyclohexanone; the cis-, trans- and cisalcohol being the major products, respectively. The catalytic activity of [Ti]-BEA was found to be much lower than that of its aluminium analogue whereas its tolerance for water was observed to be much higher. The latter property, which is related to the hydrophobic character of the aluminium free zeolite, illustrates its catalytic potential in this type of reactions.
537 4. Conclusions Heterogeneous catalysts which are active for the catalysis of the MPVO reactions include amorphous metal oxides and zeolites. Their activity is related to their surface basicity or Lewis acidity. Zeolites are only recently being developed as catalysts in the MPVO reactions. Their potential is related to the possibility of shape-selectivity as illustrated by an example showing absolute stereoselectivity as a result of restricted transition-state selectivity. In case of alkali or alkaline earth exchanged zeolites with a high aluminium content (X-type) the catalytic activity is most likely related to basic properties. For zeolite BE A (Si/Al=12), however, the dynamic character of those aluminium atoms which are only partially connected to the framework appear to play a role in the catalytic activity. Similarly, the Lewis acid character of the titanium atoms in aluminium free [Ti]-BEA explains its activity in the MPVO reactions.
Acknowledgement This work was financially supported by the Foundation for Chemical Research in the Netherlands (SON).
References [I] [2] [3] [4] [5] [6] [7] [8] [9] [10] [II] [12] [13] [14] [15] [16] [17] [18]
C.F. de Graauw, J.A. Peters, H. van Bekkum and J. Huskens, Synthesis, 10 (1994) 1007. L. Horner and U.B. Kaps, Ann. Chem., (1980) 192. G.H. Posner, A.W. Runquist and M.J. Chapdelaine, J. Org. Chem., 42 (1977) 1202. A.A. Wismeijer, A.P.G. Kieboom and H. van Bekkum, Appl. Catal., 34 (1987) 189. M. Gargano, V. D'Oranzio, N. Ravasio and M.J. Rossi, J. Mol. Catal., 58 (1990) L5. H. Kuno, M. Shibagaki, K. Takahashi and H. Matsushita, Bull. Chem. Soc. Jpn., 64 (1991) 312. J. Kaspar, A. Trovarelli, M. Lenarda and M. Graziani, Tetrahedron Lett., 30 (1989) 2705. J. Kaspar, A. Trovarelli, F. Zamoner, E. Farnetti and M. Graziani, Stud. Surf Sci. Catal., 59 (1991) 253. J. Kijenski, M. Glinski and J. Reinhercs, Stud. Surf Sci. Catal., 41 (1988) 231. J. Kijenski, M. Glinski, R. Wisniewski and S. Murghani, Stud. Surf Sci. Catal., 59 (1991) 169. J. Kijenski, M. Glinski, J. Czarnecki, R. Derlacka and V. Jarzyna, Stud. Surf. Sci. Catal., 78 (1993)631. N. Ravasio, M. Gargano, V.P. Quatraro and M. Rossi, Stud. Surf Sci. Catal., 59 (1991) 161. V.A. Ivanov, J. Bachelier, F. Audry and J.C. Lavalley, J. Mol. Catal., 91 (1994) 45. J. Shabtai, R. Lazar and E. Biron, J. Catal., 27 (1984) 35. M. Berkani, J.L. Lemberton, M. Marczewski and G. Perot, Catal. Lett., 31 (1995) 405. E.J. Creyghton, S.D. Ganeshie, R.S. Downing and H. van Bekkum, J. Chem. Soc, Chem. Commun., (1995) 1859. E.J. Creyghton, S.D. Ganeshie, R.S. Downing and H. van Bekkum, J. Mol. Catal., in press. J.C. van der Waal, K. Tan and H. van Bekkum, Catal. Lett., 41 (1996) 63.
Meerwein-Ponndorf-Verley heterogeneous catalysts
and
531
Oppenauer
reactions
catalysed
by
E.J. Creyghton, J. Huskens, J.C. van der Waal and H. van Bekkum Laboratory of Organic Chemistry and Catalysis, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands
Summary Meerwein-Ponndorf-Verley and Oppenauer reactions (MPVO) are catalysed by metal oxides which possess surface basicity or Lewis acidity. Recent developments include the application of basic alkali or alkaline earth exchanged X-type zeolites and the Lewis-acid zeolites BE A and [Ti]-BEA. The BEA catalysts show high stereoselectivity, as a result of restricted transition state selectivity, in the MPV reduction of substituted alkylcyclohexanones with z-PrOH. 1. Introduction The Meerwein-Ponndorf-Verley reduction of aldehydes and ketones and the Oppenauer oxidation of alcohols are reactions that can be performed under mild conditions. Furthermore, the MPVO reactions can be applied without the risk of reducing or oxidising other functional groups. The hydrogen donors are easily oxidisable secondary alcohols (e.g. isopropanol or 2butanol) while the oxidants are simple ketones (e.g. acetone or cyclohexanone). The MPVO reactions are usually catalysed by metal alkoxides such as Al(0z-Pr)3. The activity of these catalysts is related to their Lewis acidic character in combination with ligand exchangeability. The reaction mechanism of the homogeneous MPVO reactions proceeds via a cyclic sixmembered transition state in which both the reductant and the oxidant are coordinated to the metal centre of a metal alkoxide catalyst (Scheme 1). The alcohol reactant is coordinated as alkoxide. Activation of the carbonyl by coordination to Al(III) initiates the hydride transfer reaction from the alcoholate to the carbonyl. The formed alkoxide may leave the catalyst via an alcoholysis reaction with the bulk alcohol [1]. Industrial applications of the MPVO reactions are found in the fragrance and pharmaceutical industries. ;AI
:AI^
o ! ro
o'
^AI.
o
o
o
A H Rf R2
R3
Ri
R3
Ri
R4 R3
Scheme 1. Reaction mechanism for the MPVO reaction.
532 A major advantage of heterogeneous over homogeneously catalysed MPVO reactions is that the catalysts can easily be separated from the liquid reaction mixture. So far, several examples of heterogeneously catalysed MPVO reactions have been reported. The catalysts comprise (modified) metal oxides which exhibit either Lewis acid or basic properties. The reaction mechanisms involved have in common that the first step consists in the formation of an alkoxide-like species, while the reactions proceed via cyclic six-membered transition states, comparable to those in homogeneous systems. This paper presents a comprehensive overview of heterogeneously catalysed MPVO reactions. It includes the recent application of zeolites as new recycleable solid catalysts for the MPVO reaction. The activity of these catalysts is related to their Lewis acid and/or basic properties. Some remarkable examples of shape-selective conversions resulting in high stereoselectivities have recently been found by our group.
2. MPVO reactions catalysed by metal oxides Horner and Kaps have used chlorinated Y-AI2O3 in combination with a small amount of Al(0/-Pr)3 as the catalyst in the MPV reduction of benzaldehyde, cyclohexanone, and acetophenone by z-PrOH [2]. In the absence of Al(0z-Pr)3, no reaction occurred. A large reaction rate enhancement was found by the addition of a strong base, e.g. diisopropylamine. Analogous phenomena have been observed in the Oppenauer oxidation of several secondary alcohols. Strong bases presumably assist the deprotonation of alumina-surface coordinated /PrOH, thereby forming the required isopropoxide surface species. The modified alumina, which contained about 85 mmol chloride / 100 g alumina, was obtained by heating dry alumina in thionyl chloride during 24 h. The chloride at the surface increases the Lewis acidity of the aluminium ions and the addition of the base facilitates the deprotonation of /-PrOH. Posner et al have applied Y-AI2O3 in the MPV reduction of unsaturated carbonyl compounds by /-PrOH [3]. Dehydrated alumina was able to deprotonate /-PrOH, by which means an aluminium isopropoxide catalyst was formed in situ. However, high temperatures were needed (up to 300°C) and only low yields of alcohol were obtained due to the occurrence of several side-reactions. Wismeijer et al studied the liquid phase transfer hydrogenation of 4-tertbutylcyclohexanone by 2-propanol at 83°C over activated y-Al203 as the catalyst [4]. The activity of the catalyst was found to increase with increasing activation temperature. Selective poisoning experiments indicated that coordinatively unsaturated Al surface ions (Lewis acid sites), formed upon dehydroxylation, were essential for catalytic activity. During reaction the catalyst was found to become conditioned by irreversible alcoholysis of the initial active sites, producing less-active sites. The reaction mechanism, however, remained essentially the same as indicated by the constant ratio of c/5//rfl«^-4-^er/-butylcyclohexanol (9/91). Gargano et al. investigated the pretreatment of the alumina catalyst with hydrogen at 270°C [5]. This allowed much lower reaction temperatures, in this way increasing the selectivity towards the alcohols aimed at. Several other metal oxides have been tested and La203 proved to be the best catalyst, both with regard to conversion and to selectivity. Kuno et al. have used Zr02 as solid catalyst in the oxidation of both secondary and primary alcohols [6]. The catalyst was prepared via precipitation of an aqueous solution of
533 ZrCl20.8H20 with sodium hydroxide at room temperature. The resuhing hydroxide was calcined at 300°C. Batch-oxidations were performed at 80°C in benzene or toluene with 60 equivalents of acetone as the oxidant. These authors have also tested alumina, aluminium silicate, and zeolite NaA. These catalysts were shown to give lower yields in the oxidation of 2-octanol, whereas Si02, aqueous Ti02, and aqueous Sn203 did not show any activity at all. A continuous liquid phase system with a fixed catalyst bed in a tube reactor was applied, and the reaction temperature was optimised for several secondary alcohol substrates. Primary alcohols were oxidised by/?-benzoquinone or benzophenone with Zr02 as the catalyst at 140°C in xylene as the solvent. Kaspar et al demonstrated the reduction of a,P-unsaturated ketones to ally lie alcohols with /-PrOH in the gas phase over MgO as fixed bed catalyst at 250°C [7]. The MgO was formed in situ by heating Mg(0H)2 at 350°C in an air current during 4 hours. Regeneration of the catalyst was done in the same way. In a subsequent paper the chemoselective reduction of the carbonyl group of 4-hexen-3-one over various solid catalysts was reported [8]. MgO was found to show the highest chemoselectivity. However, as a result of its high basicity several side reactions were also observed. Doping of the MgO catalysts with HCl afforded solid catalysts with improved selectivity. Kijenski et al. studied hydrogen transfer reactions of various reactants having different functional groups [9]. The reactions were performed in the gas phase over MgO in a fixed bed, activated at either 550 or 750°C, at reaction temperatures in the range of 350-450°C. Donor molecules included alcohols and also hydrocarbons. All the (de)hydrogenation reactions studied were important for the preparation of perfumery flavours and fragrances. They included the reduction of carbonyl groups and the dehydrogenation of long chain aliphatic alcohols. Other related reactions that were screened included the hydrogenation of epoxy compounds, the hydrogenation of styrene and the reduction of aromatic nitro-compounds. Moreover, the successful hydrogenation of nitrobenzene with ethanol to aniline initiated further research in this field, leading to a separate communication [10]. Kijenski et al extended their research in the field of hydrogen transfer reactions to a series of main group and transition metal oxides, supported on silica [11]. The reductions of ethyl methyl ketone, methyl isopropyl ketone and 4-methylacetophenone with 2-propanol were chosen as model reactions. Most of the tested metal oxides exhibited catalytic activity. Catalytic titration, using poisons suppressing various types of surface sites, was applied for the identification of the active centres of the catalysts. The results indicated the participation of basic and/or one electron donor sites of the MgO surface. Acidic centres were mainly responsible for undesired consecutive dehydration. Ravasio et al. investigated the hydrogen transfer from different secondary alcohols to a steroidic conjugated enone and a saturated ketone over a CU/AI2O3 catalyst at 90°C [12]. The stereoselectivity of the transfer reaction was found to depend upon the secondary alcohol applied as hydrogen donor. Selectivities to the 5p isomer ranging from 48 and 85% were observed. In the reduction of the 3-keto group also a strong effect of the donor alcohol on the stereoselectivity was found. However, an excess of the equatorial alcohol was obtained in all cases. Ivanov et al. studied the MPV reaction between ethanol and acetone over various metal oxides having different acid-base properties [13]. Reaction was found to occur over both Lewis acid (AI2O3-CI) and base (MgO, Zr02) catalysts. Based on FT-IR and specific poisoning experiments, two mechanistic variants were proposed differing in the mode of formation of
534 the surface alkoxide species, while both mechanisms proceeded via cyclic six-membered transition states, comparable to those in homogeneous systems. The coordination of the carbonyl, however, was assumed to occur by hydrogen bonding to an acid hydroxy group instead of to the metal. For amphoteric catalysts (AI2O3) both mechanisms were possible.
3. MPVO reactions catalysed by zeolites Shabtai et al studied the potential of alkali and alkaline earth exchanged X-type zeolites in the gas-phase (100-180°C) MPV reduction of various saturated and unsaturated aldehydes and ketones, using isopropanol as reducing agent [14]. In the reduction of linear aldehydes over NaX a gradual decrease in the reduction rate was observed with increasing chain length, which was attributed to increasing diffusional limitations in the micropores. Selectivities to the corresponding 1-alcohols were generally high (> 95%). Application of Lewis-acidic CaX gave acetalisation of the aldehydes as an important side-reaction. This could be prevented, however, by applying higher reaction temperatures. Unfortunately, the X-type zeolite/isopropanol system was not capable of reducing a,P-unsaturated aldehydes. Shapeselectivity was found in the selective conversion of citronellal under MPV conditions. In NaX there was enough space for the substrate to undergo an intramolecular ring closure to isopulegol wheras over CsX reduction to the linear citronellol was observed (Scheme 2). In the reduction of methylcyclohexanone isomers at 100°C it was observed that the 4-isomer reacted relatively fast and gave a thermodynamically determined product distribution (cis:trans = 24:76). The 2- and 3-methylcyclohexanone reacted more slowly and gave a kinetically determined product distribution (cis:trans = 62.5:37.5 and 23.5:76.5 for the 2- and 3-isomer, respectively). The mechanism was proposed to involve the formation of a surface isopropoxide group attached to a cationic site (basic mechanism). It could not be excluded, however, that incompletely coordinated Si- or Al-sites contributed to the catalytic activity (Lewis-acid mechanism).
NaX sel.86% conv. 87 %
isopulegol
CsX |1 JL
O
citronellal
sel.92% conv. 77 %
citronellol
Scheme 2. Shape-selective conversions of citronellal to isopulegol or citronellol under MPV conditions, after reference 14.
535 The reaction of cyclopentanol in the presence of cyclohexanone at 350°C over amorphous metal oxides and zeolites was studied by Berkani et al (scheme 3) [15]. MgO was found to be the most active catalyst for the hydrogen transfer reaction, followed by potassium impregnated gamma alumina (Y-AI2O3-K), Y-AI2O3 and CsNaX zeolites. For the zeolites, the MPVO activity decreased with decreasing cesium content. The reverse trend was observed for the acid catalysed dehydration activity. Addition of CO2 poisoned only the hydrogen transfer reaction while the amount of cyclopentene remained constant. It was therefore concluded that hydrogen transfer occurred only on the basic sites and dehydration only on the acid sites of the catalysts.
OH MPVO
+ H2O
H2O +
Scheme 3. Reaction of the cyclopentanol/cyclohexanone mixture at 350°C over various metal oxide catalysts, from reference 15. Recently, Creyghton et al reported the application of zeolite beta (BEA) in the stereoselective (> 95%) reduction of 4-/gr/-butylcyclohexanone to cz\s'-4-rer^butyl-cyclohexanol in the liquid phase [16,17]. This zeolite-based catalyst proved to be fully regenerable without loss in activity or stereoselectivity. This is of industrial relevance, as the c/^-isomer is a fragrance-chemical intermediate. Other active solid catalysts, including zeolites, invariably gave the thermodynamically more stable trans-isomQx. The activity of the BEA catalyst was found to increase upon increasing activation temperature. Furthermore, deep-bed calcination conditions gave a higher catalytic activity than a shallow-bed procedure, indicating a relation between the catalytic activity and the extent of framework dealumination since the former method results in a greater degree of auto-steaming. However, -^^Al-NMR spectra did not show any increase in octahedral aluminium. FT-IR results indicated a relation between the catalytic activity and the amount of aluminium which is only partially bonded to the framework (Lewisacid sites). The MPV mechanism was therefore proposed to involve a six-membered transition state which is formed upon chemisorption of a secondary alcohol on a Lewis-acid aluminium site and coordination of the ketone to the same site. A base mechanism was ruled out because of the low aluminium content (Si/Al=12), the absence of alkali or alkaline earth cations in the active H-BEA catalysts and the very similar activity of the Li-, Na-, K-, Rb- and Csexchanged catalysts. Furthermore, the catalyst could be poisoned by the base piperidine.
536 The transition states which lead to the cis- or trans-3[coho\ differ substantially in spatial requirements (Figure 1). That for the c/^'-isomer is more or less linear in form and aligned with the BE A channel while the formation of the trans-2i\co\v6[ requires an axially oriented (bulkier) transition state. Although the latter might still fit in the intersections of BEA it is questionable whether there is an active site available at the required position. More coordination possibilities are available for the c/5-transition state, which can easily be accommodated within the straight channels of BEA. The observed kinetically determined product distribution is thus satisfactorily explained by true transition state selectivity.
,H.
-Al-
r
.CH3 .CH3
,0
7-7~rr77^^i^
r
.CH3 CHs
-Al
.0
zeolite
Figure 1. Transition states for the formation of c/5-4-r^r/-butylcyclohexanol (top) and trans-A^err-butylcyclohexanol (bottom). In addition to the stereoselective MPV reaction presented above, van der Waal et al. reported the catalytic activity of aluminium-free titanium beta ([Ti]-BEA) zeolite in the same MPV reaction [18]. Again, a very high selectivity of 98% to the cz^y-isomer was found which was also explained by a restricted transition state, here around a Lewis acid titanium site. The Lewis acid properties of tetrahedrally incorporated titanium in zeolite [Ti]-BEA had already become clear during catalytic studies on the epoxidation of olefins with hydrogen peroxide in alcoholic solvents. The oxophilic Lewis acidity of the titanium site was confirmed by UV-VIS which showed an increased coordination number for the originally 4-coordinated titanium atom upon adsorption of alcohols and water. Kinetically determined product distributions were also obtained in the MPV reduction of 2-, 3- and 4-methylcyclohexanone; the cis-, trans- and cisalcohol being the major products, respectively. The catalytic activity of [Ti]-BEA was found to be much lower than that of its aluminium analogue whereas its tolerance for water was observed to be much higher. The latter property, which is related to the hydrophobic character of the aluminium free zeolite, illustrates its catalytic potential in this type of reactions.
537 4. Conclusions Heterogeneous catalysts which are active for the catalysis of the MPVO reactions include amorphous metal oxides and zeolites. Their activity is related to their surface basicity or Lewis acidity. Zeolites are only recently being developed as catalysts in the MPVO reactions. Their potential is related to the possibility of shape-selectivity as illustrated by an example showing absolute stereoselectivity as a result of restricted transition-state selectivity. In case of alkali or alkaline earth exchanged zeolites with a high aluminium content (X-type) the catalytic activity is most likely related to basic properties. For zeolite BE A (Si/Al=12), however, the dynamic character of those aluminium atoms which are only partially connected to the framework appear to play a role in the catalytic activity. Similarly, the Lewis acid character of the titanium atoms in aluminium free [Ti]-BEA explains its activity in the MPVO reactions.
Acknowledgement This work was financially supported by the Foundation for Chemical Research in the Netherlands (SON).
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