Stabilizing Effect of Bulky Î²-Diketones on Homogeneous Mo Catalysts

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Stabilizing effect of bulky #-diketones on homogeneous Mo catalysts for deoxydehydration Maxime Stalpaert, and Dirk De Vos ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b02532 • Publication Date (Web): 25 Jul 2018 Downloaded from http://pubs.acs.org on July 26, 2018

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ACS Sustainable Chemistry & Engineering

Stabilizing effect of bulky β-diketones on homogeneous Mo catalysts for deoxydehydration Maxime Stalpaert† and Dirk De Vos†*

†Centre for Surface Chemistry and Catalysis, Department of Microbial and Molecular Systems (M2S), KU Leuven, Celestijnenlaan 200F, 3001 Leuven, Belgium. * Email: [email protected].

KEYWORDS: Deoxydehydration, Diols, Molybdenum, β-Diketonate, Biomass

ABSTRACT: The deoxydehydration (DODH) of vicinal diols to alkenes is a promising reaction for the deoxygenation of biomass. While most reports so far concern Re based catalysts, catalysts based on more abundant Mo and V have gained more interest in recent years. However, yields and activity are relatively low for these catalysts and, especially for Mo, little is known about the influence of ligands on the metal catalyst. Here, we study a series of β-diketones with different steric and electronic properties as ligand precursors for a Mo catalyst. We show that addition of the β-diketone 2,2,6,6-tetramethylheptanedione (TMHD), leads to a strong increase in yield for

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the desired product in DODH of a range of substrates using different reductants. The most likely explanation for this increased yield is that the sterically bulky ligand, i.e. the conjugated βdiketonate, impedes oligomerization of the Mo catalyst and hence inhibits precipitation. This hypothesis was confirmed by electrospray mass spectrometry.

Introduction Due to the highly oxygenated nature of biomass, its use as a sustainable alternative feedstock for the production of fuels and chemicals requires selective deoxygenation reactions. An example of such a reaction is the deoxydehydration (DODH) of polyols, which transforms vicinal diols into alkenes using a reducing agent (Scheme 1). Several excellent reviews on this reaction have been published.1–4 The substrate scope has been expanded to include not only challenging biobased sugar alcohols such as pentitols and hexitols, but also sugar acids and their esters and even sugars.5–12 The most performant catalysts reported thus far are Re based homogeneous compounds, in particular methyltrioxorhenium (MTO). Recently, heterogeneous Re catalysts have also been reported, some of them with other noble metals, i.e. Pd and Au, as promotor.10,13– 18

Alternatively, due to the scarcity and associated high cost of Re, some groups have

investigated the use of the more abundant metals Mo and V.8,19–24 Good to excellent yields have been achieved with these catalysts. Furthermore, the first heterogeneous Mo and V catalysts were reported very recently.18,25,26 However, the DODH activity of these non-noble metal catalysts is relatively low. One way to increase the activity of homogeneous catalysts is through the use of ligands. However, in contrast to Re and V, the effect of ligands on Mo catalysts has rarely been studied. This is likely due to the high temperatures that are required to obtain a


2

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sufficiently high activity: while for Re and V temperatures below 170°C suffice, reactions with Mo are typically performed between 200 and 250°C.8,21,22,27 At these high temperatures and in the presence of chelating diols, only strongly coordinating ligands will form stable complexes with Mo. Unsurprisingly, most tested ligands have no or even a negative effect on the Mo catalyst.8,21 Only two examples of well-defined Mo complexes have been reported so far, one of which was only used in the more facile DODH of 1-phenylethane-1,2-diol and 1,2cyclooctanediol at 110°C.20,22 As a result, little is known about the optimal chemical environment of Mo in the DODH reaction. Therefore, in this report, we investigated a series of β-diketones with different steric and electronic properties as ligand precursors for a Mo catalyst. We show that in particular the bulky 2,2,6,6-tetramethylheptanedione (TMHDH) has a strongly positive effect on the DODH reaction.

Scheme 1. General reaction scheme for deoxydehydration of a vicinal diol. Experimental section Catalytic reaction. First, 0.025 or 0.05 mmol MoO2acac2 was weighed in a glass vial. A reaction mixture containing 0.5 mmol of substrate and 0.5 mmol of the internal standards methylcyclopentane and n-tetradecane in 2 mL of solvent (mesitylene or 2-octanol) was added. In case the substrate was not soluble in the reaction solvent at room temperature, e.g. erythritol in mesitylene, the substrate was weighed separately into the vial and a liquid mixture containing


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only the two standards in the reaction solvent was added. In some reactions, the reaction mixture also contained 1.5 eq. of a reductant, e.g. PPh3, in other reactions the solvent 2-octanol functions as reductant. After addition of a magnetic stirring bar, the vial was sealed under a N2 atmosphere using a cap with septum. The mixture was then heated to 200 °C with stirring at 500 rpm. After the reaction, the mixture was cooled on ice and a sample was taken using a syringe. The samples were analyzed by gas chromatography. Two reactions were performed in a high pressure stainless steel reactor under a H2 or CO atmosphere. Except for the reaction vessel, they were performed as previously described. The reactor was cooled on ice after reaction, the pressure was released and a sample was taken. Electrospray ionization mass spectrometry (ESI-MS). Samples obtained after a typical DODH reaction were centrifuged. Next, 100 µL of each sample was diluted with 8 mL acetonitrile and injected directly with a flow of 200 µl/min into a Thermo Finnigan LCQ Advantage mass spectrometer, in full scan mode over an m/z range of 50 to 2000. The electrospray ionization was in positive mode. Results and discussion Deoxydehydration (DODH) reactions were performed using 1,2-hexanediol (HDO) as model substrate, triphenylphosphine (PPh3) as reductant and mesitylene as solvent (Scheme 2). After a preliminary investigation of different commercial Mo catalysts, bis(acetylacetonato)dioxomolybdenum(VI) (MoO2acac2) was chosen as a reference catalyst. In a typical reaction, small amounts of different side products were formed, beside the DODH product 1-hexene. These include the dehydration products hexanal and 2-hexanone, the partial deoxygenation product 1-hexanol, and the products of glycol cleavage, formaldehyde and


4

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pentanal (Scheme 2). Glycol cleavage is a typical Mo-catalyzed DODH side reaction in which the C-C bond of the diol in between the hydroxyl groups is cleaved, resulting in two aldehydes.21 The aldehyde products are generally not detected as such, but as their acetals with HDO.

Scheme 2. Deoxydehydration of 1,2-hexanediol to 1-hexene with triphenylphosphine as reducing agent and typical side reactions. The effect of different β-diketones was studied by adding 0.5 eq. of these compounds to the reaction mixture before reaction (Figure 1). The β-diketones, which are added in excess compared to Mo, can be expected to replace acetylacetonate, at least to some extent, as βdiketonate ligands. Therefore, we will refer to β-diketonates as ligands and interpret the results in that light. Interestingly, both an electronic and a steric effect of the different ligands could be


5


observed. The steric effect is most pronounced; the bulky diketones dibenzoylmethane (DBMH) and 2,2,6,6-tetramethylheptanedione (TMHDH) lead to strongly increased 1-hexene yields. The electronic effect on the other hand is most clear when considering non-bulky and bulky diketones separately. As the electron donating strength rises from 1,1,1,5,5,5-hexafluoroacetylacetonate (HFAA) over 1,1,1-trifluoroacetylacetonate (TFAA) to acetylacetonate, the 1-hexene yield increases in the same order for the respective β-diketones. A similar effect can be observed when comparing dibenzoylmethanate (DBM) to the more electron donating 2,2,6,6tetramethylheptanedionate (TMHD). As TMHD is both bulky and electron donating, addition of TMHDH leads to the highest 1-hexene yield of 36%, a significant increase compared to the 15% yield without addition of diketone. The electronic and steric effects are rationalized in the next paragraphs.

1-Hexene Hexanone 1-Hexanol

Formaldehyde/pentanal Hexanal

50 40 Yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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30 20 10 0 None

HFAAH TFAAH ACACH DBMH TMHDH

Figure 1. Effect of addition of different β-diketones to the reaction mixture for Mo catalyzed deoxydehydration of 1,2-hexanediol to 1-hexene. Conditions: 0.5 mmol 1,2-hexanediol, 0.05 mmol MoO2acac2, 0.25 mmol β-diketone, 1.5 eq. triphenylphosphine, 2 mL mesitylene, 200 °C, N2 atmosphere, 2 h reaction. HFAAH = 1,1,1,5,5,5-hexafluoroacetylacetone, TFAAH = 1,1,1trifluoroacetylacetone, ACACH = acetylacetone, DBMH = dibenzoylmethane, TMHDH = 2,2,6,6-tetramethylheptane-3,5-dione.


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The electronic properties of the ligand can have an effect on catalysis in different ways. First, groups such as –F and –Ph lower the tendency of the β-diketonate to coordinate to Mo by decreasing the electron density on the coordinating oxygen atoms. As a result, for the same molar amount of a less electron rich β-diketone, the fraction of β-diketonate coordinated to Mo is lower, so the potential positive effects are less pronounced. In this context it should be kept in mind that the coordination of Mo to 1,2-HDO is probably strong, given the high oxophilicity of Mo and the chelating nature of the diol(ate). Indeed, especially for Mo(V)- and Mo(VI), many complexes with vicinal diols have been reported.28 Secondly, a β-diketonate coordinated to Mo would influence the electron density of the metal. To rationalize the effect on catalysis, we need to consider the DODH reaction mechanism. The most accepted reaction mechanism for Mo catalyzed DODH consists of three steps, i) condensation of the diol with Mo(VI), ii) reduction of Mo(VI) to Mo(IV) and iii) extrusion of the alkene and concomitant reoxidation of Mo(IV) to Mo(VI) (Scheme 3).8,29 The order of these steps and the identity of the rate determining step depend on the combination of metal, reductant and substrate. Especially with a strong reductant like PPh3, we can expect the alkene extrusion to be the rate determining step. An electron donating ligand such as 2,2,6,6-tetramethylheptanedionate (TMHD) could facilitate this step, since it involves an electron transfer from Mo to the coordinated diol, which is facilitated by an increased electron density around Mo. Thirdly, the yield and selectivity towards hexanal and 1hexanol also decrease in the series from HFAA to acetylacetone. A more electron donating ligand would decrease the Lewis acidity of Mo, which likely plays a role in the dehydration to hexanal. In addition, the facilitated alkene extrusion step might lower the extent of partial deoxygenation to 1-hexanol.


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Scheme 3. Proposed reaction cycle for Mo catalyzed deoxydehydration based on the work of Fristrup et al.8,29 Red = reductant. RedO = oxidized reductant. L = anionic ligand, e.g. βdiketonate. Alternatively, two L’s can also represent one dianionic ligand, e.g. a diolate or terminal oxo group. Bulky β-diketones have a strongly positive effect on 1-hexene yield. A possible explanation for this observation is the stabilization of Mo in atomically dispersed form by the conjugated βdiketonate ligand. In reactions performed in the absence of a bulky diketone a black precipitate is formed. The same observation was reported by Fristrup et al.; elemental analysis showed that this precipitate contained mostly Mo and C.8 Furthermore, it showed diminished catalytic activity in a DODH with fresh substrate. By contrast, in the presence of TMHDH and DMBH the reaction mixture turns dark during reaction, but no precipitate is formed. It is therefore likely that the bulky diketonate ligands form a steric barrier that hampers oligomerization of Mo, which, if allowed to continue unimpeded, could eventually lead to precipitation. To test this hypothesis, we have analyzed samples after a 2 h reaction of HDO with MoO2acac2 at 200°C, without


8

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TMHDH and with 2 eq. TMHDH via ESI-MS. Both samples were centrifuged before analysis. For the reaction sample without TMHDH, this led to precipitation, as mentioned earlier. The ESI-MS spectrum of this sample did not contain any clear Mo peaks, which are generally easily recognized by the typical peak pattern caused by the isotopic distribution (Figure 2 top). This suggests that only a very low amount of Mo is present in solution after precipitation. The sample with TMHDH on the other hand contains several peaks which can be attributed to Mo species (Figure 2, bottom). Unambiguous identification of every peak in the mass spectrum would be very complex, especially considering the large number of possible ligands (oxo, TMHD, acac, PPh3, OPPh3, acetonitrile are most likely). However, it is clear from the molecular weights and isotopic distributions that besides mononuclear Mo, oligomeric Mo species containing 2 to 4 Mo atoms are detected. Furthermore, the peaks with m/z values between 474 and 482 can be associated with MoOTMHD2, as they correspond to the typical isotopic distribution for through

100

92

Mo

Mo (Figure S10). This compound can be produced by reduction of MoO2TMHD2 by

PPh3, so this result suggests that reduction by PPh3 occurs before condensation with 1,2hexanediol in these conditions. In conclusion, the ESI-MS spectra indicate that TMHD indeed impedes oligomerization, but does not fully avoid it. As a result, oligomeric and monomeric Mo are present in solution and precipitation is avoided.


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Figure 2: Electrospray ionization mass spectra (ESI-MS) in positive mode of samples after reaction, diluted in acetonitrile. Top: reaction without TMHDH; Bottom: reaction with 2 eq.


10

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TMHDH. Reaction conditions: 0.5 mmol 1,2-hexanediol, 0.05 mmol Mo, 1.5 eq. triphenylphosphine, 2 mL mesitylene, 200 °C, N2 atmosphere, 2 h reaction. TMHDH = 2,2,6,6tetramethylheptane-3,5-dione. Several explanations for the oligomerization of Mo oxo species can be found in literature. First, the tendency of Mo to form polymeric molybdates through condensation is well-known, for example in the formation of polyoxometalates (POMs).30 POMs are typically formed in aqueous media; however similar reactions might occur in organic solvents, especially in the presence of Lewis acidic Mo and water, which is formed in the DODH reaction. Secondly, when Mo(VI)O2 species are reduced to Mo(IV)O, which is an essential step in the DODH reaction mechanism, dimerization tends to occur through a comproportionation reaction forming µ-oxo Mo(V)2O3 (Scheme 4); only in rare cases can this dimerization be avoided.28,31,40,41,32–39 The effect of multiple redox cycles on the Mo nuclearity has not been investigated. However, it seems plausible that this could lead to further oligomerization if the µ-oxo bond is not broken upon reoxidation. In addition, MoO2acac2 is known to easily form the dimeric Mo2O3acac4, resulting in a darkening of the compound from yellow to red.42 Finally, it would even be possible that some degree of polynuclearity contributes to enhanced catalytic activity. Indeed, the recently reported heterogeneous DODH catalyst MoOx/TiO2 leads to better results if synthesized from the polynuclear precursor (NH4)6Mo7O2.4H2O (ammonium heptamolybdate hydrate, AHM), compared to the mononuclear Na2MoO4.2H2O; the authors hypothesize this might be due to Mo aggregation. Moreover, Sharkey et al. report that in MoOx on oxide catalysts, higher Mo content leads to higher activity, possibly due to more facile reduction of Mo species with a higher degree of oligomerization.18 Too strong aggregation however, would lead to only a small amount of the Mo being available for catalysis, resulting in decreased activity. Therefore,


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unhindered oligomerization should be avoided in any case, but especially if this leads to unwanted precipitation in a homogeneous system. Bulky β-diketonates, especially TMHD, prove to be suitable ligands to achieve stabilization against precipitation.

Scheme 4. Comproportionation reaction of Mo(VI)O2 and Mo(IV)O compounds. Since the best results were obtained in the presence of TMHDH, we next investigated the use of larger amounts of this compound. This results in a steady increase in 1-hexene yield leading to an excellent 93% yield at 4 eq. of TMHDH (Figure 3). At this point, a TOF of 4.7 h-1 is reached, which is the highest TOF obtained for Mo catalyzed DODH of 1,2-hexanediol, a frequently used DODH substrate. We have also tested the addition of the conjugated base of TMHDH, in the form of Li(TMHD). However, this has a clear negative effect on the 1-hexene yield (Figure S11), possibly because of the basic properties of this compound. The strongly negative effect of several bases, e.g. NaOH, CH3COONa and LiOCH3, on Mo-catalyzed DODH has been reported.8 Additionally, we tested the compound MoO2TMHD2 as catalyst. As expected, MoO2TMHD2 results in a higher 1-hexene yield than MoO2acac2. However, the yield is lower than for MoO2acac2 in the presence of 0.5 eq. TMHDH or higher. Therefore, we also performed the reaction with MoO2TMHD2 and additional TMHDH. Again, the 1-hexene yield increases, however, now an optimum is reached at 2 eq. TMHDH. These results demonstrate that a large amount of TMHDH is necessary to achieve very high yields. This is most likely because Mo has a strong affinity for diols, not only because of the oxophilicity of Mo, but also due to the chelating effect of the diol(ate). As a result, large amounts of TMHDH are required to achieve


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significant coordination to Mo. However, even at small amounts of TMHDH the effect is notably positive. In addition, since TMHDH stabilizes the molecular Mo catalyst, its effect is stronger after longer reaction times (cfr. infra). Therefore, using small amounts of TMHDH, for example 0.5 eq., in longer reactions would generally be the most economical option. If very high yields and TOFs are essential, using large amounts of TMHDH is a valid option. Importantly, TMHDH is stable in the reaction conditions; no conversion of TMHDH or appreciable formation of side products is detected. In this sense, TMHDH could be viewed as a strongly coordinating solvent, rather than as a classical ligand. Indeed, reaction in 4 eq. TMHDH without an additional solvent also leads to an excellent 1-hexene yield of 81% after 2 h. Furthermore, due to its high boiling point (202°C), it should be possible to separate the volatile DODH products from the reaction mixture by evaporation without losing a significant part of non-ligated TMHD. Finally, the positive effect of TMHDH is not limited to reactions with MoO2acac2 as catalyst precursor. Using the cheap AHM as Mo precursor in the same conditions, the 1-hexene yield increases from 12% without TMHDH to 80% using 4 eq. of TMHDH.

WithMoO2tmhd2 MoO2TMHD2 With

With MoO MoO2acac2 2acac2 100 90 80

Hexene yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


70 60 50 40 30 20 10 0 0

1

2

3

4

TMHDH (eq.)


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Figure 3. Deoxydehydration of 1,2-hexanediol: 1-hexene yield at different TMHDH contents with MoO2acac2 and MoO2TMHD2 as catalyst precursors. Conditions: 0.5 mmol 1,2-hexanediol, 0.05 mmol Mo, 1.5 eq. triphenylphosphine, 2 mL mesitylene, 200 °C, N2 atmosphere, 2 h reaction. TMHDH = 2,2,6,6-tetramethylheptane-3,5-dione, TMHD = 2,2,6,6-tetramethylheptane3,5-dionate. To investigate whether the system, MoO2acac2 stabilized by TMHDH, is generally applicable for DODH reactions, we performed reactions using other substrates, reductants, solvents and temperatures (Table 1). First, we showed that the DODH of 1,2-hexanediol can also be performed at a lower temperature (170 °C). A 23 % yield of 1-hexene is reached after a reaction time of 18 h, while the yield is only 4 % in the absence of TMHDH (Entries 3 and 4). Then, we expanded the substrate scope to cyclic and aromatic diols, but also to the bio-based diols (+)diethyl tartrate (DET), 1,4-anhydroerythritol (AHE) and erythritol. For all these compounds, acceptable to excellent yields could be achieved in the presence of TMHDH and PPh3 (Entries 59, 11). For 1-phenyl-1,2-ethanediol, high yields can even be achieved at 150°C. Next, we examined the use of other reductants using DET as substrate (Entries 10 to 15). While H2, CO and Na2SO3 are not effective reductants in these conditions, using a sec-alcohol such as 2-octanol as reductant does result in a 35 % yield of fumarate esters after only 4 h. Furthermore, 2-octanol can also be used as reductant in the DODH of AHE and erythritol (Entries 7 and 9). Unfortunately, 2,2-dimethylbutanone is formed from TMHDH in the presence of 2-octanol in up to 6% yield after 18 hours. This compound is likely formed through transfer hydrogenation of TMHDH, followed by retro aldol addition resulting in 2,2-dimethylbutanone and 2,2dimethylpropanal. The latter product is detected as its acetals with HDO and 2-octanol. The results with different reducing agents correspond well with literature, as H2, CO and Na2SO3


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have not been reported as effective reductants for Mo catalyzed DODH, in contrast to secalcohols.8,22,25 Fristrup et al. have obtained good to excellent yields using iso-propanol as reducing agent by employing a reaction temperature as high as 250°C.8 The high temperature likely increases the rate of Mo reduction by 2-octanol, which is the rate determining step in these conditions. For H2 and most likely CO and Na2SO3 even higher temperatures are necessary. However, higher temperatures would most likely necessitate even higher TMHDH concentrations to achieve sufficient coordination of TMHD to Mo. Finally, some of the aforementioned reactions were also performed in the absence of TMHDH (Entries 1, 3 and 10). Clearly, the stabilizing effect of TMHDH is present for several substrates and at different temperatures. Furthermore, using diethyl tartrate as substrate, we filtered the samples obtained after reaction in the presence of 0, 2 and 4 eq. of TMHDH and used the filtrate for a second reaction with fresh DET and PPh3. The required amount of these compounds was calculated by GC analysis. The samples with 2 and 4 eq. TMHDH lead to similar DODH yields after filtration, while without TMHDH barely any product is formed in the second reaction. This again demonstrates that most of the Mo precipitates during a reaction in the absence of TMHDH, while addition of this compound allows to stabilize the homogeneous catalyst.

Reductant

Solvent

T (°C)

TMHDH (eq.)

1

PPh3

Mesitylene

200

2

PPh3

Mesitylene

3

PPh3

4

5

Entry

Substrate

Yield (%)

Time (h)

0

15

2

200

4

93

2

Mesitylene

170

0

4

18

PPh3

Mesitylene

170

4

23

14

PPh3

Mesitylene

200

4

59

2


DODH product(s)

15


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OH

6

HO

PPh3

Mesitylene

150

4

51

2

7

2-Octanol

2-Octanol

200

4

56

18

8

PPh3

Mesitylene

200

4

3, 15

2

9

2-Octanol

2-Octanol

200

4

1, 15

18

10

PPh3

Mesitylene

200

0

9

2

11

PPh3

Mesitylene

200

4

92

2

12

Na2SO3

Mesitylene

200

4

8

18

13

CO

Mesitylene

200

4

4

2

14

H2

Mesitylene

200

4

5

2

15

2-Octanol

2-Octanol

200

4

35

2

,

,

a

Table 1. Deoxydehydration of different substrates with MoO2acac2 with and without TMHDH, using several reductants. Conditions: 0.5 mmol substrate, 0.05 mmol MoO2acac2, 1.5 eq. reductant (except CO (13 bar), H2 (40 bar), or 2-octanol (25 eq.)), 2 mL solvent, N2 atmosphere (except for reactions under CO and H2).

a

diethyl fumarate, ethyl octan-2-yl fumarate and

dioctan-2-yl fumarate. TMHDH = 2,2,6,6-tetramethylheptane-3,5-dione. Next, we investigated the DODH of the biobased substrate 1,4-anhydroerythritol (AHE) using the green solvent and reductant 2-octanol in more detail. AHE can be formed through cyclodehydration of fermentatively produced erythritol,43,44 while 2-octanol is produced from


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castor oil45 and has several characteristics of a green solvent, such as high boiling point, high flash point and relatively small effects on health (irritating to skin and eyes).46 To minimize the cost of the catalyst, a lower amount of MoO2acac2 (5 mol%) and only 0.5 eq. of TMHDH were used. DODH of 1,4-anhydroerythritol leads to 2,5-dihydrofuran (DHF) as main product (Figure 4). The only notable side reaction is double dehydration to furan; the yield for this product is only 3-4% for all reactions. The acetal of DHF with 2-octanone is also detected, but since its formation is reversible, it should not be regarded as a side product. In addition, the solvent/reductant 2-octanol is also dehydrated, leading to a yield of octenes around 3% and of 2,2’-dioctyl ether of less than 1%. As shown in Figure 4, addition of TMHDH has a strongly positive effect with 2-octanol as reductant as well. Furthermore, in the presence of TMHDH, dehydration of 2-octanol is slower, possibly due to the decreased Lewis acidity of Mo when coordinated to TMHD. As Mo-catalyzed DODH reactions with alcohol reductants typically require longer reaction times or higher temperatures, we also performed an 18 h reaction.8,22,25 Since TMHD stabilizes molecular catalysts, its positive effect is enhanced at increased reaction time. As a result, an excellent DHF yield of 72% in the presence of TMHDH is obtained, close to the best result (75%) so far obtained for Mo catalyzed DODH of 1,4-anhydroerythritol.8

No TMHD 2,5-Dihydrofuran yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.5 eq. TMHD

100 80 60 40 20 0 0

5

10 Time (h)

15


20

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Figure 4. Deoxydehydration of 1,4-anhydroerythritol to 2,5-dihydrofuran using 2-octanol as reductant: reaction scheme and yield of 2,5-dihydrofuran in function of time. Conditions: 0.5 mmol 1,4-anhydroerythritol, 0.025 mmol MoO2acac2, 0.25 mmol TMHDH, 2 mL 2-octanol, 200 °C, N2 atmosphere. TMHDH = 2,2,6,6-tetramethylheptane-3,5-dione. In conclusion, we have investigated a series of β-diketonates as ligands for Mo-catalyzed deoxydehydration and have shown that electron-donating and sterically bulky ligands have a positive effect on Mo DODH activity. Especially in the presence of 2,2,6,6tetramethylheptanedione (TMHDH) highly increased yields are achieved. We hypothesize that this is due to stabilization of molecular Mo against oligomerization by the conjugated 2,2,6,6tetramethylheptanedionate (TMHD) ligand, which results in diminishing or avoiding precipitation of the catalyst. ESI-MS of centrifuged samples after reaction confirmed this, as monomeric and oligomeric Mo could only be detected if the reaction was performed in the presence of sufficient TMHDH. Addition of larger amounts of TMHDH leads to the highest TOF reported for the DODH of 1,2-hexanediol to 1-hexene (4.7 h-1), which was obtained in an excellent 93% yield. Next, we showed that this positive effect of TMHDH on the Mo catalyst is general, occurring for DODH of several substrates, including diethyl tartrate and erythritol, using different reductants and conditions. Finally, we further investigated the use of TMHDH in the DODH of bio-based 1,4-anhydroerythritol to 2,5-dihydrofuran using 2-octanol as reductant. Also for this reaction, the catalytic performance is drastically increased in the presence of TMHDH, leading to one of the highest 2,5-dihydrofuran yields reported in Mo catalyzed DODH of 1,4anhydroerythritol. ASSOCIATED CONTENT


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Supporting Information. Experimental details. Gas chromatogram and mass spectra. Effect of Li(TMHD). Results of filtration test. AUTHOR INFORMATION Corresponding Author * Email: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT M.S. thanks Fonds Wetenschappelijk Onderzoek (FWO) for his doctoral fellowship. D.E.D.V. acknowledges the IWT and FWO for research project funding, the Flemish government for longterm structural funding through Methusalem, and EoS (Biofact) for financial support. The authors acknowledge Bart Van Huffel for his help with the ESI-MS analysis. REFERENCES (1)

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SYNOPSIS: Addition of 2,2,6,6-tetramethylheptane-3,5-dione as ligand stabilizes the molecular Mo catalyst in the green deoxydehydration reaction. For Table of Contents Use Only


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Stabilizing Effect of Bulky Î²-Diketones on Homogeneous Mo Catalysts

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