Toward Sustainable Synthesis of PA12 (Nylon-12) Precursor from

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Toward Sustainable Synthesis of PA12 (Nylon 12) Precursor from Oleic Acid Using Ring-Closing Metathesis Godwin Ameh Abel, Sridhar Viamajala, Sasidhar Varanasi, and Kana Yamamoto ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01648 • Publication Date (Web): 14 Aug 2016 Downloaded from http://pubs.acs.org on August 22, 2016

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ACS Sustainable Chemistry & Engineering

Toward Sustainable Synthesis of PA12 (Nylon 12) Precursor from Oleic Acid Using Ring-Closing Metathesis Godwin Ameh Abel, Sridhar Viamajala, Sasidhar Varanasi and Kana Yamamoto* Department of Chemical and Environmental Engineering, and School of Green Chemistry and Engineering, University of Toledo, 1640 N. Westwood Ave. Toledo, OH 43606 [email protected] RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to)

ABSTRACT

The efficient reaction conditions for ring-closing metathesis of homoallyloleamide (2) for the synthesis of PA12 (Nylon 12) precursor (4) are presented. Systematic screening of a series of commercially available metathesis catalysts identified a catalyst that promotes the reaction at low temperature (60 °C), enabling use of non-halogenated solvents, ethyl acetate and hexanes. The catalyst was adsorbed on mesoporous silica gel to aid its post-reaction recovery and reuse, which was demonstrated in hexanes up to three cycles without significant loss of reaction conversion (>

ACS Paragon Plus Environment

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90%). The identification of reaction-compatible green solvents as well as the demonstration of the potential for recycling of the supported catalyst are steps further toward establishing environmentally sustainable production of the polyamide precursors from oleic acid.

KEYWORDS Ruthenium catalyst, Olefin metathesis, Nylon, Polyamide, Fatty acids, Green chemistry, Oleic acid Introduction There has been increasing appreciation for the use of renewable bio-sourced materials to complement or replace petroleum-based specialty chemicals.1-2 In particular, olefin metathesis is regarded as an effective tool for refining bio-based lipids for this purpose.3-4 In this context, we have recently reported5-6 new approaches for synthesizing high-order polyamide (PA, nylon) precursors from oleic acid (1), an abundant feedstock available from various renewable sources. One of our approaches5 involves three steps using ring-closing metathesis to provide macrolactams having carbon length ranging from 11–13, monomers of PA11–13 (Scheme 1). Although the synthesis is concise, the ring-closing metathesis (step 2) requires a halogenated solvent (chlorobenzene),7 high temperature (120 °C), and relatively high catalyst loading (2 mol%).3


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Scheme 1. Previously demonstrated route from oleic acid (1) to PA precursors.5 While chlorinated solvents (chlorobenzene, dichloromethane, 1,2-dichloroethane) and aromatic solvents (benzene, toluene, xylene) are traditionally employed in metathesis reactions,8-9 a recent study showed that less harmful solvents could replace these traditional solvents in olefin metathesis.10 Also, significant progress has been made in improving catalyst efficiency for olefin metathesis in the past decade.11 Catalysts that possess a wide range of electronically and sterically different N-heterocyclic carbene (NHC) ligands and/or alkylidene ligands are now commercially available, allowing laboratory-scale screening of industrially-relevant catalysts and metathesis reactions.12 Studies on catalyst immobilization13 to facilitate their recovery and reuse have also been reported.14-18 In this article, we present our studies aimed at establishing improved reaction conditions for ringclosing metathesis step (step 2) in our synthesis (Scheme 1), from homoallyloleamide (2) to enelactam (3), the precursor of PA12 monomer. First, a series of commercially available ruthenium alkylidene catalysts (Figure 1) were evaluated to identify the catalysts that are sufficiently active to promote the reaction with good selectivity at lower temperatures. With low temperature conditions, formation of undesirable side-products may be minimized, and the stability and life-


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time of the catalyst improved.10 The use of the identified catalysts that enable use of low-boiling and green solvents,10

could favorably impact the process from economic as well as

environmental viewpoints. Next, the stability of each of the screened-catalysts in these reaction media was evaluated to establish the potential for its recyclability. Finally, the best catalyst was immobilized on several types of silica gel support and its recovery and reuse were demonstrated up to several cycles.

Experimental Section Materials and analytical procedures The reagents were purchased from Sigma-Aldrich, STREM chemicals, or Alfa Aesar, and used without further purification. All the solvents used for ring-closing metathesis were purchased from Sigma-Aldrich and deoxygenated by bubbling dry nitrogen gas for 20 min before being used. Reactions were carried out under inert conditions (argon) in a fume hood. Thin layer chromatography (TLC) was carried out on glass-backed silica plates purchased from Sorbent Technologies Inc., Norcross GA. The reaction products were identified by visualizing the plates under UV (254 nm) light, and also by staining with potassium permanganate followed by gentle heating. Silica gel column chromatography was carried out using 20–60 micron dry silica purchased from Sorbent Technologies Inc. 1

H- and

13

C-NMR spectra were acquired in CDCl3, on Brucker Avance 600 (600 MHz) NMR

spectrometers. Chemical shifts (δ) were reported as parts per million (ppm) with reference to tetramethylsilane (TMS) or solvent unless otherwise stated. The coupling constants (J) are reported in Hz. Mass spectra were obtained with Hewlett-Packard Esquire Ion Trap LC-MS (electrospray). GC analyses were performed with HP 5890 series II equipped with FID and an


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auto-sampler (HP controller 7672A) and Biodiesel TG column (5% diphenyl, 95% dimethyl polysiloxane, 15m, 0.33mm ID and 0.10 µm dF) or MXT biodiesel TG (Siltek – treated stainless steel) column. GC analysis of samples was carried out using the following protocol: 60 – 370 °C at 10 °C/min and 6 min hold. Representative reaction conditions for ring-closing metathesis of homoallyloleamide (2) This work, which is an effort to optimize the reaction conditions for RCM approach for the synthesis of PA12 precursors from oleic acid, builds on our previous publication in which the details regarding the product identification and characterization were provided.5 Hence, only a brief outline of the batch reaction conditions is given here. The procedure described represents the protocol for a specific catalyst (C9) in ethyl acetate. A similar procedure is implemented with all the catalysts screened and the solvents used in the study. N-(But-3-en-1-yl)oleamide (44.4 mg, 0.1323 mmol) was dissolved in ethyl acetate (31 mL) and heated to 60 °C and was maintained at this temperature for 20 min. 0.8 mg (0.00097 mmol) of catalyst C9 (M74SIPr; see Fig 1) was dissolved in ethyl acetate (1 mL) and added to the reaction mixture. The solution was kept for 15 min at this temperature, before quenching the reaction with ethyl vinyl ether. After being cooled to room temperature, the reaction mixture was concentrated usign a rotary evaporator under reduced pressure. The crude residue was purified by column chromatography using first hexanes/ethyl acetate (7/3) and then acetone/hexanes (2/8) as eluents to provide the desired ene-lactam as a white crystalline solid (19.6 mg, 75.9%). 1H NMR spectra matched those reported in the literature.5 1

H NMR (600 MHz, CDCl3) δ 5.32–5.55 (m, 4H), 3.26–3.36 (m, 2H), 2.02–2.34 (m, 8H), 1.62–

1.65 (m, 2H), 1.18–1.48 (m, 12H).


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The reaction conversions and yields were reported in GC area%. They were calculated by integrating all the peaks in the GC chromatogram, excluding the known peaks that should not be taken into mass balance (such as 1-decene, 1-decene dimers). There may be trace amounts of other oligomers that formed from isomerization and subsequent metathesis of 1-decene, which were not taken into account in the calculation. However, we believe that our approach provides data that are adequate for this qualitative study. Procedure for testing the stability of the homogeneous catalysts For establishing the catalyst stability, we used the same catalyst repeatedly in a fed-batch process. Accordingly, after the reaction is carried out for 15 minutes following the above procedure, 1 mL of the reaction mixture is withdrawn from the reactor to determine the conversion of the substrate and product yield, and a fresh batch of the substrate in 1 mL of reaction solvent is simultaneously loaded into the reactor for the next reaction cycle. Catalyst Immobilization Immobilization of metathesis catalysts on silica gel was carried out according to the following procedure from literature.14 To a round-bottom flask containing toluene (5 mL), silica gel (223 mg) and M74SiPr catalyst (9 mg) were added at room temperature and kept under N2 atmosphere for 4 h. The mixture was then vacuum-filtered and washed several times with hexane and placed under reduced pressure overnight, prior to usage. The immobilized-catalysts used in Table 4 were prepared with the following reagents: M74SiPr (13.1 mg)/SBA-15 (422 mg); M74SiPr (12.7 mg)/MCM-41 (397 mg)). Procedure for testing the stability of the immobilized catalyst The dried catalyst-loaded silica gel (31.5 mg) was placed in a three-necked flask, then ethyl acetate (15 mL) was added and the mixture was heated to 60 °C. Subsequently, N-(But-3-en-1-


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yl)oleamide (2, 11 mg) in ethyl acetate (1 mL) was charged to initiate the reaction and the mixture was maintained at 60 °C for 15 min. After 15 min, the reaction mixture was cooled to room temperature, and the clear liquid portion was removed with a syringe. The liquid was passed through a short pad of silica gel (with a mixture of ethyl acetate/hexanes and then with acetone/hexanes) to remove any leached catalyst that entrained with the liquid portion prior to GC analysis. The combined eluents from the silica pad were concentrated and re-dissolved in methanol for GC analysis. The subsequent experiments were performed by addition of a fresh solution of N-(But-3-en-1-yl)oleamide (2, 11 mg) in ethyl acetate (16 mL) to the flask containing the catalyst-loaded silica gel from the previous run, and repeating the above procedure. This approach allowed the use of the same immobilized catalyst for multiple runs for establishing the catalyst stability.

Result and discussion Rationale behind catalyst library selection for screening A series of commercially-available ruthenium catalysts were screened based on a semi-empirical rationale that is governed by (i) our own previous experience and (ii) other studies in literature on the comparative evaluation of catalysts for metathesis reactions using model experiments.12, 19 We have first excluded the catalysts without N-heterocyclic carbene (NHC) ligands because of their lower reactivity and stability.20 In addition, we have previously observed that HoveydaGrubbs II catalyst (Fig. 1, C1), but not Grubbs II catalyst, was active for the specific RCM reaction system being studied;5,

21

therefore, the boomerang-type catalysts22 as well as the

catalyst with labile ligands were selected for this study (Figure 1).


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In general, it has been recognized that catalysts with protective ligand (boomerang) that is easier to release led to faster catalyst initiation, allowing the metathesis reaction to be conducted at lower temperature. However, fast catalyst initiation does not necessarily correlate to high activity/product yield due to the potential for decomposition of the active-form of the initiated catalyst.23-27 In addition, many studies on tuning NHC ligands to improve catalyst performance have also been reported,28 and catalysts with sterically hindered NHC ligands have shown better selectivity and stability for certain ring-closing metathesis reactions.12, 27, 29 It is evident from the foregoing discussion that olefin metathesis is a rapidly advancing field of research wherein new types of catalysts are being continually developed, however there is not yet a standardized method for catalyst evaluation. Moreover, the large number of variables to consider when developing new processes makes the prediction of appropriate choice of catalyst rather difficult. We, therefore, reasoned that when developing new metathesis methodologies, the evaluation of a judiciously chosen library of catalysts can be extremely valuable10, 12, 19 The following ruthenium complexes were ultimately chosen in this study (Figure 1): (a) Benchmark catalyst: Hoveyda-Grubbs II (C1),30 (b) “Fast initiating” or “highly active”: Grela catalyst (C2);25, 31-32 GreenCat catalyst (C4);3334 Umicore M7 series:27, 35-36 M71SiPr (C6), M72SiPr (C7), M73SiPr (C8), M74SiPr (C9); (C10);26 (C11);3738 (c) Catalysts effective with unreactive or hindered olefins: Stewart-Grubbs catalyst (C3),39-40 (C5);40 (C10).26 (d) Others: (C12).41


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Figure 1. Commercially available metathesis catalysts screened in this study. The major structural elements of these catalyst systems include a ruthenium cluster with an NHC (Nheterocyclic carbene) ligand and a protective ligand (as indicated in color with the bench-mark catalyst C1). Release of the protective ligand in the reaction medium generates the active catalyst - a process known as catalyst initiation. The NHC ligand contributes to the stability of the catalyst.


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A detailed screening of this catalyst library was undertaken with the aim of answering the following questions: (1) Whether the chlorinated solvents (chlorobenzene) could be replaced with more sustainable low-boiling solvents, such as ethyl acetate or hexane; (2) The lowest temperature that allows the reaction to go to completion while maintaining reactivity and selectivity; (3) The most stable catalysts, among the ones that provide high conversion and selectivity at low temperatures; (4) The possibility of immobilizing the catalyst while retaining its activity and stability.

Optimization of reaction conditions Since one of our goals was to replace the chlorinated solvent (chlorobenzene) employed in the original procedure with greener solvents, we have evaluated two other alternative solvents10 as reaction media during the catalyst evaluation study. While more specifics regarding detailed catalyst screening were presented later, Table 1 summarizes results pertinent to reaction-solvent choice. Table 1. Ring-closing metathesis of homoallyloleamide (2) – solvent comparison.

Entry[a]

Cat.

Time (min)

Temp. (°C)[b]

Solvent

Conv. (%)[c]

3 (%)[c]

Dimers (%)[c]

1[d]

C1

15

120

PhCl

96

70

11

2

C1

15

60

EtOAc

95.1

55.1

21.1


10

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3[e]

C9

15

60

EtOAc

93.2

71.5 (76)[f]

16.5

4

C9

15

60

PhCl

42.7

–

–

5

C9

60

60

PhCl

88.9

70.2

16.3

6[e][h]

C9

15

60

Hexanes

90.3

77.4 (56)[f][h]

12.9

[a] Reaction conditions: 1 mol% of the catalyst in a solvent (1 mL) was added into a solution of homoallyloleate (0.033 mmol) in EtOAc (15 mL) in one portion (2 mM final concentration). The reaction was kept at indicated temperatures before samples were taken for GC analyses. [b] Oil bath temperature. [c] GC area%. [d] Taken from ref.5 [e] 4 mM concentration. [f] Isolated yield. [g] The sample solution was prepared by re-dissolving the concentrated reaction mixture in chloroform. [h] Isolated yield from the reaction run at 2 mM concentration.

First, low temperature reactions in ethyl acetate (60 °C) showed conversions similar to reactions performed in chlorobenzene at a much higher temperature (120 °C) with the same bench-mark catalyst (C1), although a slight increase of substrate dimerization was observed in ethyl acetate. (Table 1, entries 1–2).42 Indeed, when we compared the same two solvents with one other promising catalyst from our library (i.e., C9) we found that the reaction proceeds four times faster in ethyl acetate under otherwise identical conditions, while providing higher conversions and yields with no relative increase in substrate dimerization (entries 3–5). We also investigated hexane as prospective reaction medium because, as discussed later, the non-polar nature of hexane also proves advantageous with regards to implementing the reaction with immobilizedversions of the catalysts. As can be noted from entries 3 and 6 in Table 1, the reaction kinetics and conversions are very similar in both ethyl acetate and hexanes. Interestingly, however, in hexanes the side-products (dimers) precipitated out, presumably as a mixture of isomers, allowing their isolation by filtration.43 As seen in Table 1, the isolated and GC-based product yields agree very closely in case of ethyl acetate (entry 3) where as there is some discrepancy between these two yields in case of hexanes (entry 6). We suspect the formation of higher-order


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oligomers in hexane that precipitated out of the reaction mixture and were not accounted for in the GC area computations are the reason for the observed discrepancy. Nevertheless, this feature of oligomer precipitation in hexanes would make the product purification less cumbersome as the reaction medium has fewer soluble impurities. Overall, because of higher reaction rates, lower toxicity, and the price, both ethyl acetate and hexanes are attractive alternative solvents for our chemistry. Based on the ease of analysis, majority of the screening experiments reported below were performed using only ethyl acetate as the reaction solvent. As already noted, reaction temperature has a profound effect on the catalyst degradation rates and lower reaction temperatures could significantly extend the catalyst life. There is evidence in literature that when olefin metathesis reaction is conducted at 80 °C using immobilized Hoveyda-Grubbs II catalyst (C1), the catalyst could be recycled successfully for multiple cycles.16 Since our initial experiments with low-boiling solvents as reaction media have established the feasibility of conducting the reaction at temperatures lower than 80 °C, we wanted to verify how much further we could lower the reaction temperature without experiencing significant reduction in kinetics and yields. Table 2 compares the performance of three different catalysts (C2, C4 and C6) at 60 °C and room temperature (22 °C). Table 2. Ring-closing metathesis of homoallyloleamide (2) – temperature comparison.

Entry[a]

Cat.

Temp.

Time

Conv.

3

Dimers


12

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(°C)[b]

(min)

(%)[c]

(%)[c]

(%)[c]

1

C2

60

15

92.9

54.7

20.6

2

C2

22

300

93.1

33.8

53.9

3

C4

60

15

96.4

70.0

26.4

4

C4

22

300

95.8

33.7

61.3

5

C6

60

15

90.1

66.3

17.2

6

C6

22

300

93.7

58.9

31.7

[a] Reaction conditions: 1 mol% of the catalyst in EtOAc (1 mL) was added into a solution of homoallyloleate (0.033 mmol) in EtOAc (15 mL) in one portion (2 mM final concentration). The reaction was kept at indicated temperatures before samples were taken for GC analyses. [b] Oil bath temperature. [c] GC area%.

Two

general

trends could be seen in the data. First, at room temperature, the reaction kinetics became unacceptably slow.

Second, lower temperatures lead to increased oligomer formation, the

known trend explained by the entropy difference leading to the two products.44 Thus, it appears that 60 °C forms the threshold temperature for conducting the oleamide metathesis reaction, and further significant reduction of reaction temperatures below 60 °C may not be viable. At 60 °C, extending the reaction time beyond 15 minutes was not beneficial, as there is no further improvement in the conversion and the yield of the desired product (not shown).

Overview of optimal performance results for the catalyst library screened The results on the optimal performance (time that reached the highest conversion) of the individual members of the chosen catalyst library are compiled in Figure 2. The figure shows the selectivity towards desired ene-lactam (3) at near-complete conversion of the substrate for all the catalysts that displayed activity at 60 °C (when used fresh) in ethyl acetate. The benchmark catalyst C1 and catalyst C2 were the two least selective catalysts, while catalysts C3, C4, C5,


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and C9 showed high selectivity. It is worth noting that some of the catalysts required longer reaction times than others to achieve optimal performance. Also, unlike the rest of the catalysts in the figure, only ~50% conversion was achieved with catalyst C10, though it did display catalytic activity at the chosen reaction conditions. The results using catalyst C11 and C12 is not included in the figure, as the reaction does not occur under the said reaction conditions even after several hours. Although several of the catalysts of the library showed promise when freshly used for producing ene-lactam from oleamide, for economic viability they should also retain their stability during reuse for multiple cycles of the reaction. Accordingly, we also studied the stability of all the catalysts in the library.


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Figure 2. Ring-closing metathesis of homoallyloleamide (2) – catalyst comparison when the highest conversion is achieved. [a] Reaction conditions: See Table 2. [b] % conversion at 15 min with catalysts C5: 82.6%; C7: 51.4 %; C8: 91.6%; C9: 42.7%. [c] Reaction using C10 stalled after 15 min.

Assessment of catalyst stability We investigated the stability of each of the catalysts of our library in a “fed-batch homogeneous catalysis” mode of usage (Table 3). The experiment was conducted with successive additions of the substrate into the reaction mixture after every 15 minutes. This fed-batch approach allows the evaluation of catalyst stability over multiple runs. A portion of the reaction mixture was sampled for analysis at the start of run 1 and also at the end of each consecutive cycle (Table 3).45 The catalyst stability estimates shown in Table 4 per each cycle are based on the % conversion of the combined amounts of substrate put into the reaction medium until the completion of that cycle. In this sense, the percentages reported reflect the average activity displayed by the


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catalyst over those cycles. The effectiveness of a given catalyst is assessed based on its stability as well as its selectivity to the desired product. As our method of stability estimate of the catalyst is based on “percent of substrate converted” alone, this data has to be evaluated in combination with the selectivity data (Figure 2) for fresh catalysts, to establish the value of a given catalyst for this reaction. Since catalysts C3, C4, C5, and C9 showed high selectivity (Figure 2), a comparison of the stability of these catalysts (Table 3) reveals that catalyst C9 is the most effective of all for oleamide metathesis reaction: it retains enough activity to achieve above 70% conversion of substrate even after five cycles of reuse with a selectivity exceeding 72%. C3 and C4 appear to be the next two promising candidates. They display very similar stability and selectivity characteristics: both display high selectivity and reasonable stability. In addition, C3, C4 and C5 all have fast reaction kinetics. Thus, these catalysts could be viable candidates to consider for immobilization on supports for large-scale processes. Finally, catalysts C5, C7, and C8 also display good stability and selectivity, but would require longer reaction times (Figure 2) under the tested reaction conditions. Of all the catalysts evaluated, catalyst C10, with the lowest activity, was the least stable and deactivated after a single cycle (entry 10) and was deemed nonviable for our application. Since catalyst C9 showed the best performance characteristics in ethyl acetate, we studied its stability in hexanes as well (entry 11). It is gratifying to observe that it displayed comparable stability in this solvent as in ethyl acetate (entries 9 vs 11).


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Table 3. Stability evaluation of the selected Ru catalysts. Cycle / Conv. (%) [b] Entry[a]

Catalyst

1

2

3

4

5

1

C1

95.1

82.6

58.4

41.1

30.3

2

C2

92.9

76.2

53.9

40.1

31.7

3

C3

92.4

89.7

76.5

62.8

50.0

4

C4

96.4

92.8

84.7

69.1

52.6

5

C5

82.3

87.1

82.5

84.1

63.7

6

C6

90.1

87.5

80.8

70.5

58.7

7

C7

51.4

78.2

73.9

65.0

55.8

8

C8

91.6

87.0

85.7

80.5

74.0

9

C9

93.2

88.6

84.3

80.8

72.1

10

C10

52.4

16.3

10.8

7.6

5.9

11[c][d]

C9

95.5

88.9

82.1

70.6

61.5

[a] Homoallyloleamide (2, 0.033 mmol) in EtOAc (1 mL) was added every 15 min to a solution containing 1 mol% (initial conc.) of the catalyst in EtOAc (15 mL) at 60 °C. A portion (1 mL) of the solution was taken for GC analyses at 15 min intervals prior to addition of the next portion (1 mL) of substrate solution. [b] GC area %. [c] Hexanes were used as solvent. [d] Because of the low solubility of oligomers in hexanes, precipitated oligomers were not taken in GC samples, which were prepared from the supernatant; therefore the actual % conversions could be higher than the observed % conversions.

Structure-activity relationship Hoveyda-Grubbs II catalyst (C1) has been the most-widely used catalyst to-date for ring-closing metathesis (RCM) macrocyclizations.46 It has been established that this mechanism leads to improved catalyst initiation and stability in both CM and RCM reactions.47 More recently, Umicore M7 catalysts (in which the mesityl groups in the NHC ligand of C1 are replaced by 2,6diisopropylphenyl groups and/or H and electronegative substituents are attached to


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isopropoxystyrene ligand) (C6–C9) have been reported12 to be more effective than C1 for certain RCM macrocyclizations. It is expected that the higher steric hindrance of the NHC groups in M7 catalysts contribute to improved stability, while the higher electronegativity of the protective ligand contributes to faster catalyst initiation over C1.

Our results showed that several

structurally distinct catalysts (C3, C4, C5, and C9) out-performed the most popular HoveydaGrubbs II catalyst (C1) for oleamide to ene-lactam RCM transformation. Among them, C9 does belong to M7 class of catalysts, while C5 is structurally analogous to M7 class catalyst. An important structural feature that distinguishes the two other viable catalysts C3 and C4, which do not belong to M7 class, is the steric hindrance of its NHC ligand. Our observation that C3 is less stable than C4 (Table 3: entries 3 and 4) is consistent with this feature. Indeed, C3 has originally been developed for metathesis reactions involving hindered-olefins to provide easier access of the active site to the substrate.39-40 In our specific system, which involves non-hindered olefin (oleamide), the substrate is able to access the catalytic active site even with bulky NHC ligands, while at the same time such bulky NHC ligands impart better stability to the catalyst. Overall, our results confirmed the difficulty of predicting catalyst performance a priori. Although the literature provides useful guidance for selecting subset of catalysts through qualitative reasoning, the ultimate catalyst selection requires experimental evaluation with respect to the specific substrate.

Immobilization of the Catalyst With the best-performing catalyst identified, we have investigated the possibility of immobilizing catalyst C9 in porous supports and conducting the RCM reaction in heterogeneous catalytic mode amenable to continuous operation. Mesoporous silica molecular sieves such as


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MCM-41, SBA-15 and SBA-1 are well-characterized and highly stable supports ideally suited for catalyst immobilization. Their structure involves cage-like nano-cavities connected via nanopores, and the relative size difference between the nono-cavities and the 3-D structure of the catalyst determines the number of catalyst molecules that could fit in a cavity. As dimerization of ruthenium clusters is known to lead to loss of catalyst activity, molecular sieves that could accommodate only a single catalyst molecule per cavity were expected to provide more stable catalysts; this was demonstrated for C1 catalyst immobilized in SBA-1 molecular sieve with a few specific RCM and CM reactions.48 Based on these reports, catalyst C9 was adsorbed on two different types of mesoporous silica gels SBA-15 and MCM-41 using previously established procedures,14, 48 as described in the Experimental section. Although the immobilized catalysts were as active as the homogenous catalysts, as the immobilization mechanism involved bonding of the catalyst to silica surface only through physical forces, the catalyst would desorb from silica in polar reaction media. Accordingly, we conducted the metathesis reaction in the nonpolar hexanes, as catalyst C9 was effective in both ethyl acetate and hexanes. The reactions were performed in a batch mode, as outlined in the Experimental section, using C9 immobilized on SBA-15 as well as MCM-41 (Table 4). We did not observe any drop in activity with MCM41 supported catalyst for three cycles, while there is some drop in activity during the third cycle with the SBA-15 supported catalyst. This observation may be due to the average size of the nono-pores in MCM-41 (2.6 nm), which is known to be smaller than those in SBA-15 (6.2 nm). Thus, it is likely that the MCM-41 does not permit more than one C9 catalyst molecule per cavity on the average, while with SBA-15 multiple molecules can occupy a single cavity leading to catalyst deactivation via dimerization.48


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An issue specific to hexanes as reaction medium, as already noted, is the precipitation of substrate oligomerization products from the reaction medium. While this phenomenon simplifies the product isolation, the precipitated products could be passed on to the next reaction cycle along with the catalyst particles. Use of other non-polar solvents in which oligomers stay soluble could alleviate or eliminate this issue. In addition, catalyst leaching and decomposition could also contribute to loss of activity. On the other hand, issues with catalyst leaching from mesoporous silica by polar solvents can be circumvented by use of physical immobilization of catalyst, a more recently developed molecular sieves encapsulation method.49 Thus, while we were able to establish the feasibility of catalyst immobilization at a proof-ofconcept level, testing of other novel immobilization methods that may provide better stability to the catalyst48-50 and would also allow the use of other polar and nonpolar solvents as reaction media 9 should be investigated for future development of nylon precursor synthesis using RCM.

Table 4. Demonstration of recovery and reuse of C9 catalyst immobilized on silica gel [a] Cycle / Conv. (%) [b] Entry

Support

1

2

3

1

SBA-15

98

96

56

2

MCM-41

>99

92

93

[a] Reaction conditions: Homoallyloleamide (2, 0.033 mmol) in Hexanes (16 mL) was added to a three-neck flask containing the heterogeneous (solid) catalyst and the reaction was run at 60 °C. [b] GC area %

Conclusions


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In conclusion, improved reaction conditions for our recently developed process for the production of PA12 precursor were established. Through the screening of commercially available metathesis catalysts under various reaction conditions, we identified a catalyst with high activity, selectivity and stability for our substrate. The screening information may be useful for optimizing other ring-closing metathesis reactions generating macrocycles. Our new process features (a) lower reaction temperature (60 °C vs 120 °C) and (b) use of environmentally benign solvent (ethyl acetate/hexanes vs chlorobenzene) at (c) relatively low catalyst loading (1 mol%). The identified catalyst was adsorbed on mesoporous silica gel to aid its recovery and reuse. Its recycling was demonstrated up to three cycles using hexanes without loss of catalytic activity. Although the reaction conditions established in this study require further refinement before the process can be scaled economically, we believe they represent an important step forward toward environmentally sustainable production of polyamide precursors from oleic acid. ACKNOWLEDGMENT. We thank National Science Foundation (CHE#1230609) for funding this research. REFERENCES (1) Yelchuri, V.; Rachapudi, B. N. P.; Mallampalli, S. L. K., Synthesis of industrially important platform chemicals via olefin metathesis of palash fatty acid methyl esters. Eur. J. Chem. 2014, 5 (3), 532-535. (2) Murzin, D. Y.; Mäki-Arvela, P.; Aranda, D. A., Processing microalgae: beyond lipids. Biofuels 2014, 5 (1), 29-32. (3) Chikkali, S.; Mecking, S., Refining of Plant Oils to Chemicals by Olefin Metathesis. Angew. Chem. Int. Ed. 2012, 51 (24), 5802-5808. (4) Winkler, M.; Meier, M., Olefin cross-metathesis as a valuable tool for the preparation of renewable polyesters and polyamides from unsaturated fatty acid esters and carbamates. Green Chem. 2014, 16, 3335-3340. (5) Yapa Mudiyanselage, A.; Viamajala, S.; Varanasi, S.; Yamamoto, K., Simple RingClosing Metathesis Approach for Synthesis of PA11, 12, and 13 Precursors from Oleic Acid. ACS Sustainable Chem. Eng. 2014, 2 (12), 2831-2836.


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(6) Ameh Abel, G.; Oliver Nguyen, K.; Viamajala, S.; Varanasi, S.; Yamamoto, K., Crossmetathesis approach to produce precursors of nylon 12 and nylon 13 from microalgae. RSC Adv. 2014, 4 (98), 55622-55628. (7) Chlorinated or aromatic solvents posed regulatory issues due to safety and environmental concerns. (8) Kotha, S.; Dipak, M. K., Strategies and tactics in olefin metathesis. Tetrahedron 2012, 68 (2), 397-421. (9) Buchmeiser, M. R., In Olefin Metathesis, Grela, K., Ed. John Wiley & Sons. : 2014; pp 495-514. (10) Skowerski, K.; Białecki, J.; Tracz, A.; Olszewski, T. K., An attempt to provide an environmentally friendly solvent selection guide for olefin metathesis. Green Chem. 2014, 16 (3), 1125-1130. (11) Naresh, P.; Sirisha, M.; Vijay, K.; Srinath, N., ChemInform Abstract: Recent Advances of Olefin Metathesis and It′s Applications in Organic Synthesis. Int. J. Pharm. Technol. 2012, 43 (26), 1748-1767. (12) Caijo, F.; Tripoteau, F.; Bellec, A.; Crévisy, C.; Baslé, O.; Mauduit, M.; Briel, O., Screening of a selection of commercially available homogeneous Ru-catalysts in valuable olefin metathesis transformations. Cat. Sci. Technol. 2013, 3 (2), 429-435. (13) Buchmeiser, M. R., In Olefin Metathesis: Theory and Practice. Grela, K.; Editor, John Wiley & Sons. : 2014; pp 495-514. (14) Van Berlo, B.; Houthoofd, K.; Sels, B. F.; Jacobs, P. A., Silica Immobilized Second Generation Hoveyda-Grubbs: A Convenient, Recyclable and Storageable Heterogeneous Solid Catalyst. Angew. Chem. Int. Ed. 2008, 350 (13), 1949-1953. (15) Monge-Marcet, A.; Pleixats, R.; Cattoën, X.; Man, M. W. C., Catalytic applications of recyclable silica immobilized NHC-ruthenium complexes. Tetrahedron 2013, 69 (1), 341-348. (16) Monge-Marcet, A.; Pleixats, R.; Cattoën, X.; Man, M. W. C., Sol–gel immobilized Hoveyda–Grubbs complex through the NHC ligand: A recyclable metathesis catalyst. J. Mol. Catal. A: Chem. 2012, 357, 59-66. (17) Zhang, H.; Li, Y.; Shao, S.; Wu, H.; Wu, P., Grubbs-type catalysts immobilized on SBA15: A novel heterogeneous catalyst for olefin metathesis. J. Mol. Catal. A: Chem. 2013, 372, 35-43. (18) Yao, Q., A Soluble Polymer ‐ Bound Ruthenium Carbene Complex: A Robust and Reusable Catalyst for Ring‐Closing Olefin Metathesis. Angew. Chem. Int. Ed. 2000, 39 (21), 3896-3898. (19) Bieniek, M.; Michrowska, A.; Usanov, D. L.; Grela, K., In an attempt to provide a user's guide to the galaxy of benzylidene, alkoxybenzylidene, and indenylidene ruthenium olefin metathesis catalysts. Chem.-Eur. J. 2008, 14 (3), 806-818. (20) Nelson, D. J.; Manzini, S.; Urbina-Blanco, C. A.; Nolan, S. P., Key processes in ruthenium-catalysed olefin metathesis. Chem. Commun. 2014, 50, 10355-10375. (21) For a mechanistic study that showed a higher reactivity of the two catalysts: Bates, J. M.; Lummiss, J. A. M.; Bailey, G. A.; Fogg, D. E., Operation of the Boomerang Mechanism in Olefin Metathesis Reactions Promoted by the Second-Generation Hoveyda Catalyst. ACS Catalysis 2014, 4 (7), 2387-2394. (22) Bates, J. M.; Lummiss, J. A. M.; Bailey, G. A.; Fogg, D. E., Operation of the Boomerang Mechanism in Olefin Metathesis Reactions Promoted by the Second-Generation Hoveyda Catalyst. ACS Catalysis 2014, 4 (7), 2387-2394.


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(23) Bieniek, M.; Bujok, R.; Cabaj, M.; Lugan, N.; Lavigne, G.; Arlt, D.; Grela, K., Advanced Fine-Tuning of Grubbs/Hoveyda Olefin Metathesis Catalysts: A Further Step toward an Optimum Balance between Antinomic Properties. J. Am. Chem. Soc. 2006, 128 (42), 1365213653. (24) Borré, E.; Caijo, F.; Rix, D.; Crevisy, C.; Mauduit, M., Recent advances for controlling the activity and the recoverability of Hoveyda-Grubbs type olefin metathesis catalysts. Chimica oggi 2008, 26 (5), 89-92. (25) Olszewski, T.; Bieniek, M.; Skowerski, K.; Grela, K., A New Tool in the Toolbox: Electron-Withdrawing Group Activated Ruthenium Catalysts for Olefin Metathesis. Synlett 2013, 24 (08), 903-919. (26) Love, J. A.; Morgan, J. P.; Trnka, T. M.; Grubbs, R. H., A Practical and Highly Active Ruthenium‐Based Catalyst that Effects the Cross Metathesis of Acrylonitrile. Angew. Chem. Int. Ed. 2002, 41 (21), 4035-4037. (27) Clavier, H.; Caijo, F.; Borré, E.; Rix, D.; Boeda, F.; Nolan, S. P.; Mauduit, M., Towards Long-Living Metathesis Catalysts by Tuning the N-Heterocyclic Carbene (NHC) Ligand on Trifluoroacetamide-Activated Boomerang Ru Complexes. Liesbig Chem Annalen 2009, 2009 (25), 4254-4265. (28) Lummiss, J. A. M.; Higman, C. S.; Fyson, D. L.; McDonald, R.; Fogg, D. E., The divergent effects of strong NHC donation in catalysis. Chem. Sci. 2015, 6, 6739-6746. (29) Doppiu, A.; Caijo, F.; Tripoteau, F.; Bompard, S.; Crévisy, C.; Mauduit, M., Synthesis Optimization and Catalytic Activity Screening of Industrially Relevant Ruthenium-Based Metathesis Catalysts. Top. Catal. 2014, 57 (17-20), 1351-1358. (30) Garber, S. B.; Kingsbury, J. S.; Gray, B. L.; Hoveyda, A. H., Efficient and Recyclable Monomeric and Dendritic Ru-Based Metathesis Catalysts. J. Am. Chem. Soc. 2000, 122 (34), 8168-8179. (31) Grela, K.; Harutyunyan, S.; Michrowska, A., A Highly Efficient Ruthenium Catalyst for Metathesis Reactions. Angew. Chem. Imt. Ed. 2002, 41 (21), 4038-4040. (32) Michrowska, A.; Bujok, R.; Harutyunyan, S.; Sashuk, V.; Dolgonos, G.; Grela, K., NitroSubstituted Hoveyda−Grubbs Ruthenium Carbenes: Enhancement of Catalyst Activity through Electronic Activation. J. Am. Chem. Soc. 2004, 126 (30), 9318-9325. (33) Skowerski, K.; Kasprzycki, P.; Bieniek, M.; Olszewski, T. K., Efficient, durable and reusable olefin metathesis catalysts with high affinity to silica gel. Tetrahedron 2013, 69 (35), 7408-7415. (34) Affinity to silica gel that aids recovery. (35) Mauduit, M.; Clavier, H.; Laurent, I. US008586757B2, February 13, 2013. (36) Rix, D.; Caijo, F.; Laurent, I.; Boeda, F.; Clavier, H.; Nolan, S. P.; Mauduit, M., Aminocarbonyl Group Containing Hoveyda−Grubbs-Type Complexes: Synthesis and Activity in Olefin Metathesis Transformations. J. Org. Chem. 2008, 73 (11), 4225-4228. (37) Keitz, B. K.; Endo, K.; Patel, P. R.; Herbert, M. B.; Grubbs, R. H., Improved Ruthenium Catalysts for Z-Selective Olefin Metathesis. J. Am. Chem. Soc. 2012, 134 (1), 693-699. (38) Z-olefin selective. (39) Stewart, I. C.; Ung, T.; Pletnev, A. A.; Berlin, J. M.; Grubbs, R. H.; Schrodi, Y., Highly Efficient Ruthenium Catalysts for the Formation of Tetrasubstituted Olefins via Ring-Closing Metathesis. Org. Lett. 2007, 9 (8), 1589-1592. (40) Stewart, I. C.; Douglas, C. J.; Grubbs, R. H., Increased Efficiency in Cross-Metathesis Reactions of Sterically Hindered Olefins. Org. Lett. 2008, 10 (3), 441-444.


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(41) Ung, T.; Hejl, A.; Grubbs, R. H.; Schrodi, Y., Latent Ruthenium Olefin Metathesis Catalysts That Contain an N-Heterocyclic Carbene Ligand. Organometallics 2004, 23 (23), 5399-5401. (42) We observed many side-products in chlorobenzene, which may be due to solvent impurities. Such a case in toluene has been reported: Nicola, T.; Brenner, M.; Donsbach, K.; Kreye, P., Org. Proc. Res. Dev. 2005, 9 (4), 513-515. (43) The isolated oligomers appreared to be a mixture of cyclic dimers (head-to-head/head-totail, as well as E/Z isomers) from MS and NMR analyses. (44) A similar trend has been reported: Yamamoto, K.; Biswas, K.; Gaul, C.; Danishefsky, S. J., Effects of temperature and concentration in some ring closing metathesis reactions. Tetrahedron Lett. 2003, 44 (16), 3297-3299. (45) In this study, the sampling loss of the catalyst is 6.25% per cycle, which accounts for ~23% loss at 5th cycle. However, we think the major culprit for % conversion decrease is catalyst degrading, based on the control experiments: (1) Comparable % conversion at 0.5 and 1 mol% catalyst loading under the otherwise identical reaction conditions; (2) No product inhibition was seen when isolated products (including oligomers and impurities) were spiked in. (46) van Lierop, B. J.; Lummiss, J. A. M.; Fogg, D. E., In Olefin Metathesis: Theory and Practice, John Wiley & Sons, Inc.: Hoboken, NJ, USA, 2014; pp 85-152. (47) For a mechanistic study that showed a higher reactivity of the two catalysts: Bates, J. M.; Lummiss, J. A. M.; Bailey, G. A.; Fogg, D. E., Operation of the Boomerang Mechanism in Olefin Metathesis Reactions Promoted by the Second-Generation Hoveyda Catalyst. ACS Catalysis 2014, 4 (7), 2387-2394. (48) Yang, H.; Ma, Z.; Wang, Y.; Wang, Y.; Fang, L., Hoveyda–Grubbs catalyst confined in the nanocages of SBA-1: enhanced recyclability for olefin metathesis. Chem. Commun. 2010, 46 (45), 8659-8653. (49) Li, Q.; Zhou, T.; Yang, H., Encapsulation of Hoveyda–Grubbs 2ndCatalyst within Yolk– Shell Structured Silica for Olefin Metathesis. ACS Catalysis 2015, 5 (4), 2225-2231. (50) Yang, H.; Ma, Z.; Zhou, T.; Zhang, W.; Chao, J.; Qin, Y., Encapsulation of an Olefin Metathesis Catalyst in the Nanocages of SBA-1: Facile Preparation, High Encapsulation Efficiency, and High Activity. ChemCatChem 2013, 5 (8), 2278-2287.


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For Table of Contents Use Only

Toward Sustainable Synthesis of PA12 (Nylon 12) Precursor from Oleic Acid Using RingClosing Metathesis Godwin Ameh Abel, Sridhar Viamajala, Sasidhar Varanasi and Kana Yamamoto* Synopsis The

environmentally

benign

reaction

conditions

for

ring-closing

metathesis

of

homoallyloleamide for the synthesis of PA12 (Nylon 12) precursor are reported.


25

Toward Sustainable Synthesis of PA12 (Nylon-12) Precursor from

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