Selective long-range isomerization carbonylation of a complex

Research Article Cite This: ACS Catal. 2018, 8, 9232−9237

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Selective Long-Range Isomerization Carbonylation of a Complex Hyperbranched Polymer Substrate Ye Liu, Kaiwu Dong,† Matthias Beller,† and Stefan Mecking* Chair of Chemical Materials Science, Department of Chemistry, University of Konstanz, 78464 Konstanz, Germany

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S Supporting Information *

ABSTRACT: Hyperbranched oligoethylenes are rewarding and extremely challenging substrates for functionalization. They contain one double bond per molecule which offers itself for long-range isomerization carbonylation; however, these are present in various substitution patterns and high degrees of substitution render them unreactive, and furthermore different branch sites need to be passed in an isomerizing functionalization approach. Studies of Pd(II) catalysts with different bulky-substituted diphosphines on hyperbranched oligomers (Mn 1000−3000 g mol−1) amended by low-molecular-weight model substrates show that with 1,2-bis(4phosphorinone)xylene alkoxycarbonylation occurs with high selectivity at the ends of branches to form primary esters with high conversions. The key to this is the retention of selectivity in long-range isomerization/carbonylation even at elevated temperatures that promote reaction rates. By a tandem cross-metathesis/long-range isomerization functionalization approach, difunctional esters were obtained. As a demonstration of their synthetic utility, these were polycondensed to semicrystalline allaliphatic polyesters in which they serve as soft segments. KEYWORDS: carbonylation, ethylene oligomer, long-range isomerization, hyperbranched, polyethylene functionalization, polyester

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INTRODUCTION Carbonylation of olefins is one of the most important reactions for the introduction of functional groups. Ethylene or 1-olefins are hydroformylated, or hydroxy- or alkoxycarbonylated, to the linear aldehyde or carboxylic acid (ester), respectively, on a large scale.1,2 The aldehydes are usually an intermediate for the corresponding linear carboxylic acids or alcohols. High selectivities for the linear products and high rates can be achieved in these carbonylations. A much more challenging situation is terminal functionalization from internal double bonds. This requires the conversion of less reactive disubstituted or multiple substituted double bonds, and in particular a carbonylation in a different position than the original double bond. An example of substrates of interest are 2-butene and higher internal olefins present in waste streams of crackers.3 A more demanding substrate are unsaturated fatty acids in which the double bond is typically eight carbon atoms away from the desired terminal site of functionalization and which contain a functional group in the substrate. The latter results in hydrogenation as an undesired side reaction in isomerizing hydroformylation,4 and to a lesser extent in tandem isomerizing hydroformylation/reduction.5 However, by isomerizing hydroxy- and alkoxycarbonylation with appropriate catalysts, very high selectivities of 95% to the α,ω-difunctional linear product can be achieved.6−9 This is enabled by rapid reversible insertion of olefins into a Pd−H and of CO into the resulting Pd−alkyl. On this low-barrier landscape the metal can access all positions of the fatty acid substrate chain, and all possible Pd−alkyls and Pd−acyls are present. Alcoholysis or hydrolysis of the metal acyl are rateand selectivity-determining.8−11A metal-catalyzed carbonyla© XXXX American Chemical Society

tion of tri- or tetra-substituted double bonds has long remained elusive, and conversion of such substrates has been restricted to carbocationic Koch chemistry that yields highly branched products. Only recently, the trisubstituted double bond of citronellic acid was converted in a Pd-catalyzed methoxycarbonylation to yield dimethyl-3,7-dimethylnonanedioate.12 Even tetramethyl ethylene could be carbonylated, to yield the primary ester exclusively.13 These findings encourage pushing the limit to complex polymeric substrates that are challenging in containing a multitude of structural branching motifs and in which isomerization can occur over long ranges to the final site of functionalization. We now report on carbonylation of hyperbranched low-molecular-weight polyethylenes and their conversion to reactive difunctional building blocks, as probed by polycondensation to thermoplastic elastomeric polyesters.

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RESULTS AND DISCUSSION Beyond the well-known applications of polyethylenes as thermoplastic materials, low-molecular-weight (Mn) ethylene oligomers can also serve numerous functions. Branched, noncrystallizing amorphous oligomers can serve, for example, as viscosity modifiers or interface active agents. For the latter example and other applications, functionalization is desirable. To this end, hyperbranched oligoethylenes which contain one double bond per molecule as a result of chain transfer by ß-H elimination are a readily accessible starting material. Beyond Received: August 4, 2018 Revised: August 11, 2018 Published: August 14, 2018 9232

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Figure 1. Diphosphine ligands used and functionalization scheme pursued.

this practical interest, they are also an instructive probe to stake out and extend the scope of catalytic carbonylation. Because of extensive chain-walking processes that occur during polymerization, they contain numerous types of highly substituted double bonds, and of reactive sites where (remote) functionalization can occur in the form of linear units, branch points, and particularly chain ends.14 As the established diphosphine for selective isomerization of Pd(II)-catalyzed alkoxycarbonylation of linear internal alkenes including unsaturated fatty acids, bis(di-tert-butyl)phosphinoo-xylene (dtbpx) was studied. Furthermore, 1,2-bis((tertbutyl(pyridin-2-yl)phosphanyl)methyl)benzene (pytbpx) has proven to be exceptionally active and capable of reacting highly substituted, usually problematic, olefinic substrates, though the selectivity is generally somewhat lower than that for dtpbx.13 Also, the less prominent 1,2-bis(4-phosphorinone)xylene (bpx) was included. The reactivity and substrate scope of this diphosphine has been less studied, but it has proven useful in the alkoxycarbonylation of pentenoate isomers (Figure 1).15 The hyperbranched oligomer substrate contains internal 1,2disubstituted double bonds for the largest part (ca. 80%), in addition to trisubstituted double bonds (1-alkyl-1-methyl-2alkyl), terminal double bonds, and trace amounts of vinylidene and trisubstituted double bonds with an adjacent branch (1alkyl-1-methyl-2-iso-alkyl). As an additional challenge, the concentration of double bonds is comparatively low (about 0.8 mmol double bond in 1 g of substrate, compared to 11.9 mmol for 1-hexene and 3.3 mmol for methyl oleate) and it is insoluble in methanol, the most common and most reactive solvent and substrate for alkoxycarbonylation. Thus, in preliminary studies using a low-molecular-weight oligomer of about 850 g mol−1 and employing a solvent mixture (pentane and ethanol, 1:1), the highest conversion was limited to only 50% with a dtbpx-coordinated Pd(II) catalyst.14 As models for the oligomers’ various double-bond motifs, the methoxycarbonylation of 1-heptene, 4-methylhex-2-ene, and 2,5-dimethylhex-2-ene was mapped out with the different catalysts (Figure 2 and Supporting Information). With all three diphosphines, the linear ester is the major product. This is generally desirable because of the high reactivity of primary esters compared to secondary branched esters. However, while dtbpx-coordinated catalysts are very

Figure 2. Functionalization site selectivities (%) and conversion to esters (%, in brackets) with dtbpx (blue), bpx (red), and pytbpx (magenta) in methoxycarbonylation (for methoxycarbonylation of 1heptene, 90 °C for 12 h, method A; 4-methylhex-2-ene and 2,5dimethylhex-2-ene, 110 °C for 18 h, method D; for more details see Supporting Information). For sites equivalent in terms of the product formed, only one site is marked.

selective for the linear product, with pytbpx also significant amounts of branched esters are formed, as expected. On the other hand, pytbpx-based catalysts are significantly more active, and this difference is more pronounced with increasing degree of substitution. While the trisubstituted olefin 2,5-dimethylhex2-ene is fully converted with pytbpx, conversion was low with dtbpx and ether from methoxy addition is formed as a side product. This is very relevant for the functionalization of oligomers, as not only small amounts of trisubstituted olefins are present in the starting material, but chain walking during the carbonylation will form such olefin intermediates whenever the catalyst walks to a branch. Indeed, exposing the oligomers to catalyst precursors under conditions similar to those encountered in carbonylation but in the absence of CO resulted in rapid isomerization with dtbpx already at room temperature (Supporting Information, Figures S44 and S45). Isomerization was considerably slower with bpx and pytbpx and required ca. 10 h at 50 °C; however, at the temperatures of carbonylations (≥90 °C) significant isomerization to trisubstituted olefins can also be expected. Remarkably, the carbonylation studies with the model smallmolecule olefins reveal that bpx combines both a high linear selectivity and reactivity, even for highly substituted double 9233

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ACS Catalysis Table 1. Ethoxycarbonylation of Polyethylene Oligomer for Monofunctionalized Ester Synthesisa entry

catalyst (molar ratio)

oligomer typeb

temperature (°C)

time (h)

con (%)c

sel. (%)d

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

[(dtbpx)Pd(OTf)2]:oligomer (1:25) [(dtbpx)Pd(OTf)2]:(dtbpxH2)(OTf)2:oligomer (1:3:25) Pd(OAc)2:dtbpx:MSA:oligomer (1:2:10:25) Pd(OAc)2:bpx:MSA:oligomer (1:2:10:25) Pd(OAc)2:bpx:MSA:oligomer (1:2:10:25) Pd(OAc)2:bpx:MSA:oligomer (1:2:10:25) Pd(OAc)2:bpx:MSA:oligomer (1:2:10:25) Pd(OAc)2:bpx:MSA:oligomer (1:2:10:25) Pd(OAc)2:bpx:MSA:oligomer (1:2:10:25) Pd(OAc)2:bpx:MSA:oligomer (1:5:20:25) Pd(OAc)2:bpx:MSA:oligomer (1:5:20:25) Pd(OAc)2:bpx:MSA:oligomer (1:5:20:25) Pd(OAc)2:pytbpx:MSA:oligomer (1:2:10:25) Pd(OAc)2:pytbpx:MSA:oligomer (1:2:10:25) Pd(OAc)2:pytbpx:MSA:oligomer (1:2:10:50) Pd(OAc)2:pytbpx:MSA:oligomer (1:2:10:100) Pd(OAc)2:pytbpx:MSA:oligomer (1:2:10:250)

A A A A B C A A A A B C A A A A A

90 90 90 90 90 90 90 120 120 120 120 120 90 90 120 120 120

96 192 96 96 96 96 192 48 96 96 96 96 48 96 24 24 96

36 40 45 65 53 24 69 57 78 92 90 45 92 98 88 87 92

99 97 98 99 98 97 98 95 95 95 96 95 42 38 50 49 49

a Reaction conditions:1 g of oligomer, 10 mL of pentane/EtOH (1/1), 20−25 bar CO pressure. bOligomer with different branch parameters and molecular weights. A: Mn = 1000 and 74 branches per 1000 carbon atoms (68% methyl branches and 10% sec-butyl branches. For complete branch pattern, see Supporting Information). B: Mn = 2100 and 76 branches per 1000 carbon atoms (80% methyl branches and 4% sec-butyl branches). C: Mn = 3000 and 66 branches per 1000 carbon atoms (91% methyl branches and 1% sec-butyl branches). Further information about the oligomers is in the Supporting Information. cTo the ester, and the isomerized olefins are seen as unreacted materials. dThe primary ester selectivity, calculated by integral 13C NMR, and no ether structure was observed in all examples.

bonds (Figure 2). This is facilitated by good stability at high reaction temperatures (>100 °C). The functionalization of highly branched oligomers was probed, employing substrates with varied molecular weights, degrees of branching, and branch patterns (Tables S1 and S2). With dtbpx, functionalization occurred with high selectivity to the primary ester (Table 1, entries 1−3). Conversion was limited to 36−45%. This was also not overcome by employing [(dtbpx)Pd(OTf)2] as a defined diphosphine-coordinated metal precursor rather than the in situ catalyst system based on Pd(OAc)2. Frequent observation of Pd-black indicated a limited stability especially at very long reaction times at the elevated reaction temperature of 90 °C. Advantageously, under identical conditions bpx resulted in a higher conversion of 65% with the high primary ester selectivity maintained (Table 1, entry 4). Considering the impact of substrate microstructure, oligomer A is a viscous oil with a number-average molecular weight of Mn = 1000 g mol−1, and 74 branches per 1000 carbon atoms (68% methyl branches and 10% sec-butyl branches) compared to solid semicrystalline C with Mn = 3000 g mol−1, dominated by 91% methyl branches and with only 1% sec-butyl branch content (total 66 branches/1000 C; cf. Table S1 for complete data). The above studies of model small-molecule olefins suggest that branches will rather impede the overall functionalization rate as the trisubstituted olefin intermediate formed upon chain walking is comparatively stable and less reactive and from sec-butyl branches even tetrasubstituted olefin may be formed. This is, however, not reflected by the reactivity of the different oligomers (Table 1, entries 4−6). Rather, the oligomer solubility in the reaction media appears more crucial. Optimization of the alkoxycarbonylation conditions revealed that an additional increase of reaction time was not very effective, but the conversion can be improved to 78% by an increased reaction temperature of 120 °C (Table 1, entries 7−

9). An enhanced diphosphine ligand and methanesulfonic acid (MSA) loading proved beneficial, providing a conversion of up to 92% without a significant sacrifice of primary ester selectivity (Figures S19 and S20), and equally important, a higher acid loading did not lead to undesired ether formation in all cases (Table 1, entry 10). Higher acid and bpx ligand loadings also gave a conversion of 90% for oligomer B with 76 branches per 1000 carbon atoms (80% methyl branches and 4% sec-butyl branches). The alkoxycarbonylation of the solid oligomer C was less effective, likely because of its lower solubility (entries 11 and 12). The microstructure of the ester monofunctionalized oligomer products was further elaborated. The dtbpx- or bpx-catalyzed ethoxycarbonylation at 90 °C is highly selective, and only the primary ester is formed exclusively, as confirmed by 2D NMR spectroscopy, and heteronuclear single quantum coherence spectroscopy (HSQCed, Figures S23 and S24). We did not observe other types of esters in the final product. However, when the ethoxycarbonylation was performed at 120 °C, a new doublet resonance at 2.15 ppm (J = 6.8 Hz) was found. From HMBC and correlation spectroscopy (COSY), this resonance is correlated with 34.40 ppm (carbon) and 1.80 ppm (proton) signals; the latter is also identified as a CH group by HSQCed (Figures S25−S27). That is, the ethoxycarbonylation can take place at a methyl branch, giving β-alkyl primary esters, but this amounted to only about 4%. Moreover, the HSQCed suggests the formation of a secondary ester structure, which amounts to only ca. 3−4% according to the 13C NMR integral (cf. Supporting Information; the products from ethoxycarbonylation with pytbpx, vide infra, were used for identification of the minor products). With pytbpx, oligomers could be functionalized quantitatively already at 90 °C (Table 1, entries 13 and 14). At 120 °C, high conversions could be maintained also at lower catalysts loadings of 0.4 mol % (entry 17). Analysis of the products by 2D NMR spectroscopy showed that two types of structures, 9234

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Figure 3. Cross metathesis of ethylene oligomer and further carbonylation for difunctional products (only terminal functionalization shown, secondary ester products, which account for ca. 5% of the ester groups not shown).

primary and secondary esters, were formed (Figures S28 and S29). From the integrals in 13C NMR spectra, the content of different types of esters can be determined from the carbonyl (CO) or methylene group (−OCH2−) resonances (Figures S33 and S34). The main primary ester was identical to the product obtained with dtbpx- or bpx-coordinated catalysts. Further, we observed two minor sets of peaks, one of which can be identified as the α-methyl ester by COSY, HMBC, and HSQCed spectroscopy (Figures S30−S33). The other was assigned to a β-alkyl primary ester, which was also observed as a minor product with bpx at 120 °C. Notably, the functionalized oligomers can be separated from catalyst residues straightforwardly by flash chromatography. Beyond these monofunctionalizations, an introduction of two ester groups of the oligomers would be of interest, for example, to enable their utilization as highly branched (macro)monomers for polycondensation. It has been shown that olefin cross-metathesis of the unsaturated oligomers with acrylate can afford the corresponding α,ß-unsaturated esters (cf. Figure 3).14,16 Note that this occurs without a significant loss in molecular weight. Likely, the double bond in the oligomers as obtained from ethylene oligomerization is preferably relatively close to a chain end.14 In this functionalization scheme a double bond is retained that offers itself for carbonylation to a second ester group, albeit α,ßunsaturated esters (which are also substituted in the ßposition) are rather demanding substrates here. Cross metathesis of a branched oligomer (sample type A) was performed with 10 equiv of ethyl acrylate in the presence of 0.1 mol % of Hoveyda-Grubbs (II) catalyst at 80 °C, resulting in quantitative conversion to the α,β-unsaturated ester (Figures S46 and S47). This was further subjected to isomerizing alkoxycarbonylation. In comparison to the ethoxycarbonylation of the nonfunctionalized oligomer (vide supra), the reactivity is considerably lower; only 48% conversion was obtained after 192 h with a bpx-based catalyst at 90 °C (Table 2, entry 1). Notably, the low reactivity did not compromise the primary ester selectivity. With optimized diphosphine and acid loading, improved conversion of 74% could be achieved, retaining a high primary ester selectivity up to 95% (Table 2, entries 2 and 3, and Figures S35−S39). No significantly improved ethoxycarbonylation yields could be achieved from screening of other solvents, such as heptane, toluene, and MeOH mixtures (Table 2, entries 4−6, and Figures S40 and S41). However, with the highly reactive pytbpx-based catalyst even this less reactive functionalized oligomer could be converted completely to the diester, while the microstructure is complex based on 13C NMR analysis (Table 2, entry 8, and Figures S42 and S43). To achieve both high linear selectivities and conversions in this demanding functionalization, a tandem approach was employed. Subsequent to a Pd(II)-bpx catalyzed alkoxycarbonylation of the α,β-unsaturated functionalized oligomer

Table 2. Further Alkoxycarbonylation of Unsaturated EsterFunctionalized Oligomer to Diestersa entry 1 2 3 4c 5d 6e 7 8

catalyst (molar ratio) Pd(OAc)2:bpx:MSA:oligomer (1:2:10:25) Pd(OAc)2:bpx:MSA:oligomer (1:5:20:25) Pd(OAc)2:bpx:MSA:oligomer (1:10:20:25) Pd(OAc)2:bpx:MSA:oligomer (1:5:20:25) Pd(OAc)2:bpx:MSA:oligomer (1:5:20:25) Pd(OAc)2:bpx:MSA:oligomer (1:5:20:25) Pd(OAc)2:pytbpx:MSA:oligomer (1:2:10:25) Pd(OAc)2:pytbpx:MSA:oligomer (1:2:10:50)

temperature (°C)

time (h)

con. (%)b

90

192

48

120

96

74

120

192

74

120

96

76

120

96

45

120

96

78

90

48

92

120

72

99

a Reaction conditions: 1 g of α,β-unsaturated ester functionalized oligomer (obtained after metathesis with ethyl acrylate using oligomer A: Mn = 1000, and 74 branches per 1000 carbon atoms (68% methyl branches and 10% sec-butyl branches; for complete branch pattern see Supporting Information), 10 mL of pentane/EtOH (1/1), 20−25 bar CO pressure. bTo the ester, and the isomerized olefin is seen as unreacted material. cReaction was performed in pentane/MeOH (1/ 1). dReaction was performed in heptane/MeOH (1/1). eReaction was performed in toluene/MeOH (1/1).

(according to Table 2, entry 2), in the pressure reactor an ethanol solution of the most reactive pytbpx-based catalyst and Pd(OAc)2 (Pd(OAc)2:pytbpx:oligomer = 1:2:25, molar ratio, 1 mL of ethanol) was injected into the reaction mixture and further heated to 120 °C for 24 h to carbonylate remaining double bonds. This afforded difunctionalized product in quantitative yield, with a primary ester selectivity up to 90%. As a probe of their utility and purity, the novel difunctional hyperbranched polyethylenes were further converted to diols and polyesters. Reduction of the esters with LiAlH4 resulted in quantitative conversion to the diols. Also, saponification with KOH yielded the dicarboxylic acids cleanly (Figure S48). The above diols were probed as (macro)monomers for all-aliphatic polyester elastomers. In addition to these noncrystallizing soft blocks, crystallizable linear long-chain α,ω-diesters MeOOC− (CH2)n−COOMe of variable length (C19 (n = 17), C26 (n = 24), and C48 (n = 46)) were employed as the second component for A2 + B2 polycondensation (Figure 4). The latter were derived from isomerizing carbonylation,7 olefin metathesis/hydrogenation, or chain doubling,17 respectively, of fatty acids. Titanium alkoxide-catalyzed melt polycondensation afforded the novel polyesters with number-average molecular weights around Mn ≈ 104 g mol−1 as determined by size exclusion chromatography (SEC; Table 3 and Figures S49− S60). This is within the molecular weight regime typical for 9235

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Figure 4. Synthesis of polyesters from difunctionalized ethylene oligomer and various α,ω-diesters.

Table 3. Synthesis of Polyester from Ethylene Oligomersa entry 1 2 3

linear diester

Mn (g/mol)

Mw/ Mnb

(°C)

ΔHm (J/g)

(°C)

ΔHc (J/g)

1,19diester 1,26diester 1,48diester

1.5 × 104

1.6

27

37

17

−38

4

1.1 × 10

1.7

51

51

44

−51

9.0 × 103

1.8

90

81

76

−80

b

Tmc

c

Tcc

accessible by a successive olefin cross metathesis with acrylate and isomerizing carbonylation of the formed unsaturated ester monofunctionalized branched oligoethylene. Because of the α,β-unsaturated ester nature of the latter, carbonylation is particularly challenging but complete conversion to the branched diester with a high selectivity for primary ester groups can be achieved by a tandem approach, taking advantage of the high selectivity of bpx-modified catalysts and the high reactivity of pytbpx-modified catalysts. Further conversions to the branched polyethylene diol and to an allaliphatic segmented polyester illustrate the utility of the products. An attractive feature of this overall approach is that it is based entirely on catalytic, scalable steps including the generation of the starting oligomers. Our findings significantly enhance the synthetic scope of long-range isomerization reaction.19,20

c

a

1 equiv of linear diester, 1 equiv of branched diol oligomer, 0.0005 equiv of [Ti(OnBu)4] in toluene solution with 0.002 mol/L concentration. Purging nitrogen for 1 h at 120 °C, and 140 °C for 2 h, and vacuum at 140 °C for 2 h, 160 °C for 2 h, and 180 °C for 12 h. bDetermined by GPC in THF at 50 °C vs polystyrene standards. c Determined by DSC with a heating/cooling rate of 10 K min−1.

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polycondensates. The linear long-chain and ultra-long-chain diester monomers afford crystallizable blocks as evidenced by the observation of melt transitions. These melt temperatures increase strongly with the length of these segments, with a peak Tm of 90 °C for the polyester derived from hyperbranched macrodiol and linear C48 diester.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.8b03117.

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CONCLUSION Hyperbranched oligoethylenes represent a particularly challenging probe for the synthetic scope of isomerizing functionalizations. They contain multiple types of branch sites that can form difficult to react tri- or even tetrasubstituted olefinic intermediates. Notwithstanding, a complete and highly selective conversion can be achieved. Compared to previously studied low-molecular weight substrates like methyl oleate or tetramethylethylene, not only is the substrate structure more complex here but isomerization occurs over much more extended ranges and a multitude of challenging branch motives need to be overcome in “chain walking”. The key is utilization of a bpx-coordinated Pd(II) catalyst. Its temperature stability allows for ethoxycarbonylation at elevated temperature to achieve sufficient reaction rates, while at the same time selectivity is retained. This is superior to the two benchmark catalysts studieddtbpx being a gold standard for high linear selectivity in internal olefin carbonylation and pytbpx affording very high reactivity even with unreactive multiple substituted double bonds. Possibly, the temperature stability of a bpxcoordinated catalyst is related to a hindering of deactivation via CH activation on the tert-butyl groups.18 Compared to other types of catalytic reactions, this highly selective functionalization at a remote site is unique. This approach affords hyperbranched polyethylene structures precisely functionalized with one primary ester functionality. Difunctional products are

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Details of experimental methods, further NMR characterization of functionalized esters, and polyesters synthesis (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ye Liu: 0000-0003-3143-3629 Matthias Beller: 0000-0001-5709-0965 Stefan Mecking: 0000-0002-6618-6659 Present Address †

Leibniz-Institut für Katalyse e.V. Albert Einstein Strasse 29a, 18059 Rostock, Germany

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Y.L. is grateful to the Alexander von Humboldt Foundation for a postdoctoral research fellowship.

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REFERENCES

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