Introduction

Palladium-catalyzed reactions of propargylic carbonates with nucleophiles attract much attention because of a useful method for the formation of heterocyclic compounds such as dihydrofurans, 1,4-benzodioxins and β-lactams in organic synthesis.1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13 In these reactions, two consecutive nucleophilic attacks occur. A nucleophile attacks at the central carbon of the η3-allenyl/propargyl intermediate generated by reaction of a propargylic carbonate with Pd(0) to yield a π-allylpalladium intermediate, which is successively attacked by a second nucleophile. This suggests that propargylic carbonates can be regarded as monomers for polycondensation with bifunctional nucleophiles. We have recently reported that a Pd(0)-catalyzed polycondensation of methyl propargyl carbonate (1a) with bifunctional oxygen nucleophiles such as bisphenols proceeded successfully to afford polyethers having exomethylene groups (Scheme 1).14, 15 Exomethylene-containing polymers are expected to undergo polymer reactions. The polycondensation was capable of proceeding under stoichiometrically imbalanced conditions because the propagating species always have two hydroxy groups at the polymer termini through two consecutive nucleophilic attacks. The polycondensation between bisphenols and an excess of 1a yielded the corresponding polyethers with high molecular weights.

Although we found the successful Pd(0)-catalyzed polycondensation of 1a with bisphenols, the number-average molecular weights (Mn) of the obtained polyethers were not high (Mn=1500–5000).14, 15 The model reaction of 1a with p-hydroxyacetophenone suggested that the methanol liberated from 1a caused termination reactions.15 Three by-products formed by a reaction between 1a and methanol were isolated in the model reaction. The formation of such by-products implies that methanol is nucleophilic enough to hinder polycondensation. Therefore, we anticipated that propargyl carbonates having a bulky alkyl ester group would yield polyethers with higher Mn as the bulky alcohols liberated are low nucleophilic. Isopropyl and tert-butyl propargyl carbonates (1b and 1c) were prepared and their polymerization behaviors with bisphenols examined under palladium catalysis. In this paper, we report that propargyl carbonates 1b and 1c lead to formation of the corresponding polyethers with high molecular weights, compared with the polycondensation with 1a.

Experimental procedure

Materials

All commercially available chemicals were used as received. Propargyl carbonate 1a was synthesized by the reported method.4 Tetrahydrofuran (THF) and ether were distilled from sodium/benzophenone ketyl under nitrogen before use. Pyridine was distilled over calcium hydride under nitrogen.

Measurement

Both 1H nuclear magnetic resonance (NMR) (300 MHz) and 13C NMR (75.5 MHz) spectra were recorded on Jeol-AL300 (Jeol, Tokyo, Japan) with CDCl3 as the solvent. Chemical shifts are reported in p.p.m. (δ) with reference to Me4Si or CDCl3. Infrared (IR) spectra were recorded on Jasco FT-IR 460 Plus (Jasco, Tokyo, Japan). Gel-permeation chromatographic analyses were performed on a Tosoh HLC-8220 system (Tosoh, Tokyo, Japan) equipped with three columns (TSK gel, Super HM-M, Super HM-N and Super H3000), and THF was used as an eluent at 40 °C The GPC was calibrated with standard polystyrene samples. Elemental analyses were carried out using a Thermo-Finnigan Flash EA1112 CHN-O analyzer (Thermo-Finnigan, Milan, Italy).

Synthesis of Isopropyl Propargyl Carbonate 1b

To a solution of propargyl alcohol (9.7 g, 173 mmol) and dry pyridine (31 ml) in dry ether (150 ml), isopropyl chloroformate (25.6 g, 200 mmol) was added dropwise at 0 °C over 40 min under Ar. After being stirred at room temperature for 3 h, 1 M hydrochloric acid (30 ml) was added to the mixture at 0 °C. The ethereal layer was separated and the aqueous layer extracted with ether (30 ml × 2). The combined ethereal layers were washed with saturated sodium chloride aqueous solution two times and dried over anhydrous magnesium sulfate. After filtration, the ethereal solution was evaporated to dryness and the residue distilled under reduced pressure to obtain 1b (18.42 g, 75%): bp 98–99 °C/77 mm Hg; IR (neat): 3295, 2986, 2943, 1748, 1380, 1260, 1094, 962, 918 cm–1 1H NMR (CDCl3): δ 1.31 (CH3, d, J=6.34 Hz, 6H), 2.52 (CHC, t, J=2.4 Hz, 1H), 4.71 (CH2, d, J=2.4 Hz, 2H), 4.91 (CH(CH3)2, sept, J=6.3 Hz, 1H). 13C NMR (CDCl3): δ 21.72 (CH3), 54.86 (CH2), 72.76 (CH), 75.43 (CHC), 77.15 (CHC), 154.00 (C=O). Anal. calcd for C7H10O3: C, 59.14; H, 7.09. Found: C, 59.37; H, 7.08.

Synthesis of tert-butyl propargyl carbonate 1c

This compound was synthesized according to the method reported by Haight et al.16 To a solution of propargyl alcohol (12.43 g, 222 mmol) in dry THF (200 ml), 2.64 M n-butyl lithium n-hexane solution (84 ml, 222 mmol) was added dropwise at 0 °C over 30 min under Ar. After being stirred for 30 min, di-tert-butyl dicarbonate was added to the THF solution at 0 °C. The mixture was warmed to room temperature and stirred for 24 h. Distilled water (100 ml) was added slowly to the reaction mixture at 0 °C, and the solution was extracted with ether (100 ml × 2). The combined ethereal layers were washed with saturated sodium chloride aqueous solution and dried over anhydrous magnesium sulfate. After filtration, the ethereal solution was evaporated to dryness. The residue was distilled under reduced pressure to obtain 1c (28.95 g, 84%): bp 71–74 °C/18 mm Hg. IR (neat): 3294, 2985, 2941, 1750, 1371, 1280, 1255, 1160, 1097, 957, 856 cm–1. 1H NMR (CDCl3): δ 1.50 (CH3, s, 9H), 2.50 (CHC, t, J=2.4 Hz, 1H), 4.66 (CH2, d, J=2.4 Hz, 2H). 13C NMR (CDCl3): δ 27.65 (CH3), 54.27 (CH2), 75.20 (CHC), 77.32 (CHC), 83.00 (CMe3), 152.69 (C=O). Anal. calcd for C8H12O3: C, 61.52; H, 7.74. Found: C, 61.89; H, 7.78.

Pd(0)-catalyzed reaction of carbonate 1a with p-methoxyphenol: isolation of 6

A large-scale reaction was carried out to isolate by-product 6. To a yellow solution of Pd(PPh3)4 (1.156 g, 1.0 mmol) in THF (55 ml), p-methoxyphenol (4; 4.964 g, 40 mmol) was added. A solution of methyl propargyl carbonate 1a (2.282 g, 20 mmol) in THF (5 ml) was added to the mixture. After being stirred at 60 °C for 24 h under Ar, the reaction mixture was evaporated to dryness. The residue was subjected to flash column chromatography on silica gel (4:1 hexane/ethyl acetate) to yield the desire 1:2 product 515(3.72 g, 61%) and by-product 6 (0.03 g, 1%).

5, 1H NMR (CDCl3): δ 3.74 (CH3, s, 3H), 3.76 (CH3, s, 3H), 4.11 (CH2=C, d, J=2.1 Hz, 1H), 4.47 (CH2=C, d, J=2.1 Hz, 1H), 4.57 (CH2, s, 2H), 6.81–6.87 (ArH, m, 4H), 6.94–7.00 (ArH, m, 4H).

6, IR (neat): 3049, 2996, 1504, 1247, 1212, 1038, 964, 841 cm–1. 1H NMR (CDCl3): δ 1.98 (CH3, s, 3H), 3.79 (OCH3, s, 3H), 3.81 (CH2=C, s, 1H), 4.05 (CH2=C, s, 1H), 6.86 (ArH, d, J=8.7 Hz, 2H), 7.12 (ArH, d, J=8.3 Hz, 2H). 13C NMR (CDCl3): δ 20.21 (CH3), 55.57 (OCH3), 87.58 (CH2=C), 114.52 (Ar), 122.12 (Ar), 148.70, 156.13, 160.65. Anal. calcd for C10H12O2: C, 73.15; H, 7.37. Found: C, 73.48; H, 7.59.

Pd(0)-catalyzed polycondensation of carbonates 1 with bisphenols: a typical procedure

To an orange solution of Pd2(dba)3·CHCl3 (in which dba is dibenzylideneacetone; 0.026 g, 0.025 mmol) and 1,1′-bis(diphenylphosphino)ferrocene (DPPF; 0.056 g, 0.1 mmol) in THF (2 ml) was added 4,4′-dihydroxydiphenyl ether (2a; 0.182 g, 0.9 mmol). To the mixture, a solution of propargyl carbonate 1c (0.156 g, 1.0 mmol) in THF (1 ml) was added. After being stirred at 60 °C for 0.5 h under an Ar atmosphere, the reaction mixture was poured into methanol (100 ml) to precipitate the polymer (run 6 in Table 3). The resulting polymer was filtered off, washed with methanol and dried in vacuo (0.127 g, 59%) yielding polymer 3a: IR (KBr): 3045, 2927, 2865, 1654, 1491, 1198, 830 cm–1. 1H NMR (CDCl3): δ 4.22 (CH2=C, d, J=6.0 Hz, 1H), 4.55 (CH2=C, d, J=6.0 Hz, 1H), 4.62 (CH2, s, 2H), 6.93–7.06 (ArH, m, 8H).

3b, IR (neat): 3060, 2925, 2869, 1654, 1597, 1221, 1161, 927, 851 cm–1. 1H NMR (CDCl3): δ 4.57 (CH2=C, s, 1H), 4.74 (CH2, s, 2H), 4.85 (CH2=C, 1H), 7.04–7.06 (ArH, m, 2H), 7.18–7.20 (ArH, m, 2H), 7.80–7.84 (ArH, m, 4H).

3c, IR (KBr): 3036, 2966, 2934, 2871, 1508, 1223, 1181, 828 cm–1. 1H NMR (CDCl3): δ 1.64 (CH3, br s, 6H), 4.25 (CH2=C, s, 1H), 4.58 (CH2 and CH2=C, s, 3H), 6.85–6.97 (ArH, m, 4H), 7.12–7.19 (ArH, m, 4H).

3d, IR (KBr): 3056, 2938, 2875, 1658, 1609, 1510, 1246, 1172, 968, 827 cm–1. 1H NMR (CDCl3): δ 4.47 (CH2=C, s, 1H), 4.64 (CH2, s, 2H), 4.75 (CH2=C, s, 1H), 6.92–6.96 (ArH, m, 2H), 7.05–7.09 (ArH, m, 2H), 7.30–7.38 (ArH, m, 4H). 13C NMR (CDCl3): δ 67.28, 94.01, 114.43, 119.54, 126.09, 129.10, 131.72, 155.68, 156.65, 158.47.

Results and Discussion

Isopropyl carbonate 1b was prepared by reaction of propargyl alcohol with isopropyl chloroformate in the presence of pyridine. tert-Butyl carbonate 1c was synthesized from di-tert-butyl dicarbonate and propargyl alcohol. These carbonates were examined for polycondensation with 4,4′-dihydroxydiphenyl ether 2a having an ether linkage in the presence of the palladium catalyst. The polymerization was carried out in THF at 60 °C for 17 h. Various phosphine ligands such as PPh3, 1,2-bis(diphenylphosphino)ethane, 1,3-bis(diphenylphosphino)propane and 1,4-bis(diphenylphosphino)butane were used. Among them, DPPF and bis[(2-diphenylphosphino)phenyl]ether (DPEphos) were effective. Table 1 shows the results with DPPF and DPEphos. The effect of the alkyl ester moiety of 1a–c on the polycondensation with 2a appeared. As expected, the Mn value of polyether 3a15 increased with increasing bulkiness of the alkyl ester moiety. The molecular weight of 3a was 2100 (run 1) in the polycondensation using methyl carbonate 1a and DPPF as a ligand. Isopropyl carbonate 1b afforded higher-molecular-weight 3a than 1a (Run 2). Polyether 3a with the highest Mn was obtained from tert-butyl carbonate 1c (Mn=5000; run 3). The polycondensation using DPEphos was similar to that using DPPF. The Mn value of 3a increased in the order of 1a<1b<1c (runs 4–6). tert-Butyl carbonate 1c was found to be most effective for the formation of high-molecular-weight polyether 3a. A monodentate phosphine ligand, PPh3, also yielded polyether 3a from 1a–c, but the difference between the Mn values was not observed (ca. 3000). In this case, polydispersities were relatively large (Mw/Mn>3.0) compared with those of 3a shown in Table 1.

Table 1 Pd(0)-catalyzed polycondensation of carbonates 1 with 2a

Pd(0)-catalyzed condensation between 1a–c and p-methoxyphenol (4) was conducted as a model reaction. The results are shown in Table 2. Methyl carbonate 1a was treated with 4 in the presence of Pd2(dba)3·CHCl3/DPPF to yield the desired 1:2 product 5 (71%) and by-product 6 (4%). The yield of 5 increased with increasing bulkiness of the alkyl ester moiety. The reaction using 1c yielded 84% of 5 (runs 1–3).16, 17 On the other hand, the yield of 6 decreased as bulkiness increased. By-product 6 seems to be obtained by reductive elimination of the hydride palladium intermediate generated by reaction of 1 with Pd(0).15 It is known that a palladium alkoxide causes β-hydrogen elimination to afford a hydride palladium complex.18, 19 The formation of 6 was diminished by the use of 1c (run 3) because β-hydrogen elimination of the palladium tert-butoxide generated from 1c cannot take place. The large-scale reaction of 1a with 4 was also carried out, but other by-products, except 6 could not be detected.

Table 2 Model reaction of 1 with p-methoxyphenol

We next conducted polycondensation of 1a–c with 2a for a shorter time because longer reaction time causes a decrease in the Mn value of polyethers obtained from 1a and bisphenols.15 The results are shown in Table 3. A large increase in Mn was observed when polymerization with 1c was conducted for 0.5 h (Mn=13 500; run 3), whereas methyl carbonate 1a yielded a low molecular weight of 3a (run 1). Polyether 3a with high Mn was also formed from isopropyl carbonate 1b (run 2).

Table 3 Effects of feed ratio and time on the polycondensation of 1 with 2a

A characteristic of the Pd(0)-catalyzed condensation between methyl carbonate 1a and bisphenols is that the corresponding polyethers are formed efficiently even under stoichiometrically imbalanced conditions. This is because the propagating termini always have two nucleophilic sites.20, 21, 22, 23, 24, 25 We reported that the use of an excess of 1a was required for formation of higher-molecular-weight polyethers because 1a itself was slightly consumed by side reactions under the polymerization conditions.14, 15 Consequently, the consumption of 1a interferes with polycondensation because of the existence of excess bisphenols when 1 equiv. of bisphenols toward 1a was used. Therefore, polycondensation was next investigated using slightly excess 1a–c toward 2a (runs 4–6 in Table 3). When 0.9 equiv. of 2a to 1c was used, the molecular weight of 3a increased from 13 500 to 16 100 (runs 3 and 6). The same trend was also observed in the polycondensation with 1b. The Mn value was 15 800 (run 5). Methyl carbonate 1a yielded the lowest Mn of 3a (run 4). Prolonged reaction time (17 h) caused a decrease in the molecular weight (run 7). Thus, high-molecular-weight 3a could be obtained when polycondensation using propargyl carbonates 1b and 1c was carried out for 0.5 h using 0.9 equiv. of 2a.

A plausible polymerization mechanism is shown in Scheme 2. Propargyl carbonate 1 reacts with Pd(0) to generate η3-allenyl/propargyl intermediate A.26, 27, 28 The central carbon of A is attacked by bisphenol 2 to produce intermediate B followed by protonation to yield η3-allylpalladium intermediate C. A second 2 attacks intermediate C to afford 4, which is equivalent to 2 and acts as a nucleophile.

Polyether 3a with low Mn was formed in the polycondensation with methyl carbonate 1a. It is possible that the methanol liberated from 1a attacked intermediate A and/or C as a nucleophile during polymerization and interfered with the polycondensation.15 We detected the formation of 6 as a by-product in the model reaction; consequently, reductive elimination of hydride palladium intermediate D would also be a possible termination in the polycondensation with 1a as shown in Scheme 3.

Polycondensation with 4,4′-dihydroxybenzophenone (2b) having a carbonyl group was next investigated using carbonates 1b and 1c to compare with the results with 2a having an ether linkage. We already reported that polycondensation of methyl carbonate 1a with 0.5 equiv. of 2b was carried out in THF at 60 °C in the presence of benzenesulfonamide as an additive and Pd(PPh3)4 (5 mol%) to yield polyether 3b15 (Mn=3300). In this case, the resulting polyether 3b was gradually decomposed under polymerization conditions. Polycondensation of tert-butyl carbonate 1c with 2b was conducted under the same conditions and produced 3b, but the Mn value was rather low (Mn=2900). We explored the polymerization conditions and found that polycondensation with 1c proceeded successfully in dioxane at 100 °C for 10 min. Table 4 summarizes the results with 1a–c and 2b. Isopropyl carbonate 1b yielded the desired polymer 3b in a good yield, but the molecular weight was about equal to that of 3b from 1a (runs 1 and 2). tert-Butyl carbonate 1c was the most effective. The Mn value of 3b reached 11 000 (run 3).

Table 4 Pd(0)-catalyzed polycondensation of 1 with 2b

The relationship between the Mn value of 3b and polymerization time was examined in the polycondensation using 1a and 1c under the same conditions of Table 4. Figure 1 shows the results. The molecular weight of 3b reached a maximum after 10 min in each case. After 3 h, methyl carbonate 1a yielded oligomeric 3b, the Mn value of which was 400. Signals based on 3b were observed in the 1H NMR spectrum. In addition, a small signal due to the methoxy group was observed (3.62 p.p.m.). These results suggest that the methanol liberated from 1a reacted with the resulting polymer 3b under the polymerization conditions to afford the oligomeric product 3b. We recently reported that the methoxylated product 8 was obtained in a good yield by the Pd(0)-catalyzed reaction of 1a with p-hydroxyacetophenone (7) and methanol as shown in Scheme 4 (1).29 Furthermore, we already found that model compound 9 obtained from 1a and 2 equiv. of 7 reacted with the methoxide ion liberated from 1a to produce 8 as shown in Scheme 5 (2).15 Therefore, the sharp decrease in the Mn value would be caused by reaction between the resulting 3b and the methanol liberated from 1a. A plausible mechanism is shown in Scheme 5. Oxidative addition of polyether 3b to Pd(0) generates a π-allylpalladium intermediate, subsequently in which the central carbon is attacked by the methanol. The resulting palladacyclobutane intermediate isomerizes to the methoxylated π-allylpalladium intermediate followed by reaction with the aryloxide liberated from 3b to yield the decomposed 3b. Organ reported a Pd(0)-catalyzed reaction of 1,2-dibromo-2-propene with phenol to afford 1,2-diphenoxy-2-propene and proposed a reaction mechanism similar to Scheme 5.30 The presence of the vinyl carbon having an aryloxy group seems to cause the initial nucleophilic attack at the central carbon of the π-allylpalladium intermediate.

Figure 1
figure 1

Plots of Mn of 3b vs time.

Polycondensation with tert-butyl carbonate 1c also afforded oligomer 3b after 3 h, but the molecular weight was high (Mn=1000) compared with that of 3b from 1a. No signal due to the tert-butoxy group was detected in the 1H NMR spectrum. These results indicate that the tert-butanol liberated from 1c did not react with the resulting polymer 3b. Consequently, the decomposition of 3b would be retarded. The observed decrease in Mn might be caused by reaction of 3b with trace amounts of water because it is known that allyl carbonates react with water in the presence of the Pd(0) catalyst.16, 17

To examine the difference in decomposition behavior between polyethers 3a and 3b, we conducted polycondensation using 1c and 4,4′-dihydroxydiphenyl ether 2a in dioxane at 100 °C in the presence of Pd2(dba)3·CHCl3 and DPPF. The Mn value of 3a reached 16 000 after 5 min and decreased to 2800 after 3 h. These results show that polyether 3b is decomposed easier than 3a, probably due to the higher reactivity of 3b toward the Pd(0) catalyst.

Bisphenol A (2c) and 4,4′-(hexafluoroisopropylidene)diphenol (2d) also afforded higher-molecular-weight polyethers 3c15 and 3d in the polycondensation with tert-butyl carbonate 1c. The results are shown in Table 5. Polycondensation was carried out at 60 °C for 17 h in the presence of Pd(PPh3)4. A monodentate ligand, PPh3, was superior to DPPF and DPEphos as the ligand in this case. Prolonged reaction time (17 h) required to obtain high-molecular-weight polymers 3c and 3d, contrary to the polycondensation using 2a and 2b. In the polycondensation with 2c, tert-butyl carbonate 1c yielded higher-molecular-weight 3c. The Mn values of the polyether 3c obtained from 1a and 1c were 4800 and 5500, respectively (runs 1 and 3). Similarly, polycondensation of 1c with 2d produced high-molecular-weight polyether 3d (Mn=13 000; run 6). Bisphenol 2d having trifluoromethyl groups showed high reactivity and afforded 3d with high Mn values compared with 2c.

Table 5 Pd(0)-catalyzed polycondensation of carbonates 1 with 2a

Conclusions

We investigated isopropyl and tert-butyl propargyl carbonates 1b and 1c for the Pd(0)-catalyzed polycondensation with bisphenols and found that propargyl carbonates having a bulky alkyl ester group could lead to the formation of polyethers 3 with high molecular weight. The Mn values of the obtained polyethers increased in the order of 1a (methyl ester)<1b (isopropyl ester)<1c (tert-butyl ester). The results of the model reaction of 1a–c with phenol 4 were consistent with those of the polycondensation. The yield of the model compound 5 increased in the order of 1a<1b< 1c. Polyether 3b was decomposed easier than polyether 3a.

scheme 1

Pd(0)-catalyzed polycondensation of 1a with bisphenols 2.

scheme 2

Possible mechanism.

scheme 3

Possible termination.

scheme 4

Formation of methoxylated product 8.

scheme 5

Decomposition of 3b.