Photo-on-Demand Phosgenation Reactions with Chloroform for Selective Syntheses of N-Substituted Ureas and Isocyanates.

Two new reaction processes involving the in situ oxidative photochemical conversion of CHCl3 to COCl2 allowed selective syntheses of N-substituted ureas and isocyanates from amines. (I) A CHCl3 solution containing an amine and an organic base under O2 bubbling provided the urea derivative under exposure to UV light generated from a low-pressure mercury lamp at 20-40 °C. (II) A two-step reaction involving the oxidative photodecomposition of CHCl3 at lower temperatures and subsequent sequential injections of an amine and organic base into the sample solution provided the isocyanate in high yield. The reaction processes of (I) and (II) capitalize on the solution conditions of [COCl2] < [amine] and [COCl2] > [amine], respectively, to result in 1:2 and 1:1 reactions. In general, isocyanates, especially aromatic and haloalkyl ones, readily undergo hydrolysis in the presence of an organic base. However, with the advantage of synthesizing the isocyanates in CHCl3 solvent, direct addition of monoalcohols and diols to the as-prepared sample solution containing the diisocyanate allowed the one-pot syntheses of biscarbamates and polyurethanes, respectively. The reactions developed in this study are simple, safe, and inexpensive methods of synthesizing N-substituted ureas and isocyanates, and derivatives of isocyanates such as carbamates and polyurethanes. The present new methods can replace current synthetic methods using COCl2 in both academia and industry.

Introduction

Isocyanates, having a −N=C=O group, are typically synthesized from amines and phosgene (COCl2) in industry (Scheme , reaction a).[1−3] This chemical reaction was first reported by Hentschel in 1884[4] and has been used widely in manufacturing isocyanate derivatives. N-Substituted ureas are also formed through the reaction of isocyanates and amines, but they can be reconverted to the isocyanates upon heating with COCl2 (reaction b).[5,6] Although these reactions use COCl2, a highly toxic gaseous compound at room temperature, they have enormous advantages in terms of reactivity, product quality, and cost in comparison with other developed methods.[7] Hence, this method has been used for a century without major modifications. For safe use of COCl2, bis(trichloromethyl)carbonate (BTC), generally called triphosgene, a solid under standard conditions, is preferred for synthesizing isocyanates in laboratory-scale experiments and small-scale industrial production.[8] BTC generates COCl2in situ upon mixing with an organic base in the solution, and its subsequent rapid reaction with an amine gives the isocyanate (reaction c). However, Cotarca and co-workers recently reported that BTC is also a highly toxic compound, whose vapor pressure is sufficiently high to reach toxic concentrations at room temperature.[9] This background motivated us to develop safe, convenient, and inexpensive phosgenation reactions for synthesizing N-substituted ureas and isocyanates without the use of highly toxic corrosive reagents, expensive reagents, or a complicated special apparatus. We herein report two novel reaction methods that allow their selective syntheses by applying the in situ photochemical conversion of CHCl3 to COCl2. In these reactions, CHCl3 bifunctionally serves as a solvent and a COCl2 precursor (reactions d and e).

Scheme 1

Syntheses of N-Substituted Ureas and Isocyanates

(a–c) Conventional reactions. (d, e) New reactions developed in this study.

We previously reported that CHCl3 undergoes oxidative photodecomposition under exposure to UV light to efficiently produce COCl2 (Scheme ).[10] Recently, we then successfully developed in situ photo-on-demand phosgenation reactions.[11−13] As a representative example, a CHCl3 solution containing a mixture of an alcohol and an organic base such as pyridine upon exposure to UV-C light provided carbonate esters in high yield via the chloroformate as an intermediate.[13] Although it is known that the alcohol serves as a stabilizer to decelerate the decomposition of CHCl3, and further, that the organic base absorbs UV light,[14,15] the phosgenation reaction occurred to give the product through an in situ base-catalyzed reaction of the alcohol and COCl2 generated by the oxidative photodecomposition of CHCl3. In this study, with the same procedures, we succeeded in synthesizing N-substituted ureas through an in situ photo-on-demand phosgenation reaction of amines with CHCl3.[16] Furthermore, we found that a two-step reaction, involving oxidative photodecomposition of CHCl3 at low temperatures and subsequent sequential injections of the amine and base into the solution, allowed the practical synthesis of isocyanates.[17] The reaction facilitated one-pot conversions of the isocyanates to carbamates and polyurethanes (PU) upon mixing with alcohols.

Scheme 2

Photo-on-Demand Syntheses of (a) Phosgene, (b) Chloroformate, and (c) Carbonate Ester with CHCl3 as Reported Previously by Our Group

Results and Discussion

Photochemical Reaction System

A low-pressure mercury lamp generates UV light mainly at wavelengths of 184.9 and 253.7 nm, which cover the electronic absorption bands of CHCl3, originating from σ–σ* and/or n−σ* transitions. The lamp has an advantage in terms of its relatively low electric power consumption compared with other light sources including light-emitting diodes (LEDs) capable of emitting short-wavelength UV-C light.[10a,18] The lamp (20 W, ⌀24 mm × 120 mm), whose illuminance at 185 and 254 nm is 2.00–2.33 and 5.96–8.09 mW/cm2, respectively, at 5 mm from the lamp, was inserted into the solution via a quartz glass jacket (⌀28 mm) fixed in the center of a cylindrical flask (⌀42 mm). This setup allows for transmission of UV-C light, while protecting the lamp from corrosive gases such as HCl and COCl2, which are generated by photochemical reactions. The photoreactor is further connected to a gas trap system containing alkaline water such as NaHCO3 aq. and NaOH aq., which can decompose unreacted COCl2 and HCl.

Photo-on-Demand Synthesis of N-Substituted Ureas

With this reaction system, we initially demonstrated the in situ photo-on-demand phosgenation reaction with cyclohexylamine, an aliphatic amine, by reference to the procedures developed in our previous study.[13] When a CHCl3 solution (20 mL) containing 10 mmol of amine was exposed to UV light under O2 bubbling (0.1 L/min) with vigorous stirring at 20 °C, the sample solution immediately became a white suspension due to the formation of the HCl salt of the amine (Scheme , entry 1). The resulting sample solution contained 1,3-dicyclohexylurea, which is expected from the reaction of the amine and COCl2, but it was obtained only in 4% yield. To trap the generated HCl, which most likely decelerates the nucleophilic substitution of the amine to COCl2, the photochemical reaction was then carried out upon mixing with triethylamine (TEA). Although the urea yield was slightly increased to 16%, the reaction stopped in a short time, probably due to the decomposition of TEA that resulted in dark coloration of the sample solution (entry 2). The yield was also increased slightly when using pyridine, whose UV light resistance and basicity are relatively higher and lower, respectively, than those of TEA (entry 3). Based on the hypothesis that aliphatic amines having higher basicity dominantly react with HCl rather than COCl2, we next performed the photochemical reaction with aniline, an aromatic amine, having lower basicity. The reaction of aniline without a base also mainly provided the HCl salt of aniline with the corresponding urea only in 3% yield (entry 4). However, the reaction with a CHCl3 solution containing a mixture of aniline and 5 equiv amounts of pyridine provided 1,3-diphenylurea in 51% isolated yield (entry 5). 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU), having higher basicity than pyridine, also allowed the formation of urea but caused its photodecomposition, decreasing the yield to 38% (entry 6). Pentafluoroaniline, having lower basicity than aniline, in the presence of pyridine provided the corresponding urea in 21% yield (entry 7). When this reaction was applied to the synthesis of 1,1′-carbonyldiimidazole (CDI),[19] which is widely used as a phosgene substitute in organic syntheses, it was obtained in 31% yield (>99% in 1H NMR yield) from a CHCl3 solution of imidazole (IZ) and 2,6-lutidine (Scheme ). The lower isolated yield than that estimated by 1H NMR analysis (>99%) may be responsible to that CDI readily undergoes hydrolysis under air in the presence of base.

Scheme 3

Photochemical Syntheses of N-Substituted Ureas with CHCl3 Solutions Containing an Amine and an Organic Base

Reaction procedures and conditions: Irradiation by UV light of 20 mL of CHCl3 containing 10 mmol of amine and 0–5 equiv of base under O2 bubbling (0.1 L/min) with a 20 W low-pressure mercury lamp.

Scheme 4

Photochemical Synthesis of CDI with CHCl3 Solutions Containing Imidazole and 2,6-Lutidine

Yield was determined by 1H NMR analysis.

Reaction procedures and conditions: Irradiation by UV light of 20 mL of CHCl3 containing 10 mmol of IZ and 5 equiv of 2,6-lutidine under O2 bubbling (0.1 L/min) with a 20 W low-pressure mercury lamp.

Synthesis of Isocyanates by a Two-Step Procedure

The N-substituted ureas described above might form under the condition of [amine] > [COCl2], which allows an immediate reaction of COCl2 with two molecules of the amine via the formation of an isocyanate. Thus, it can be expected that the condition of [amine] < [COCl2], which allows a preferential reaction of the amine with COCl2 rather than the isocyanate formed in the reaction, would provide mainly the corresponding isocyanate through a 1:1 reaction. Considering the low boiling point of COCl2 (8.3 °C), a two-step procedure by preparation of a CHCl3 solution containing COCl2 was conducted at low temperatures, with subsequent injection of the amine into the solution. Prior to performing this experiment, we first examined whether CHCl3 undergoes oxidative photodecomposition at 0 °C to give a CHCl3 solution of COCl2. With the same reaction system as that for the preparation of N-substituted ureas, 20 mL of CHCl3 after exposure to UV light at 0 °C for 1 h under O2 bubbling at a flow rate of 0.1 L/min was found to contain 18.5 mmol of COCl2, which could be estimated from the amounts of the products formed upon addition of ethanol (chloroformate and carbonate ester). By reference to this result, 10 mmol of aniline was added to 50 mL of a CHCl3 solution containing COCl2, as prepared upon photoirradiation at 0 °C for 3 h. The aniline was slightly converted to the corresponding isocyanate but was mainly obtained as its HCl salt. The HCl originated from what remained in the CHCl3 solution and what was generated by the reaction of aniline with COCl2. However, the HCl salt of aniline could be further reacted with unreacted COCl2 upon elevating the temperature to 60 °C to give the isocyanate in 80% yield (Scheme , entry 1). The HCl salts of amines are known to react with COCl2 at high temperatures (vide ante) to give isocyanates,[2b] and this method has actually been used for industrial production. However, one may expect under these conditions that COCl2 would not be retained in the sample solution but would vaporize to the gas phase. This may reduce the yield of the product and is also unfavorable in terms of the safe handling of the reaction, especially in laboratory-scale experiments. To achieve the synthesis of the isocyanate at low temperatures, preferably below the boiling point of COCl2, we then added an organic base such as pyridine or 2,6-lutidine to the above CHCl3 solution containing the HCl salt of aniline and COCl2 without elevating the temperature. The corresponding isocyanate was obtained in >99 and 97% yields, respectively (entries 2 and 3).

Scheme 5

Two-Step Procedure for Synthesizing Phenyl Isocyanate through the Photochemical Conversion of CHCl3 to COCl2 and Subsequent Addition of Aniline

Yields were determined by 1H NMR analysis.

Reaction procedures and conditions: [Step 1] irradiation by UV light of 50 mL of CHCl3 under O2 bubbling (0.1 L/min) at 0 °C for 3 h. [Step 2] (1) Addition of 10 mmol of amine. (2) Addition of 5 equiv of base.

The reaction was further available for the synthesis of a variety of isocyanates (Scheme ). Hexylamine, having higher basicity than aniline, also provided the corresponding isocyanate in >99% yield. Fluorine-substituted aniline and an alkyl amine, having lower basicity, provided the corresponding isocyanates in 62 and 66% yields, respectively. 3-(Triethoxysilyl)propylamine and 3-(trimethoxysilyl)propylamine, which readily undergo hydrolysis condensations with water under acidic conditions, also provided the corresponding isocyanates in 62 and 56% yields, respectively. This reaction was further applicable to the synthesis of a series of diisocyanates that are used in the manufacturing of polyurethanes in industry (Scheme ).[6] Aromatic diisocyanates such as toluene diisocyanate (TDI), methylenediphenyl diisocyanate (MDI), benzene diisocyanate (BDI), and naphthalene diisocyanate (NDI) were successfully synthesized in 80, 90, 47, and 67% yields, respectively. Alkyl diisocyanates such as hexamethylene diisocyanate (HDI) and pentamethylene diisocyanate (PDI) were also obtained in 96 and 83% yields, respectively. Aliphatic diisocyanates including a cyclic component such as m-xylylene diisocyanate (XDI), 1,4-bis(isocyanatomethyl) cyclohexane (H6XDI), isophorone diisocyanate (IPDI), and norbornanediisocyanate (NBDI) were obtained in 63, 58, 42, and 65% yields, respectively.

Scheme 6

Two-Step Procedure for Synthesizing Monoisocyanates through the Photochemical Conversion of CHCl3 to COCl2 and Subsequent Addition of Amine

Reaction procedures and conditions: [Step 1] Irradiation by UV light of 50 mL of CHCl3 under O2 bubbling (0.1 L/min) at 0 °C for 3 h. [Step 2] (1) Addition of 10 mmol of amine. (2) Addition of 5 equiv of base. Yields were determined by 1H NMR analysis.

Scheme 7

Two-Step Procedure for Synthesizing Diisocyanates through the Photochemical Conversion of CHCl3 to COCl2 and Subsequent Addition of Diamines

Reaction procedures and conditions: [Step 1] Irradiation by UV light of 50 mL of CHCl3 under O2 bubbling (0.1 L/min) at 0 °C for 3 h. [Step 2] (1) Addition of 10 mmol of diamine. (2) Addition of 5 equiv of base. Yields were determined by 1H NMR analysis.

One-Pot Syntheses of Biscarbamates and Polyurethanes

These isocyanates and diisocyanates can be isolated by vacuum distillation, and the HCl salt of pyridine remaining as a byproduct can also be reused by treating it with an inorganic base such as aqueous NaOH. However, since isocyanates are generally moisture sensitive, especially in the presence of organic bases, the isocyanates produced using this method readily undergo hydrolysis under air to give the corresponding amines. The amine then further reacts with isocyanate to give the corresponding urea derivative. With the advantage of preparing the isocyanates in CHCl3 solvent, we further demonstrated the one-pot synthesis of carbamates without their isolation (Scheme ). When EtOH was added to the CHCl3 solution containing MDI, as prepared by the above two-step procedure, the corresponding biscarbamate was obtained in 78% yield (91% by 1H NMR). 1,1,1,3,3,3,-Hexafluoro-2-propanol (HFIP), whose nucleophilicity is lower than that of EtOH, also allowed the reaction upon mixing with additional pyridine to give the corresponding biscarbamate in 71% yield (76% in 1H NMR yield). The pyridine might allow the catalytic substitution of HFIP to MDI. HDI, whose reactivity is relatively low compared with MDI, also provided the corresponding biscarbamate through the reaction with HFIP, but the yield was reduced to 37% (47% by 1H NMR). We further demonstrated the one-pot synthesis of polyurethane upon addition of 1,6-hexanediol (HD) to the CHCl3 solution containing MDI. The corresponding polyurethane [poly(MDI-HD)] was obtained in 50% yield and had an average molecular weight of Mw = 4100 and Mn = 2200 (Mw/Mn = 1.87). This observed low yield and relatively low average molecular weight most likely originated from the poor solubility of the PU formed in the reaction. Poly(propylene glycol) (PPG) with an average molecular weight of 400 that can increase the solubility of PU provided poly(MDI-PPG) in 89% yield with Mw = 6200 and Mn = 3600 (Mw/Mn = 1.72).

Scheme 8

One-Pot Syntheses of Biscarbamates and Polyurethanes from Diamines through Their Photochemical Conversions to the Diisocyanates

Yields were determined by 1H NMR analysis.

Reaction procedures and conditions: (1) irradiation by UV light of 50 mL of CHCl3 under O2 bubbling (0.1 L/min) at 0 °C for 3 h. (2) Addition of 5 mmol of diamine. (3) Addition of 10 equiv of base. (4) Addition of [A] 20 mmol of alcohol with or without 25 mmol of base or [B] 5 mmol of diol.

With similar procedures, we next demonstrated the one-pot syntheses of a biscarbonate and a polyurethane, including a fluoroalkyl backbone, from the HCl salt of 2,2,3,3,4,4,5,5-octafluorohexane-1,6-diamine (8FHDA·2HCl) (Scheme ). Photoirradiation of a CHCl3 solution containing insoluble 8FHDA·2HCl at 0 °C, and subsequent slow injection of 2,6-lutidine into the sample solution, quantitatively provided the corresponding diisocyanate (8FHDI), which was characterized by 1H NMR analysis. Although 8FHDI formed in the solution was highly moisture sensitive, which would cause its hydration, the one-pot procedure upon addition of HFIP to the sample solution afforded the corresponding biscarbamate in 37% yield (68% by 1H NMR). The addition of HD and PPG provided the corresponding polyurethanes in 91% yield with Mw = 4000 and Mn = 2400 (Mw/Mn = 1.66) and 86% yield with Mw = 8800 and Mn = 3400 (Mw/Mn = 2.59), respectively.

Scheme 9

One-Pot Syntheses of Biscarbamates and Polyurethane from a HCl Salt of Fluoroalkylene Diamine through Photochemical Conversion to Diisocyanate

Yields were determined by 1H NMR analysis.

Reaction procedures and conditions: (1) irradiation by UV light of 30 mL of CHCl3 containing 2 mmol of 8FHDA·2HCl under O2 bubbling (0.1 L/min) at 0 °C for 3 h. (2) Addition of 10 equiv of 2,6-lutidine. (3) Addition of [A] 8 mmol of HFIP with 8 mmol of 2,6-lutidine or [B] 2 mmol of diol.

Conclusions

In this study, N-substituted ureas and isocyanates were selectively synthesized from amines by two new reaction processes, involving photo-on-demand phosgenation reactions with CHCl3. Direct UV irradiation of a CHCl3 solution containing an amine under O2 bubbling provided N-substituted ureas. This reaction allowed for a higher concentration of the amine than that of COCl2, which was constantly generated from the CHCl3 solution, resulting in a 2:1 reaction. A two-step reaction, involving oxidative photodecomposition of CHCl3 at low temperatures and subsequent slow injection of an amine and organic base into the sample solution, resulted in dominant formation of the isocyanate through the base-catalyzed 1:1 reaction of amine and COCl2. This process allowed a lower concentration of the amine than that of COCl2. The isocyanates formed under these conditions readily underwent hydrolysis under air. However, with the advantage of synthesizing isocyanates in CHCl3 solvent, this reaction system allowed for one-pot syntheses of biscarbamates and polyurethanes upon direct addition of monoalcohols and diols, respectively, to the as-prepared CHCl3 solution of the diisocyanate. The obtained biscarbamates bearing electron-withdrawing fluoroalkyl groups are also interesting as blocked isocyanates applicable for synthesizing polyurethanes.[20] The photochemical reactions reported herein are simple and safe methods of synthesizing N-substituted ureas and isocyanates and are expected to be an innovative method that can replace current phosgenation reactions.

Experimental Section

General Information

Unless otherwise noted, all reagents were purchased from commercial suppliers and used without further purification. 2,2,3,3,4,4,5,5-Octafluoro-1,6-diaminohexane hydrochloride (8FHDA·2HCl) was synthesized according to the literature method.[21] For vacuum distillation, a glass tube oven (Sibata Scientific Technology, model GTO-1000) was used with an oil rotary vacuum pump. 1H and 13C NMR spectra were recorded on a Bruker AVANCE 400 spectrometer or Bruker AVANCE 500 spectrometer, where chemical shifts (δ in ppm) were determined with respect to tetramethylsilane as an internal standard. 19F NMR spectra were recorded on a Bruker AVANCE 400 spectrometer, where chemical shifts (δ in ppm) were determined with respect to hexafluorobenzene as an external standard. Fourier transform infrared (FTIR) spectroscopy was performed on a JASCO FT/IR 4200. Fourier transform mass spectrometry was performed on a Thermo Fisher Scientific LTQ Orbitrap. Elemental analysis was carried out on a Yanaco CHN corder MT-5 analyzer. Size-exclusion chromatography (SEC) measurements were performed at 40 °C on TOHSOH TSKgel G5000HHR and TSKgel G4000HHR columns using a JASCO Type PU-2089 quaternary gradient pump, equipped with a JASCO type RI-4030 refractive index detector with tetrahydrofuran (THF) as an eluent. Polystyrene standards (123 000, 50 000, 25 000, 13 502, 4000, 1300 Da, 0.56% w/v) were used for calibration of the molecular weights of the polymers.

General Procedure for the Synthesis of N-Substituted Ureas

A cylindrical flask (⌀42 × 120 mm2), equipped with a low-pressure mercury lamp (Sen Light Co., UVL20PH-6, 20 W, ⌀24 × 120 mm2) and a magnetic stirring bar, was charged with a CHCl3 solution (20 mL) containing an amine (10 mmol) and an organic base (1.5 or 5.0 equiv). If necessary, an aluminum block bath or water bath was used to heat the sample solution. The sample solution was vigorously stirred upon bubbling with O2 (0.1 L/min) under exposure to UV light for 0.5–2.0 h. After the photochemical reaction, the lamp was turned off, and the sample solution was stirred at 50 °C for 0.5–1.0 h to remove the unreacted COCl2 and generated HCl dissolved in the solution. The resulting sample solution was washed with water and extracted with CH2Cl2. The combined organic layer was dried over anhydrous Na2SO4 and evaporated to dryness. The residue was recrystallized to give the N-substituted urea.

1,3-Dicyclohexylurea

CHCl3 (20 mL), cyclohexylamine (10 mmol, 1.15 mL), and triethylamine (15 mmol, 2.1 mL) were mixed in a cylindrical flask. The sample solution was vigorously stirred under O2 bubbling (0.1 L/min) and exposed to UV light at 20 °C for 0.5 h. After the photochemical reaction, the lamp was turned off, and the sample solution was stirred at 50 °C for 1 h. It was then washed with water and extracted with CH2Cl2. The combined organic layer was dried over anhydrous Na2SO4 and evaporated to dryness. 1,3-Dicyclohexylurea was obtained through recrystallization with CHCl3/n-hexane as a white solid in 16% yield (0.18 g, 0.8 mmol). 1H NMR (400 MHz, dimethyl sulfoxide (DMSO)-d6, 293 K) δ/ppm: 5.56 (d, 2H, J = 8.0 Hz), 1.70–1.74 (m, 4H), 1.61 (dt, 4H, J = 3.6, 13.2 Hz), 1.50 (dt, 2H, J = 3.2, 12.0 Hz), 1.03–1.27 (m, 10H); 13C NMR (100 MHz, DMSO-d6, 293 K) δ/ppm: 47.4, 33.2, 25.2, 24.4.

1,3-Diphenylurea

CHCl3 (20 mL), aniline (10 mmol, 0.91 mL), and pyridine (50 mmol, 4.0 mL) were mixed in a cylindrical flask. The sample solution was vigorously stirred under O2 bubbling (0.1 L/min) and exposed to UV light at 20 °C for 2 h. After the photochemical reaction, the lamp was turned off, and the sample solution was stirred at 50 °C for 1 h. It was then washed with water and extracted with CH2Cl2. The combined organic layer was dried over anhydrous Na2SO4 and evaporated to dryness. 1,3-Diphenylurea was obtained through recrystallization with EtOAc/n-hexane as a white solid in 51% yield (0.54 g, 2.5 mmol). 1H NMR (400 MHz, DMSO-d6, 293 K) δ/ppm: 7.45 (d, 4H, J = 7.6 Hz), 7.28 (t, 4H, J = 7.8 Hz), 6.97 (t, 2H, J = 7.4 Hz); 13C NMR (100 MHz, DMSO-d6, 293 K) δ/ppm: 152.5, 139.6, 128.7, 121.7, 118.1.

1,3-Bis(perfluorophenyl)urea

CHCl3 (20 mL), 2,3,4,5,6-pentafluoroaniline (10 mmol, 1.83 g), and pyridine (50 mmol, 4.0 mL) were mixed in a cylindrical flask. The sample solution was vigorously stirred under O2 bubbling (0.1 L/min) and exposed to UV light at 40 °C for 2 h. After the photochemical reaction, the lamp was turned off, and the sample solution was stirred at 50 °C for 1 h. It was then washed with water and extracted with CH2Cl2. The combined organic layer was dried over anhydrous Na2SO4 and evaporated to dryness. 1,3-Bis(perfluorophenyl)urea was obtained through recrystallization with EtOAc/n-hexane as a white solid in 21% yield (0.41 g, 1.0 mmol). 1H NMR (400 MHz, DMSO-d6, 293 K) δ/ppm: 9.04 (br, 2H); 13C NMR (100 MHz, DMSO-d6, 293 K) δ/ppm: 152.1, 114.3, 141.9, 140.1, 138.5, 137.6, 136.0, 113.6; 19F NMR (376 MHz, DMSO-d6, 293 K) δ/ppm = −146.4 (d, 4F, J = 20.3 Hz), −158.8 (br, 2F), −163.9 (t, 4F, J = 21.8 Hz).

1,1-Carbonyldiimidazole

CHCl3 (20 mL), imidazole (10 mmol, 0.68 g), and 2,6-lutidine (50 mmol, 5.80 mL) were mixed in a cylindrical flask. The sample solution was vigorously stirred under O2 bubbling (0.1 L/min) and exposed to UV light at 20 °C for 2 h. After the photochemical reaction, the lamp was turned off, and the sample solution was stirred at 50 °C for 0.5 h. 1,2-Dichloroethane (5 mmol, 0.40 mL) was then added to the sample solution as an internal standard, and the yield of 1,1-carbonyldiimidazole produced was estimated to be 99% in the 1H NMR analysis. The solvent was evaporated to dryness under reduced pressure. The residue was recrystallized with THF to give a white solid in 31% yield (0.25 g, 1.5 mmol). 1H NMR (400 MHz, CDCl3, 293 K) δ/ppm: 8.20 (s, 2H), 7.53 (t, 2H, J = 1.6 Hz), 7.26 (s, 2H); 13C NMR (100 MHz, CDCl3, 293 K) δ/ppm: 138.3, 132.3, 118.9.

General Procedure for the Synthesis of Monoisocyanates

A cylindrical flask (⌀42 × 120 mm2), equipped with a low-pressure mercury lamp (Sen Light Co., UVL20PH-6, 20 W, ⌀24 × 120 mm2) and a magnetic stirring bar, was charged with CHCl3 (20 mL). The sample was cooled at 0 °C with an ice bath and vigorously stirred upon bubbling with O2 (0.1 L/min) under exposure to UV light for 3 h. After the photochemical reaction, the lamp was turned off, and amine (10 mmol) and organic base (5 equiv) were sequentially added to the sample solution with a syringe pump. The sample solution was stirred at 0 °C for 1 h and then further stirred at 60 °C for 1.5 h to remove the unreacted COCl2 dissolved in the solution. C2H4Cl2 or CH2Cl2 was then added to the sample solution as an internal standard, and the yield of the isocyanate produced was determined by 1H NMR analysis.

General Procedure for the Synthesis of Diisocyanates

A cylindrical flask (⌀42 × 120 mm2), equipped with a low-pressure mercury lamp (Sen Light Co., UVL20PH-6, 20 W, ⌀24 × 120 mm2) and a magnetic stirring bar, was charged with CHCl3 (50 mL). The sample was cooled at 0 °C with an ice bath and vigorously stirred upon bubbling with O2 (0.1 L/min) under exposure to UV light for 3 h. After the photochemical reaction, the lamp was turned off, and diamine (5 mmol) and organic base (10 equiv) were sequentially added to the sample solution with a syringe pump at −20 to 0 °C. The sample solution was further stirred for 1.5 h at 60 °C or subjected to vacuum distillation to remove the unreacted COCl2 dissolved in the solution. The yields of the diisocyanates were then determined by 1H NMR analysis with CH2Cl2 or C2H4Cl2 as an internal standard.

Synthesis of Biscarbamates and Polyurethane

MDI-2EtOH

A cylindrical flask, equipped with a low-pressure mercury lamp and a magnetic stirring bar, was charged with CHCl3 (50 mL). The sample was cooled at 0 °C with an ice bath and vigorously stirred upon bubbling with O2 (0.1 L/min) under exposure to UV light for 3 h. After the photochemical reaction, the lamp was turned off, and the sample solution was cooled to −20 °C. 4,4′-Diaminodiphenylmethane (5.0 mmol, 1.00 g) dissolved in CHCl3 (10 mL) and pyridine (50 mmol, 4.0 mL) was sequentially added to the sample solution with a syringe pump at this temperature. The sample solution was then evaporated at 100 °C to remove the solvent. Ethanol (20 mmol, 1.17 mL) was injected into the sample with a syringe pump, and the sample solution was stirred at 30 °C for 2 h. The sample was washed with water and extracted with CH2Cl2. The combined organic layer was dried over anhydrous Na2SO4 and evaporated to dryness under reduced pressure. The residue was dried under vacuum at 50 °C for 2 h to give MDI-2EtOH in 78% yield (1.33 g) as a dark brown colloidal solid. 1H NMR (400 MHz, DMSO-d6, 293 K) δ/ppm: 9.52 (s, NH), 7.37 (d, 4H, J = 8.4 Hz), 7.09 (d, 4H, J = 8.4 Hz), 4.11 (q, 4H, J = 7.2 Hz), 3.79 (s, 2H), 1.23 (t, 6H, J = 7.2 Hz); 13C NMR (100 MHz, DMSO-d6, 293 K) δ/ppm: 153.5, 137.1, 135.3, 128.7, 118.3, 59.9, 14.5. IR (attenuated total reflectance (ATR)) ν (cm–1): 3320, 3123, 3045, 2979, 2931, 1697, 1595, 1525, 1478, 1443, 1414, 1390, 1365, 1315, 1230, 1067, 922, 858, 815, 758, 668. High-resolution mass spectrometry (HRMS) (electrospray ionization (ESI) orbitrap) m/z: [M + Na]+ calcd for C19H22N2O4Na 365.1477; found 365.1475.

MDI-2HFIP

A cylindrical flask, equipped with a low-pressure mercury lamp and a magnetic stirring bar, was charged with CHCl3 (50 mL). The sample was cooled at 0 °C with an ice bath and vigorously stirred upon bubbling with O2 (0.1 L/min) under exposure to UV light for 3 h. After the photochemical reaction, the lamp was turned off, and the sample solution was cooled to −20 °C. 4,4′-Diaminodiphenylmethane (5.0 mmol, 1.00 g) dissolved in CHCl3 (10 mL) and pyridine (50 mmol, 4.0 mL) was sequentially added to the sample solution with a syringe pump at this temperature. The sample solution was then evaporated at 100 °C to remove the solvent. A mixture of 1,1,1,3,3,3-hexafluoro-2-propanol (20 mmol, 2.31 mL) and pyridine (25 mmol, 2.0 mL) was injected into the sample with a syringe pump, and the sample solution was stirred at 30 °C for 2 h. The sample was washed with water and extracted with CH2Cl2. The combined organic layer was dried over anhydrous Na2SO4 and evaporated to dryness under reduced pressure. The residue was dried under vacuum at 50 °C for 2 h to give MDI-2HFIP in 71% yield (2.07 g) as light brown powder. 1H NMR (400 MHz, CDCl3, 293 K) δ/ppm: 7.33 (d, 4H, J = 8.0 Hz), 7.16 (d, 4H, J = 7.6 Hz), 5.75 (sep, 2H, J = 3.6 Hz), 3.94 (s, 2H); 13C NMR (100 MHz, DMSO-d6, 293 K) δ/ppm: 149.6, 136.8, 135.4, 139.1, 120.1 (q, J = 278.5 Hz), 119.0, 116.7, 66.4 (m); 19F NMR (376 MHz, DMSO-d6, 293 K) δ/ppm: −72.5 (d, J = 6.8 Hz). IR (ATR) ν (cm–1): 3308, 3126, 3045, 2977, 1736, 1597, 1539, 1414, 1382, 1357, 1247, 1222, 1190, 1105, 1005, 906, 888, 848, 809, 781, 757, 688, 658. HRMS (ESI orbitrap) m/z: [M + Na]+ calcd for C21H14F12N2O4Na 609.0660; found 609.0639.

HDI-2HFIP

A cylindrical flask, equipped with a low-pressure mercury lamp and a magnetic stirring bar, was charged with CHCl3 (50 mL). The sample was cooled at 0 °C with an ice bath and vigorously stirred upon bubbling with O2 (0.1 L/min) under exposure to UV light for 3 h. After the photochemical reaction, the lamp was turned off, and the sample solution was cooled to −20 °C. 1,6-Hexanediamine (5.0 mmol, 0.58 g) dissolved in CHCl3 (10 mL) and pyridine (50 mmol, 4.0 mL) was sequentially added to the sample solution with a syringe pump at this temperature. The sample solution was then evaporated at 100 °C to remove the solvent. A mixture of 1,1,1,3,3,3-hexafluoro-2-propanol (20 mmol, 2.31 mL) and 2,6-lutidine (25 mmol, 2.9 mL) was injected into the sample with a syringe pump, and the sample solution was stirred at 40 °C for 65 h. The sample was washed with water and extracted with CH2Cl2. The combined organic layer was dried over anhydrous Na2SO4, and evaporated to dryness under reduced pressure. The residue was dried under vacuum at 50 °C for 2 h to give HDI-2HFIP in 37% yield (0.86 g) as white powder. 1H NMR (400 MHz, CDCl3, 293 K) δ/ppm: 5.67 (sep, 2H, J = 6.0 Hz), 5.12 (br, NH), 3.27 (q, 4H, J = 8.8 Hz), 1.57 (t, 4H, J = 6.8 Hz), 1.37 (m, 4H); 13C NMR (100 MHz, DMSO-d6, 293 K) δ/ppm: 152.4, 121.1 (q, J = 282.9 Hz), 66.5 (m), 40.8, 28.8, 25.7; 19F NMR (376 MHz, CDCl3, 293 K) δ/ppm: −73.6 (d, J = 5.3 Hz). IR (ATR) ν (cm–1): 3336, 3072, 2971, 2937, 2865, 1731, 1548, 1383, 1355, 1252, 1193, 1145, 1102, 1066, 1007, 944, 905, 878, 747, 691, 652. ESI HRMS m/z: [M + Na]+ calcd for C14H16F12N2O4Na 527.0816; found 527.0812.

Poly(MDI-HD)

A cylindrical flask, equipped with a low-pressure mercury lamp and a magnetic stirring bar, was charged with CHCl3 (50 mL). The sample was cooled at 0 °C with an ice bath and vigorously stirred upon bubbling with O2 (0.1 L/min) under exposure to UV light for 3 h. After the photochemical reaction, the lamp was turned off, and the sample solution was cooled to −20 °C. 4,4′-Diaminodiphenylmethane (5.0 mmol, 1.00 g) dissolved in CHCl3 (10 mL) and pyridine (50 mmol, 4.0 mL) was sequentially added to the sample solution with a syringe pump at this temperature. The sample solution was then evaporated at 90 °C to remove the solvent. 1,6-Hexanediol (5 mmol, 0.59 g) dissolved in CHCl3 (7 mL) was injected into the sample with a syringe pump, and the sample solution was stirred at −20 °C for 12 h. The sample was washed with water and extracted with CH2Cl2. The combined organic layer was dried over anhydrous Na2SO4 and evaporated to dryness under reduced pressure. The residue was dried under vacuum at 50 °C for 2 h to give poly(MDI-HD) in 50% yield (0.92 g) as brown powder. The average molecular weight was estimated by gel permeation chromatography (GPC) in comparison with the polystyrene standard: Mw = 4100, Mn = 2200, Mw/Mn = 1.87. 1H NMR (400 MHz, DMSO-d6, 293 K) δ/ppm: 9.50 (s, NH), 7.35 (d, 4H, J = 8.0 Hz), 7.08 (d, 4H, J = 8.0 Hz), 4.05 (s, 4H), 3.78 (s, 2H), 3.39 (t, 4H, J = 6.4 Hz), 1.62–1.21 (m, 9H). 13C NMR (100 MHz, DMSO-d6, 293 K) δ/ppm: 153.5, 137.0, 135.3, 128.7, 118.2, 63.9, 60.5, 32.3, 28.4, 25.1. IR (ATR) ν (cm–1): 3308, 3120, 3045, 2934, 2857, 1698, 1596, 1530, 1413, 1310, 1230, 1069, 815, 769, 678.

Poly(MDI-PPG)

A cylindrical flask, equipped with a low-pressure mercury lamp and a magnetic stirring bar, was charged with CHCl3 (50 mL). The sample was cooled at 0 °C with an ice bath, and vigorously stirred upon bubbling with O2 (0.1 L/min) under exposure to UV light for 3 h. After the photochemical reaction, the lamp was turned off, and the sample solution was cooled to −20 °C. 4,4′-Diaminodiphenylmethane (5.0 mmol, 1.00 g) dissolved in CHCl3 (10 mL) and pyridine (50 mmol, 4.0 mL) was sequentially added to the sample solution with a syringe pump at this temperature. The sample solution was then evaporated at 90 °C to remove the solvent. PPG with the average molecular weight of 400 (5 mmol, 2.00 g) was injected into the sample with a syringe pump, and the sample was stirred at 50 °C for 13 h. The sample was washed with water and extracted with CH2Cl2. The combined organic layer was dried over anhydrous Na2SO4 and evaporated to dryness under reduced pressure. The residue was dried under vacuum at 60 °C for 2 h to give poly(MDI-PPG) in 89% yield (2.91 g) as dark brown solid. The average molecular weight was estimated by GPC in comparison with the polystyrene standard: Mw = 6200, Mn = 3600, Mw/Mn = 1.72. 1H NMR (400 MHz, CDCl3, 293 K) δ/ppm: 7.37 (m, 4H), 7.08 (m, 4H), 4.87 (s, 1H), 3.53–3.31 (m, 29H), 1.91–1.02 (m, 24H); 13C NMR (100 MHz, DMSO-d6, 293 K) δ/ppm: 153.2, 137.8, 137.2, 135.4, 135.3, 129.2, 118.3, 79.2, 74.6, 72.4, 72.2, 71.2, 65.4, 20.2, 17.4. IR (ATR) ν (cm–1): 3275, 3188, 3114, 3059, 2970, 2923, 1723, 1597, 1533, 1509, 1456, 1411, 1374, 1308, 1224, 1079, 927, 817, 754, 681.

8FHDI-2HFIP

A cylindrical flask, equipped with a low-pressure mercury lamp and a magnetic stirring bar, was charged with CHCl3 (20 mL) and 8FHDA·2HCl (2 mmol, 0.68 g). The sample was cooled at 0 °C with an ice bath and vigorously stirred upon bubbling with O2 (0.1 L/min) under exposure to UV light for 3 h. After the photochemical reaction, the lamp was turned off, and 2,6-lutidine (20 mmol, 2.3 mL) was slowly added to the sample solution with a syringe pump. The sample solution was then evaporated at 100 °C to remove the solvent. A mixture of 1,1,1,3,3,3-hexafluoro-2-propanol (8 mmol, 0.92 mL) and 2,6-lutidine (8 mmol, 0.92 mL) was injected into the sample with a syringe pump, and the sample solution was stirred at 50 °C for 20 h. The sample was washed with water and extracted with CH2Cl2. The combined organic layer was dried over anhydrous Na2SO4 and evaporated to dryness under reduced pressure. The residue was dried under vacuum at 30 °C for 16 h to give 8FHDI-2HFIP in 37% yield (0.48 g) as white powder. 1H NMR (400 MHz, CDCl3, 293 K) δ/ppm: 5.68 (sep, 2H, J = 6.0 Hz), 5.40 (t, NH), 3.98 (td, 4H, J = 15.2, 6.4 Hz); 13C NMR (100 MHz, DMSO-d6, 293 K) δ/ppm: 153.3, 121.0 (q, J = 282.8 Hz), 115.6 (m), 111.0 (m), 67.1 (m), 41.1 (m); 19F NMR (376 MHz, CDCl3, 293 K) δ/ppm: −73.6 (d, J = 5.3 Hz), −118.8, −123.5 (m). IR (ATR) ν (cm–1): 3342, 3094, 2981, 1736, 1560, 1438, 1382, 1360, 1257, 1201, 1155, 1106, 1081, 1014, 924, 906, 872, 751, 734, 691, 650. HRMS (ESI orbitrap) m/z: [M – H]− calcd for C14H7F20N2O4 647.0092; found 647.0083.

Poly(8FHDI-HD)

A cylindrical flask, equipped with a low-pressure mercury lamp and a magnetic stirring bar, was charged with CHCl3 (30 mL) and 8FHDA·2HCl (2 mmol, 0.68 g). The sample was cooled at 0 °C with an ice bath and vigorously stirred upon bubbling with O2 (0.1 L/min) under exposure to UV light for 2 h. After the photochemical reaction, the lamp was turned off, and 2,6-lutidine (20 mmol, 2.3 mL) was slowly added to the sample solution with a syringe pump. The sample solution was then evaporated at 90 °C to remove the solvent. 1,6-Hexanediol (2 mmol, 0.24 g) dissolved in CHCl3 (10 mL) was injected into the sample with a syringe pump, and the sample solution was stirred at 50 °C for 110 h. The sample was washed with water and extracted with CH2Cl2. The combined organic layer was dried over anhydrous Na2SO4 and evaporated to dryness under reduced pressure. The residue was dried under vacuum at 60 °C for 2 h to give poly(8FHDI-HD) in 91% yield (0.81 g) as brown powder. The average molecular weight was estimated by GPC in comparison with the polystyrene standard: Mw = 4000, Mn = 2400, Mw/Mn = 1.66. 1H NMR (400 MHz, DMSO-d6, 293 K) δ/ppm: 3.99 (t, 4H, J = 6.4 Hz), 3.91 (td, J = 18.0, 6.4 Hz), 3.79 (td, J = 16.4, 6.0 Hz), 1.40–1.55 (m, 4H), 1.32 (m, 4H); 13C NMR (100 MHz, DMSO-d6, 293 K) δ/ppm: 156.7, 154.7, 124.6, 106.9, 67.0, 64.4, 60.6, 45.2, 32.4, 32.4, 31.9, 27.9, 25.1; 19F NMR (376 MHz, DMSO-d6, 293 K) δ/ppm: −118.1, −123.4. IR (ATR) ν (cm–1): 3322, 3072, 2944, 2868, 1698, 1543, 1474, 1425, 1267, 1221, 1155, 1125, 979, 862, 781, 734. Anal. calcd for [C14H18F8N2O4]: C, 39.08; H, 4.22; N, 6.51. Found: C, 39.53; H, 4.39; N, 6.23.

Poly(8FHDI-PPG)

A cylindrical flask, equipped with a low-pressure mercury lamp and a magnetic stirring bar, was charged with CHCl3 (30 mL) and 8FHDA·2HCl (2 mmol, 0.68 g). The sample was cooled at 0 °C with an ice bath and vigorously stirred upon bubbling with O2 (0.1 L/min) under exposure to UV light for 2 h. After the photochemical reaction, the lamp was turned off, and 2,6-lutidine (20 mmol, 2.3 mL) was slowly added to the sample solution with a syringe pump. The sample solution was then evaporated at 100 °C to remove the solvent. PPG with the average molecular weight of 400 (2 mmol, 0.8 g) was injected into the sample with a syringe pump, and the sample was stirred at 50 °C for 20 h. The sample was washed with water and extracted with CH2Cl2. The combined organic layer was dried over anhydrous Na2SO4 and evaporated to dryness under reduced pressure. The residue was dried under vacuum at 60 °C for 2 h to give poly(8FHDI-PPG) in 86% yield (1.18 g) as a light brown solid. The average molecular weight was estimated by GPC in comparison with the polystyrene standard: Mw = 8800, Mn = 3400, Mw/Mn = 2.59. 1H NMR (400 MHz, CDCl3, 293 K) δ/ppm: 4.96 (br, 1H), 3.91 (br, 2H), 3.49 (m, 22H), 1.13–1.25 (m, 26H); 13C NMR (100 MHz, DMSO-d6, 293 K) δ/ppm: 156.1, 79.1, 74.7, 72.3, 71.0, 70.8, 70.2, 70.0, 65.2, 20.2, 17.0; 19F NMR (376 MHz, DMSO-d6, 293 K) δ/ppm: −119.0, −123.7. IR (ATR) ν (cm–1): 3314, 2973, 2933, 2873, 1723, 1541, 1453, 1375, 1256, 1161, 1119, 942, 863, 827, 753, 680. Anal. calcd for [C14H18F8N2O5(C3H6O)5]: C, 47.24; H, 6.52; N, 3.80. Found: C, 46.86; H, 6.51; N, 3.89.