Gold-catalysed process for manufacturing chromenes intended for the preparation of thermosetting resins

Abstract

A process for manufacturing chromenes intended for the preparation of thermosetting resins, includes transforming an aromatic propargyl ether of general formula (I) into a chromene by homogeneous gold(I) catalysis with the catalyst (acetonitrile)[(2-biphenyl)di-tert-butylphosphine]gold(I) hexafluoroantimonate in an organic solvent under an inert or non-inert atmosphere. Moreover, a process for preparing a material made of thermoset resin, includes successively a) implementation of the above process; polymerization of the reaction product obtained in step a) so as to obtain the material made of thermoset resin; c) recovery of the material made of thermoset resin obtained in step b).

Claims

1. A process for manufacturing chromenes which are intended for the preparation of thermosetting resins, comprising the step of transforming an aromatic propargyl ether of general formula (I) below ##STR00013## in which: R.sub.1 and R.sub.5 represent, independently of each other, a hydrogen atom, a C.sub.2-C.sub.6 alkene, C.sub.2-C.sub.6 alkyne, O—(C.sub.1-C.sub.6)alkyl, O—(C.sub.2-C.sub.6)alkene or O—(C.sub.2-C.sub.6)alkyne group, on condition that at least one from among R.sub.1 and R.sub.5 represents a hydrogen atom and that the groups R.sub.1 and R.sub.5 do not represent an O-propargyl group; R.sub.2 and R.sub.1 represent, independently of each other, a hydrogen atom, a C.sub.2-C.sub.6 alkene, C.sub.2-C.sub.6 alkyne, O—(C.sub.1-C.sub.6)alkyl, O—(C.sub.2-C.sub.6)alkene or O—(C.sub.2-C.sub.6)alkyne; and R.sub.3 represents a hydrogen atom or a C.sub.2-C.sub.6 alkene group, the alkene group being optionally substituted with a group of general formula (II) below ##STR00014## in which: R.sub.6 and R.sub.9 represent, independently of each other, a hydrogen atom, a C.sub.2-C.sub.6 alkene, C.sub.2-C.sub.6 alkyne, O—(C.sub.1-C.sub.6)alkyl, O—(C.sub.2-C.sub.6)alkene or O—(C.sub.2-C.sub.6)alkyne group, on condition that at least one from among R.sub.6 and R.sub.9 represents a hydrogen atom; and R.sub.7 and R.sub.8 represent, independently of each other, a hydrogen atom, a C.sub.2-C.sub.6 alkene, C.sub.2-C.sub.6 alkyne, O—(C.sub.1-C.sub.6)alkyl, O—(C.sub.2-C.sub.6)alkene or O—(C.sub.2-C.sub.6)alkyne; on condition that at least one from among R.sub.1, R.sub.2, R.sub.3, R.sub.1 and R.sub.5 does not represent a hydrogen atom or a O—(C.sub.1-C.sub.6)alkyl group; and the cis/trans isomers thereof and the optical isomers thereof and the racemic mixtures thereof into a chromene by homogeneous gold(I) catalysis with the catalyst (acetonitrile)[(2-biphenyl)di-tert-butylphosphine]gold(I) hexafluoroantimonate in an organic solvent under an inert or non-inert atmosphere wherein the aromatic propargyl ether of general formula (I) is chosen from the group consisting of propargylated resorcinol, propargylated eugenol, propargylated coupled eugenol, propargylated coupled isoeugenol, propargylated isoeugenol and mixtures thereof and the cis/trans isomers thereof and the optical isomers thereof and the racemic mixtures thereof.

2. The process according to claim 1, wherein the molar percentage of residual propargyl functions in the chromene is less than 11% when the aromatic propargyl ether of general formula (I) is propargylated resorcinol, the molar percentage of residual propargyl functions in the chromene is less than 39% when the aromatic propargyl ether of general formula (I) is propargylated coupled eugenol and the molar percentage of residual propargyl functions in the chromene is less than 35% when the aromatic propargyl ether of general formula (I) is propargylated coupled isoeugenol.

3. The process according to claim 1, wherein the organic solvent is chosen from tetrahydrofuran and 2-methyltetrahydrofuran.

4. The process according to claim 1, wherein the content of catalyst in the reaction medium is between 0.1 mol % and 2 mol %.

5. The process according to claim 4, wherein the content of catalyst in the reaction medium is between 0.1 mol % and 0.5 mol %.

6. The process according to claim 1, wherein the aromatic propargyl ether of general formula (I) is propargylated resorcinol and wherein the chromene obtained has the formula C and/or D below ##STR00015##

7. The process according to claim 1, wherein the catalyst is removed from the reaction medium at the end of the process.

8. A process for preparing a material made of thermoset resin, comprising the following successive steps: a) implementation of the process according to claim 1; b) polymerization of the reaction product obtained in step a) so as to obtain the material made of thermoset resin; c) recovery of the material made of thermoset resin obtained in step b).

9. The process according to claim 8, wherein the enthalpy of polymerization of step b) is less than 500 J/g.

10. The process according to claim 8, wherein the coke content of the thermoset resin obtained in step c) is greater than 50%.

11. The process according to claim 1, wherein the aromatic propargyl ether of general formula (I) is propargylated resorcinol.

12. The process according to claim 6, wherein the chromene obtained is a mixture of the formulae C and D.

Description

EXAMPLE 1

Conversion of Propargylated Resorcinol and Preparation of the Resin According to the Invention

(1) Synthesis of Propargylated Resorcinol

(2) 10 g (0.091 mol) of resorcinol (Alfa Aesar) are dissolved in 50 mL of dimethyl sulfoxide (DMSO), 50 g (0.363 mol) of potassium carbonate (K.sub.2CO.sub.3) are ground and then added with magnetic stirring, and the mixture is heated to 70° C. (ext). 14.45 mL (2.2 eq.) of propargyl chloride (ABCR) are added dropwise. The reaction is monitored by TLC with a 7/3 (volume) petroleum ether/diethyl ether eluent. After filtration and dilution in 100 mL of ethyl acetate, the medium is extracted with 3×100 mL of brine. The organic phase is dried over MgSO.sub.4, filtered and concentrated under reduced pressure. The compound is purified by vacuum distillation (T° C.=120° C., 4.5 Pa). The yield is 77.4%.

(3) Conversion of Propargylated Resorcinol

(4) Weigh out 0.0415 g (1 mol %) of catalyst (acetonitrile)[(2-biphenyl)di-tert-butylphosphine]gold(I) hexafluoroantimonate (ABCR) in a pill bottle without any air exclusion precautions. 1 g (0.0054 mol) of propargylated resorcinol obtained previously is introduced into a Schlenk tube and then sealed. The medium is placed under inert atmosphere by successive emptying/filling with argon, at least three times. 50 mL of dry dichloromethane (DCM) (purified with a PureSolv MD7 device) are added by syringe. The catalyst is introduced against an argon counter-current using the pill bottle, and the medium is then conserved under argon. The reaction is monitored by TLC with a 9/1 (volume) petroleum ether/ethyl acetate eluent. After completion of the reaction (time: less than 1 minute), the medium is filtered through a thin bed of silica to remove the catalyst. The proportion of residual propargyl ether functions is estimated by .sup.1H NMR at 0%. The yield is 65.8%. The molar proportion of chromene of formula C in the mixture is 50%. The molar proportion of chromene of formula D in the mixture is 50%. 6% of the compound 2-methylbenzofuran are present in the product. The product was not purified further, and is used in crude form. The proportion of residual propargyl ether functions of less than 11% complies with the set specifications.

(5) Polymerization of the Chromene Obtained from Propargylated Resorcinol

(6) The polymerization of the propargyl-chromene mixtures is performed by gradual raising of the temperature. In the case of a propargyl resorcinol-chromene mixture with a proportion of residual propargyl functions of less than 11% as obtained previously, the heat treatment applied is as follows: 2 hours at 80° C., 2 hours at 100° C., 2 hours at 110° C., 2 hours at 120° C., 2 hours at 130° C. and 2 hours at 150° C.

(7) Annealing at 220° C. may be performed to increase the thermomechanical properties.

(8) The coke content obtained before annealing is 63%.

EXAMPLE 2

Conversion of Propargylated Resorcinol

(9) Weigh out 0.0207 g (0.5 mol %) of catalyst (acetonitrile)[(2-biphenyl)di-tert-butylphosphine]gold(I) hexafluoroantimonate (ABCR) in a pill bottle without any air exclusion precautions. 1 g (0.0054 mol) of propargylated resorcinol obtained according to the process indicated in Example 1 is introduced into a Schlenk tube. 50 mL of dichloromethane (DCM) are added without taking any air exclusion precautions. The catalyst is introduced using the pill bottle without taking any air exclusion precautions. The reaction is monitored by TLC with a 9/1 (volume) petroleum ether/ethyl acetate eluent. After completion of the reaction, the medium is filtered through a thin bed of silica to remove the catalyst. The proportion of residual propargyl ether functions is estimated by .sup.1H NMR at 8%. The yield is 74%. 4.7% of the compound 2-methylbenzofuran are present in the product. The product was not purified further, and is used in crude form. The proportion of residual propargyl ether functions of less than 11% complies with the set specifications.

EXAMPLE 3

Conversion of Propargylated Resorcinol

(10) Weigh out 0.0124 g (0.3 mol %) of catalyst (acetonitrile)[(2-biphenyl)di-tert-butylphosphine]gold(I) hexafluoroantimonate (ABCR) in a pill bottle without any air exclusion precautions. 1 g (0.0054 mol) of propargylated resorcinol obtained according to the process indicated in Example 1 is introduced into a Schlenk tube and then closed. The medium is placed under inert atmosphere by successive emptying/filling with argon, at least three times. 10 mL of dry tetrahydrofuran (THF) (purified with a PureSolv MD7 device) are added by syringe. The catalyst is introduced against an argon counter-current using the pill bottle, and the medium is then conserved under argon. The reaction is monitored by TLC with a 9/1 (volume) petroleum ether/ethyl acetate eluent. After completion of the reaction, the medium is filtered through a thin bed of silica to remove the catalyst. However, this catalyst is not retained. The proportion of residual propargyl ether functions is estimated by .sup.1H NMR at 0%. The yield is 99.9%. The molar proportion of chromene of formula C in the mixture is 50%. The molar proportion of chromene of formula D in the mixture is 50%. 1.9% of the compound 2-methylbenzofuran are present in the product. The product was not purified further, and is used in crude form. The proportion of residual propargyl ether functions of less than 11% complies with the set specifications.

EXAMPLE 4

Conversion of Propargylated Resorcinol

(11) Weigh out 0.0124 g (0.3 mol %) of catalyst (acetonitrile)[(2-biphenyl)di-tert-butylphosphine]gold(I) hexafluoroantimonate (ABCR) in a pill bottle without any air exclusion precautions. 1 g (0.0054 mol) of propargylated resorcinol obtained according to the process indicated in Example 1 is introduced into a Schlenk tube and then closed. The medium is placed under inert atmosphere by successive emptying/filling with argon, at least three times. 10 mL of methyltetrahydrofuran (Me-THF) are added by syringe. The catalyst is introduced against an argon counter-current using the pill bottle, and the medium is then conserved under argon. The reaction is monitored by TLC with a 9/1 (volume) petroleum ether/ethyl acetate eluent. After completion of the reaction, the medium is filtered through a thin bed of silica to remove the catalyst. However, this catalyst is not retained. The proportion of residual propargyl ether functions is estimated by .sup.1H NMR at 1.5%. The yield is 86%. The molar proportion of chromene of formula C in the mixture is 55%. The molar proportion of chromene of formula D in the mixture is 45%. 7.1% of the compound 2-methylbenzofuran are present in the product. The product was not purified further, and is used in crude form. The proportion of residual propargyl ether functions of less than 11% complies with the set specifications.

EXAMPLE 5

Conversion of Propargylated Resorcinol

(12) Weigh out 0.1244 g (0.3 mol %) of catalyst (acetonitrile)[(2-biphenyl)di-tert-butylphosphine]gold(I) hexafluoroantimonate (ABCR) in a pill bottle without any air exclusion precautions. 10 g (0.054 mol) of propargylated resorcinol obtained according to the process indicated in Example 1 are introduced into a Schlenk tube and then closed. The medium is placed under inert atmosphere by successive emptying/filling with argon, at least three times. 100 mL of dry tetrahydrofuran (THF) (purified with a PureSolv MD7 device) are added by syringe. The catalyst is introduced against an argon counter-current using the pill bottle, and the medium is then conserved under argon and heated to 50° C. The reaction is monitored by TLC with a 9/1 (volume) petroleum ether/ethyl acetate eluent. After reaction for 4 hours 30 minutes, the medium is filtered through a thin bed of silica to remove the catalyst. However, this catalyst is not retained. The proportion of residual propargyl ether functions is estimated by .sup.1H NMR at less than 1%. The yield is greater than 97%. The molar proportion of chromene of formula C in the mixture is 50%. The molar proportion of chromene of formula D in the mixture is 50%. 3.3% of the compound 2-methylbenzofuran are present in the product. The product was not purified further, and is used in crude form. The proportion of residual propargyl ether functions of less than 11% complies with the set specifications.

EXAMPLE 6

Conversion of Propargylated Eugenol and Preparation of the Resin According to the Invention

(13) Synthesis of Propargylated Eugenol

(14) Eugenol (Sigma-Aldrich) (200 g), K.sub.2CO.sub.3 (211 g) and dimethylformamide (DMF) (2000 mL) are placed in a 6 L round-bottomed flask and heated to 75° C. with mechanical stirring. Propargyl chloride (ABCR) at 70% in toluene (158.5 mL) is added dropwise via a dropping funnel, and the reaction medium is heated and stirred at 75° C. overnight. The reaction is monitored by TLC, eluting with 7/3 (volume) petroleum ether/diethyl ether. After the reaction, the reaction medium is filtered and then diluted and rinsed with ethyl acetate. The organic phase is rinsed with water until the aqueous phase has decolourized (four times). The organic phase is dried over MgSO.sub.4 and concentrated under vacuum. The yield of crude product is 93%. The compound is purified by vacuum distillation (p=4.5 Pa and T° C.=60° C.). The yield of the distilled compound is 90%.

(15) Conversion of Propargylated Eugenol

(16) Weigh out 0.076 g (0.1 mol %) of catalyst (acetonitrile)[(2-biphenyl)di-tert-butylphosphine]gold(I) hexafluoroantimonate (ABCR) in a pill bottle without any air exclusion precautions. 20 g (0.0054 mol) of propargylated eugenol obtained previously are introduced into a 250 mL three-necked round-bottomed flask and then sealed. The medium is placed under an inert atmosphere with a gentle stream of argon. 100 mL of dry dichloromethane (purified with a PureSolv MD7 device) are added by syringe. The catalyst is introduced against an argon counter-current using the pill bottle, and the medium is then conserved under argon and placed in an oil bath preheated to 40° C. The reaction is monitored by TLC with a 9/1 (volume) petroleum ether/ethyl acetate eluent. After reaction for 1 hour, the medium is filtered through a thin bed of silica to remove the catalyst. The proportion of residual propargyl ether functions is estimated by .sup.1H NMR at less than 1%. The yield is greater than 95%. 11% of the compound 2-methylbenzofuran are present in the product. The eugenol chromene is isolated by column chromatography with a 9/1 (volume) petroleum ether/ethyl acetate eluent. The yield of the pure product is 65%.

(17) Polymerization of the Chromene Obtained from Propargylated Coupled Eugenol

(18) The process performed is identical to that described in Example 1 for propargylated resorcinol, except for the fact that the heat treatment applied is as follows: 1 hour at 100° C., 1 hour 30 minutes at 150° C., 3 hours 30 minutes at 210° C. and 1 hour at 200° C.

EXAMPLE 7

Conversion of Propargylated Coupled Eugenol and Preparation of the Resin According to the Invention

(19) Synthesis of the Coupled Eugenol

(20) 26.675 g (0.1625 mol) of eugenol (Sigma-Aldrich) are placed in a 50 mL three-necked round-bottomed flask with a magnetic stirrer. 0.1337 g (0.1 mol %) of first-generation Grubbs catalyst (Sigma-Aldrich) is introduced using a pill bottle against a counter-current of argon. The medium is directly placed under high vacuum (3 kPa) and left stirring for 12 hours. A .sup.1H NMR spectrum of the crude product is taken to determine the conversion of the eugenol. The conversion is 67 mol % of coupled eugenol compound, the remaining 37 mol % being a mixture of unreacted eugenol and of its isomer, isoeugenol. The stoichiometric proportion is evaluated by 1H NMR at 34.3%/65.7% for the cis- and trans-coupled eugenol compounds, respectively. The medium is dissolved in a minimum amount of refluxing ether and then left to stand at room temperature. The solid is filtered off under reduced pressure on a No. 4 porosity sinter, and then washed with 4×20 mL of cyclohexane. The solid is dried under high vacuum using a vane pump, for 10 hours. In order to remove the first-generation Grubbs catalyst, the solid is dissolved in dichloromethane (DCM) and then filtered through Celite. A black deposit is observed on the Celite. The organic phase is dried under reduced pressure. The yield obtained is 25%.

(21) Synthesis of Propargylated Coupled Eugenol

(22) 3 g (0.010 mol) of coupled eugenol obtained previously are dissolved in 30 mL (10 eq.) of DMF. 5.52 g (4 eq.) of finely ground potassium carbonate (K.sub.2CO.sub.3) are added with magnetic stirring. 2.78 mL (2.5 eq.) of propargyl bromide (Alfa Aesar) (80 mol % in toluene) are added via a dropping funnel. Magnetic stirring is continued for 12 hours. Completion of the reaction is monitored by TLC, eluting with 50/50 (volume) petroleum ether/ethyl acetate. After filtering off the K.sub.2CO.sub.3 and washing with DMF, an excess of distilled water (200 mL) is added to precipitate the product. The solid is recovered. 200 mL of ethyl acetate are added to the medium for the extraction. The aqueous phase is discarded. The solid recovered previously is redissolved in the organic phase. The organic phase is washed three times with distilled water (3×100 mL) and once with brine (1×100 mL). The organic phase is dried over MgSO.sub.4, filtered and concentrated under reduced pressure. The yield is 90%.

(23) Conversion of Propargylated Coupled Eugenol

(24) Weigh out 0.0308 g (1 mol %) of catalyst (acetonitrile)[(2-biphenyl)di-tert-butylphosphine]gold(I) hexafluoroantimonate (ABCR) in a pill bottle without any air exclusion precautions. Introduce 1.5 g (0.0040 mol) of propargylated coupled eugenol obtained previously into a 100 mL Schlenk tube and perform three vacuum/argon cycles. Next, add 10 mL of dry dichloromethane (purified with a PureSolv MD7 device) by syringe. Introduce the catalyst against a counter-current of argon and then seal the medium under argon. The reaction is monitored by TLC with a 9/1 (volume) petroleum ether/ethyl acetate eluent. After reaction for 5 minutes, the medium is filtered through a No. 4 porosity sinter with a short bed of silica, using DCM. The DCM is evaporated off under reduced pressure on a rotary evaporator. The conversion is 100%. The proportion of residual propargyl ether functions is estimated by .sup.1H NMR at 0%. The yield of crude product is 77.6%. The crude reaction product is composed of 15% of the compound 2-methylbenzofuran. The product is purified by column chromatography with a 9/1 (volume) petroleum ether/ethyl acetate eluent. The pure coupled eugenol chromene is obtained in a yield of 40%. The proportion of residual propargyl ether functions of less than 39% complies with the set specifications.

(25) Polymerization of the Chromene Obtained from Propargylated Coupled Eugenol

(26) The process performed is identical to that described in Example 1 for propargylated resorcinol, except for the fact that the heat treatment applied is as follows: 1 hour at 80° C., 2 hours at 150° C., 2 hours at 220° C. The coke content obtained before annealing is 56%.

EXAMPLE 8

Conversion of Propargylated Coupled Isoeugenol and Preparation of the Resin According to the Invention

(27) Synthesis of the Coupled Isoeugenol

(28) 0.0181 g (0.017%) of Grubs II catalyst (Umicore M2a) is placed in a 50 mL round-bottomed flask with a magnetic stirrer, and 20 g (0.122 mol) of isoeugenol (Sigma-Aldrich) are then added. The medium is placed under a stream of argon and heated at 90° C. The medium turns solid after 3 minutes of reaction. After cooling, a .sup.1H NMR spectrum of the crude product is taken to determine the conversion of the isoeugenol to stilbene. The degree of conversion is 90%. Only the trans-compound is observed. The product is recovered by suspension in 4 volumes of DCM. The medium is refluxed for 1 hour until dissolution is complete, and is then left to stand at room temperature overnight. The suspension is filtered through a sinter and washed with 1 volume of cyclohexane. The isolated yield is 67%.

(29) Synthesis of Propargylated Coupled Isoeugenol

(30) 10 g (0.037 mol) of eugenol stilbene obtained previously are dissolved in 100 mL (10 eq.) of DMF. 25 g (4.5 eq.) of finely ground potassium carbonate (K.sub.2CO.sub.3) are added with magnetic stirring. 10.23 mL (2.5 eq.) of propargyl bromide (Alfa Aesar) (80 mol % in toluene) are added by syringe. Magnetic stirring is continued for 12 hours. Completion of the reaction is monitored by TLC, eluting with 50/50 (volume) petroleum ether/ethyl acetate. The conversion is total after reaction overnight. The K.sub.2CO.sub.3 is filtered off and then washed with ethyl acetate. The compound is extracted with 2×100 mL of ethyl acetate. The organic phases are washed with 4×100 mL of brine. The organic phases are dried over MgSO.sub.4, filtered and concentrated under reduced pressure. The yield of the product is 30%.

(31) Conversion of the Propargylated Coupled Isoeugenol

(32) Weigh out 0.0054 g (0.5 mol %) of catalyst (acetonitrile)[(2-biphenyl)di-tert-butylphosphine]gold(I) hexafluoroantimonate (ABCR) in a pill bottle without any air exclusion precautions. Introduce 0.4864 g (0.0014 mol) of propargylated eugenol stilbene obtained previously into a 100 mL Schlenk tube and perform three vacuum/argon cycles. Next, add 10 mL of dry DCM (purified with a PureSolv MD7 device) by syringe. Introduce the catalyst against a counter-current of argon. Add a balloon filled with argon. The medium is heated to between 30 and 40° C. to fully dissolve the propargylated eugenol stilbene in the DCM. The reaction is monitored by TLC with a 5/5 (volume) petroleum ether/ethyl acetate eluent. After the reaction, the medium is filtered through a No. 4 porosity sinter with a short bed of silica, using DCM. The DCM is evaporated off under reduced pressure on a rotary evaporator. The proportion of residual propargyl ether functions is estimated by .sup.1H NMR at less than 1%. The yield of crude product is 56%. The crude reaction product is composed of 14% of compound of the 2-methylbenzofuran type, estimated by .sup.1H NMR. The product was not purified further, and is used in crude form. The proportion of residual propargyl ether functions of less than 35% complies with the set specifications.

(33) Polymerization of the Chromene Obtained from Propargylated Coupled Isoeugenol

(34) The process performed is identical to that described in Example 1.

(35) The enthalpy of reaction is 170 J/g.

EXAMPLE 9

Conversion of Propargylated Isoeugenol and Preparation of the Resin According to the Invention

(36) Synthesis of Propargylated Isoeugenol

(37) 20 g (0.130 mol) of isoeugenol (Sigma-Aldrich) are dissolved in 100 mL (5 eq.) of DMF. 33.67 g (2 eq.) of finely ground potassium carbonate (K.sub.2CO.sub.3) are added with magnetic stirring. 20.35 mL (1.5 eq.) of propargyl bromide (Alfa Aesar) (80 mol % in toluene) are added via a dropping funnel. Magnetic stirring is continued for 12 hours. Completion of the reaction is monitored by TLC, eluting with 70/30 (volume) petroleum ether/ethyl acetate. After filtering off the K.sub.2CO.sub.3 and washing with ethyl acetate, 100 mL of ethyl acetate are added to the medium for the extraction. The organic phase is washed three times with distilled water (3×100 mL) and once with brine (1×100 mL). The organic phase is dried over MgSO.sub.4, filtered and concentrated under reduced pressure. The yield is 91%.

(38) The compound is purified by vacuum distillation in a Kugelrohr glass oven (p=15 Pa and heating T° C.=140° C.). The compound is recovered in the form of white crystals. The overall yield after purification is 73%.

(39) Conversion of the Propargylated Isoeugenol

(40) Weigh out 0.3818 g (0.0005 mol) of catalyst (acetonitrile)[(2-biphenyl)di-tert-butylphosphine]gold(I) hexafluoroantimonate (ABCR) in a pill bottle without any air exclusion precautions. Introduce 10 g (0.05 mol) of propargylated isoeugenol obtained previously into a 100 mL three-necked round-bottomed flask on which is mounted a condenser. The medium is placed under argon via one of the inlets of the three-necked flask. 40 mL of dry DCM (purified with a PureSolv MD7 device) are added by syringe. The catalyst is introduced against an argon counter-current using the pill bottle by the second inlet of the three-necked flask and the medium is conserved under argon. The reaction is monitored by TLC with a 9/1 (volume) petroleum ether/ethyl acetate eluent. After 5 minutes, the conversion is total: the proportion of the compound 2-methylbenzofuran is estimated by .sup.1H NMR at 13.6%. The medium is concentrated under reduced pressure and then purified by column chromatography by solid deposition, with a 9/1 (volume) petroleum ether/ethyl acetate eluent. The yield of isoeugenol chromene is 69%.

(41) It is possible to reduce the amount of 2-methylbenzofuran formed by performing the reaction at low temperature (0° C.). After 2 hours, the conversion is total: the proportion of the compound 2-methylbenzofuran is estimated by .sup.1H NMR at 4.2%.

(42) Polymerization of the Chromene Obtained from Propargylated Isoeugenol

(43) The process performed is identical to that described in Example 1.