The reaction between diphenylmethanols and substituted benzenes is useful to yield triarylmethane derivatives which are important skeletons in various functional materials and biologically relevant substances. The Reactions were carried out under microwave irradiation as environmentally-friendly method. In cyclohexane, the reaction was accelerated under microwave irradiation as compared to under conventional heating. Also, when more than 0.8 equivalents of iron(III) chloride were used, the acceleration was observed. Notably, when iron(III) chloride and arenes were combined, the temperature of the reaction solution rose to 40 °C. It is considered that a chemical species was formed upon coordination of iron(III) chloride to the diphenylmethanols or arenes.
Recently, environmental problems such as global warming caused by energy consumption have garnered concern [
Diarylmethane and triarylmethane derivatives are important skeletons in various functional materials and biologically relevant substances [
Scheme 1. The reaction between benzyl derivatives and arenes.
reported [
In regard to viable catalysts for Friedel-Crafts reactions, various Lewis acids [
In this study, a Friedel-Crafts diphenylmethylation reaction using iron(III) chloride as a catalyst was developed for the environmentally friendly synthesis of triarylmethane derivatives. The reactions were carried out under conventional heating and microwave irradiation, and a mechanism was proposed.
GC data were acquired using a Shimadzu GC-14B with FID detector. Separation of the compounds was carried out using a Shimadzu HiCap-CBP10 (0.2 mm I.D. × 25 m, 0.25 μm film thickness) capillary column, and the carrier gas was nitrogen. The GC oven was programmed at 240˚C - 270˚C.
The substrates were purchased from Wako Chemicals, Ltd. The synthesized triarylmethanes are shown in Scheme 2.
Diphenylmethanols (0.2 mmol) and substituted benzenes (0.2 mmol) were dissolved in 20 mL of cyclohexane in the presence of iron(III) chloride (0.2 mmol). The reaction flask was irradiated with 300 W microwaves. The reaction temperature increased to 35˚C. After the reaction, 30 mL of water was added to the flask, and the reaction mixture was extracted with 20 mL of ethyl acetate. The reaction yields were determined by GC.
Scheme 2. The synthesized triarylmethanes.
For the reactions with diphenylmethanol and 4-chlorodiphenylmethanol, dichloromethane was used as the solvent. In this solution, the temperature reached the boiling point of the solvent.
The reaction temperatures under conventional heating conditions were 35˚C in cyclohexane and reflux in dichloromethane.
The formed triarylmethanes were identified by 1H and 13C NMR. The yield was determined using GC.
Reactions between diphenylmethanol and alkylbenzenes were carried out in dichloromethane (
Therefore, the reactions were carried out in cyclohexane, which has a lower polarity than dichloromethane. However, no reaction occurred with the alkylbenzenes, so the reaction was attempted with several alkoxybenzenes. The yields of the reactions between diphenylmethanol and alkoxybenzenes are shown in
With anisole and phenetol, the product yields under conventional heating were 48% and 50%, respectively. Under the microwave conditions, the yields increased to 76% and 62%, respectively. These results indicated the existence of a microwave irradiation effect.
On the other hand, with 1,2-dimethoxybenzene and 1,4-dimethoxybenzene, there was no difference in the yield, perhaps due to the lower reactivity of these alkoxybenzenes.
The results of the reactions with 4-chlorodiphenylmethanol are summarized in
Entry | Substrate 2 | Conditions | Yields of 3 (%)b |
---|---|---|---|
1 2 3 4 5 6 | R1 = H R2 = H R3 = H R1 = Me R2 = H R3 = H R1 = Me R2 = Me R3 = Me | Irradiation of MW (300 W) Conventional Heating Irradiation of MW (300 W) Conventional Heating Irradiation of MW (300 W) Conventional Heating | 4 5 63 (p:o = 5:1) 60 (p:o = 5:1) 69 70 |
a. [
Entry | Substrate 2 | Conditions | Yields of 3 (%) |
---|---|---|---|
1 2 3 4 5 6 7 8 | R1 = OMe R2 = H R3 = H R1 = OEt R2 = H R3 = H R1 = OMe R2 = OMe R3 = H R1 = H R2 = OMe R3 = OMe | Irradiation of MW (300 W) Conventional Heating Irradiation of MW (300 W) Conventional Heating Irradiation of MW (300 W) Conventional Heating Irradiation of MW (300 W) Conventional Heating | 74 (p:o = 3:1) 48 (p:o = 3:1) 62 (p:o = 4:1) 50 (p:o = 4:1) 65 60 27 25 |
a. [
Entry | Substrate 2 | Conditions | Yields of 3 (%) |
---|---|---|---|
1 2 3 4 5 6 7 8 | R1 = OMe R2 = H R3 = H R1 = OEt R2 = H R3 = H R1 = OMe R2 = OMe R3 = H R1 = H R2 = OMe R3 = OMe | Irradiation of MW (300 W) Conventional Heating Irradiation of MW (300 W) Conventional Heating Irradiation of MW (300 W) Conventional Heating Irradiation of MW (300 W) Conventional Heating | 55 (p:o = 5:1) 40 (p:o = 5:1) 54 (p:o = 4:1) 38 (p:o = 4:1) 70 67 25 21 |
a. [
difference in the yields was found under conventional heating or microwave conditions.
There was no difference in the yields in the reactions with 4-methlydiphe-nylmethanol under conventional heating or microwave conditions, owing to the high reactivity caused by the high electron donating effect of the methyl group (
The microwave irradiation effect was observed in the reactions with 4,4’-dichlorodipenylmethanol, as well as for the reactions with diphenylmethanol and 4-chloromethanol (
The amounts of iron(III) chloride were varied in the reaction between diphenylmethanol and anisole (
Entry | Substrate 2 | Conditions | Yields of 3 (%) |
---|---|---|---|
1 2 3 4 5 6 7 8 | R1 = OMe R2 = H R3 = H R1 = OEt R2 = H R3 = H R1 = OMe R2 = OMe R3 = H R1 = H R2 = OMe R3 = OMe | Irradiation of MW (300 W) Conventional Heating Irradiation of MW (300 W) Conventional Heating Irradiation of MW (300 W) Conventional Heating Irradiation of MW (300 W) Conventional Heating | 71 (p:o = 7:1) 60 (p:o = 7:1) 55 (p:o = 10:1) 57 (p:o = 10:1) 55 52 26 29 |
a. [
accelerating effect of the microwave was observed. Based on these results, an equivalent amount of iron(III) chloride was necessary for the accelerating effect.
The reaction temperature versus irradiation time plot is shown in
substrates alone or iron(III) chloride alone, the reaction temperature was 24˚C; no temperature increase was observed (
In cyclohexane, which is a low polarity solvent, microwave irradiation was found to accelerate the reaction between diphenylmethanol and alkoxybenzenes. However, when methylene chloride, which has a higher polarity, was used, no differences in the product yields were observed under conventional heating or microwave irradiation, perhaps due to the fact that methylene chloride absorbs the microwaves, and the substrates were not affected by the microwave energy. Furthermore, when more than 0.8 equivalents of iron(III) chloride were used, an accelerating effect was observed under microwave irradiation. Moreover, when
Entry | Substrate 2 | Conditions | Yields of 3 (%) |
---|---|---|---|
1 2 3 4 5 6 7 8 | R1 = OMe R2 = H R3 = H R1 = OEt R2 = H R3 = H R1 = OMe R2 = OMe R3 = H R1 = H R2 = OMe R3 = OMe | Irradiation of MW (300 W) Conventional Heating Irradiation of MW (300 W) Conventional Heating Irradiation of MW (300 W) Conventional Heating Irradiation of MW (300 W) Conventional Heating | 38 (p:o = 5:1) 21 (p:o = 7:1) 42 (p:o = 5:1) 32 (p:o = 5:1) 52 54 26 20 |
a. [
the arenes and iron(III) chloride were present, the reaction temperature increased to 40˚C. Therefore, iron(III) chloride plays an important role in accelerating the reaction. Namely, the chemical species formed upon coordination of iron(III) chloride to the arenes selectively absorbs the microwaves. Therefore, local heating around these chemical species would occur, and the acceleration effect would be affected by the increased temperature.
Okada, Y. and Yamabe, M. (2018) Effect of Microwave Irradiation on Friedel-Crafts Diphenylmethylation of Arenes. Green and Sustainable Chemistry, 8, 130-138. https://doi.org/10.4236/gsc.2018.81009