Versatile CO₂ Transformations into Complex Products: A One-pot Two-step Strategy

This protocol is significant because we can control the full electron reduction of CO2 and we can use the product form as a reactive and versatile intermediate in one-pot cascade reactions. Thanks to the crucial CO2 reduction step and cascade strategy, this technique and reverse probing of the intermediate reactivity leading to the synthesis of carbon compound from CO2 for the first step. The reaction mixture preparation is a meticulous process that involves multiple additions to a reaction vessel inside the glove box, CO2 pressurization, and a strict time and temperature control.

This cascade strategy requires subsequent reaction steps, the use of a glove box and gas handling, all of which are easier to understand through visual demonstration. For synthesis of compound two, charge an NMR tube with 15.9 milligrams of nine BBN, 130 microliters of a 10%molar hexamethyl benzene as an internal standard, and 100 microliters of a 1%molar iron catalyst solution in deuterated tetrahydrofuran. Add 0.37 milliliters of additional deuterated tetrahydrofuran to the tube before closing the tube and connecting it to a gas system outside of the glove box.

Place the NMR tube in a 25 degree Celsius bath for 15 minutes. Then add one atmosphere of carbon dioxide for three minutes before closing the tube for a 45-minute incubation at 25 degree Celsius. During the equilibration, prepare a stock solution by mixing 189 microliters of freshly distilled 2, 6-diisopropylethylamine, and 811 microliters of fresh deuterated tetrahydrofuran.

When compound one has been generated, open the NMR tube inside the glove box, and add 55 microliters of the prepared 2, 6-diisopropylethylamine solution to the tube. Then close and shake the tube for 10 seconds. After 20 minutes, confirm the formation of imine 2 by proton NMR analysis and determine the NMR yield by comparison of the signal to the internal standard.

For synthesis of compound three, charge the reaction vessel containing a magnetic stirring bar with 9.4 milligrams of iron catalyst, 320 milligrams of 9-BBN, And 10 milliliters of tetrahydrofuran inside a glove box. After closing the reaction vessel, remove it from the glove box for connection to the gas system. Add one atmosphere of carbon dioxide to the vessel for three minutes before closing the vessel for a 45-minute stirring incubation at 25 degree Celsius.

At the end of the reaction, open the reaction vessel in the glove box, and add a solution 380 milligrams of triazol-5-ylidene in six milliliters of tetrahydrofuran to the mixture. Then charge the reaction with three atmospheres of carbon dioxide and stirring for 60 minutes at 60 degrees Celsius. At the end of the reaction, when the solution has cooled to room temperature, remove the volatiles under vacuum and wash the residue three times with two milliliters of diethyl ether at zero degrees Celsius to obtain carbon dioxide adduct three as a white powder.

For compound four synthesis in a glove box, charge a new reaction vessel containing a magnetic stir bar with 159 milligrams of 9-BBN, 10%molar hexamethyl benzene as an internal standard, and 4.7 milligrams of iron catalyst followed by the addition of five milliliters of tetrahydrofuran. Place the closed vessel outside of the glove box for a 15-minute equilibration at 25 degree Celsius before connecting the vessel to the gas system for three minutes under a dynamic pressure of one atmosphere carbon dioxide. At the end of the pressurization, close the vessel for a 45-minute stirring incubation at 25 degrees Celsius.

When compound one has been generated, open the reaction vessel in the glove box, and add 54 milligrams of triazol-5-ylidene. Outside of the glove box, stir the solution at 80 degrees Celsius for 40 minutes to generate mixture of compounds containing compound four, and remove the solvent under vacuum. Then dissolve part of the residue in 0.6 milliliters of deuterated tetrahydrofuran, and strain the solution through a syringe equipped with a 0.2 micrometer polytetrafluoroethylene filter into an NMR tube for analysis.

A successful generation of bis(boryl)acetal compound one, as assessed by proton NMR analysis, results in a characteristic methylamine peak at 5.4 parts per million in deuterated tetrahydrofuran. A successful generation of compound two presents a characteristic AB peak set at 7.30 parts per million for the two inequivalent methylamine protons in deuterated tertrahydrofuran. In a successful generation of compound three, the most notable signals are the methene peak connected to the carbene at 5.34 parts per million and the methene peaks of the BBN fragment at 0.26 and minus 0.65 parts per million.

A successful synthesis of compound four in situ generated from carbon dioxide is notably characterized by a doublet at 4.64 parts per million in a pseudo-triplet at 3.36 parts per million. The aliphatic C3 chain of the isolated compound four from DL-glyceraldehyde is characterized in proton NMR by four proton signals, and in carbon 13 NMR by three carbon signals. The reduction steps are sensitive to changes in the protocol, and the reproducibility must be probed notably by the reproductive synthesis of compound two in good yield.

Please be sure to check the safety sheets for any hazardous material and gases to take precaution regarding handling of gases, and to avoid the building of overpressure.