Method for Preparing Methoxyboranes and for Producing Methanol

Abstract

The present disclosure relates to a method for preparing methoxyboranes by dismutation of formic acid or at least one of the derivatives thereof or a mixture of formic acid and at least one of the derivatives thereof, in the presence of an organoborane, and optionally an organic or inorganic base.

Claims

1. A method for preparing methoxyboranes according to formula (I) ##STR00012## wherein R.sup.1 and R.sup.2, independently of one another, are chosen in the group formed by a hydroxyl group, an alkoxy group, an alkyl group, an alkenyl group, an alkynyl group, an aryl group, a heteroaryl group, a heterocyclic group, a halogen group, a silyl group, a siloxy group, a phosphino group, and an amino group, said alkyl, alkenyl, alkynyl, alkoxy, silyl, siloxy, aryl, heteroaryl, heterocyclic, phosphino and amino groups being optionally substituted; or R.sup.1 and R.sup.2 taken together with the boron atom to which they are bound, form an optionally substituted heterocycle; by dismutation of formic acid or of at least one of the derivatives thereof having the formula HCO.sub.2M wherein M is chosen in the group formed by Na.sup.+, K.sup.+, Li.sup.+, Cs.sup.+, NH.sub.4.sup.+, triethylammonium (HNEt.sub.3.sup.+), tetraphenylphosphonium (PPh.sub.4.sup.+), tetramethylammonium (NMe.sub.4.sup.+), tetraethylammonium (NEt.sub.4.sup.+), tetrabutylammonium (NBu.sub.4.sup.+) and tetraphenylammonium (NPh.sub.4.sup.+), or of a mixture of formic acid and of at least one of the derivatives thereof, in the presence of an organoborane according to formula (II) ##STR00013## wherein R.sup.1 and R.sup.2 are as defined for the methoxyborane compounds according to formula (I); X is chosen in the group formed by a hydrogen atom, a halogen atom, a carboxylate group, a sulphonate group, a hydroxyl, an alkoxy group, an alkyl group, an alkenyl group, an alkynyl group, an aryl group, a heteroaryl group, a heterocyclic group, a silyl group, a siloxy group, a phosphino group, an amino group, said alkyl, alkenyl, alkynyl, alkoxy, silyl, siloxy, aryl, heteroaryl, heterocyclic, phosphino and amino groups being optionally substituted; and optionally of an organic or inorganic base.

2. The method according to claim 1, wherein the formic acid and derivatives thereof are prepared by 2 e.sup. electro-reduction or catalytic hydrogenation of CO.sub.2.

3. The method according to claim 1, wherein in the methoxyborane according to formula (I) and the organoborane according to formula (II), R.sup.1 and R.sup.2, independently of one another, are chosen in the group formed by an alkyl group comprising 1 to 12 carbon atoms; an aryl comprising 6 to 20 carbon atoms, said alkyl and aryl groups being optionally substituted.

4. The method according to claim 1, wherein in the methoxyborane according to formula (I) and the organoborane according to formula (II), R.sup.1 and R.sup.2 taken together with the boron atom to which they are bound, form a heterocycle comprising 5 to 10 members, said heterocycle being optionally substituted.

5. The method according to claim 1, wherein in the organoborane according to formula (II), X is chosen in the group formed by a hydrogen atom; a carboxylate group having the formula OCOR.sup.8 wherein R.sup.8 is chosen from a hydrogen atom, an alkyl group comprising 1 to 12; a halogen atom; an alkyl group comprising 1 to 12; a sulphonate group having the formula OSO.sub.2R.sup.7, wherein R.sup.7 is chosen from a methyl group (CH.sub.3), a trifluoromethyl group (CF.sub.3), a toluene group (p-CH.sub.3C.sub.6H.sub.4) or a benzene group (C.sub.6H.sub.5).

6. The method according to claim 1, wherein the organoborane according to formula (II) is chosen in the group formed by tributylborane, dicyclohexylborane, iododicyclohexylborane, dibutylborane methanesulphonate (n-Bu.sub.2BOSO.sub.3Me), B-iodo-9-borabicyclo[3.3.1]nonane, the dimer of 9-borabicyclo[3.3.1]nonane, B-benzyl-9-borabicyclo[3.3.1]nonane, Me-TBD-BBN.sup.+I.sup..

7. The method according to claim 1, wherein the base is an organic base chosen from: nitrogen-containing organic bases chosen in the group formed by triethylamine, trimethylamine, N-diisopropylethylamine (DIPEA), diethylisopropylamine (DIEA), 7-Methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD), 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), 1,4-diazabicyclo[2.2.2]octane (DABCO), and N-methylpiperidine; phosphorus-containing organic bases chosen in the group formed by triphenylphosphine, 2,2-bis(diphenylphosphino)-1,1-binaphthyl (BINAP), triisopropylphosphine, 1,2-bis(diphenylphosphino)ethane (dppe), tricyclohexylphosphine (PCy.sub.3); aza-phosphines chosen in the group formed by 2,8,9-triisopropyl-2,5,8,9-tetraaza-1-phosphabicyclo[3.3.3]undecane (BV.sup.Me) and 2,8,9-triisobutyl-2,5,8,9-tetraaza-1-phosphabicyclo[3.3.3]undecane (BV.sup.iBu); carbon-containing bases chosen from the N-heterocyclic carbenes derived from an imidazolium salt, said carbenes being chosen in the group formed by the salts of 1,3-bis(2,6-diisopropylphenyl)-1H-imidazol-3-ium (also referred to as IPr), 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydro-1H-imidazol-3-ium (also referred to as s-IPr), 1,3-bis(2,4,6-trimethylphenyl)-1H-imidazol-3-ium (also referred to as IMes), 1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydro-1H-imidazol-3-ium (also s-IMes), 4,5-dichloro-1,3-bis(2,6-diisopropylphenyl)-1H-imidazol-3-ium (also referred to as Cl.sub.2IPr), 1,3-di-tert-butyl-1H-imidazol-3-ium (also referred to as ItBu), and 1,3-di-tert-butyl-4,5-dihydro-1H-imidazol-3-ium (also referred to as s-ItBu), said salts being in the form of chloride or tetraphenylborate salts.

8. The method according to claim 1, wherein the organic base is a nitrogen-containing organic base chosen in the group formed by triethylamine, trimethylamine, N-diisopropylethylamine (DIPEA), diethylisopropylamine (DIEA), 7-Methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD), 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), 1,4-diazabicyclo[2.2.2]octane (DABCO), and N-methylpiperidine.

9. The method according to claim 1, wherein the dismutation of formic acid, or of at least one of the derivatives thereof or of a mixture of formic acid and at least one of the derivatives thereof as defined in claim 1, further takes place in the presence of an additive chosen from: crown ethers in the group formed by 12-crown-4, 15-crown-5, 18-crown-6, dibenzo-18-crown-6, benzo-18-crown-6, benzo-15-crown-5, and dibenzo-15-crown-5; aza-crowns in the group formed by 1,4,7,10-tetraazacyclododecane (cyclene), 1,4,7,10,13,16-hexaazacyclooctadecane (hexacyclene), and diaza-18-crown-6; crown thioethers in the group formed by 1,5,9,13-tetrathiacyclohexadecane (16-Ane-S.sub.4), and 1,4,7,10,13,16-hexathiacyclooctadecane (18-Ane-S.sub.6).

10. The method according to claim 1, wherein the quantity of the organoborane according to formula (II) is 0.05 to 1 molar equivalents, inclusive, with respect to formic acid or the derivative(s) thereof, or a mixture of formic acid and at least one of the derivatives thereof.

11. The method according to claim 1, wherein the base quantity is 0.05 to 3 molar equivalents, inclusive, with respect to the formic acid derivative(s), or a mixture of formic acid and at least one of the derivatives thereof.

12. The method according to claim 1, wherein the quantity of additive is 1 to 2 molar equivalents, inclusive, with respect to the formic acid derivative(s), or a mixture of formic acid and at least one of the derivatives thereof.

13. A method for preparing methanol wherein it comprises (A) a step for preparing a methoxyborane according to formula (I) by dismutation of formic acid, or of at least one of the derivatives thereof having the formula HCO.sub.2M wherein M is chosen in the group formed by Na.sup.+, K.sup.+, Li.sup.+, Cs.sup.+, NH.sub.4.sup.+, triethylammonium (HNEt.sub.3.sup.+), tetraphenylphosphonium (PPh.sub.4.sup.+), tetramethylammonium (NMe.sub.4.sup.+), tetraethylammonium (NEt.sub.4.sup.+), tetrabutylammonium (NBu.sub.4.sup.+), and tetraphenylammonium (NPh.sub.4.sup.+), or of a mixture of formic acid and at least one of the derivatives thereof, in the presence of an organoborane according to formula (II); and optionally of an organic or inorganic base, according to claim 1; and (B) a step for the hydrolysis or protonolysis of the methoxyborane according to formula (I) obtained following the dismutation step (A) into methanol.

14. A method for producing methanol wherein it comprises (i) a step for preparing formic acid or the derivatives thereof having the formula HCO.sub.2M wherein M is chosen in the group formed by Na.sup.+, K.sup.+, Li.sup.+, Cs.sup.+, NH.sub.4.sup.+, triethylammonium (HNEt.sub.3.sup.+), tetraphenylphosphonium (PPh.sub.4.sup.+), tetramethylammonium (NMe.sub.4.sup.+), tetraethylammonium (NEt.sub.4.sup.+), tetrabutylammonium (NBu.sub.4.sup.+), and tetraphenylammonium (NPh.sub.4.sup.+), by 2e.sup. reduction of CO.sub.2, or by catalytic hydrogenation of CO.sub.2; (ii) a step for preparing a methoxyborane according to formula (I) by dismutation of formic acid or of at least one of the derivatives thereof, or of a mixture of formic acid and at least one of the derivatives thereof, in presence of an organoborane according to formula (II), and optionally an organic or inorganic base, according to claim 1; and (iii) a step for the hydrolysis or protonolysis of the methoxyborane according to formula (I) obtained following the dismutation step (ii) into methanol.

Description

[0164] Further advantages and features of the present invention will emerge on reading the examples hereinafter given by way of illustration and not limitation and the appended figures:

[0165] FIG. 1 represents the hexaelectronic (with 6 electrons i.e. 3 hydrides) reduction of CO.sub.2 into methanol by catalytic hydrogenation (Equation (1)), and the hexaelectronic electro-reduction of CO.sub.2 into methanol (Equation (2));

[0166] FIG. 2 represents the dismutation of formic acid into CO.sub.2 and methanol;

[0167] FIG. 3 represents two formic acid breakdown reactions which are in competition with the dismutation reaction of formic acid into methanol and CO.sub.2. These two reactions are: dehydrogenation of formic acid into CO.sub.2 and H.sub.2 (Equation (4)) and dehydration of formic acid into CO and H.sub.2O (Equation (5));

[0168] FIG. 4 represents examples of N-heterocyclic carbenes;

[0169] FIG. 5 represents the preparation of formic acid by 2 e.sup. electro-reduction or catalytic hydrogenation of CO.sub.2;

[0170] FIG. 6 represents the method for producing methanol comprising (i) a step for preparing formic acid or one of the derivatives thereof by 2e reduction of CO.sub.2, or by catalytic hydrogenation of CO.sub.2; (ii) a step for preparing a methoxyborane according to formula (I) by dismutation of formic acid or at least of one of the derivatives thereof, or of a mixture of formic acid and at least one of the derivatives thereof; and (iii) a step for the hydrolysis or protonolysis of the methoxyborane according to formula (I) obtained following the dismutation step (ii) into methanol;

[0171] FIG. 7 represents the reactions involved in calculating the yields for obtaining methoxyboranes.

EXAMPLES

[0172] A set of results is presented hereinafter, giving examples of dismutation of formic acid into methoxyborane according to formula (I) and CO.sub.2 in the presence of an organoborane according to formula (II). The yields are obtained by .sup.1H NMR by integrating the signals of the methoxyborane according to formula (I) or the methanol obtained after hydrolysis or protonolysis of the methoxyborane according to formula (I) with respect to those of an internal standard (mesitylene). The yields are calculated by observing the stoichiometry of the dismutation reaction, that is to say that, at best, 3 moles of formic acid produce not more than maximum 1 mol of methanol (see equation 3 in FIG. 2).

[00001]$\left({\mathrm{MeOBR}}^1.Math.R^2\right)=\frac{3n\left({\mathrm{MeOBR}}^1.Math.R^2\right)}{n_0\left(\mathrm{HCOOZ}\right)}$

[0173] n.sub.0(MeOBR.sup.1R.sup.2): quantity of methoxyborane substance determined by .sup.1H NMR with respect to mesitylene

[0174] n.sub.0(HCOOZ)=n.sub.0(HCOOH)+n.sub.0(HCOOM): quantity of total substance in formic acid (HCOOH) and the derivatives thereof (HCOOM, M as defined above) introduced initially.

[0175] In our case, the use of organoboranes requires equations 6 and 7 (FIG. 7). The yields are nonetheless calculated on the basis of equation 3.

[0176] The hydrolysis yield being quantitative, the yields of methoxyboranes according to formula (I) and methanol are equal (FIG. 6).

[0177] The dismutation reaction of formic acid may be carried out according to the following experimental protocol: [0178] 1. In an inert atmosphere, in a glovebox, the organoborane according to formula (II), the formic acid or one of the derivatives thereof, or a mixture of formic acid and at least one of the derivatives thereof and optionally, the solvent and/or the base and/or the additive are introduced into a Schlenk tube which is subsequently sealed with a J. Young valve. The order of introduction of the reagents is of no importance. [0179] 2. The Schlenk is then heated to a temperature between 25 and 150 C. (preferentially >110 C.) until the complete conversion of the formic acid (from 5 minutes to 72 hours of reaction). The reaction is monitored by .sup.1H and/or .sup.13C and/or .sup.11B proton NMR. [0180] 3. When the reaction is complete (which corresponds to the disappearance of the characteristic signals of the protons of the formiate HCOO.sup. in .sup.1H NMR), the pressure in the tube is released and the solvent as well as the volatile compounds are evaporated in a vacuum (10.sup.2 mbar). [0181] It should be noted that the gases generated by the reaction (essentially CO.sub.2 and less quantities of H.sub.2) may be retrieved and reused to generate formic acid by catalytic hydrogenation of CO.sub.2. [0182] 4. The residue obtained after evaporating the volatile compounds is then hydrolysed or protonolysed respectively with H.sub.2O or anhydrous HCl (in solution in diethyl ether) to release the free methanol which may be collected readily by distillation. In the case where HCl is used, it is also possible to retrieve chloroorganoboranes by distillation, the latter being suitable for reuse in the dismutation reaction according to the method.

Example 1: Preparation of MeOBBN from BBNI, Formic Acid and Triethylamine

[0183] ##STR00004##

[0184] According to the general protocol described above, BBNI (commercial, 1 M in hexane, 100 L, 0.1 mmol) is added to a mixture of formic acid (7.4 L, 0.2 mmol, 2 equiv.) and triethylamine NEt.sub.3 (27.8 L, 0.2 mmol, 2 equiv.) in acetonitrile. The tube is then sealed and shaken vigorously to solubilise the borane adhering to the walls of the tube. Once the mixture is homogenous (<1 min), it is heated to 130 C. for 19 hours. The reaction mixture is then analysed by .sup.1H NMR and a 19% MeOBBN yield is obtained. The latter may then be hydrolysed to obtain free methanol. For this purpose, the tube is opened and 18 L of water (1 mmol, 5 equiv.) is added. The tube is then stirred for 30 min at ambient temperature to quantitatively obtain methanol.

Example 2: Preparation of MeOBBu.SUB.2 .from Bu.SUB.2.B-OTf, Formic Acid and Triethylamine

[0185] ##STR00005##

[0186] Using the same protocol as that of example 1, replacing BBNI by Bu.sub.2B-OTf, a 15% yield of methanol borate MeOBBu.sub.2 is obtained.

Example 3: Preparation of MeOCy.SUB.2 .from Cy.SUB.2.BCl, Formic Acid and Triethylamine

[0187] ##STR00006##

[0188] Using the same protocol as that of example 1, replacing BBNI by Cy.sub.2BCl, a 17% yield of methanol borate McOBCy.sub.2 is obtained.

Example 4: Preparation of MeOBBu.SUB.2 .from Bu.SUB.3.B, Formic Acid and Triethylamine

[0189] ##STR00007##

[0190] Using the same protocol as that of example 1, replacing BBNI by Bu.sub.3B (commercial, 1 M in Et.sub.2O), a 25% yield of methoxyborane MeOBBu.sub.2 is obtained.

Example 5: Preparation of MeOBBN from BBNH, Formic Acid and Triethylamine

[0191] ##STR00008##

[0192] According to the general protocol described above, (BBNH).sub.2 (0.5 equiv.) is added to a mixture of formic acid (2 equiv.) and triethylamine NEt.sub.3 (1 equiv.) in acetonitrile. The tube is then sealed and heated slightly (approx. 50 C.) to solubilise the solid reagents. A significant release of hydrogen is then observed, which once complete leaves a homogenous mixture (approx. 10 min). The latter is heated to 130 C. for 19 hours. The reaction mixture is then analysed by .sup.1H NMR and a 53% MeOBBN yield is obtained.

Example 6: Preparation of MeOBBN from BBNH, Formic Acid and Diisopropylethylamine

[0193] ##STR00009##

[0194] According to the general protocol described above, (BBNH).sub.2 (0.5 equiv.) is added to a mixture of formic acid (2 equiv.) and diisopropylethylamine i-Pr.sub.2NEt (1 equiv.) in acetonitrile. The tube is then sealed and heated slightly (approx. 50 C.) to solubilise the solid reagents. A significant release of hydrogen is then observed, which once complete leaves a homogenous mixture (approx. 10 min). The latter is heated to 130 C. for 7 hours. The reaction mixture is then analysed by .sup.1H NMR and a 49% MeOBBN yield is obtained.

Example 7: Preparation of MeOBBN from BBNH, Formic Acid and Sodium Formiate, in the Presence of the Crown Ether 15-C-5

[0195] ##STR00010##

[0196] According to the general protocol described above, (BBNH).sub.2 (0.5 equiv.) is added to a mixture of formic acid (1 equiv.) and sodium formiate HCO.sub.2Na (1 equiv.) in acetonitrile. The crown ether 15-C-5 (1 equiv.) is then added with a syringe and the tube heated slightly (approx. 50 C.) to solubilise the solid reagents. The latter is heated to 130 C. for 24 hours. The reaction mixture is then analysed by .sup.1H NMR and a 58% MeOBBN yield is obtained.

Example 8: Preparation of MeOBCy.SUB.2 .from Cy.SUB.2.BH, Formic Acid and Triethylamine

[0197] ##STR00011##

[0198] According to the general protocol described above, Cy.sub.2BH (1 equiv.) is added to a mixture of formic acid (2 equiv.) and triethylamine NEt.sub.3 (1 equiv.) in acetonitrile. The tube is then sealed and heated slightly (approx. 50 C.) to solubilise the solid reagents. A significant release of hydrogen is then observed, which once complete leaves a homogenous mixture (approx. 5 min). The latter is heated to 120 C. for 7 hours. The reaction mixture is then analysed by .sup.1H NMR and a 31% MeOBCy.sub.2 yield is obtained.

Example 9: Production of Methanol

[0199] The MeOBBN obtained in example 5 may then be hydrolysed to obtain free methanol:

[0200] Once the reaction is complete, the volatile compounds are evaporated in a vacuum (10.sup.1 to 10.sup.2 mbar), the solid residue obtained is then dissolved in THF and H.sub.2O (5-10 equiv. with respect to the borane initially introduced) is added to the reaction mixture. The solution is stirred for 30 minutes to 1 hour at ambient temperature (20+5 C.). The volatile methanol is then retrieved in another Schlenk tube by transfer under reduced pressure. An aqueous methanol solution with a 50% yield is thereby obtained.