Process for preparing symmetric pincer ligands from the group of the M-terphenyl compounds

1. A process for preparing compounds of the formula (ABA)

by reacting a compound of the formula (A)

with a compound of the formula (B)

with X=—OH or —NR′R″;

and R′=H or a protecting group function;

and R″=H or a protecting group function;

or R′ and R″ may together form covalently bonded protecting group radicals

and R 1 , R 2 , R 3 , R 4 , R 6 , and R 8 are each independently selected from the group consisting of hydrogen, (C 1 -C 12 )-alkyl, —O—(C 1 -C 12 )-alkyl, —N[(C 1 -C 12 )-alky, ] 2 , —NH(C 1 -C 12 )-alky, (C 6 -C 20 )-aryl, O—(C 6 -C 20 )-aryl, and halogen;

and R 5 and R 7 are each independently selected from the group consisting of (C 1 -C 12 )-alkyl, —O—(C 1 -C 12 )-alkyl, —N[(C 1 -C 12 )-alkyl] 2 , —NH(C 1 -C 12 )alkyl, (C 6 -C 20 )-aryl, —O—(C 6 -C 20 )-aryl, and halogen;

where adjacent radicals from the group of the R 1 to R 8 radicals may be joined via a covalent bond,

characterized in that

the reaction is conducted electrochemically.

2. The process according to claim 1 ,

characterized in that

the electrochemical process step comprises:

aa) a mixture of at least one solvent and at least one conductive salt is produced,

bb) the compounds to be converted are added to this mixture, with addition of the compound of the formula (A) in a molar excess based on the compound (B),

cc) at least two electrodes are introduced into the reaction solution obtained in bb) and a voltage is applied to the electrodes,

dd) the compounds (A) and (B) are converted to the compound (ABA),

ee) the voltage is switched off and optionally

ff) the compound (ABA) is isolated and/or purified.

3. The process according to claim 1 ,

characterized in that

the molar ratio of the compound (A) to the compound (B) is in the range from 1.5:1 to 4:1.

4. The process of claim 3 , wherein the molar ratio of the compound (A) to the compound (B) is in the range from 1.8:1 to 2.5:1.

5. The process of claim 4 , wherein the molar ratio of the compound (A) to the compound (B) is 2:1.

6. The process according to claim 1 ,

characterized in that

compounds of the formula (A)are compounds in which X=NHR′.

7. The process according to claim 1 ,

characterized in that

compounds of the formula (A) are compounds in which X=—NH 2 .

8. The process according to claim 1 ,

characterized in that

R 6 and R 8 are identical.

9. The process of claim 8 , wherein R 6 and R 8 are hydrogen.

10. The process according to claim 1 ,

characterized in that

the reaction is conducted in the presence of a solvent and the solvent is selected from the group consisting of acetonitrile, propylene carbonate, methyl carbonate, nitromethane, ethylene glycol dimethyl ether, methanesulphonic acid, benzene, toluene, water, methanol, ethanol, propanol, isopropanol, halogenated solvents, halogenated or non-halogenated acids, and mixtures thereof.

11. The process according to claim 10 ,

characterized in that

the solvent is methanol, formic acid, trifluoroacetic acid, hexafluoroisopropanol or mixtures thereof.

12. The process of claim 11 , where the solvent is methanol, hexafluoroisopropanol, or mixtures thereof.

13. The process of claim 12 , wherein the solvent is hexafluoroisopropanol.

14. The process according to claim 1 ,

characterized in that

the reaction is conducted without the use of organic oxidizing agents.

15. The process according to claim 1 ,

characterized in that

the electrochemical process step is conducted in the presence of at least one conductive salt.

16. The process according to claim 15 ,

characterized in that

the counterions of the conductive salts are selected from the group consisting of arsenate, sulphate, hydrogensulphate, alkylsulphate, alkylphosphate, perchlorate, fluoride, aryisulphate, hexafluorophosphate, and tetrafluoroborate.

17. The process according to claim 15 ,

characterized in that

the conductive salt is selected from the group consisting of quaternary ammonium borates, ammonium fluoroalkylphosphates, ammonium fluoroalkylarsenates, ammonium trifluoromethylsulphonates, ammonium bis(fluoromethanesulphon)imides, ammonium tris(fluoromethanesulphonyl)methides, methyltributylammonium methylsulphate, methyltriethylammonium methylsulphate, tetrabutylammonium hexafluorophosphate, tetraethylanunonium tetrafluoroborate, lithium hexafluorophosphate, and tetraethylanunonium tetrafluoroborate.

18. The process of claim 17 , wherein the conductive salt is methyltriethylammonium methylsulphate or Bu 3 NMe +MeOSO 3 − (methyltributylammonium methylsulphate).

19. The process of claim 18 , wherein the conductive salt is methyltributylammonium methylsulphate.

20. The process of claim 15 , wherein the at least one conductive salt is a tetra(C 1 -C 6 -alkyl)ammonium or 1,3-di(C 1 -C 6 -alkyl)imidazolium salt, with the proviso that the alkyl groups may be halogen-substituted.

21. The process of claim 1 , wherein protecting group function of R′ and R″ is selected from the group consisting of tert-butyloxycarbonyl, methyloxycarbonyl, benzyloxycarbonyl, phenyloxycarbonyl, acetyl, trifluoroacetyl, benzoyl, sulphonyl, and sulphenyl.

22. The process of claim 1 , wherein R′ and R″ may together form radicals of the formula PGa or PGb

23. The process of claim 1 , wherein R 1 , R 2 , R 3 ,R 4 ,R 6 and R 8 are each independently selected from the group consisting of hydrogen, (C 1 -C 12 )-alkyl, —O—(C 1 -C 12 )-alkyl, —N[(C 1 -C 12 )-alkyl] 2 , and halogen.

24. The process of claim 23 , wherein R 1 ,R 2 ,R 3 ,R 4 ,R 6 and R 8 are each independently selected from the group consisting of hydrogen, (C 1 -C 12 )-alkyl, and —O—(C 1 -C 12 )-alkyl.

25. The process of claim 1 , wherein R 5 and R 7 are each independently selected from the group consisting of (C 1 -C 12 )-alkyl, —O—(C 1 -C 12 )-alkyl, —N[(C 1 -C 12 )-alkyl] 2 , and halogen.

26. The process of claim 25 , wherein R 5 and R 7 are each independently selected from the group consisting of (C 1 -C 12 )-alkyl, —O—(C 1 -C 12 )-alkyl, and —N[(C 1 -C 12 )-alkyl] 2 .