Process for the alkoxycarbonylation of olefins in a medium having a low Brønsted acid concentration

1. An acid-free alkoxycarbonylation process comprising the following process steps:

a) introducing an ethylenically unsaturated compound;

b) adding a ligand-metal complex comprising Pd and a bidentate phosphine ligand, or adding a bidentate phosphine ligand and a compound which comprises Pd;

c) adding an alcohol;

d) supplying CO;

e) heating the reaction mixture, the ethylenically unsaturated compound being reacted to form an ester in the presence of a Brønsted acid at low concentration,

where the reaction mixture has less than 0.1 mol %, based on the amount of substance of the ethylenically unsaturated compound, of Brønsted acids having an acid strength of pKa≤3 or an acid strength of a pKa≤5, where the Brønsted acid results from the reaction course and

wherein the bidentate phosphine ligand is selected from a compound according to one of the formulae (I) and (II)

where

R 1 , R 2 , R 3 , R 4 , R 1′ , R 2′ , R 3′ , R 4′ are each independently selected from —(C 1 -C 12 )-alkyl, —(C 3 -C 12 )-cycloalkyl, —(C 3 -C 12 )-heterocycloalkyl, —(C 6 -C 20 )-aryl, —(C 3 -C 20 )-heteroaryl;

at least one of the R 1 , R 2 , R 3 , R 4 radicals or at least one of the R 1′ , R 2′ , R 3′ , R 4′ radicals is a —(C 3 -C 20 )-heteroaryl radical;

and

R 1 , R 2 , R 3 , R 4 , R 1′ , R 2′ , R 3′ , R 4′ , if they are —(C 1 -C 12 )-alkyl, —(C 3 -C 12 )-cycloalkyl, —(C 3 -C 12 )-heterocycloalkyl, —(C 6 -C 20 )-aryl or —(C 3 -C 20 )-heteroaryl,

may each independently be substituted by one or more substituents selected from —(C 1 -C 12 )-alkyl, —(C 3 -C 12 )-cycloalkyl, —(C 3 -C 12 )-heterocycloalkyl, —O—(C 1 -C 12 )-alkyl, —O—(C 1 -C 12 )-alkyl-(C 6 -C 20 )-aryl, —O—(C 3 -C 12 )-cycloalkyl, —S—(C 1 -C 12 )-alkyl, —S—(C 3 -C 12 )-cycloalkyl, —COO—(C 1 -C 12 )-alkyl, —COO—(C 3 -C 12 )-cycloalkyl, —CONH—(C 1 -C 12 )-alkyl, —CONH—(C 3 -C 12 )-cycloalkyl, —CO—(C 1 -C 12 )-alkyl, —CO—(C 3 -C 12 )-cycloalkyl, —N—[(C 1 -C 12 )-alkyl] 2 , —(C 6 -C 20 )-aryl, —(C 6 -C 20 )-aryl-(C 1 -C 12 )-alkyl, —(C 6 -C 20 )-aryl-O—(C 1 -C 12 )-alkyl, —(C 3 -C 20 )-heteroaryl, —(C 3 -C 20 )-heteroaryl-(C 1 -C 12 )-alkyl, —(C 3 -C 20 )-heteroaryl-O—(C 1 -C 12 )-alkyl, —COOH, —OH, —SO 3 H, —NH 2 or halogen.

2. The process according to claim 1 ,

where R 1 , R 2 , R 3 , R 4 , R 1′ , R 2′ , R 3′ and R 4′ , if they are —(C 1 -C 12 )-alkyl, —(C 3 -C 12 )-cycloalkyl, —(C 3 -C 12 )-heterocycloalkyl, —(C 6 -C 20 )-aryl or —(C 3 -C 20 )-heteroaryl,

may each independently be substituted by one or more substituents selected from —(C 1 -C 12 )-alkyl, —(C 3 -C 12 )-cycloalkyl, —O—(C 1 -C 12 )-alkyl, —O—(C 1 -C 12 )-alkyl-(C 6 -C 20 )-aryl, —O—(C 3 -C 12 )-cycloalkyl, —(C 6 -C 20 )-aryl, —(C 6 -C 20 )-aryl-(C 1 -C 12 )-alkyl, —(C 6 -C 20 )-aryl-O—(C 1 -C 12 )-alkyl, —(C 3 -C 20 )-heteroaryl, —(C 3 -C 20 )-heteroaryl-(C 1 -C 12 )-alkyl or —(C 3 -C 20 )-heteroaryl-O—(C 1 -C 12 )-alkyl.

3. The process according to claim 1 ,

where at least two of the R 1 , R 2 , R 3 , R 4 radicals or at least two of the R 1′ , R 2′ , R 3′ , R 4′ radicals are a —(C 3 -C 20 )-heteroaryl radical.

4. The process according to claim 3 ,

where the R 1 and R 3 radicals or the R 1′ and R 3′ radicals are each a —(C 3 -C 20 )-heteroaryl radical.

5. The process according to claim 4 ,

where the R 1 and R 3 or R 1′ and R 3′ radicals are each a —(C 3 -C 20 )-heteroaryl radical;

and the R 2 and R 4 or R 2′ or R 4′ radicals are each independently selected from —(C 1 -C 12 )-alkyl, —(C 3 -C 12 )-cycloalkyl, —(C 3 -C 12 )-heterocycloalkyl or —(C 6 -C 20 )-aryl.

6. The process according to claim 1 ,

where the radicals R 1 , R 2 , R 3 and R 4 and/or R 1′ , R 2′ , R 3′ and R 4′ , if they are a heteroaryl radical, are a heteroaryl radical having five or six ring atoms.

7. The process according to claim 1 ,

where the radicals R 1 , R 2 , R 3 and R 4 and/or R 1′ , R 2′ , R 3′ and R 4′ , if they are a heteroaryl radical, are selected from 2-furyl, 2-thienyl, N-methyl-2-pyrrolyl, N-phenyl-2-pyrrolyl, N-(2-methoxyphenyl)-2-pyrrolyl, 2-pyrrolyl, N-methyl-2-imidazolyl, 2-imidazolyl, 2-pyridyl or 2-pyrimidyl, the stated heteroaryl radicals not being further substituted.

8. The process according to claim 1 ,

where the bidentate phosphine ligand is selected from a compound according to one of the formulae (8) and (4)

9. The process according to claim 1 ,

wherein the ethylenically unsaturated compound is selected from the group consisting of ethene, propene, 1-butene, cis- and/or trans-2-butene, isobutene, 1,3-butadiene, 1-pentene, cis- and/or trans-2-pentene, 2-methyl-1-butene, 3-methyl-1-butene, 2-methyl-2-butene, hexene, tetramethylethylene, heptene, 1-octene, 2-octene, di-n-butene, and mixtures thereof.

10. The process according to claim 1 ,

wherein the compound comprising Pd in process step b) is selected from palladium dichloride, palladium(II) acetylacetonate, palladium(II) acetate, dichloro(1,5-cyclooctadiene)palladium(II), bis(dibenzylideneacetone)palladium, bis(acetonitrile)dichloropalladium(II) or palladium(cinnamyl) dichloride.

11. The process according to claim 1 ,

wherein the alcohol in process step c) is selected from the group consisting of methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, 1-hexanol, 2-propanol, tert-butanol, 3-pentanol, cyclohexanol, phenol, and mixtures thereof.

12. The process according to claim 1 ,

where the reaction mixture is admixed with less than 0.1 mol %, based on the amount of substance of the ethylenically unsaturated compound, of Brønsted acids having an acid strength of pKa≤5.