Methods for preparing alkylfurans
Provided herein are methods for preparing alkylfurans, such as 2,5-dialkylfurans and 2-alkylfurans. Furfural or 5-alkylfurfural can be reacted with aniline or diaminobenzene, or derivatives thereof, to form the corresponding imine, which can be reduced to form alkylfurans and to regenerate the aniline or diaminobenzene, or derivatives thereof. The alkylfuran may be, for example, 2,5-dimethylfuran or 2-methylfuran.
1. A method for preparing an alkylfuran of formula (I):
wherein R 1 is H or C x alkyl, wherein x is an integer equal to or greater than 1;
wherein R 2 is C y alkyl, wherein y is an integer equal to or greater than 1; and
wherein the method comprises:
i) providing an imine of formula (C-1) or (C-2):
wherein:
R 1 is as defined in formula (I);
R 1a is H or C x-1 alkyl, wherein x is as defined in formula (I), provided that R 1a is hydrogen when x is 1;
R 2a is H or C y-1 alkyl, wherein y is as defined in formula (I), provided that R 2a is hydrogen when y is 1;
m is an integer equal to or greater than 0; and
each R 3 is independently halo, alkyl, haloalkyl, cycloalkyl, alkoxy, or —C(O)O-alkyl; and
ii) reacting the imine of formula (C-1) or (C-2) with a reducing agent in the presence of a catalyst to produce the alkylfuran of formula (I).
2. The method of claim 1 , further comprising reacting a compound of formula (A-1) or (A-2) with an aniline of formula (B) to provide the imine of formula (C-1) or (C-2), wherein:
the compound of formula (A-1) or (A-2) is:
wherein:
R 1 is as defined in formula (I);
R 1a and R 2a are each as defined in formula (C-1) or (C-2); and
the aniline of formula (B) is:
wherein:
m and R 3 are as defined in formula (C-1) or (C-2).
3. The method of claim 1 , wherein x and y are independently 1 to 10.
4. The method of claim 1 , wherein each R 3 is independently chloro, fluoro, methyl, ethyl, propyl, butyl, —CF 3 , —CHF 2 , —CH 2 F, methoxy, ethoxy, propoxy, butoxy, —CO 2 -methyl, —CO 2 -ethyl, —CO 2 -propyl, or —CO 2 -butyl.
5. The method of claim 1 , wherein the catalyst is a palladium catalyst.
6. The method of claim 1 , wherein the catalyst is palladium on carbon.
7. The method of claim 1 , wherein the reducing agent is hydrogen or a hydrogenation donor.
8. The method of claim 1 , wherein the reducing agent is hydrogen, and wherein the imine of formula (C-1) or (C-2) is reacted with the hydrogen in the presence of the catalyst and a solvent.
9. The method of claim 8 , wherein the solvent has a pKa between 2 and 6.5.
10. The method of claim 1 , wherein the compound of formula (A-1) or (A-2) is reacted with the aniline of formula (B) at a temperature of at least 40° C.
11. The method of claim 1 , wherein:
the alkylfuran of formula (I) is:
and
the imine of formula (C-1) or (C-2) is:
12. A method for preparing 2,5-dimethylfuran, comprising:
reacting an unsubstituted or substituted ((5-methylfuran-2-yl)methylene)aniline with a reducing agent in the presence of a catalyst to produce 2,5-dimethylfuran.
13. The method of claim 12 , wherein the ((5-methylfuran-2-yl)methylene)aniline is:
unsubstituted ((5-methylfuran-2-yl)methylene)aniline; or
((5-methylfuran-2-yl)methylene)aniline substituted on the aniline moiety with one to five substituents independently selected from the group consisting of halo, alkyl, haloalkyl, cycloalkyl, alkoxy, and —C(O)O-alkyl.
14. The method of claim 12 , wherein the catalyst is a palladium catalyst, a platinum catalyst, a ruthenium catalyst, a copper catalyst or any combination thereof.
15. The method of claim 12 , wherein the catalyst is palladium on carbon.
16. The method of claim 12 , wherein the unsubstituted or substituted ((5-methylfuran-2-yl)methylene)aniline is reacted with the reducing agent in the presence of the catalyst and a solvent.
17. The method of claim 16 , wherein the solvent has a pKa between 2 and 6.5.
18. The method of claim 12 , wherein the unsubstituted or substituted ((5-methylfuran-2-yl)methylene)aniline is provided by:
reacting 5-methylfurfural with an unsubstituted or substituted aniline to provide the unsubstituted or substituted ((5-methylfuran-2-yl)methylene)aniline.
19. The method of claim 18 , wherein the 5-methylfurfural is reacted with the unsubstituted or substituted aniline at a temperature of at least 40° C.
20. The method of claim 18 , wherein the aniline is:
unsubstituted aniline; or
aniline substituted with one to five substituents independently selected from the group consisting of halo, alkyl, haloalkyl, cycloalkyl, alkoxy, and —C(O)O-alkyl.
21. The method of claim 1 , wherein R 1 is H, and R 2 is methyl.
22. The method of claim 1 , wherein R 1 and R 2 are both methyl.