Second cycle asymmetric dihydroxylation reaction
A process for catalyzing asymmetric dihydroxylations of olefins employs an Os(VI) complex as a catalytic intermediate in the formation of chiral vicinal diol products. The process requires a chiral bidentate ligand that favors diol formation in the “second cycle” of asymmetric dihydroxylation.
1. A process for catalyzing a second-cycle asymmetric dihydroxylation reaction for converting an olefin substrate to a chiral vicinal diol product, said process comprising the following steps:
Step A: mixing the olefin substrate with a catalytic amount of a chiral bidentate ligand in an aqueous solvent, the aqueous solvent optionally being an alcohol/water solvent mixture, the chiral bidentate ligand being represented by the following structure:
wherein:
R is a radical selected from the group consisting of hydrogen, carboxylic acid, phenyl, 1-naphthyl, 2-naphthyl, alkyl(C 1 -C 12 ), cyclo-alkyl(C 3 -C 12 ), carbamoyl, N-alkyl(C 1 -C 12 ) carbamoyl, and N,N-alkyl(C 1 -C 12 ) carbamoyl, wherein the phenyl, 1-naphthyl, and 2-naphthyl radicals ay optionally have substituents in any available position;
R 1 is a radical selected from the group consisting of carboxylic acid, carbamoyl, N-alkyl(C 1 -C 12 ) carbamoyl, and N,N-dialkyl(C 1 -C 12 ) carbamoyl;
R 2 is a radical selected from the group consisting of hydrogen , alkyl(C 1 -C 12 ), aryl, and heteroaryl; and
X and Y are radicals independently selected from the group consisting of hydroxyl, amino, N-alkyl(C 1 -C 12 )sulfonylamino, N-arylsulfonylamino, and N-heteroarylsulfonylamino;
with the following provisos:
if X is not a hydroxyl group, then Y must be a hydroxy group;
if Y is not a hydroxyl group then X must be a hydroxyl group; and
at least one of the following must be a carboxylic acid group: R, R 1 or R 2 ; and then
Step B: adding to the mixture of said Step A a stoichiometric amount or a stoichimetric excess of N-methyl morpholine N-oxide with respect to the olefin substrate and a catalytic amount of osmium tetraoxide under reaction conditions for catalyzing the second-cycle asymmetric dihydroxylation reaction for converting the olefin substrate to the chiral vicinal diol product.
2. A process according to claim 1 where the chiral bidentate ligand is selected from the group represented by the following structures:
3. A process according to claim 1 wherein the chiral ligand is an α-hydroxy-β-N-sulfonylamino carboxylic acid.
4. A process according to claim 1 wherein the chiral ligand is an α-N-sulfonylamino-β-hydroxy carboxylic acid.
5. A process according to claim 1 wherein chiral bidentate ligand has a concentration between 1 mol % and 10 mol %, with respect to the olefin substrate.
6. A process according to claim 5 wherein, in said Step A, the chiral bidentate ligand has a concentration of 5 mol %, with respect to the olefin substrate.
7. A process according to claim 6 wherein, in said Step B, the osmium tetraoxide has a concentration of between 0.1 mol % and 1 mol %, with respect to the olefin substrate.
8. A process according to claim 7 wherein, in said Step B, the osmium tetraoxide has a concentration of approximately 0.2 mol %, with respect to the olefin substrate.
9. A process according to claim 8 , wherein the olefin substrate is an α,β-unsaturated carboxylic acid ester.
10. A process according to claim 9 wherein the pH of the reaction mixture of said Step B is approximately 5.
11. A process according to claim 10 wherein the alcohol used in the alcohol/water solvent mixture of said Step A is tertiary butyl alcohol.