Process to purify dialkyl sulfides
The invention relates to new processes to prepare low odor dialkyl sulfides, the low odor dialkyl sulfides obtainable by these processes and to methods of using these low odor dialkyl sulfides. Moreover, the invention relates to a process to prepare dialkyl sulfide borane complexes of high purity, the dialkyl sulfide borane complexes obtainable by this process and to a process for enantioselective reductions employing these dialkyl sulfide borane complexes of high purity as reducing agent.
1. A process to prepare a dialkyl sulfide borane complex comprising the steps of
a) bringing a dialkyl sulfide into contact with at least one base and/or at least one alkali or alkaline earth metal, wherein the base is a metal alcoholate, a metal oxide, a metal hydroxide, alkylammonium hydroxide, a metal or alkylammonium carbonate, a metal enolate, a metal amide or a metal hydride, wherein the metal is selected from the group, consisting of the alkali metals, the alkaline earth metals and the metals of the groups IIIa to VIIIa, Ib and IIb of the periodic table of elements, and
b) reacting the product of step a) with an alkyl or benzyl halide,
c) distilling out the dialkyl sulfide, and
d) reacting the dialkyl sulfide purified according to steps a) to c) with diborane.
2. The process according to claim 1 , wherein the dialkyl sulfide is dimethyl sulfide.
3. A process for enantioselective reductions which comprises using as reducing agent the dialkyl sulfide borane complex obtained by a process by the process according to claim 1 .
4. The process according to claim 3 , wherein the enantioselective reduction is the reduction of a prochiral ketone or imine.
5. The process according to claim 3 , wherein the enantioselective reduction is carried out in the presence of a tetrahydro-1-methyl-3,3-diphenyl-1H,3H-pyrrolo[1,2-c][1,3,2]oxazaborole or a 2-[(1,3,2-dioxaborolan-2-yloxy)diphenylmethyl]pyrrolidine as a catalyst.
6. The process according to claim 4 , wherein the enantioselective reduction is carried out in the presence of a tetrahydro-1-methyl-3,3-diphenyl-1H,3H-pyrrolo[1,2-c][1,3,2]oxazaborole or a 2-[(1,3,2-dioxaborolan-2-yloxy)diphenylmethyl]pyrrolidine as a catalyst.
7. The process according to claim 1 , wherein the step d) is carried out at a temperature from −10 to 50° C. and the diborane is in gaseous form.
8. The process according to claim 7 , wherein the step d) is carried out at ambient temperature.
9. The process according to claim 1 , wherein the dialkyl sulfide borane has a content of each of the impurities carbon disulfide, methanethiol and dimethyldisulfide below 0.01 wt. % respectively.