Reports: B1
46222-B1 Design of Nitrogen, Phosphorus and/or Sulfur-based Catalysts for the Enantioselective Hydrosilylation of Prochiral Ketones
Catalytic Asymmetric Hydrosilylation of Acetophenone with New Chiral Thiourea Ligands Containing the (S)-a-Phenylethyl Group.
We prepared new chiral thioureas containing 1,2-ethylendiamine or trans-1,2-diaminocyclohexane as carbon skeleton, and containing (S)-a-phenylethyl group have been prepared (79–98% yield). The thioureas were used as ligands for the zinc-based catalyzed asymmetric hydrosilylation of acetophenone with polymethylhydrosiloxane (PMHS). Enantiomeric excess up to 75% was obtained in toluene with a catalyst load of 5 mol%. A manuscript was submitted to Tetrahedron: Asymmetry (TETASY-D-09-00444).
Mario A. Penagos-Puig, an undergraduate student, who received a PRF-scholarship work at this project. We first prepared the (1R,2R)-trans-11,12-diamino-9,10-dihydro-9,10-etanoanthracene following the procedure described by Fox et. al.1 The rac-trans-1,2-diamines were prepared by a Diels-Alder cycloaddition of anthracene and fumaroyl chloride affording the rac-1,2-dicarboxylic acid followed by a Curtius rearrangement in 87% yield. The enantiopure trans-1,2-(1R,2R)-diamine was resolved with (S)-mandelic acid with 37% yield.
We used the enantiopure diamine as the starting material for secondary diamine ligands. First, diimines were prepared by condensation of diamine with aldehydes (i.e. benzaldehyde, 1-naphthylaldehyde, 9-anthracenylaldehyde, p-t-butyl-benzaldehyde, salilcylaldehyde). Diimines were reduced with sodium borohydride in MeOH to afford the secondary diamines. Yields were up to 75% after purification by column chromatography.
This work was accepted for the poster session at the 44th National Mexican Chemistry Conference. The conference took place during September at Puebla, Mexico.
Asymmetric hydrosilylation of acetophenone in the presence of the N,N-ligands and diethylzinc was performed. Unfortunately low yields and enantioselectivities were obtained.
Preparation of chiral trans-2-(alkylthio)cyclohexanamines.
Paulina González Moreno was the Summer Scholar who participate in this part of the project. First, we prepared the cyclohexene aziridine from cyclohexene oxide. Second, the aziridine was opened with thiols: benzenthiol and (1S,4R)-10-mercaptomethyl-7,7-dimethyl-bicyclo[2.2.1]-heptan-2-one.2 On one hand, rac-trans-aminothioethers derived from benzenthiol were resolved with tartaric acid. On the other hand the camphor based diasteroisomeric mixture was tried to be separated by column chromatography, nevertheless it decomposed. We are now preparing the Boc-cyclohexene aziridine as starting material for the preparation of the N,S-ligands.
References.
1. Fox, M. E.; Gerlach, A.; Lennon, I. C.; Meek, G.; Praquin, C. Synthesis 2005, 19, 3196.
2. Gayet, A.; Bolea, C. Andersson, P. G. Org. Biomol. Chem. 2004, 2, 1887-1893.
