Journal
EUROPEAN JOURNAL OF ORGANIC CHEMISTRY
Volume 2011, Issue 28, Pages 5649-5658Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/ejoc.201100726
Keywords
Helical structures; Heterohelicenes; Sulfur heterocycles; Phosphanes; Thiaheliphos; Rhodium; Asymmetric catalysis; Hydrogenation
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Funding
- Ministero dell'Universita e della Ricerca (MIUR)
- University of Milan [2007XFA27F 004]
- CASPUR
- Engineering and Physical Sciences Research Council (EPSRC) [EH/C009922]
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Tetrathia[7]helicene-based phosphanes thiaheliphos (2a), nPr-thiaheliphos (2b) and di-nPr-thiaheliphos (2c) have been prepared from the 2,13-dilithio derivatives of thiahelicenes 1a-c by reaction with an excess of Ph2PCl. Protection of the air-sensitive products as BH3 adducts, from which the phosphanes 2a-c are easily regenerated on heating with ethanol, is described. New rhodium(I) complexes 8 and 9 were obtained by reaction of 2c with [Rh(COD)(2)](+)[BF4] and [BARF], respectively, which were converted by oxidation into the chelating phosphane-phosphane oxide Rh-I complexes 10 and 11. The monophosphane 5 was similarly prepared, and reaction of the dilithio species with ClP(O)(OEt)(2) gave the diphosphonate 17, which was characterized by X-ray crystallography. Comparison of data from DFT calculations (B3LYP/SVP) on 2c, 8 and 10 and X-ray data for 17 reveals a strong tension within the thiaheliphos ligands that is partially relaxed by increasing the Rh-P distances and the bite angle of the chelating ligand. Compound (P)-(+)-1c, obtained by chromatographic resolution, was transformed into optically pure (P)-(+)-2c, which was used in the asymmetric hydrogenation of itaconic acid ester and methyl 2-acetamidoacrylate (product ees up to 40%).
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