Homogeneous Hydrogenation of Tri- and Tetrasubstituted Olefins: Comparison of Iridium-Phospinooxazoline [Ir-PHOX] Complexes and Crabtree Catalysts with Hexafluorophosphate (PF6) and Tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BArF) as Counterions
✍ Scribed by Bettina Wüstenberg; Andreas Pfaltz
- Publisher
- John Wiley and Sons
- Year
- 2008
- Tongue
- English
- Weight
- 183 KB
- Volume
- 350
- Category
- Article
- ISSN
- 1615-4150
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✦ Synopsis
Abstract
Four iridium complexes with achiral phosphino‐oxazoline (PHOX) ligands were readily prepared in two steps starting from commercially available phenyloxazolines. The air‐stable complexes with tetrakis[3,5‐bis(trifluoromethyl)phenyl]borate (BAr~F~) as counterion showed high reactivity in the hydrogenation of a range of tri‐ and tetrasubstituted olefins. The best results were obtained with an iridium complex (11) derived from a dicyclohexylphosphino‐oxazoline ligand bearing no additional substituents in the oxazoline ring. With several substrates, which gave only low conversion with the Crabtree catalyst, [Ir(Py)(PCy~3~)(COD)]PF~6~, full conversion was observed. The productivity of the Crabtree catalyst could be strongly increased by replacing the hexafluorophosphate anion with tetrakis[3,5‐bis(trifluoromethyl)phenyl]borate. In one case, in the hydrogenation of a tetraalkyl‐substituted CC bond, [Ir(Py)(PCy~3~)(COD)]BAr~F~ gave higher conversion than catalyst 11. However, with several other substrates complex 11 proved to be superior.