Enantioselective Ir<sup>I</sup>‐Catalyzed Carbocyclization of 1,6‐Enynes by the Chiral Counterion Strategy
抄録
<jats:title>Abstract</jats:title><jats:p>Enantioenriched bicyclo[4.1.0]hept‐2‐enes were synthesized by Ir<jats:sup>I</jats:sup>‐catalyzed carbocyclization of 1,6‐enynes. No chiral ligands were used, CO and PPh<jats:sub>3</jats:sub> were the only ligands bound to iridium. Instead, the stereochemical information was localized on the counterion of the catalyst, generated in situ by reaction of Vaska’s complex (<jats:italic>trans</jats:italic>‐[IrCl(CO)(PPh<jats:sub>3</jats:sub>)<jats:sub>2</jats:sub>]) with a chiral silver phosphate. Enantiomeric excesses up to 93 % were obtained when this catalytic mixture was used. <jats:sup>31</jats:sup>P NMR and IR spectroscopy suggest that formation of the <jats:italic>trans</jats:italic>‐ [Ir(CO)(PPh<jats:sub>3</jats:sub>)<jats:sub>2</jats:sub>]<jats:sup>+</jats:sup> moiety occurs by chlorine abstraction. Moreover, density functional theory calculations support a 6‐<jats:italic>endo</jats:italic>‐dig cyclization promoted by this cationic moiety. The chiral phosphate anion (OP*) controls the enantioselectivity through formation of a loose ion pair with the metal center and establishes a CH⋅⋅⋅OP* hydrogen bond with the substrate. This is a rare example of asymmetric counterion‐directed transition‐metal catalysis and represents the first application of such a strategy to a CC bond‐forming reaction.</jats:p>
収録刊行物
-
- Chemistry – A European Journal
-
Chemistry – A European Journal 17 (49), 13789-13794, 2011-11-03
Wiley