Diastereoselective propargylations with planar chiral chromiumcarbonyl arene complex substituted propargyl cations.

Abstract

The ionization of planar chiral ortho-substituted (arene)Cr(CO)(3)-substituted alpha-propargylic acetates 3 with Lewis acids results in the formation of stable (arene)Cr(CO)(3)-substituted alpha-propargyl cations 4. Subsequent additions of sulfur, nitrogen, oxygen, and pi-carbon nucleophiles to these organometallic electrophiles give rise to the regio- and highly diastereoselective formation of propargyl derivatives 5 in good yields (44-90%; dr = 70:30 to >99:1). The relative stereochemistry of the propargyl acetates 3 and the trapping products 5 was established by several crystal structure analyses, indicating that the cationic propargylations occurred under retention of configuration at the propargylic center. Most important for the diastereoselectivity of the nucleophilic trapping reaction is the configurational stability of the diastereotopic cation 4 as reflected by substituent effects. In situ ionizations according to an S(N)1-mechanism not only result in a considerable loss but also in an inversion of diastereoselectivity.