Understanding the steric control of stereoselective olefin binding in cyclopentadienyl complexes of rhenium: An application of de novo ligand design

Gladysz has demonstrated that the [(eta (5)-C5H5)Re(NO)(PPh3)](+) fragment can stereoselectively bind prochiral olefins. The origins of selectivity ar e thought to be rooted in the steric interaction between the substituent on the olefin and ligands on the Re. Using Brown's ligand repulsive energy me thodology, we have verified that stereoselectivity toward prochiral alpha - olefins in [(eta (5)-C5H5)Re(NO)(PPh3)](+) can be understood in terms of a steric argument. Using molecular mechanics, we examine how the fragment [(e ta (5)-C5R5)Re(NO)(L)](+) (R = H, Me; L = PMe3, PPh3) stereoselectively bin ds prochiral alpha -olefins, CH2=CHR (R = Me, n-Pr, CH2Ph, Ph, i-Pr, t-Bu, and SiMe3). We have used the molecular mechanics results to rate the relati ve impact on stereoselectivity toward alpha -olefins by the size of the cyc lopentadienyl ring versus the size of the phosphine.