D.A. Evans Transition State Torsional Effects: Olefin Additions Chem 206 Olefin Addition Reactions: Case one Olefin Addition Reactions: Case two How do we account for the high exo selectivities in addition How do we account for the high selectilvities in the oxidation of ctions to norbornene? the indicated olefin? exo Highly exo selective for electrophilic, Oso nucleophilic and cycloaddition reactions mCPBA Rate enhancement due to strain ndo 98:2 diastereoselectivity diastereoselectivity The Controversy over origin of high exoselectivities Steric effects Nitrogen protecting group does not affect selectivities Least nuclear motion A Orbital distortion Schleyer: torsional steering Favored Schleyer, P. R.J. Amer. Chem. Soc. 1967, 89, 701 Martinelli et al. Tett. Lett. 1989. 30. 3935 Addition from exo face avoids eclipsing A &B hydrogens Addition from indicated olefin face avoids eclipsing a&BHs tter hy perconjugative stabilization of transition state) (better hyperconjugative stabilization of transition state Martinelli has carried out further studies on related structuresSteric effects Least nuclear motion Orbital distortion Nitrogen protecting group does not affect selectivities H H N Ph O OH OH H OsO4 N Ph O H A B A N Ph O O H B D. A. Evans Transition State Torsional Effects: Olefin Additions Chem 206 ■ Olefin Addition Reactions: Case one How do we account for the high exo selectivities in addition reactions to norbornene? exo endo Highly exo selective for electrophilic, nucleophilic and cycloaddition reactions The Controversy over origin of high exoselectivities Schleyer: torsional steering Rate enhancement due to strain Schleyer, P. R. J. Amer. Chem. Soc. 1967, 89, 701. Addition from exo face avoids eclipsing A & B hydrogens (better hyperconjugative stabilization of transition state) 98 : 2 diastereoselectivity 99 : 1 diastereoselectivity Martinelli, et.al. Tett. Lett. 1989, 30, 3935 mCPBA ■ Olefin Addition Reactions: Case two Favored Addition from indicated olefin face avoids eclipsing A & B H's (better hyperconjugative stabilization of transition state) How do we account for the high selectilvities in the oxidation of the indicated olefin? Martinelli has carried out further studies on related structures