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Clarify ENDO and EXO.pdf

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CHEM 282
Robert Britton

Clarification of ENDO and EXO The Diels-Alder reaction can proceed via an ENDO or EXO transition structure. When there is a double bond (π bond) ATTACHED to the dieneophile the ENDO transition structure is favored. For example, consider the following dieneophiles: O O O OCH OCH 3 3 dieneophiles with π group dieneophiles without π group IF THE DIENEOPHILE HAS A π GROUP then consider the two possible orientations of the dieneophile in the transition structure. In one arrangement the π group is underneath the diene and this is termed the ENDO transition structure and consequently gives rise to the ENDO product. In arrangement B, the π group is pointed away (not underneath) the diene and this is termed an EXO transition structure and consequently gives rise to the EXO product. While the EXO transition structure has less steric interactions, the ENDO TRANSITION STRUCTURE IS FAVORED DUE TO A FAVORABLE INTERACTION BETWEEN THE π GROUP AND THE p-ORBITALS ON THE DIENE. O O CH 3 O H CO H H3CO 3 H OCH 3 + H H3C ENDO transition structure H O H3C H OCH 3 H ENDO PRODUCT 3CH H C O 3 O CH 3 O O
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