The “cieplak effect”: Hyperconjugative interactions in diels alder reactions.

✍ Scribed by James M. Coxon; D.Quentin McDonald

• Publisher: Elsevier Science
• Year: 1992
• Tongue: French
• Weight: 295 KB
• Volume: 33
• Category: Article
• ISSN: 0040-4039
• DOI: 10.1016/s0040-4039(00)92334-7

✦ Synopsis

Hyperconjugative stabilization in Diels-Alder re:lction< of 5-substituted cyclopentadienes by o-bonds antiperiplanar to the bond forming centers hii'; recently been proposed to account for the observed facial selectivity.1 The explanation was first propostd by CizplakZ to ;~ccount for the directing effects of remote substituents in addition reactions to substittlted cyclt)llr*~~noncs. The basis of the "Cieplnk effect" is that an incoming nucleophile will add to LI cnrbonyl from the t';~ce thilt ILIICWS the rreuter anti-periplanar hyperconjugative stabilization from an djacent o-bond. By an:~iogy Diels-Alder reactions of cyclopentadiene would be predicted to occur urlti to the adjacent C7 a-bond which is the better hq Iperco~l~iugative donor,~ 4 as shown below for the diem HOMO and the developing incipent bonds with the LUMO of'tllc di~nophilt: (il. This C7 o-bond will also best be _-X X X able to stabilize the vacant cr"'-ouhitals of the incipient electron: deficient o-bonds forming nt the transition state (ii and iii). This qualitative argument rationalist\ the cxperitnental observations and the present study seeks to