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Rearrangement and cycloreversion of Diels‐Alder adducts of cyclic 1,3‐dienes to norbornadienes. A novel homo ‐cope rearrangement. Evidence for competitive pericyclic and diradical processes
Author(s) -
Hochstrate Dirk,
Klärner FrankGerrit
Publication year - 1995
Publication title -
liebigs annalen
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 0947-3440
DOI - 10.1002/jlac.1995199505110
Subject(s) - pericyclic reaction , chemistry , diradical , cope rearrangement , adduct , computational chemistry , medicinal chemistry , organic chemistry , physics , excited state , nuclear physics , singlet state
On thermolysis of endo,endo ‐ 5 , endo,endo ‐ 17 , and endo,exo ‐ 17 (the endo,endo and endo,exo Diels‐Alder adducts of cyclopentadiene 7 or dimethylfulvene 15 to norbornadiene 8 or 7‐isopropylidenenorbornadiene 20 ), a novel type of homo ‐Cope rearrangement leading to 6, 21 , and 23a , respectively, competes with the retro ‐Diels‐Alder reactions. According to a force‐field analysis of the kinetic parameters, the competitive reactions (rearrangement and retro ‐Diels‐Alder reaction) of endo,endo ‐ 5 occur in a pericyclic fashion whereas in the isopropylidene‐substituted systems stepwise processes compete with the corresponding pericyclic reactions. Indirect experimental evidence for this assumption comes from a stereochemical analysis of the refro ‐Diels‐Alder reaction in the cis ‐5,6‐dideuterionorbornene derivatives exo ‐ 36 ‐ d 2 , endo ‐ 36 ‐ d 2 and exo ‐ 39 ‐ d 2 , endo ‐ 39 ‐ d 2 occurring stereospecifically in the case of exo ‐ 36 ‐ d 2 , endo ‐ 36 ‐ d 2 and non‐stereospecifically in the case of exo ‐ 39 ‐ d 2 , endo ‐ 39 ‐ d 2 . The change in mechanism is rationalized by a different (allyl vs. pentadienyl) stabilization of the potential diradical intermediates in the stepwise reactions.