| Abstract |
This work includes the mechanistic studies of the reactions of four different electophiles with alkenes, substituted in allylic position (e.g. allylic alcohols and enones), conjugated dienes, conjugated trienes and p-methoxy arylalkenes. These electrophiles are the following:
a) Tetracyanoethylene (TCNE, Chapter 1).
b) 4-Aklyl-triazoline-3.5-dione (PTAD, Chapter 2).
c)Singlet oxygen (1O2, Chapter 2).
d) Buckminsterfullerene (C60, Chapter 3).
The investigation of the reaction mechanisms was accomplished by the aid of kinetic isotope effect measurements, stereochemical and steroisotopic studies. The principal findings are as follows:
a) The [2+2] pathway of TCNE addition to conjugated dienes and trienes includes the formation of an open dipolar intermediate in the rate-determining step of the reaction, while the mechanism of the [4+2] pathway is concerted. The isotope effects measured, are the most remote isotope effects reported in the literature so far, and can be attributed to the hyperconjugation effect in the corresponding transition states.
b) In the [2+2] addition of β,β-dimethyl p-methoxy styrene to TCNE a significant inverse intermolecular β-secondary kinetic isotope effect was measured. The steroisotopic study of the above cycloaddition is consistent with the formation of an open intermediate, free to rate.
c) A mechanistic comparison of the additions RTAD and 1O2 (ene, [2+2], methanol trapping) to 2,5-dimethyl-2,4-hexadiene (DMHD) and (Z,Z)-2,4-hexadiene have been reported. The primary kinetic isotope effects in reaction of MTAD and 1O2 with the DMHD, reveal that the ene pathways procees through the reversible formation an aziridinium imide and perepoxide intermediate, respectively. The pathways leading to [2+2] and methanol trapping adducts, proceed through a zwitterionic intermediate which is formed by rearrangement of the initially formed aziridinium imide and perepoxide, respectively.
d) The diastereoselectivity in conjuction with primary isotope effect, in the photooxygenation and PTAD ene reaction of the selectively lebeled and optical active (R,R)-cis-3-hexane-2,5-d2 and (S,S)-cis-1,4-diphenyl-2-butene-1,4-d2 were studied. The resuts are in agreement with a suprafacial mechanism.
e) Both electrophiles, 1O2 and RTAD, behave at similar manner in their ene reactions with secondary allylic alcolols. The interaction between the hydroxyl group and the electrophiles (steering effect) in the rate-determining step is significant in the distribution of the diastereomeric ene products. Intermoleculaar kinetic isotope effects support the change in the energy profile of the ene reaction of triazolinediones, on going from secondary to tertiary allylic alcohols.
f) The mechanism of the photochemical [2+2] cycloaddition of unsaturated substrates to C60 was also examined. These substrates are the following:
i) p-Methoxy-aryl alkenes
ii) Alkyl substituted conjugated dienes
iii) α,β-Enones and α,β-dienones
The stereochemical, stereoisotopic studies and the secondary kinetic isotope effects of the photochemical [2+2] additions of the above unsaturated molecules to C60, show that the first step of the reaction includes the formation of a biradical intermediate followed by a rapid collapse of the initial ion pair to the [2+2] cycloadducts.
|
Μελέτη των αντιδράσεων προσθήκης των ηλεκτρονιοφίλων C60,1O2 TCNE και RTAD σε ακόρεστα υποστρώματα : μηχανιστική σύγκριση
