| Abstract |
This dissertation describes the reaction mechanism of photochemical cycloadditions of alkenes to C60 as well as thermal addition of diphenylmethane to (C59N)2. The principle findings are as follows: In the First Chapter, the mechanism of photochemical [2+2] cycloaddition of β,β- dimethyl-p-methoxystyrene to C60 was investgated. The stereochemistry and kinetic secondary isotope effects are in good agreement with a stepwise mechanism which proceeds through the formation of an open intermediate (dipolar or biradical) in the first and rate determining step, followed by a rapid ring closure in the second step. In the Second Chapter, the chemical reactivity of vinylcyclopropanes to the electrophilic reagent tetracyanoethylene (TCNE), which is known to procced through a dipolar mechanism, was examined. This study was used as a ‘model-study’ for cycloadditions of alkenes to C60. The results show that vinylcyclopropanes provide a powerfull mechanistic tool for studying not only biradical intermediates but also: a) radical cations of alkenes and, b) dipolar intermediates. In the Third Chapter, the chemical reactivity of vinylcyclopropanes to the electrophile C60 was examined. The photochemical reaction of 1-isopropylene-2-phenylcycloropane to C60 and the stereochemistry of the reaction are consonant with an electron transfer mechanism which includes the rearrangement of the radical cation of vinylcyclopropane and the formation of synvinylcyclopentane derivative. Additional cyclopropane derivatives were designed and prepared in order to study the mechanism of this new functionalization of C60. The results are in agreement with the non-reversible formation of the corresponding vinylcyclopentane derivatives through an electron transfer mechnism from the vinylcyclopropane to the excited triplet state of C60. The mechanism is stepwise and includes the formation of a biradical intermediate after combination of the resultant ion pairs. Eventually, the biradical intermediate cyclizes to the corresponding vinylcyclopentane derivative. In the Fourth Chapter, the mechanism of hydrogen transfer from diphenylmethane to aza[C60]fullerene radical in the thermal reaction of diphenylmethane to (C59N)2 was investigated. For this purpose, suitable deuterated substrates were prepared and primary isotope effects were measured. The results are in agreement with the C-H(D) bond cleavage in the first and ratedetermining step followed by the coupling of diphenylmethyl radical and aza[C60]fullerene monomer.
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Μελέτη του μηχανισμού της Φωτοχημικής Κυκλοπροσθήκης του Φουλερενίου C60 με αλκένια
