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Mechanistic insights into the thermal C2-C6 cyclization of enyne allenes and enyne-carbodiimides

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  1. Tez No: 403328
  2. Yazar: MEHMET EMİN ÇINAR
  3. Danışmanlar: Prof. Dr. MİCHAEL SCHMITTEL
  4. Tez Türü: Doktora
  5. Konular: Kimya Mühendisliği, Chemical Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 2010
  8. Dil: İngilizce
  9. Üniversite: Universität Siegen (University of Siegen)
  10. Enstitü: Yurtdışı Enstitü
  11. Ana Bilim Dalı: Belirtilmemiş.
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 188

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Özet (Çeviri)

In the present doctoral thesis, enyne–allenes and -carbodiimides with various substituents at the allene and alkyne terminus were synthesized and their thermolyses were investigated. A product study at different temperatures, a helpful tool for the identification of non-statistical dynamics, was used to shed more light on the thermal Schmittel (C2-C6)/Diels-Alder cyclization of enyneallenes and –carbodiimides. The influence of radical stabilizing substituents was probed with a series of specially designed benzannulated enyne-allenes and -carbodiimides possessing a m- or o-tolyl subunit at the allene terminus giving rise to two different regioisomeric products. A constant ratio as a function of temperature indicates non-statistical dynamic behaviour. The two other substituents were chosen as; (i) only radical non-stabilizing substituents, (ii) a single radical stabilizing group (an aryl group), (iii) in the case of enyne-allene, two radical stabilizing groups at both the allene and alkyne termini. For enyne-allenes, lacking any radical stabilizing subunit except for the m-tolyl group or being substituted by only one radical stabilizing subunit, constant product ratios were observed over a wide range of temperature. Such ratios point to the occurrence of dynamic effects in concerted or stepwise diradical mechanisms. In contrast, the thermolyses of enyneallenes with a radical non-stabilizing group at the alkyne terminus provided temperature dependent product ratios, in full agreement with a classical mechanism. Comprehensive studies including trapping of the intermediate, correlation of the rate constants with Hammett -values, solvent effect studies, temperature dependence of product ratios, 13C kinetic isotope effects and computational efforts were used to get more insight in the mechanism of the thermal cyclization of enyne-carbodiimides. Dynamic effects are observed with electron withdrawing substituents (-CN, -NO2) at the alkyne terminus, whereas electron donating group (-OMe) lead to a classical mechanism. In contrast, a phenyl substituent places the system at the boundary between a classical stepwise mechanism at lower temperatures and a non-statistical dynamic mechanism at higher temperatures. On the other hand, the cyclization of an alkyl substituted enyne-carbodiimide takes place in a concerted manner.

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