טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentPerl Olshvang Sharon
SubjectAromatic Strained Systems: Aspects of Basic Research and
Aspects of Apllied Research
DepartmentDepartment of Chemistry
Supervisor Professor Amnon Stanger
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Abstract

Strained Molecules show bond angles and/or lengths that deviated by considerably from those in unstrained systems. In aromatic molecules this chemical strain may distort the symmetry of the cyclic delocalization of the electrons. The direction of the distortion in strained aromatic molecules is either out-of-plane (π frame), in-plane (σ frame), or a combination of the two.

This research focuses on strained aromatic systems from the points of view of both applied and basic research. The applied research concentrates on systems in which the annulation is a four-membered ring, especially the trans- 7, 8-dibromo derivatives. In a previous study it was found that trans,trans- 7,8,9,10 tetrabromobicyclobutabenzene is destroyed under the conditions of its preparation. The first main goal of this research was to improve the yields of its preparation. This was accomplished by developing a procedure where trans,trans- 7,8,9,10 tetrabromobicyclobutabenzene was continuously removed from the reaction mixture. The synthesis was conducted inside a column containing immobilized Ni. The substrate was dissolved in hot DMF and passed through a column containing immobilized Ni(0). The column was heated to 120°C to allow for efficient cyclization.

 In addition to the above applied research, basic computational research that was aimed to obtain a general understanding of these strained aromatic systems was also pursued. The SIBL (Strain Induced Bond Localization) approach was found to yield the most general explanation. Numerous studies of the influence of small ring annulation on aromatic systems revealed a correlation between bond localization and strain.

One such project focused on the oxidation reaction of Vitamin E, one of the most efficient antioxidants in the human body. This study was carried out in collaboration with Prof. Rosenau and his group, who carried out the experimental part. The oxidation of different derivatives that are structural analogs to a-tocopherol, having different annulated moieties with different bond angles (the sum of which are 219°- 242°) were studied. Correlating theory and experiment provides mechanistic insight and reveals the basis of the regioselectivity of the oxidation reaction, namely:

  1. The regioselectivity is determined by the annulation bond angles.
  2. A planar benzilic cation intermediate is the key intermediate which is formed after the rate determining step of the oxidation of vitamin E by silver ions.