טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentVictoria Molev
SubjectSilyl Radicals Preparation and Reactivity of Novel
Conpounds with Si=C Bond
DepartmentDepartment of Chemistry
Supervisor ? 18? Apeloig Yitzhak


Abstract

This thesis studied two main topics: silyl radicals and silenes.

  1. Silyl radicals

    We studied the homolytic cleavage of polysilanes R2R’Si-SiR2R’, with R = R’ = (Me3Si)3Si (3a), (Me3SiMe2Si)3Si (4a), and (t-Bu2HSi)(Me3Si)2Si (5a), and with R = t-Bu, R’ = Me3Si (6a), and R = t-Bu R’ = Me3SiMe2Si (7a), All polysilanes were successfully crystallized and their structures were determined by X-ray crystallography. By following the rates of decay of the produced radicals of type R2R’Si• we could determine their dimerization rates. The dimerization rate constants span over a range of six orders of magnitude. A Me3SiMe2Si substituent kinetically stabilizes the radical by a factor of 100 relative to a b-Me3Si substituent. Two Me3SiMe2Si substituents kinetically stabilize the radical by a factor of 40 relative to two t-Bu groups. The bulky silyl substituents are responsible for the significant kinetic stabilization of (Me3SiMe2Si)3Si• (4b) and Bu2HSi(Me3Si)2Si• (5b) relative to (Me3Si)3Si• (3b). In two pairs of oligosilanes: [(Me3Si)3Si]2 vs. [(Me3SiMe2Si)3Si]2 and [t-Bu2(Me3Si)Si]2 vs. [t-Bu2(Me3SiMe2Si)Si]2 the b-silyl substituents do not cause elongation of the central Si-Si bond, being 2.41Å and 2.54Å respectively, but in [(t-BuMe2Si)3Si]2 it is calculated to be very long, 2.63Å, and the central Si-Si bond energy is estimated to be only 8 kcal/mol.
  2. Silenes

    Three novel silenes, RR’Si=2-Ad (2-Ad = 2-adamantantyl), with RR’Si with R = R’ = (Me3SiMe2Si)2Si (1) and R = R’ = (t-BuMe2Si)2Si (3), and with R = (t-BuMe2Si), R’ = (Me3SiMe2Si)Si, (2) were synthesized using the silaolefination reaction (reaction of 2-adamantanone with the corresponding lithiosilane). The following order was found for the relative rate of elimination of the R3SiOLi group as a function of R, from the intermediate silanolate: Me3Si (120) > t-BuMe2Si (6) > Me3SiMe2Si (1).
    In the cycloaddition reactions of 2,6-di-t-Bu-para-quinone  and of 3,6-di-t-Bu-ortho-quinone to silene 3 triplet biradicals were detected as intermediates by EPR spectroscopy. This is the first clear evidence for a radical mechanism in cycloaddition reactions of silenes. The reaction of silene 3 with 3,6-di-t-Bu-ortho-quinone leads to an unusual [1+4] cycloaddition product, having a five-member ring. A radical mechanism was also found in the addition of (t-BuMe2Si)2Hg to 3.