Generation of Silyl Cations by Hydroalumination Reactions

• Applicant: Professor Dr. Werner Uhl
• Subject Area: Inorganic Molecular Chemistry - Synthesis and Characterisation
• Term: from 2008 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 82408704

In the first period of this project we synthesized highly functionalized silicon and germanium compounds which had in one molecule Lewis basic donor atoms (X) bound to the group 14 element and Lewis acidic acceptor atoms (Al, Ga) in beta-position. Intramolecular interactions between the opposite functionalities yielded adducts with the concomitant activation of the E-X bonds. Complete transfer of the groups X to the aluminum and gallium atoms and cleavage of the E-X bonds may result in the formation of compounds with silyl or germyl cations which due to their zwitterionic nature should be soluble in non-polar organic solvents. The unique structures and reactivity of cations of this type based on separate ions initiated intense current research activities. In our compounds X represents nitrogen or chlorine atoms and the alpha-carbon atoms of alkynes. The activated alkynes gave an interesting rearrangement to yield sila- or germacyclobutenes which showed a remarkable fluorescent behavior. A first silyl cation may be formed as an intermediate in an unsymmetric dyotropic rearrangement. These compounds with activated Si-X or Ge-X bonds represent the first successful step for the synthesis of our target molecules, and we have learned to synthesize a large variety of compounds with E-X bond activation. These results clearly help to develop a reasonable strategy for our future work and the generation of cationic species. Complete cleavage of the E-X bond may be favored by the presence of electron donating substituents bound to silicon or germanium atoms, the modification of the leaving groups with respect to their hard- or softness and an increased acceptor strength of the aluminum or gallium atoms which may be achieved by electron withdrawing groups. The activated compounds obtained so far show a unique reactivity which resembles that one of the famous and intensely investigated Frustrated Lewis Pairs and may make an important contribution to the transition-metal-free activation of organic and inorganic substrates. Insertion reactions into the activated Si-X and Ge-X bonds or substituent exchange reactions are facilitated.

• DFG Programme: Research Grants