The Formation of CH2-Linked 1,3,5,7-Tetrasilaadamantanes

7-tetrasilaadamantane structural unit itself has proved to be relatively stable against the effects of AIX3 (X = Br, Cl). Reaction between Si-methylated 1,3,5,7-tetrasilaadamantane 39 and AlBr3 occurs very slowly in benzene (30% converted in 8 days at 80 °C), but without solvent, the rate was much faster (30 % converted after 4 hrs at 90 °C). Apart from unconverted compound mono-brominated 1,3,5,7- 

Through H-NMR spectroscopy and mass spectrometry, the methylene-linked tetrasilaadamantane chains in compounds 218 and 2, as well as their brominated analogues in 219, 22J and 222 were characterized. 

An extension of time for the reaction between 1,3,5,7-tetrasilaadamantane and AlBr3 showed at first the formation of pentane-insoluble oligomers which were soluble in THF or benzene (reaction time 4 to 7 hrs at 90 °C). Reaction times exceeding 7 hrs result in the synthesis of polymers insoluble in commonly used solvents, polymers which are constructed in the same way as those previously encountered. An IR spectroscopic investigation revealed the presence of absorption bands which are comparable to those of 1,3,5,7-tetrasilaadamantane or which are characteristic for oligo-tetrasilaadamantanes containing two to four tetrasilaadamantane structural units. 

As a result of this investigation, the insoluble polymers were shown to contain three-dimensional methylene-bridged tetrasilaadamantane units. Their skeletal structure was almost entirely resistant to attack by AlBr3, and on preservation of their skeletal structure a partial CH3/Br exchange on silicon resulted, producing brominated derivatives [59]. 

The formation of the CH2-bridged oligo-tetrasilaadamantanes can be clarified by consideration of mechanism 2 formulated for cyclization reactions. Here, methane is released by action of AlBr3 through an intermolecular and not through an intramolecular process. The presence of methane was proved experimentally [59]. 

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