Silacyclopropenes stability

In the reaction of 1 with alkynes possessing electron-withdrawing substituents, the corresponding silacyclopropene derivatives 66 and 67 are formed, as described in Scheme 23.29 An unexpected pathway was observed in the reaction with the electron-poor hexafluorobutyne(2) the X-ray characterized heterocycle 68 was most likely obtained by nucleophilic attack of 1 at the triple bond. A subsequent shift of a fluorine atom from carbon to silicon creates an allene-type molecule which was stabilized by a [2 + 2] cycloaddition process involving a double bond from the pentamethylcyclopentadienyl unit, as described in Scheme 24.33... 

Silylium ions stabilized by Si-X-Si three-center bonds (X = halogen or hydrogen) have been synthesized by the reaction of the corresponding l-tris(trimethylsilyl)methyl-2,3-bis(trimethylsilyl)-l-silacyclopropene derivative with triethylsilylbenzenium tetrakis(pentaflourophenyl)borate <2004CL530>. 

The structures for silacyclopropenes (24)-(28) were calculated using CNDO/2. The small C—Si—C bond angle and the short C=C bond length suggest that the n bond interacts with acceptor orbitals on silicon. This interaction leads to a net stabilization, described as aromatic character <78JOM(i54)C33>. 

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