On the silaethene tBu(2)Si=C(SiMe(3))(2), and the structure and reactivity of its [4+2]cycloadduct with Ph(2)C=O

From tBu(2)SiF-CLi(SiMe(3))(2) by LiF elimination only Me(2)Si=C(SiMe(3))(SiMetBu(2)) (Ib) instead of tBu(2)Si=C(SiMe(3))(2) (la) is isolated. With benzophenone, silaethene Ib (from its THF adduct 1b . THF) forms at -78 degrees C a yellow [4 + 2] cycloadduct (5) and a colorless [2 + 2] cycloadduct (6). With increasing temperature 5 transforms into 6, and 6 transforms - more slowly - into the yellow [4 + 2] cycloadduct (7) of la and Ph(2)CO. It follows from this that the migration equilibrium la reversible arrow Ib really exists. The structure of 7 shows a planar -O-CPh=C6H5- framework with the cycloadded atoms Si and C from -tBu(2)Si-C(SiMe(3))(2)- located above the plane. The =C6H5-C bond (1.62 Angstrom) is exceptionally long. 7 is a source for la which is trapped by acetone (formation of an ene product) and by benzaldehyde [formation of a [2 + 2] cycloadduct, unstable against cycloreversion into PhHC=C(SiMe(3))(2) and tBu(2)SiO-containing substances]. Reaction of 7 with trimethylsilyl azide, less reactive than acetone or benzaldehyde, leads - by way of la which transforms into Ib - to a trapping product of Ib (formation of a [3 + 2] cycloadduct, unstable against transformation into (RMe(2)Si)(Me(3)Si)CN2 with R=(Me(3)Si)(2)N, or cycloreversion into (tBu(2)MeSi) (Me(3)Si)CN2 and Me(2)SiNSiMe(3)-containing substances). Reaction of Ib or 1b . THF with acetone leads only to an ene reaction product of Ib. This means, that only undetectable amounts of la or 1a . THF are in equilibrium with Ib or 1b . THF. A minor substance which is shown by NMR at low temperatures besides Ib or Ib THF is interpreted as a rotamer of Ib or 1b . THF.