REACTIVITY OF THE SILAETHENE PH(2)SI=C(SI ME(3))(2)

Silaethene Ph(2)Si=C(SiMe(3))(2) (3), generated as a reaction intermed iate by the thermal elimination of LiX from Ph(2)SiX-CLi(SiMe(3))(2) ( X=Br, F) or by the thermal cycloreversion of the [4 + 2] cycloadduct o f 3 and Ph(2)C=NSiMe(3), forms adducts 3 donor, the stability of which increases in the order of donor=Et(2)O < Br- < THF < NEtMe(2) < F-, a nd combines with the reactants a-b (e.g. R-Li; R=H, Me, nBu, Ph) with insertion into the a-b bond, with a=b-c-H [e.g. O=CtBu-CH2-H; CH2=CR-C H2-H, R=Me, CMe=CH2, CH(2)SiPh(2)CH(SiMe(3))(2)] according to an ene r eaction, with a=b (e.g. CH2=CHOMe; Ph(2)C=NSiMe(3)) or a=b=c (e.g. tBu (2)RSiN(3), R=Me, tBu) or a=b-c=d [e.g. CH2=CR-CR=CH2, R/R=H/H, Me/Me, Me/CH(2)SiPh(2)CH-(SiMe(3))(2); Ph(2)C=Y, Y=O, NSiMe(3)] with [2 + 2] or [3 + 2] or [4 + 2] cycloaddition. The following order of relative reactivity of trapping reagents for Ph(2)Si=C(SiMe(3))(2) was found: P h(2)CO > tBu(2)MeSiN(3) > butadiene > 2,3-dimethylbutadiene > Ph(2)CNS iMe(3) > tBu(3)SiN(3) > anthracene. Summing up, it may be said that go ing from Me(2)Si=C(SiMe(3))(2) to Ph(2)Si=C-(SiMe(3))(2) there is only a gradual but not principal change of silaethene reactivity. This cha nge is due to increasing polarity and overcrowding of the double bond, that is increasing Lewis acidity and steric hindrance of the unsatura ted silicon atom. Certainly, the former silaethene stabilizes thermall y by dimerization, the latter by isomerization.