Isolable 2,3-Disila-1,3-butadiene from a Double Sila-Peterson Reaction

A new 2,3-disila-1,3-butadiene (3) was synthesized as pale yellow crystals by a double sila-Peterson reaction of 1,1,2,2-tetrasilyl-1,2-dilithiodisilane with two equivalents of 2-adamantanone at 0 degrees C. In the solid state, the two Si=C bonds adopt a synclinal conformation with a dihedral angle of 76.8(1)degrees. UV/Vis spectra show two distinct absorption bands, assignable to the pi(Si=C)->pi*(Si=C) transition, at 371 and 322 nm and the former is considerably redshifted compared with that of structurally similar silenes. The 2,3-disilabutadiene isomerizes to the corresponding 1,3-disilabicyclo-[1.1.0] butane with the activation parameters of Delta H = 74.5(+/-) +/- 5.4 kJmol(-1) and Delta S-+/- = -71.1 +/- 7.1 Jmol(-1)1K(-1). DFT studies suggest that the isomerization proceeds through a conrotatory route rather than a disrotatory route. H2O and 9,10-phenanthrenequinone added across each Si= C bond in the 2,3disila- 1,3-butadiene. The UV/Vis spectrum and reactivity of 3 suggest that the interaction between the two Si= C bonds in 3 would be significant but rather small compared with that between Si= Si bonds in a synclinal tetrasilabutadiene.