SYNTHESIS AND STRUCTURES OF EXTREMELY HINDERED AND STABLE DISILENES

Extremely hindered and stable disilenes Tbt(Mes)Si=Si(Mes)Tbt, ((Z)- a nd (E)-6; Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phnyl, Mes = 2,4, 6-trimethylphenyl) were synthesized by a reductive coupling reaction o f the corresponding overcrowded dibromosilane Tbt(Mes)SiBr2 (4) with l ithium naphthalenide in THF. The molecular structures of (Z)- and (E)- 6 were determined by X-ray crystallographic analysis at 120 K. Compoun d (Z)-6 crystallizes in the monoclinic space group P2(1)/n with a = 22 .212(3) Angstrom, b = 13.368(2) Angstrom, c = 29.725(3) Angstrom, beta = 91.344(9)degrees, V = 8824(2) Angstrom(3), and Z = 4. Compound (E)- 6 C10H8 crystallizes in the monoclinic space group P2(1)/c with a = 18 .130(3) Angstrom, b = 18.466(3) Angstrom, c = 28.522(2) Angstrom, beta = 95.976(9)degrees, V = 9497(2) Angstrom(3), and Z = 4. The X-ray str uctures show remarkable pyramidalization around silicon atoms and elon gation of the Si-Si double bond (2.195(4) Angstrom for (Z)-6 and 2.228 (2) Angstrom for (E)-6), the values of which are the longest ones repo rted so far for disilenes having carbon substituents on the silicon at oms. Both disilenes were found to be stable for weeks in the open air even in a microcrystalline form, and they were gradually oxidized to a fford stereospecifically the (Z)- and (E)-1,3,2,4-dioxadisiletane comp ounds 10. Compound (Z)-10 crystallizes in the monoclinic space group P 2(1)/n with a = 22.420(5) Angstrom, b = 13.458(6) Angstrom, c = 30.417 (4) Angstrom, beta = 90.96(1)degrees, V = 9176(4) Angstrom(3), and Z = 4. Reaction of Tbt(t-Bu)SiBr2 (5) with lithium naphthalenide proceede d in a different way to afford the benzosilacyclobutene 11, which was most likely an intramolecular C-H insertion product of the intermediar y silylene Tbt(t-Bu)Si: (12).