LITHIUM-INDUCED CYCLIZATION OF TRIBENZOCYCLYNE - SYNTHESIS AND STRUCTURAL CHARACTERIZATION OF A NOVEL HELICENE DIANION, ITS PROTONATED FORM, SILYLATED ISOMERS OF A SUBSTITUTED FULVALENE LIGAND, AND A NOVEL DIMERIC LITHIUM COMPLEX

A novel lithium-induced cyclization reaction of tribenzocyclotriyne (TBC) (1) leads to the formation of a helical fulvalene dianion derivative Li2(eta5,eta5-C24H14) (2) in high yield. The helical nature of the dianion is observed in the crystal structure of the 1:2 adduct with TMEDA (2a), which crystallizes in the triclinic space group P1 with a = 12.379 (3) angstrom, b = 16.043 (3) angstrom, c = 16.902 (4) angstrom, alpha = 95.42 (2)-degrees, beta = 96.64 (2)-degrees, gamma = 90.27 (2)-degrees, V = 3318.6 (13) angstrom3, and Z = 4. Reaction of the fulvalene dianion 2 with methanol gives a protonated neutral compound C24Hc, (3) which crystallizes in the orthorhombic space group P2(1)2(1)21 with a = 5.460 (9) angstrom, b = 13.675 (2) angstrom, c = 20.552 (3) angstrom, V = 1534.74 (4) angstrom3, and Z = 4. Reaction of the fulvalene dianion 2 with trimethylchlorosilane leads to the formation of bis(trimethylsilyl) trans and cis isomers Of C24H14-(SiMe3)2. The structures of the two isomers have been determined by single crystal X-ray diffraction. The trans isomer (4a) crystallizes in the monoclinic space group P2(1)/a with a = 12.066 (4) angstrom, b = 18.129 (4) angstrom, c = 13.253 (4) angstrom, beta = 116.41 (2)-degrees, V = 2596.6 (13) angstrom3, and Z = 4. The cis isomer (4b) crystallizes in the triclinic space group P1BAR with a = 8.3694 (13) angstrom, b = 11.446 (2) angstrom, c = 14.807 (3) angstrom, a = 110.058 (13)-degrees, beta = 96.901 (13)-degrees, gamma = 99.810 (13)-degrees, V = 1288.0 (4) angstrom3, and Z = 2. The trans-bis(trimethylsilyl) compound 4a has the most pronounced helical character. Dimetalation of the disilylated isomers 4a and 4b with 2 equiv of n-butyllithium results in the formation of a new dimeric organolithium compound {Li2(THF)2[eta2:eta5,eta5-C24H12]-(SiMe3)2}2 (5) which shows unusual modes of coordination for lithium atoms. Compound 5 crystallizes in the monoclinic space group P2(1)/n with a = 13.663 (3) angstrom, b = 14.509 (3) angstrom, c = 18.494 (4) angstrom, beta = 107.50 (3)-degrees, V = 3496.5 (13) angstrom3, and Z = 4. This is the first example of a crystal structure of an organolithium having lithium atoms bound in an eta5,eta5 fashion to cyclopentadienyl (Cp) moieties forming a sandwich system and is the only inverse sandwich structure with Li-Cp-Li-Cp interactions.