THE CHEMISTRY OF TITANACYCLOPENTADIENE RINGS SUPPORTED BY 2,6-DIPHENYLPHENOXIDE LIGATION - STOICHIOMETRIC AND CATALYTIC REACTIVITY

The sodium amalgam (two Na per Ti) reduction of hydrocarbon solutions of [Ti(OAr'')2Cl2] (OAr'' = 2,6-diphenylphenoxide) in the presence of EtC=CEt, Bu(t)C=CH, and MeC=CPh (greater-than-or-equal-to2 equiv) produces the corresponding titanacyclopentadiene complexes [Ar''O)2Ti(C4Et4)] (1), [(Ar''O)2(C4H2Bu2t)] (2), and [(Ar''O)2Ti(C4Me2Ph2)] (3). The uses of less bulky substituents on the alkyne substrate in such reactions leads to mixtures of aromatic compounds due to cyclotrimerization reactions. The solid state structure of 1 shows a pseudotetrahedral environment about the titanium metal center with a planar titanacyclopentadiene ring. The H-1 and C-13 NMR spectra of solutions of 1-3 show no exchange of alpha- and beta-positions of the metallacycle rings. Only the 2,4-regioisomer and 2 and 3 was detected in solution. Reaction of 1 with PhC=CPh produced [(Ar''O)2Ti(c4Ph4)](4). reaction of 1-4 with protic reagents yielded the corresponding diene derivatives while iodination of 1 and 2 yielded 1,4-diiodo-1,3-butadienes. Compounds 1-4 will catalyze the cyclotrimerization of a range of alkynes. Terminal alkynes with small substituents produce the 1,2,4-trisubstituted benzene preferentially in an exothermic reaction. The more bulky substrates ButC=CH and Me3SiC=CH react more slowly and only the symmetrical 1,3,5-isomer is produced. The reaction of the titanacyclopentadiene rings in 1 and 2 with a ariety of unsaturated organic molecules has been investigated. Reaction of 1 with ButNC leads to a new organometallic compound 5 containing an eta2-C,N-bound cyclopentadiene-imine which was structurally characterized as a pyridine adduct (7). The solid state structure of 7 showed a structure related to other titanium eta2-C,N-bound imine complexes, but with a long Ti-C distance of 2.262(3) angstrom. Reaction of 1 or 2 with benzonitrile leads to the elimination of 1 equiv of the corresponding pyridine. The organometallic product of these reactions was identified as a dimeric material [(Ar''O)2Ti(mu-PhCN)2Ti(OAr'')2] (8) containing two bridging benzonitrile ligands. The solid state structure of 8 showed the bridging PhCN unit to be highly reduced and strongly bound to the titanium metal centers. The lack of reactivity of 8 precluded the catalytic formation of pyridines. Both compounds 1 and 2 undergo ring expansion with Ph2CO at 20-degrees-C to form the corresponding 2-oxatitanacyclohepta-4-6-diene derivatives 9 and 10. In 10 the ketone was found to insert into the side of the titanacyclopentadiene ring containing the less bulky substituent, leading to a single regioisomer. Reaction of 1 with Ph2CO at 100-degrees-C led to the 2-oxatitanacyclopent-4-ene complex [(Ar''O)2Ti(OCPh2CEtCEt)] (11) along with 1 equiv of 3-hexyne. Attempts to interconvert 9 and 11 failed. Further elaboration of the seven membered ring in 9 by reaction with ButNC yielded the eta2-iminoacyl derivative 12. The conformation of the large metallacycle rings in 9 and 12 was analyzed by carrying out single crystal X-ray diffraction analyses. Crystal data: at 20-degrees-C for TiO2C48H46 (1) a = 12.627(3) angstrom, b = 17.378(4) angstrom, C = 17.739(3) angstrom, alpha = 90.41(2)-degrees, gamma = 92.89(2)-degrees, Z = 4, d(calcd) = 1.205 g cm-3 in space group P1BAR; for TiO2N2C58H60 (7) at -105-degrees-c a = 12.554(4) angstrom, b = 17.934(5) angstrom, c = 21.567(6) angstrom, beta = 102.38(2)-degrees, Z = 4, d(calcd) = 1 211 g cm-3 in space group P2(1)/n; for Ti2O4N2C98H74 (8) at -50-degrees-c a = 23.100(4) angstrom, b = 12.656(3) angstrom, c= 27.486(7) angstrom, beta = 109.09(2)-degrees, Z = 4, d(calcd) = 1.259 g cm-3 in space group C2/c; for TiO3C68H64 (9) at -50-degrees-C a = 12.147(3) angstrom, b = 12.527(3) angstrom, c = 20.363(3) angstrom, alpha = 80.92(2)-degrees, beta = 80.94(2)-degrees, gamma = 61.78(2)-degrees, Z = 2,d(calcd) = 1.204 g cm-3 in space group P1BAR for TiO3NC66H65 (12) at -20-degrees-C a = 11.774(1) angstrom, b = 22.775(3) angstrom, c = 20.137(4) angstrom, beta = 98.320(9)-degrees, Z = 4, d(calcd) = 1.203 cm-3 in space group P2(1)/NBAR.