The chemistry of titanacyclopentadiene rings supported by 2,6-diphenylphenoxide ligation: Stoichiometric and catalytic reactivity

Article abstract of DOI:10.1021/om00032a012

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^tC≡CH, and MeC≡CPh (≥2 equiv) produces the corresponding titanacyclopentadiene complexes [(Ar″O)₂Ti(C₄Et₄)] (1) [(Ar″O)₂(C₄H₂Bu^t₂)] (2), and [(Ar″O)₂Ti(C₄Me₂Ph₂)] (3). The use 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 ¹H and ¹³C NMR spectra of solutions of 1-3 show no exchange of α- and β-positions of the metallacycle rings. Only the 2,4-regioisomer of 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 Bu^tC≡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 variety of unsaturated organic molecules has been investigated. Reaction of 1 with Bu^tNC leads to a new organometallic compound 5 containing an η²-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 η²-C,N-bound imine complexes but with a long Ti-C distance of 2.262(3) A?. 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(μ-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 25°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°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 Bu^tNC yielded the η²-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°C for TiO₂C₄₈H₄₆ (1) a = 12.627(3) A?, b = 17.378(4) A?, c = 17.739(3) A?, α = 90.41(2)°, β = 94.68(2)°, γ = 92.89(2)°, Z = 4 d_calcd = 1.205 g cm^-3 in space group P1; for TiO₂N₂C₅₈H₆₀ (7) at -105°C a = 12.554(4) A?, b = 17.934(5) A?, c = 21.567(6) A?, β = 102.38(2)°, Z = 4, d_calcd = 1.211 g cm^-3 in space group P2₁/n; for Ti₂O₄N₂C₉₈H₇₄ (8) at -50°C a = 23.100(4) A?, b = 12.656(3) A?, c = 27.486(7) A?, β = 109.09(2)°, Z = 4, d_calcd = 1.259 g cm^-3 in space group C2/c; for TiO₃C₆₈H₆₄ (9) at -50°C a = 12.147(3) A?, b = 12.527(3) A?, c = 20.363(3) A?, α = 80.92(2)°, β = 80.94(2)°, γ = 61.78(2)°, Z = 2, d_calcd = 1.204 g cm^-3 in space group P1; for TiO₃NC₆₆H₆₅ (12) at -20°C a = 11.774(1) A?, b = 22.775(3) A?, c = 20.137(4) A?, β = 98.320(9)°, Z = 4, d_calcd = 1.203 g cm^-3 in space group P2₁/n.