Synthesis and Chemistry of Titanacyclopentane and Titanacyclopropane Rings Supported by Aryloxide Ligation

Treatment of the titanacyclopentadiene compound [Ti(OC₆H₃Ph₂-2,6)₂(C₄Et₄)] (3) (OC₆H₃Ph₂-2,6 = 2,6-diphenylphenoxide) with olefins leads to the formation of a variety of stable titanacyclopentane derivatives along with one equivalent of substituted 1,3-cyclohexadiene. The structural and spectroscopic properties of the ethylene product [Ti(OC₆H₃Ph₂-2,6)₂(CH₂)₄] (4) show a ground state titanacyclopentane structure, but facile fragmentation on the NMR time scale to form a bis(ethylene) complex has been detected. The substituted products [Ti(OC₆H₃Ph₂-2,6)₂(C₄H₆R₂)] (R = Me, 5; Et, 6; Ph, 7) formed from α-olefins RCHCH₂ exist as a mixture of regio- and stereoisomers in hydrocarbon solution. Analysis of a crystal obtained from solutions of 7 showed a trans-2,5-diphenyl-titanacyclopentane ring to be present in the solid state. Alternative routes to the titanacyclopentane compounds involve treatment of the dichlorides [Ti(OC₆H₃Ph₂-2,6)₂Cl₂] (1) or [Ti(OC₆HPh₄-2,3,5,6)₂Cl₂] (2) with either sodium amalgam (2 Na per Ti) or 2 equiv of BuⁿLi in the presence of the substrate olefin. Using these conditions the titanabicyclic compounds [(ArO)₂Ti{CH₂CH(C₄H₈)CHCH₂}] (ArO = OC₆H₃Ph₂-2,6, 10; OC₆HPh₄-2,3,5,6, 11) can be obtained by intramolecular coupling of 1,7-octadiene. The spectroscopic properties of 10 and 11 as well as a single-crystal X-ray diffraction analysis of 11 show an exclusive trans stereochemistry is present. Thermolysis of 10 or 11 in the presence of excess 1,7-octadiene leads to the catalytic formation of 2-(methylmethylene)cyclohexane (80%) along with E,Z isomers of 2,6-octadiene (20%). A kinetic study shows the reaction to be zero order in diene with activation parameters, ΔH^⧧ = +18.7(5) kcal mol^-1 and ΔS^⧧ = −26(5) eu. The diphenyltitanacyclopentane 7 will catalyze the dimerization of styrene to trans-1,3-diphenylbut-1-ene followed by isomerization to 1,3-diphenylbut-2-ene. This result shows that although a 2,5-diphenyl regiochemistry was observed in the solid state, styrene dimerization occurs via the 2,4-diphenyltitanacyclopentane intermediate. The facile fragmentation of these titanacyclopentane compounds accounts for the products observed in a number of reactions. Addition of phosphine donor ligands (L) leads to a series of titanacyclopropane compounds [Ti(OC₆H₃Ph₂-2,6)₂(η²-CHRCH₂)(L)] (R = H, 14; Me, 15; Et, 16; Ph, 17) along with 1 equiv of olefin. The solid-state structure of the ethylene complex 14 shows the C₂H₄ unit lies approximately coplanar with the Ti−PMe₃ bond. This structure is not only maintained in solution but slow olefin rotation is observed on the NMR time scale. In the case of the α-olefin products, two isomers are detected by ¹H, ¹³C, and ³¹P NMR spectroscopy. Addition of Ph₂CO or PhCHNR (R = Ph, CH₂Ph) to the titanacyclopentane and titanacyclopropane compounds leads to different products depending upon the reagent and reaction conditions. These can be classified as 2-oxa(aza)titanacycloheptanes, 2-oxa(aza)titanacyclopentanes, 2,5-dioxa(diaza)titanacyclopentanes, and examples of 2-oxatitanacyclopropane (η²-ketone) and 2,7-dioxatitanacycloheptane compounds. The 2-azatitanacyclopentane compounds [Ti(OC₆H₃Ph₂-2,6)₂{(PhCH₂)NCH(Ph)CH₂CH₂}] (30) and trans-[Ti(OC₆H₃Ph₂-2,6)₂{(Ph)NCH(Ph)CH₂CH(Ph)}] (31) react with alkynes to produce the corresponding 2-azatitanacyclopent-4-ene which hydrolyze to produce a stoichiometric equivalent of allylamine. Reaction of 30 with benzonitrile produces the 2,5-diazatitanacyclopent-2-ene [Ti(OC₆H₃Ph₂-2,6)₂(NCPhCHPhNR)] (35) along with ethylene.