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

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journal contribution
posted on 17.09.1997, 00:00 by Matthew G. Thorn, John E. Hill, Steven A. Waratuke, Eric S. Johnson, Phillip E. Fanwick, Ian P. Rothwell
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)22-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.

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