161271-50-7Relevant articles and documents
Selective cross-coupling of 2,3-dimethylbutadiene and isoprene with α-olefins catalyzed by titanium aryloxide compounds
Balaich, Gary J.,Hill, John E.,Waratuke, Steve A.,Fanwick, Phillip E.,Rothwell, Ian P.
, p. 656 - 665 (2008/10/08)
The titanacyclopentane complex [(Ar′O)2Ti(CH2)4] (Ar′O = 2,6-diphenylphenoxide) (1) reacts with butadiene or isoprene to form the π-allyl complexes [(Ar′O)2Ti(CH2CH=CHCH2CH 2-CH2)] (2) and [(Ar′O)2Ti(CH2CMe=CHCH2CH 2CH2)] (3). In contrast, 1 reacts with 2,3-dimethylbutadiene to initially form the titanacyclopent-3-ene complex [(Ar′O)2Ti-(CMe=CMeCH2)] (4) and free ethylene. The solid state structure of 4 shows a bent titanacyclic ring with a fold angle of 75°. The 1H NMR spectrum of 4 shows nonequivalent α-CH2 protons, indicating that the bent structure is maintained in solution and that flipping of the metallacycle ring is slow on the NMR time scale. Sodium amalgam reduction of the dichloride complex [(Ar″O)2TiCl2] (Ar″O = 2,6-diisopropylphenoxide) in the presence of 2,3-dimethylbutadiene has been shown to produce the titanacyclopent-3-ene complex [(Ar″O)2Ti-(CHCMe=CHMe)] (7) which has similar spectroscopic (1H, 13C NMR) properties to those of 4. Complex 4 reacts with ethylene to produce the titanacyclohept-3-ene complex [(Ar′O)2Ti-(CH2CMe=CMeCH2CH 2CH2)] (5). It has been shown that 5 consists of a mixture of cis-and trans-titanacyclohept-3-ene complexes in solution, 5a and 5b, with 5b as the major (80%) isomer. In the absence of ethylene or 2,3-dimethylbutadiene, the isomeric mixture of 5a and 5b slowly converts to the titanacyclopent-3-ene complex [(Ar′O)2Ti(CH2-CMe=CMeCHEt)] (6). In the presence of an excess of 2,3-dimethylbutadiene and 1 atm of ethylene at 70°C, 5 produces the cross-coupled product 4,5-dimethylhexa-1,4-diene catalytically with a turnover rate of ～8 Ti-1 h-1. The titanacyclopentadiene complexes [(Ar′O)2Ti-(C4Et4)] (8) and [(Ar″O)2Ti{C2(SiMe3)2C 6H8}] (9) as well as 7 have been shown to be catalyst precursors in the cross-coupling of 2,3-dimethylbutadiene with α-olefins (CH2=CHR, R = Me, ET, Ph, Bun, SiMe3). These reactions catalytically produce mixtures of substituted acyclic 1,4-diene products, which have been identified by 1H and 13C NMR, GC, and MS. The cross-coupling of isoprene and styrene to form 1,4-diene products has also been carried out using complexes 8 and 9 as precursors. In the isoprene/styrene cross-coupling reaction, isoprene dimerization was also observed to form vinylcyclohexenes and the linear dimer, 2,7-dimethyl-1,3,6-octatriene. Kinetic studies of the cross-coupling reaction have been carried out. Plots of concentration of substrates vs time show that there is an approximate zero-order dependence on diene or α-olefin concentration and a first-order dependence on [Ti]. Comparison of the observed product distributions from the cross-coupling reactions of 2,3-dimethylbutadiene with proteo and perdeuterated styrene provides evidence that isomerization of an initially formed β-phenyl titanacyclohept-3-ene to an a-phenyl titanacyclohept-3-ene occurs prior to elimination of the organic products. Further evidence for the isomerization was obtained from the results of a crossover experiment, in which a 50/50 mixture of proteo and perdeuterated styrene was reacted with 2,3-dimethylbutadiene in the presence of a catalytic amount of 8. The observed products from isoprene dimerization can be accounted for by invoking β-hydrogen abstraction and ring closure reactions of titanacyclohept-3-ene intermediates. Crystal data: at -60°C for TiC42H36O2 (4) a = 35.351-(3) A?, b = 37.927(3) A?, c = 9.755(1) A?, Z = 16, dcalcd = 1.261 g cm-3 in space group Fdd2.
