Ti Group Metallocene-Catalyzed Synthesis of 1-Hexene Dimers and Tetramers

Abstract

1-Hexene transformations in the catalytic systems L2MCl2–XAlBui2 (L = Cp, M = Ti, Zr, Hf; L = Ind, rac-H4C2[THInd]2, M = Zr; X = H, Bu i) and [Cp2ZrH2]2-ClAlR2 activated by MMAO-12, B(C6F5)3, or (Ph3C)[B(C6F5)4] in chlorinated solvents (CH2Cl2, CHCl3, o-Cl2C6H4, ClCH2CH2Cl) were studied. The systems [Cp2ZrH2]2-MMAO-12, [Cp2ZrH2]2-ClAlBui2-MMAO-12, or Cp2ZrCl2-HAlBui2-MMAO-12 (B(C6F5)3) in CH2Cl2 showed the highest activity and selectivity towards the formation of vinylidene head-to-tail alkene dimers. The use of chloroform as a solvent provides further in situ dimer dimerization to give a tetramer yield of up to 89%. A study of the reaction of [Cp2ZrH2]2 or Cp2ZrCl2 with organoaluminum compounds and MMAO-12 by NMR spectroscopy confirmed the formation of Zr,Zr-hydride clusters as key intermediates of the alkene dimerization. The probable structure of the Zr,Zr-hydride clusters and ways of their generation in the catalytic systems were analyzed using a quantum chemical approach (DFT).

Transition Metal–(μ-Cl)–Aluminum Bonding in α-Olefin and Diene Chemistry

“…During these experimental studies, increases in the selectivity of dimerization when adding R 2 AlCl were detected. A similar ‘chlorine effect’ was previously demonstrated by the chemists Idemitsu Kosan [ 182 , 183 ], and later by Parfenova et al [ 184 ]. As was shown by Parfenova’s group, Zr 2 bimetallic species are also active in selective dimerization [ 185 ]; however, the use of these in situ forming species is not of interest in practice.…”

Catalytic Properties of Zirconocene-Based Systems in 1-Hexene Oligomerization and Structure of Metal Hydride Reaction Centers

“…We studied the catalytic effect of complexes 1a – j in the oligomerization reaction in the presence of HAlBu i 2 and the activators MMAO-12 (modified methylaluminoxane) and [Ph 3 C][B(C 6 F 5 ) 4 ]. We used di(isobutyl)alane HAlBu i 2 as a co-catalyst similar to AlBu i 3 [ 6 , 7 , 26 , 27 , 28 , 30 ] to initiate the formation of zirconocene hydrides, which act as catalytically active sites in these reactions [ 29 , 30 , 33 , 34 , 35 ]. The reactant ratios were chosen on the basis of our earlier studies [ 33 , 34 , 35 ].…”

“…We used di(isobutyl)alane HAlBu i 2 as a co-catalyst similar to AlBu i 3 [ 6 , 7 , 26 , 27 , 28 , 30 ] to initiate the formation of zirconocene hydrides, which act as catalytically active sites in these reactions [ 29 , 30 , 33 , 34 , 35 ]. The reactant ratios were chosen on the basis of our earlier studies [ 33 , 34 , 35 ]. During the reaction under the selected conditions, the conversion of 1-hexene was approximately 99% in 1–3 h in most of the experiments.…”

“…The difference between the behaviors of the complexes is primarily determined by the composition and reactivity of the intermediates. It was assumed that hydride bimetallic intermediates may act as the active sites of dimerization and oligomerization reactions [ 29 , 30 , 33 , 34 , 35 ]. Our previous NMR study of the reaction of Cp 2 ZrCl 2 or [Cp 2 ZrH 2 ] 2 with OACs showed the existence of Zr,Zr-hydride complexes, which are activated by MMAO-12 or organoboron compound ([Ph 3 C][B(C 6 F 5 ) 4 ] or B(C 6 F 5 ) 3 ), being thus converted to active species responsible for dimer formation [ 32 , 33 , 34 , 35 ].…”

“…Our interest in the structural features and reactivity of group 4B metal hydrides is due to the recent data on the alkene transformation reactions catalyzed by zirconocene dichloride Cp 2 ZrCl 2 or zirconocene dihydride [Cp 2 ZrH 2 ] 2 and OACs in the presence of aluminum- or boron-containing activators [ 32 , 33 , 34 , 35 ]. The formation of the bimetallic Zr,Zr-hydride intermediate in these systems was established, and its ability to serve as the precursor of the specific and selective catalytically active sites for alkene dimerization was demonstrated.…”