Titanium complexes butadiene

Figure 1.25 Minimum-energy diastereoisomeric monomer free intermediates for butadiene polymerization catalyzed by titanium complexes presenting Cp group as ancillary ligand. Chiralities of coordination of allyl groups (assumed to be si) and back-biting double bonds (si or re) are indicated, in order to easily visualize possible stereoregularity (iso or syndio) of model chains. In fact, like and unlike chiralities would possibly lead to isotactic and syndiotactic enchainments, respectively.

The outstanding properties of binaphthol (BINOL) as a ligand in chiral Lewis acidic metal complexes were also demonstrated highly successfully by Mikami [108, 109] using a binol-titanium complex 2-69a. Even in the cycloaddition of methyl glyoxylate 2-66 to 1 -methoxy-1,3-butadiene 2-65 which usually shows only a low selectivity, a reasonable cis/trans-selectivity and an excellent enantioselectivity could be obtained in the presence of catalytic amounts of this complex. 

Mixed cyclopentadienyl-diene titanium complexes, Cp TiX(diene)(X = Cl, Br, I), have been prepared in 30-60% yield by the stoichiometric reaction of CpTiXs with (2-butene-l,4-diyl)magnesium derivatives or by the reduction of CpTiXs with RMgX (R = i-Pr, f-Bu, Et X = Cl, Br, I) in the presence of conjugated dienes, as shown in Scheme 4. The butadiene, 1,3-pentadiene, and 1,4-diphenylbutadiene complexes of Cp TiX exhibit a unique prone (endo) conformation (13), while the isoprene, 2,3-dimethylbutadiene, and 2,3-diphenylbutadiene complexes prefer the supine (exo) conformation (14). Reduction of Cp TiX(diene) with RMgX or Mg gives a low-valent species, which catalyzes a highly selective (>99%) tail-to-head linear dimerization of isoprene and 2,3-dunethylbutadiene. " ... 

Many soluble catalysts are known which will polymerize ethylene and butadiene. High activity soluble catalysts are employed commercially for diene polymerization but most soluble types are inefficient for olefin polymerization. A few are crystalline and of known structure such as blue (7r-C5H5)2TiCl. AlEtaCl [49] and red [(tt-CsHs )2TiAlEt2 ] 2 [50]. The complex (tt-CsHs )2TiCl2. AlEt2Cl polymerizes ethylene rapidly but decomposes quickly to the much less active blue trivalent titanium complex. Soluble catalysts are obtained from titanium alkoxides or acetyl acetonates with aluminium trialkyls and these polymerize ethylene and butadiene. Several active species have been identified, dependent on the temperature of formation and the Al/Ti ratio. Reduction to the trivalent state is slow and incomplete and maximum activity for ethylene polymerization occurs at about 25% reduction to Ti [51]. 

If the rate of anti-syn isomerization is relatively low, then the cis-trans selectivity can be determined by the formation of the anti- or the 5y/i-butenyl structure, for example from the t] -cis or the if-trans coordinated butadiene, in the catalyst complex. This is the mechanism of stereoregulation which was suggested in the mid-1960s by Cossee and Arlman [34, 35] for titanium-catalyzed butadiene polymerization, and which was reconsidered more recently for the allylne-odymium complex catalysts to explain their cis-trans selectivity [39], But it is also possible that the difference in reactivity between the anti and the syn structure of the catalytically active butenyl complex can determine the cis-trans selec-... 

Crystal structures of titanium complexes 36a, 36b and 36c and of hafnium complex 36d indicate that the diene-metal interaction is best characterized by the <7, n bonding mode. In general, the diene ligand adopts an s-cis supine conformation with respect to the Cp ligand (e.g. 36c and 36d). However, Nakamura and coworkers have noted that the Cp TiCl complexes of butadiene (36a), 1,3-pentadiene and 1,4-diphenyl-... 

Molecular sieves (MS) can have a detrimental effect on the enantioselectivity when dienophiles with a free hydroxyl group e.g., juglone (lib) are used. The ee-value drops from 85% for the addition of 1,4-naphlhoquinone (11a) to 1-acetoxy-l,3-butadiene (3c) to 9% for addition of juglone (lib). The latter reaction proceeds with the MS-free binaphthol-titanium complex 21 with 96% ee37. 

After the Ziegler-Natta classical catalysts, other kind of metallic complexes were reported for the polymerization of butadienes, such as Ni-based catalysts, halfsandwich titanium complexes, or transition-metal imido compounds activated with Bp30Et2, AlEt3, B(CgF5)4, or MAO [3]. 

The patent literature contains several references to the use of sulfoxide complexes, usually generated in situ, as catalyst precursors in oligomerization and polymerization reactions. Thus, a system based upon bis(acrylonitrile)nickel(0> with added Me2SO or EtgSO is an effective cyclotrimerization catalyst for the conversion of butadiene to cyclo-1,5,-9-dodecatriene (44). A similar system based on titanium has also been reported (407). Nickel(II) sulfoxide complexes, again generated in situ, have been patented as catalyst precursors for the dimerization of pro-pene (151) and the higher olefins (152) in the presence of added alkyl aluminum compounds. 

It can be seen that both the solvent and the catalyst affect the structure of the polymer produced. For example, the structure of the polyisoprene differs strongly with the alkali metal, even when used in the same solvent medium. Experiments with a typical organometallic complex catalyst, consisting of trialkyl-aluminum and titanium tetrachloride, show that the same initiator can lead to quite different structures in the products of polymerization of isoprene and of butadiene. 

Alkene, Alkyne, Alkylidene,m and Carbonyl Complexes. While titanium al-kene complexes are unquestionably involved in polymerizations, relatively few have been isolated. Interactions of TiCL,(dmpe)2 and butadiene under reducing conditions give Ti(T7-C4H6)2(dmpe), which can be converted by CO and PF3 to Ti(CO)2(PF3)-(dmpe)2 and Ti(CO)3(dmpe)2. The latter reacts with K in the presence of biphenyl or naphthalene and then with CO to give Ti(CO) - species which are isolable as K(cryptate)+ salts 102... 

The dimerization and cyclotrimerization of butadiene are also catalyzed by titanium but are less well investigated. The Tiw Tj -octadienyl complex (22-XXXV) is isolable it catalyzes the dimerization of butadiene to the thermodynamically favored product, vinylcyclohexene.134... 

The homoleptic (see Homoleptic Compound) titanium diene complex (j " -r-BuCH=CHCH=CHBu-t)2Ti (11) has been prepared by the condensation ofelectron-beam vaporized titanium with an excess of 1,4-di-t-butylbuta-1,3-diene" (see Metal Vapor Synthesis of Transition Metal Compounds). The bis-diene dmpe complex Ti(dmpe)( " -C4H6)2Ti (12) has been prepared by the reduction of TiCl4(dmpe) with Na/Hg in the presence of butadiene, and by the reduction of TiCLi with CH2=CHLi in the presence of dmpe. ... 

Prompted by the X-ray studies of the acryloyl lactate-TiCU complex (380a), commercially available pantolactone was chosen as an auxiliary aiming to facilitate entropically the formation of a seven-mem-bered titanium chelate (385). Indeed, using 0.1-0.75 mol equiv. of TiCU the acrylate and crotonate of (/ )-pantolactone (384) underwent smooth addition of cyclopentadiene, butadiene and isoprene to give adducts (386) and (387) in ratios of >93 <7 (Scheme 94, Table 24, entries a-d). Opposite product ratios were obtained using dienophiles (388) derived from (S)-pantolactone (entiy e) or (390) derived from the more readily available A -methyl-2-hydroxysuccinimide (entries f, g). The major products were purified by crystallization and saponified without epimerization (LiOH, THF/water, r.t.) to furnish the corresponding carboxylic acids. 

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