Cyclotrimerization titanium

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. 

Finally, it has been reported that the titanium-catalyzed (TiCl4/Et2AlCl) cyclotrimerization of l-phenyl-2-(trimethylsilyl)acetylene affords the ter-phenyl silane l,3,5-triphenyl-2,4,6-tris(trimethylsilyl)benzene in trace amounts.101... 

Transformations to the cyclotrimeric boiazines and cyclotetrameric tetraza-2,4,6,8,l,3,5,7-tetraboracanes also occur. The rate of dimerization for amino iminoboranes has been shown to be stabilized by bulky substituents (76,79,83). This stabilization through dimerization is essentially a [2 + 2] cycloaddition. There are a number of examples of these compounds forming cycloadducts with other unsaturated polar molecules (78). Iminoboranes can add to electron-deficient carbene complexes of titanium such as (C5H5)2Ti(CH2) [84601-70-7] by [2 + 2] cyclo addition, yielding the metallacycle shown in equation 26 (84). 

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... 

Subsequent protic workup releases the aromatic compound. The metalative Reppe reaction can also be used to prepare iodo-substituted or homologated aromatics by treatment of the titanium aryl compound with iodine or an aldehyde, respectively. This procedure has recently been extended to include pyridine derivatives (254 and 255), where the titanacyclopentadiene intermediate can be treated with sulfonylnitriles to afford pyridines after protic workup.192 As with the alkyne cyclotrimerizations, treatment with the appropriate electrophiles affords iodo- and homologated pyridines. 

H. Shirakawa and S. Ikeda, Cyclotrimerization of acetylene by the tris (acetylacetonato)titanium(III)-diethylaluminum chloride system, J. Poylm. Sci. Polym. 

Oligophenylenes with acetylene end groups also form branched polymers. These compounds can be cross-linked, presumably by cyclotrimerization of the acetylene groups, with catalysts such as titanium (IV) chloride/diethyl aluminum chloride. The prepolymers are soluble and can be moulded with simultaneous hardening. They are suitable for corrosion-resistant coatings. 

In 1958, Natta tried to synthesize PA by bubbling acetylene gas through a titanium/trialkyl aluminum catalyst solution while stirring. One of the products in the reaction was a black, semicrystalline powder that was completely insoluble and unstable in the presence of air and water [4]. Although no analyses could be made, Natta assumed that he had made a high-molecular-weight, mostly trans, PA. Subsequently, PA was observed as a side product in the attempts to cyclotrimerize and cyclotetramerize acetylene [5]. 

Considerable work has also been devoted to catalytic properties of aromatic titanium-aluminumhalide complexes (CXL). These complexes have been reported to promote polymerization of ethylenes (Martin and Vohwinkel, 1961 Zimmer, 1966) and cyclotrimerization of butadienes (Vohwinkel, 1964) and isoprenes (Zakharkin and Akhmedov, 1969). They are able to lead to linear or cyclic polymers depending on the experimental conditions. For example, when butadiene is bubbled through a solution of CgHgTiCljAljCls in benzene, a vigorous reaction occurs. The yields of 1,5,9-cyclododecatriene (CDT), at the optimum temperature (60°-75°), are in the region of 60-65%. 

The cyclotrimerization of alkynes catalyzed by transition metals is a general method for building substituted benzenes from aliphatic precursors. Multiple bonds are formed in these reactions in a single operation. Although the reaction of thermal trimerization relates to allowed electro-cyclic processes, it is catalyzed by several transition metals, such as Co, Ni, Rh, Pd, Rh, and Ru [38]. Most recent publications show promise for the participation of transition metal complexes in [2+2+2] cycloaddition reactions based on zirconium, titanium, and indium [9]. This reaction has synthetic potential for using metallocyclopentadienes as intermediates in the cyclotrimerization of alkynes. The reaction mechanism is shown in Scheme 2.1 [3, 38]. Two alkyne molecules coordinated to the metal, that is, complex 2.1, couple to form cyclopentadiene 2.2. Next there is either addition of the alkyne to the metallocycle 2.3 to form the metallocycle... 

Although in principle the thermal [2-I-2-I-2] cycloaddition process is allowed by orbital symmetry rules, there are problems with the entropy component, which may be overcome by using transition metal catalysis. This approach (Scheme 2.35) is one of the most convenient for the synthesis of pyridines 2.100. Metal-induced cycloaddition of two alkyne and one nitrile molecules has been described in general reviews of cycloaddition reactions [3,4]. However in some reviews on heterocycles the nitriles are considered as equivalent to alkyne in the [2+2+2] cyclotrimerization reaction [76], in particular, for the synthesis of pyridines and pyridinones in the reactions catalyzed by cobalt, ruthenium, titanium, and zirconium. 

---