Titanium metallocycle

Titanium metallocycles as intermediates in the synthesis of acyclic and heterocyclic compounds 92SL13. 

Diphenylacetylene reacts regiospecifically with bis(cyclopenta-dienyl)( >-fluorobenzyne)titanium, which was prepared by thermal pyrolysis of Cp2Ti(o-fluorophenyl)2, to give the corresponding titanole. Treatment of the titanium metallocycle with Cr(CO)3(NCMe)3 afforded the... 

The diamagnetic, monomeric thorium derivative has been completely characterized by its 1h and 13C nuclear magnetic resonance spectra. Data are shown in Table II. In particular the NMR spectrum, proton-coupled, shows a triplet pattern clearly due to splitting by two equivalent hydrogen atoms. The uranium metallocycle is paramagnetic (yg = 2.7 B.M.) and we have only observed its NMR spectrum. A titanium metallocycle,... 

Titanium and zirconium chemistry has some unique features the chair conformation of the M2N4 metallocycle in the exobidentate complexes and endo-bidentate coordination. 

The metallocycle 1,4-tetramethylene bis(cyclopentadienyl)titanium-(IV), obtained by reacting CpTiCl2 with Li(CH2)4Li, decomposes at -30°C, yielding a mixture of 1-butene and ethylene (43). The pathway leading to 1-butene may involve a hydride intermediate, although whether this is formed by a- or /3-hydrogen atom elimination is uncertain, e.g.,... 

The moisture- and air-sensitive dark-green titanium derivative (dec. > 180°) was also obtained in 54% yield when bis[cyclopentadienyl]diphenyltitanium in heptane and elemental tellurium were refluxed for 12 days. The compound was purified by recrystallization from dry toluene3. The activation energy of the ring inversion of the metallocycle was determined by H-NMR spectroscopy to be 51kJ mol-1. 

The aldol reactions of the titanium Z-enolates proceeded smoothly with various aldehydes precomplexed with titanium chloride at -78° C. The diastereose-lectivity is high to excellent, with the single exception of benzaldehyde. The high degree of diastereoselection associated with this current asymmetric anti-aldol process can be rationalized by a Zimmerman-Traxler type of six-membered chairlike transition state Al9fl (Scheme 2.2r). The model is based on the assumptions that the titanium enolate is a seven-membered metallocycle with a chairlike conformation, and a second titanium metal is involved in the transition state, where it is chelated to indanolyloxy oxygen as well as to the aldehyde carbonyl in a six-membered chairlike transition-state structure. 

The diastereoselectivity for the reaction of 157, which possesses a 2-methyl-1,3-dioxolanyl group, with enolates generated from fert-butyl acetate was found to be highly dependent on the reaction conditions.89 For example, the lithium enolate gave (Ss,S)-158 (72% de) while the titanium enolate afforded (Ss,/ )-158 (92% de). A non-chelation-controlled transition state was used to explain the preferential formation of (Ss,S)-158 while a six-membered chairlike transition state containing a four-membered metallocycle and/or a seven-membered counterpart was proposed for the formation of the (Ss,R)-158. Treatment of (Ss,S)-158 with TFA gave P-amino acid 159 in 70% yield. 

In addition, cyclopentanone is formed from ethylene and carbon monoxide via a titanium(IV) metallocycle (McDermott and Whitesides, 1974). 

The second type of single-site catalyst, designated as the constrained geometry catalyst (CGC), was developed by James C. Stevens and coworkers in the late 1980s at Dow Chemical Company in Freeport, Texas. This catalyst type contains one Cp ligand as one component in forming a metallocycle structure. The active site is based on titanium [8]. 

Cp2Ti(CO)2], a precursor of titanocene, undergoes an unusual reaction with diphenylketene (Scheme 3). The product dimer, characterized by crystal structure determination, contains diphenylketene j -bonded to the titanium through the CO moiety. Previously such jr-interaction of a ketonic C=0 group was only known for complexes of hexafluoroacetone with transition metals. An A -ray crystal structure has been determined for the dimer, further reaction of which affords a metallocycle (Scheme 3). 

In a very nice titanium-mediated annulation cascade, Cheng and Micalizio [80] synthesized functionalized, bicychc [3] dendralenes 133 in situ as intermediates that were trapped via a subsequent DA reaction (Scheme 1.18). The report includes one example of a DA dimerization product, seven examples of intermolecular metallocycle-mediated annulation followed by intermolecular [4-1-2] cycloaddition reaction to afford (135), and one example of an isolated, acyclic substituted [3]dendralene 137. 

Titanium and zirccoiium diemistiy has some imiqne features die diair con-formaticm of die M2N4 metallocycle in die exobidentate ccmplexes and endo-bidentate cotHdinaticm. 

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... 

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