Titanium compounds alkenes

Other transition-metal oxidants can convert alkenes to epoxides. The most useful procedures involve /-butyl hydroperoxide as the stoichiometric oxidant in combination with vanadium, molybdenum, or titanium compounds. The most reliable substrates for oxidation are allylic alcohols. The hydroxyl group of the alcohol plays both an activating and a stereodirecting role in these reactions. /-Butyl hydroperoxide and a catalytic amount of VO(acac)2 convert allylic alcohols to the corresponding epoxides in good yields.44 The reaction proceeds through a complex in which the allylic alcohol is coordinated to... 

Fiirstner reported the first McMurry-type reactions working with 5-10 mol% of titanium trichloride and stoichiometric amounts of zinc powder in the presence of chlorotrimethylsilane. The amount of TiCl3 could be reduced to 2 mol% when (ClMe2SiCH2)2 was used as a reagent [125, 131]. At the same time, Burton and coworkers reported atom transfer radical additions of perfluoroalkyl iodides 39 to alkenes 40 catalyzed by 20 mol% of a low-valent titanium compound generated from TiCLt and zinc powder affording 41 in 10-85% yield (Fig. 13). A tandem radical addition/5-exo cyclization/iodine transfer reaction with diallyl ether proceeded in 66% yield [132]. 

Ziegler-Natta catalyst (Section 31.2) A catalyst of an alkylaluminum and a titanium compound used for preparing alkene polymers. 

The active species in the Tebbe olefination is believed to be the nucleophilic (Schrock-type) titanocene methylidene, which is formed from the Tebbe reagent upon coordination of the aluminum with a Lewis base (e.g., pyridine). This methylidene in its uncomplexed form, however, has never been isolated or observed spectroscopically owing to its extreme reactivity. The same intermediate can also be generated by other means." The titanocene methylidene reacts with the carbonyl group to form an oxatitanacyclobutane intermediate that breaks down to titanocene oxide and the desired methenylated compound (alkene). The driving force is the formation of the very strong titanium-oxygen bond. 

These titanium compounds can be described as an alkene 7r-complex or a metallacyclopropane, which is of practical importance. According to several computational studies, it has been concluded that the alkene titanium complexes are best represented as titanacyclopropane derivatives. The synthesis of titanium-alkyne complexes Ti(Me3SiC=CG6H13)(OR)2 from reaction between l-(trimethylsilyl)oct-l-yne with achiral or chiral alkoxo titanium compounds Ti(OR)4 has been described (Scheme 95).184 A series of organotitanium compounds (Scheme 96) are obtained by metathesis reactions.41... 

The synthesis, structural determination, and reactivity of ( -symmetric ethylene-bridged titanium compounds has been investigated (Scheme 645). The complexes have been characterized by X-ray crystallography and applied as catalysts for the enantioselective isomerization of alkenes, though they prove to be less enantioselective than the known chiral ansa-bis-Ind titanium catalyst.1667... 

The dichloro bis(binaphthyl-Cp) titanium compound (catalyst 1 in Scheme 757) has shown excellent ability to catalyze the asymmetric epoxidation of unfuctionalized alkenes with virtually the same selectivity as previously... 

Addition of the A1—H bond to C=C double bonds in alkenes is greatly accelerated by catalytic quantities of certain titanium compounds [e.g., TiCU, Ti(OC4H9)4]. For example, when propene is passed into dipropyl-aluminum hydride at room temperature no reaction is observed, whereas small amounts of TiCl4 (R2A1H TiCl4, 100 1) lead to rapid addition with the evolution of heat. Small amounts of polypropylene are formed simultaneously, but this can be avoided by using titanium (IV) butylate (256). 

The mechanism of this complex reaction involves the titanium compound acting as a clamp, holding the alkene, peroxide, and (5,5)-tartaric ester together. Because the ester is asymmetric, the clamped combination of molecules is also asymmetric. In one cluster, the ox irane oxygen is delivered from one side, whereas in the enantiomeric cluster formed from (R,R)-tartaric ester, the oxygen comes from the other side. These and other epoxides can then be transformed into all manner of compounds, as we win now see. 

In the other reactions with Cp2TiCl, Cp2TiCl2/LiAlH4 or CpTi(CO)2, the elimination of oxygen from epoxides gives alkenes, and elimination of oxygen from sulfoxide give a thioether [57]. As a reduction with low valence titanium compounds, for example, cis-olefin is prepared from disubstituted acetylene [57]. 

The hydroalumination of terminal alkenes by LiAlH4 catalyzed by titanocene and zirconocene dichloride, Cp2 IiCl2 and CpjZrClj, respectively, has been reported by Isagawa [16] and Sato [14]. Again, the titanium compound proved to be more active... 

Among the appHcations of lower valent titanium, the McMurry reaction, which involves the reductive coupling of carbonyl compounds to produce alkenes, is the most weU known. An excellent review of lower valent titanium reactions is available (195). Titanium(II)-based technology is less well known. A titanium(II)-based complex has been used to mediate a stetio- and regio-specific reduction of isolated conjugated triple bonds to the corresponding polyenes (196). 

Alkenes and alkynes can also add to each other to give cyclic products in other ways (see 15-61 and 15-63). The first exclusive exo-dig carbocyclization was reported using HfCU as a catalyst. Alkynes also add to alkenes for form rings in the presence of a palladium catalyst or a zirconium catalyst. " Carbocyclization of an alkene unit to another alkene unit was reported using an yttrium catalyst and alkenes add to alkynes to give cyclic compounds with titanium catalysts. ... 

Unsymmetrical alkenes can be prepared from a mixture of two ketones, if one is in excess. " The mechanism consists of initial coupling of two radical species to give a 1,2-dioxygen compound (a titanium pinacolate), which is then deoxygenated. " ... 

Hydroalumination of terminal alkenes using EtjAl as the hydride source must be carried out with titanium catalysts [24], since zirconium compounds lead to the formation of alumacyclopentanes [60, 61] (Scheme 2-11) and carbometallated products [62]. Suitable substrates for hydroalumination include styrene, allylnaphthalene and vinylsilanes. Only one of the ethyl groups in EtjAl takes part in these reactions, allowing the synthesis of diethylalkylalanes, which are difficult to obtain by other methods. 

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