Titanacyclic compounds

Titanacycle compounds supported by aryloxo ligands catalyze cross-coupling and oligomerization reactions involving 1,3-cyclohexadiene, 1,3-cyclooctadiene, and a-olefins. Mechanistic aspects are reported.244 The regio- and the... 

As well as undergoing carbonyl addition, titanacyclopentadiene intermediates generated from two unsymmetrical acetylenes have been shown to react with ethynyl para-tolyl sul-fone to afford an aryltitanium compound of the structure shown in Scheme 9.6 [34], The reaction may proceed according to path a or path b, as shown in Scheme 9.7. In path a, the first step should be regioselective [4+2] cycloaddition of the titanacyclopentadiene with the sulfonylacetylene to afford the bicyclic titanacycle, at least in an equilibrium concen-... 

Titanacyclobutanes also serve as useful synthetic intermediates the titanacycle 43, prepared by the intramolecular reaction of the alkenylidene complex 44, affords the a-dike-tone 45 and the other functionalized cyclic compounds by further transformations (Scheme 14.20) [35]. 

The titanaallene compound [(77 -Cp )2Ti=C=CH2] reacts with cyclohexylisonitrile to give titanacycle 150 <20010M1354>. With 2,6-Me2C6H3NC, product 151 (Ar = 2,6-Me2C6H3) is formed. One of the products of interaction of [(77 -Cp)2Ti(PMe3)2] with 3-methyl-l,2-butadiene is titanacyclopentane 152 <2000POL879>. 

The most useful procedure for the preparation of early transition metal complexes of type 13 is the thermal elimination of benzene or methane from diphenyl or arylmethyl compounds,respectively (12 and 14 in Scheme 1) [5-9]. At the end of the 1960s, the first studies of the reactivity of such a complex, specifically the aryne-titanocene complex 15, showed that it reacts with species such as alkynes, CO2, N2 or selenium to form titanacycles [7]. Particularly interesting for the aim of this chapter is the insertion of alkynes to form titana-cyclopentadienes 16. This reaction takes place in moderately good yields with both electron-rich and electron-poor alkynes (Scheme 2) [13-15]. 

Methylenation of carboxylic acid derivatives The Tebbe reagent 3 is extremely valuable as a reagent for the methylenation of carboxylic acid derivatives, which is generally unsuccessful using phosphorus ylides. Esters and lactones are readily transformed into enol ethers (Table 4.3), especially when a Lewis base such as THE is present in the reaction mixture. In the methylenation of a,j8-unsaturated esters, the internal olefin is not involved in the reaction, and the configuration of the double bond is maintained (entry 4). When carbonyl compounds bearing a terminal double bond are subjected to the methylenation, significantly lower yields are observed (entries 6 and 11), which may be attributable to competitive formation of a titanacycle from titanocene-methylidene 4 and the terminal olefin. 

Carbocycles are important compounds in natural products and functional materials. The traditional preparative methods for cyclic compounds include the Diels-Alder addition reaction, and cyclization mediated by Lewis acid and base. With the aid of metallacycles, carbocycles are often prepared in high yields with high selectivity from different components (Scheme 1). Among many metallacycles, zirconacycles and titanacycles with Cp ligands have been most widely used in stoichiometric reactions to prepare carbocycles. 

There have been several excellent reviews and books dealing with preparation methods of zirconacycles and titanacycles, especially on the formation of 3- and 5-membered zirconacycles [ 1-5]. Therefore, this paper will not describe details of preparation of 3- and 5-membered zirconacycles, which are the most popular metallacycles of zirconocene and titanocene. In order for readers to find appropriate references, a brief introduction into preparative methods of these metallacycles is given. Applications of zirconacycles and titanacycles in the construction of useful carbocyclic compounds from multi-components are summarized. 

Four-membered titanacycle intermediates have been often involved in titanium carbene mediated olefin metathesis and related reactions (Eqs. 12-13). Both ti-tanacyclobutanes and titanacyclobutenes have been reported and are useful intermediates for the synthesis of various organic compounds. Since many reviews have appeared [13], no details will be described in the paper. 

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