Titanium-methylidene complex

Another approach to synthetically useful olefin metathesis involves the utilization of higher homologues of titanium-methylidene 15, as shown in Scheme 14.11. If the resulting titanium carbene complex 20 is more stable than the starting alkylidene complex 15, this reaction can be employed for the generation of various titanocene-alkylidenes and as a method for the preparation of unsaturated compounds. 

Although the reaction of a titanium carbene complex with an olefin generally affords the olefin metathesis product, in certain cases the intermediate titanacyclobutane may decompose through reductive elimination to give a cyclopropane. A small amount of the cyclopropane derivative is produced by the reaction of titanocene-methylidene with isobutene or ethene in the presence of triethylamine or THF [8], In order to accelerate the reductive elimination from titanacyclobutane to form the cyclopropane, oxidation with iodine is required (Scheme 14.21) [36], The stereochemistry obtained indicates that this reaction proceeds through the formation of y-iodoalkyltitanium species 46 and 47. A subsequent intramolecular SN2 reaction produces the cyclopropane. 

Similarly to alkenes, alkynes react with various titanium-methylidene precursors, such as the Tebbe reagent [13,63], titanacydobutanes [9b, 64], and dimethyltitanocene [65] to form the titanium-containing unsaturated cyclic compounds, titanacydobutenes 67 (Scheme 14.29). Alternatively, 2,3-diphenyltitanacydobutene can be prepared by the reaction of the complex titanocene(II) bis(trimethylphosphine) with 1,2-diphenylcyclopropene [66]. Substituent effects in titanacydobutenes [67], the preparation of titanocene-vinylke-tene complexes by carbonylation of titanacydobutenes [68], and titanacyclobutene-vinylcar-bene complex interconversion [69] have been investigated. 

Since the hybridization and structure of the nitrile group resemble those of alkynes, titanium carbene complexes react with nitriles in a similar fashion. Titanocene-methylidene generated from titanacyclobutane or dimethyltitanocene reacts with two equivalents of a nitrile to form a 1,3-diazatitanacyclohexadiene 81. Hydrolysis of 81 affords p-ketoena-mines 82 or 4-amino-l-azadienes 83 (Scheme 14.35) [65,78]. The formation of the azati-tanacyclobutene by the reaction of methylidene/zinc halide complex with benzonitrile has also been studied [44]. 

More recently, Petasis has shown that titanocene dimethyl, " which is easily generated from titanocene dichloride and methyllithium, also generates the same titanium methylidene upon thermolysis (Scheme 13.5, bottom). Thermolysis of titanocene dimethyl is a convenient way to generate the titanocene-methylidene complex because titanocene dimethyl is formed in high yield and is stable to water, -although it is sensitive to light and can undergo autocatalytic decomposition. This complex has been used in synthesis... 

A few years later, Tebbe and co-workers found that the methylene-bridged metallacycle 3, which has become known as the Tebbe reagent, is useful for the methylenation of ketones and aldehydes [5]. Titanocene-methylidene 4, the active species of this olefination, also transforms carboxylic acid derivatives into heteroatom-substituted olefins. Because the carbene complex 4 is much less basic than conventional olefination reagents such as phosphorus ylides, it can be employed for the olefination of carbonyl compounds possessing highly acidic a-protons or of highly hindered ketones, and has become an indispensable tool in organic synthesis. Various methods for the preparation of titaniumcarbonyl olefination. This chapter focuses on the use of metal-carbene complexes and some related species in carbonyl olefination (Scheme 4.2). 

By the use of di- or trithioorthoformates, heteroatom-substituted olefins are obtained. The active organotitanium species formed in these preparations are assumed to be the corresponding methoxy- and phenylthio-methylidene carbene complexes, SO and 51, respectively (Scheme 4.44). The success of olefinations using these species indicates that the a-heteroatom substituent does not affect the nucleophilic nature of the titanium carbene complex [103]. 

Very recently, Eisch and co-workers have developed new alkylidene-group IV metal complexes such as methylidene titanium dichloride 67, readily accessible from titanium(iv) chloride and an excess of methyllithium at low temperature (Scheme 24).53 The new methylenating agent 67 can easily convert benzophenone at low temperature into 1,1-diphenylethylene in quantitative yield. 

As mentioned already, new methylidene-group IV metal complexes have been prepared and were subsequently used in nucleophilic additions to carbonyl electrophiles (Scheme 43).53 In contrast to titanium and zirconium, the reaction of methylidene hafnium dichloride 97 benzophenone stopped at the first stage (i.e., addition). The tertiary alcohol was obtained in 73% yield, while the corresponding alkene was formed only as minor product. 

Although the molybdenum and ruthenium complexes 1-3 have gained widespread popularity as initiators of RCM, the cydopentadienyl titanium derivative 93 (Tebbe reagent) [28,29] can also be used to promote olefin metathesis processes (Scheme 13) [28]. In a stoichiometric sense, 93 can be also used to promote the conversion of carbonyls into olefins [28b, 29]. Both transformations are thought to proceed via the reactive titanocene methylidene 94, which is released from the Tebbe reagent 93 on treatment with base. Subsequent reaction of 94 with olefins produces metallacyclobutanes 95 and 97. Isolation of these adducts, and extensive kinetic and labeling studies, have aided in the eluddation of the mechanism of metathesis processes [28]. 

Complexes with /i -methylidenes are also known. Complex 2.41 in which methylidene and chloride bridges are present between titanium and aluminum is known as Tebbe s complex. Note that had we not known the structure, Tebbe s complex might have been formulated as Cp TiCCH ) plus Me AlCl. As will be seen, Tebbe s complex has relevance in alkene polymerization and metathesis reactions. Complex 2.42 is a unique example of a monometallic complex with a single carbon atom as one of the ligands. From the earlier examples it is clear that representative organometallic complexes with M-CR (n = 0-3) have all been isolated and fully characterized. 

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