Carbenes titanium

Schrock carbene complexes may be identified by their alkyl substituents on the carbene carbon and an electropositive, early-transition metal. As shown in Figure 2, the Schrock complexes are considered a result of spin-coupling between the carbene triplet state and the two electrons on the metal [8]. The reactivity of the Schrock carbene carbon is nucleophilic [4]. The reaction involving bis(cyclopentadienyl)carbene titanium(IV) and the electrophile acyl chloride in Eq. (3) [9], for example, illustrates the nucleophilicity of... 

Reactions of Titanium Carbene Complexes with Carbon Carbon Double Bonds... 

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. 

The alkyl-substituted titanium carbene complex 18 reacts with norbornene 24 to form a new titanacycle 25, which can be employed for the ROMP of 24 (Scheme 14.13). The titanacycle generated by the reaction of the Tebbe reagent with 24 is also used as an initiator for the same polymerization [23]. These preformed titanacyclobutanes also initiate ROMP of various other strained olefin monomers [24],... 

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. 

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]. 

The potential synthetic utility of titanium-based olefin metathesis and related reactions is evident from the extensive documentation outlined above. Titanium carbene complexes react with organic molecules possessing a carbon—carbon or carbon—oxygen double bond to produce, as metathesis products, a variety of acyclic and cyclic unsaturated compounds. Furthermore, the four-membered titanacydes formed by the reactions of the carbene complexes with alkynes or nitriles serve as useful reagents for the preparation of functionalized compounds. Since various types of titanium carbene complexes and their equivalents are now readily available, these reactions constitute convenient tools available to synthetic chemists. 

The Reaction of Titanium Carbene Complexes with Nitriles 495 ... 

Titanium enolates.1 This Fischer carbene converts epoxides into titanium enolates. In the case of cyclohexene oxide, the product is a titanium enolate of cyclohexanone. But the enolates formed by reaction with 1,2-epoxybutane (equation I) or 2,3-epoxy butane differ from those formed from 2-butanone (Equation II). Apparently the reaction with epoxides does not involve rearrangement to the ketone but complexation of the epoxide oxygen to the metal and transfer of hydrogen from the substrate to the methylene group. 

The expected intermediate for the metathesis reaction of a metal alkylidene complex and an alkene is a metallacyclobutane complex. Grubbs studied titanium complexes and he found that biscyclopentadienyl-titanium complexes are active as metathesis catalysts, the stable resting state of the catalyst is a titanacyclobutane, rather than a titanium alkylidene complex [15], A variety of metathesis reactions are catalysed by the complex shown in Figure 16.8, although the activity is moderate. Kinetic and labelling studies were used to demonstrate that this reaction proceeds through the carbene intermediate. 

Diaminocarbene complexes were reported as early as 1968 [152], Preparation and applications of such complexes have been reviewed [153], Because of 7t-electron donation by both nitrogen atoms, diaminocarbenes are very weak tt-acceptors and have binding properties towards low-valent transition metals similar to those of phosphines or pyridines [18,153]. For this reason diaminocarbenes form complexes with a broad range of different metals, including those of the titanium group. Titanium does not usually form stable donor-substituted carbene complexes, but rather ylide-like, nucleophilic carbene complexes with non-heteroatom-substituted carbenes (Chapter 3). 

A further way of generating carbon-metal double bonds is based on the [2 -I- 2] cycloreversion of metallacyclobutanes. This method has proven particularly useful for the generation of synthetically valuable titanium and zirconium carbene complexes (Section 3.1.7). 

Cycloreversion of four-membered metallacycles is the most common method for the preparation of high-valent titanium [26,27,31,407,599-606] and zirconium [599,601] carbene complexes. These are usually very reactive, nucleophilic carbene complexes, with a strong tendency to undergo C-H insertion reactions or [2 -F 2] cycloadditions to alkenes or carbonyl compounds (see Section 3.2.3). Figure 3.31 shows examples of the generation of titanium and zirconium carbene complexes by [2 + 2] cycloreversion. 

Fig. 3.34. Cyclopropanation with titanium carbene complexes generated in situ [33].

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