Titanium complexes, reaction with carbon

A mechanistically obscure transformation occurs upon treatment of the tetramethylfulvene titanium complex 161 with methallyl Grignard, producing bridged titanacyclobutane complex 162. This reaction is proposed to proceed by intramolecular alkylation at the central carbon of an 7]4-fulvene, 7]3-methallyl intermediate, but with due consideration... 

Imido zwitterionic titanium complexes, Ti=NAr[CH3B(C6F5)3], undergo reaction with carbon dioxide to give isocyanates and symmetrical carbodiimides via a ligand metathesis... 

Titanium-acetylene complexes 29 generated in situ from acetylenes, Ti(0-i-Pr)4 and /-PrMgX react with imines to form azatitanacyclopentenes 30 which then react with carbon monoxide under atmospheric pressure to provide pyrroles 31 <96TL7787>. This reaction, which utilizes commercially available reagents is an improvement over a related procedure via the corresponding zirconium complexes under 1500 psi CO <89JA776>. 

Based on the established mechanism for titanium-catalyzed hydroamination, the authors propose a reversible reaction between a titanium imide complex and the alkyne to form metalloazacyclobutene 86, which in turn undergoes 1,1-insertion of the isonitrile into the Ti-C bond. The generated five-membered ring iminoacyl-amido complex 87 with the new C-C bond is protonated by the primary amine to afford the desired three-component coupling product, with regeneration of the catalytic imidotitanium species. Very recently, titanium-catalyzed carbon-carbon bond-forming reactions have been reviewed.122... 

Similar to the reaction of zirconacyclopropene 1, titanacyclopropene 14 reacted with C02 to give titanacycle 15 (Scheme 5) I0,i0a-i0c j-[owever5 the reaction of Cp TiC /Mg with l,4-bis(trimethylsilyl)-l,3-butadiyne did not afford a titanacyclocumulene species, but yielded titanacyclopropene instead 16, which on reaction with C02 gave the titanacycle complex 17.7 In the case of the titanium half-metallocene complex 18, the five-membered titanacyclocumulene 19 was obtained but the insertion of C02 took place only at one of the two Ti-carbon bonds, leading to the formation of 20 (Scheme 5),11 which is in contrast with what was observed in the case of the Zr analog 3. The... 

The bisfunctionalization of alkynes by both C02 and another electrophile can also be achieved, as shown in Scheme 9.17,17a The titanium-carbon bond in the titanacycle complex 31, which was formed by reaction of C02 with the titanacyclopropene 30, can be substituted with various electrophiles. For example, its reaction with NBS or I2 afforded the synthetically useful vinyl bromide or iodide 32, respectively, while the reaction with D20 yielded the /3-deuterated a,/ -unsaturated carboxylic acid. When an aldehyde such as PhCHO was used as an electrophile, butenolide 33 was produced after acidic workup. 

Addition of the (l-silylalkyne)titanium complex to carbonyl compounds and imines occurs at the (3-position to the silyl group, as shown in Fig. 9.2. However, the reaction with sBuOH takes place exclusively at the carbon—titanium bond a to the silyl group to give the (P-silylalkenyl)titanium species, as in Eq. 9.5 (values in square brackets denote the regioselectivity) [24], where the vinyl—titanium bond is visualized by the outcome of the iodi-nolysis. The overall reaction can therefore be regarded as the hydrotitanation of silylace-... 

The first synthetically useful reaction of titanium complexes of type 4, leading to the formation of two new carbon—carbon bonds, was developed by Kulinkovich et al. [55]. They found that treatment of a carboxylic acid ester with a mixture of one equivalent of titanium tetraisopropoxide and an excess of ethylmagnesium bromide at —78 to —40 °C affords 1-alkylcyclopropanols 9 in good to excellent yields (Scheme 11.2) [55,56], This efficient transformation can also be carried out with sub-stoichiometric amounts of Ti(OiPr)4 (5—10 mol%) [57,58]. In this case, an ethereal solution of two equivalents of EtMgBr is added at room temperature to a solution containing the ester and Ti(OiPr)4. Selected examples of this transformation are presented in Table 11.1 (for more examples, see ref. [26a]). 

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

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. 

In 1977, an article from the authors laboratories [9] reported an TiCV mediated coupling reaction of 1-alkoxy-l-siloxy-cyclopropane with aldehydes (Scheme 1), in which the intermediate formation of a titanium homoenolate (path b) was postulated instead of a then-more-likely Friedel-Crafts-like mechanism (path a). This finding some years later led to the isolation of the first stable metal homoenolate [10] that exhibits considerable nucleophilic reactivity toward (external) electrophiles. Although the metal-carbon bond in this titanium complex is essentially covalent, such titanium species underwent ready nucleophilic addition onto carbonyl compounds to give 4-hydroxy esters in good yield. Since then a number of characterizable metal homoenolates have been prepared from siloxycyclopropanes [11], The repertoire of metal homoenolate reactions now covers most of the standard reaction types ranging from simple... 

The chemical reactivities of such titanium homoenolates are similar to those of ordinary titanium alkyls (Scheme 2). Oxidation of the metal-carbon bond with bromine or oxygen occurs readily. Transmetalations with other metal halides such as SnCl4, SbClj, TeCl4, and NbCls proceed cleanly. Reaction with benzaldehyde gives a 4-chloroester as the result of carbon-carbon bond formation followed by chlorination [9]. Acetone forms an addition complex. No reaction takes place with acid chloride and tm-alkyl chlorides. 

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