Reactions with organotitanium compounds

The above transformation has two different goals 21,22) 1) To increase chemo-, regio-, diastereo- and enantioselectivity in the reaction with carbonyl compounds (Sections C-E), and 2) to make certain reaction types amenable which do proceed readily with classical reagents, e.g., methylation of tertiary alkyl halides, alcohols and ethers, and direct geminal dialkylation of ketones (Section F). It turns out that organotitanium compounds are usually complementary to Li, Mg, Zn, Fe, Ni, Cu and Pd reagents. So far, experience in the above two areas points to the following positive aspects ... 

This review describes some novel reaction types (Section F), but the more extensive area of activity has to do with variable adjustment of carbanion reactivity and selectivity in reactions with carbonyl compounds (Sects. C-E). Organotitanium... 

With organotitanium compounds, especially carbometalation, substitution and insertion reactions tend to proceed. Carbometalation, as described in the reaction mechanisms of the Ziegler-Natta catalyst in Chapter 7, contains olefin polymerization with titanium compounds. Namely, titanium reacts with organoalumi-num compounds and the titanium atom forms a bridged structure (Ti—R—Al) with an alkyl group of organoaluminum compounds and this Ti—R adds to a carbon-carbon double bond. 

The organotitanium compounds produced by desulfurization of the diphenyl thioacetals of aldehydes 28 with the titanocene(II) species Cp2Ti[P(OEt)3]2 29 react with carbon—carbon double bonds to form the olefin metathesis-type products. Thioacetals 28 may be transformed into terminal olefins by desulfurization with 29 under an ethene atmosphere (Scheme 14.15) [27]. This reaction is believed to proceed through a titanacyclobutane intermediate, formed by cycloaddition of the titanocene-alkylidene with ethene. 

The use of organotitanium compounds in the synthesis of allenes involves mainly Wittig-type olefmation reactions of carbonyl compounds [86] with titanium ylides. The formation of allenes according to the scheme Q + Q + Q was described by... 

D. Diastereoselectivity in Reactions of Organotitanium Reagents with Carbonyl Compounds... 

Organotitanium compounds have been intensively studied, initially mainly because of the discovery by Ziegler and Natta that ethylene and propylene can be polymerized by TiCl3-aluminum alkyl mixtures in hydrocarbons at 25°C and 1 atm pressure84 (Section 22-9). Organic compounds have been found to react with N2 and to act as catalysts in a number of other reactions. 

In addition to the aforementioned X-ray analysis to disclose the structure of a few crystalline titanium chelates, and NMR studies have been performed to provide evidence for the chelation structure of a- and /1-oxycarbonyl compounds in solution [33-35]. Approximate solution structures for -alkoxyaldehydes are as shown in Fig. 7 [34]. The mechanism of chelation-controlled reactions of organotitanium reagents has been investigated experimentally [5] and theoretically [36], and the subject has been reviewed [10]. The formation of a chelate structure with titanium metal at the center plays a pivotal role in determining the reactivity and selectivity [37] in many synthetic reactions as shown in the following discussion. 

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 reaction rate with Cp2TiEtCl/AlEtCl2, is faster than that with Cp2TiCl2/ AlEtCb or Cp2TiCl2/AlEt2Cl, it is 15 dm moP s at 10 °C [46]. However, as reviews of Ishioka [47] and Kashiwa [48-51] show, the titanium component of Ziegler-Natta type catalysts show only TiCb and TiCb. Therefore it is a problem whether organotitanium compounds are used industrially. 

As reactions with tetravalent organotitanium compounds, the following four reactions are particularly noted. 

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