Organotitanium reagents reactivity

Reetz, M. T. Organotitanium Reagents in Organic Synthesis. A Simple Means to Adjust Reactivity and Selectivity of Carbanions. 106, 1-53 (1982). 

Hesse s report8S) that 72 is smoothly methylated by 6 (85 % 73) also demonstrates that incompatibility of aliphatic nitro groups with organotitanium reagents is not general. In this case a more reactive aldehyde function is involved. As already stressed, ketones react much more slowly than aldehydes, so that side reactions... 

Why are organotitanium reagents more chemoselective than the lithium and magnesium counterparts It is clear from the above presentation that the rate of reaction of classical carbanions is considerably higher than those of the titanated species. Generally, more reactive species are less selective. However, this does not really answer the above question, since the phenomenon of different rates remains unclear. Apart from steric factors, we believe it has to do with the different polarity of the carbon-metal bond. Whereas C—Li bonds are highly polar (and ionic in certain cases) 57,86,87) anaj0gS appear to be considerably less so (Sect. B). 

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. 

As previously discussed for organotitanium reagents, the nature of the added Lewis acid influences the geometiy of the reactive conformer. For example, TiCl4 prefers hexacoordinated complexes, and it induces cycloadditions of benzyl ester of IV-acryloylproline 31 through an s-cis chelate whose Si face is not shielded by a chlorine bound to titanium. The opposite facial selectivity is observed under EtAlCl2 catalysis, probably because the s-trans monodentate complex is the reac-... 

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