Catalyst generation from precatalysts

Although metallocene silyl and hydride complexes are the active species, many researchers have sought to develop more convenient precatalysts. The original dimethyltitanocene system reported by Harrod is reasonably easy to prepare, but Corey140 and others have shown that in situ catalyst generation from metallocene dichlorides (Ti, Zr, and Hf) and "BuLi is both simpler and equally effective. However, Harrod and Dioumaev have shown... 

Recently, Ishihara and co-workers developed a more powerful hypervalent iodine catalyst, generated in situ from 2-iodobenzoic acid and furthermore demonstrated that its sulfonic acid analog 2 is more reactive as a precatalyst (Equation 10.3) [8]. They reported that it was not necessary to isolate hypervalent iodine compounds, which are potentially explosive oxidants, and furthermore that more powerful oxidants could be generated in situ. 

A particularly active catalyst for the hydrogenation of polycylic arenes has been generated from the anionic hydridoruthenium precatalyst in Figure 15.18, discovered by Grey and Fez. This system catalyzes the hydrogenation of antliracene to 1,2,3,4-tetrahydroanthracene under relatively mild conditions with a turnover frequency of > 50 h" and a turnover number of > 100 (Equation 15.96). Both anthracene and naphthalene are partially hydrogenated in the presence of tlais catalyst, but monocyclic arenes do not react. 

Highly reactive palladium catalysts based on this type of bulky biphenylphos-phines can be readily generated from phenylethylamine-derived paUadacycles [122] or, even more conveniently, from biarylamine-derived precatalysts [123]. Complexes Pdl2 can also be conveniently prepared in situ by the reaction of Pd(r -l-PhC3H4)(r -C5H5) with tertiary phosphines L [124]. 

Recently, iron(II) complexes with chiral N2P2 and N2P4 macrocychc hgands were introduced by Mazzetti and Gao for apphcations in ATH catalysis. In 2004, Gao and co-workers reported the ATH of ketones with the catalysts in situ generated from iron(ll) complexes and chiral PNNP ligands [154], ligands that were later incorporated into the well-defined Fe-PNNP complexes by Morris et al., vide supra. The 22-membered macrocycles 174 and 175 (Fig. 55) synthesised by Gao et al. were used to in situ generate precatalysts from different iron sources (e.g. 

All three catalysts are illustrated as LnM=CHR in the mechanism below. Generation of the catalyst from the precatalysts ... 

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