Organometallic Approaches

For symmetrical coupling of allylic and vinylic halides or alcohols, see also page 77, Section 1. 

See also page 420, Section 2.6 page 506, Section 3.22 and page 514, Section 3.24. 

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Although not of industrial importance, many organometallic approaches have been developed (38). A one-pot synthesis of vitamin has been described and is based on the anionic [4 + 2] cycloaddition of three-substituted isoben2ofuranones to l-phytyl-l-(phenylsulfonyl)propene. Owing to the rather mild chemical conditions, the (H)-stereochemistry is retained (39). 

In summary, we have described our approach towards the synthesis of novel nano-objects consisting of a metal core and a surface that may be functionalized by addition of organic ligands. TEM pictures of the metal core of these nanoparticles appear similar to those of particles commonly used in heterogeneous catalysis or to colloids prepared by well-known reduction methods. However, the organometallic approach displays several specificities which can be summarized as follows ... 

The organometallic approach is a low temperature approach, which means that in general synthesis of the particles, and therefore many characteristics of the nano-objects, can be controlled ... 

Finally, the groups of Chaudret and Choukroun have demonstrated that PVP-protected native Rh nanoparticles synthesized by an organometallic approach are active in the hydrogenation of benzene in a biphasic mixture. 

Although the role of MAO remains still speculative, organometallic approaches have led to progress in distinguishing catalytically active species... 

An alternative organometallic approach for functionalizing C-H bonds is by means of metal carbene- or metal nitrene-induced C-H insertions (Equations (1) and (2)).35 36 A major advantage of this approach over other methods is that the reaction is routinely catalytic and by using chiral catalysts, high enantioselectivity can be achieved. One of the major challenges with the metal carbene- and metal nitrene-induced C-H insertion is controlling the... 

An alternative approach to stabilize nanoparticles is to use polyoxoanions (see Scheme 9.5). Finke and coworkers described polyoxoanion- and ammonium-stabilized rhodium zerovalent nanoclusters for the hydrogenation of classical benzene compounds [95, 108]. This organometallic approach allows reproducible preparation of stable nanoparticles starting from a well-defined complex in terms of composition and structure (see Section 9.3.5). 

Among the several strategies developed for carbazole syntheses, organometallic approaches to carbazoles have received special attention, since they have led, in many cases, to highly convergent total syntheses of carbazole alkaloids (8,10, 570-572). The iron-mediated retrosynthesis of 1-oxygenated carbazoles 7-11 leads to cyclohexadiene (597) and the corresponding arylamines 598-600 as synthetic precursors (Scheme 5.32). 

Difluoroethylene and trifluorobromoethylene are also starting molecules to access difluorinated molecules through organometallic approaches (e.g., metallation, palladium-catalyzed coupling) vide infra)... 

In a related organometallic approach, treatment of the diiodoarylformamidine 878 with isopropylmagnesium bromide gave an organomagnesium intermediate which reacted with a variety of alkyl, allyl, and aryl isocyanates to give 3-substituted-8-iodo-4(37/)-quinazolinones 879 <20020L1819>. 

The nucleophilic addition of a trifluoromethyl anion or its equivalent to an activated carboxylic acid derivative is another potential method for the synthesis of trifluoromethyl ketones (Scheme 8). Due to the well-known instability of the trifluoromethyl anion, the organometallic approach (Section 15.1.4.3.3) is often difficult to utilize. However, a trifluoromethyl anion equivalent, (trifluoromethyl)trimethylsilane (CF3TMS), was developed in 1984 by Ruppert and co-workers.[30] This reagent, known as Ruppert s reagent, is stable and does not undergo fluoride elimination like other equivalent trifluoromethyl anions. An obvious limitation to this method is that it is only useful for the synthesis of a-amino trifluoromethyl ketones unless other fluoroalkyl analogues of Ruppert s reagent are developed. 

The activation and transformation of C-H bonds in alkanes by homogeneous transition metal catalysts should to be a topic of further research. No processes are close to commercialization on a technical scale. So far the organometallic approach by Shilov is one of the most promising ways of producing a practical system [55]. 

Triprotection of the 12-16-membered tetraazacycles was also achieved through organometallic approaches. Chromium(O) or molybdenum(O) tricarbonyl triamino complexes (e.g., 163-165) <1995JOM215> are substituted with acylhalogenides or aldehydes and N-monosubstituted products are obtained after oxidative deprotection in strong acid and acylamide/enamine reduction to amines <1995TL79>. 

In summary, Dupont and coworkers have developed an organometallic approach for the stabilization of various zero-valent nanoparticles in the ionic liquid BMI PFe. Transition metal nanoparticles of 2.0-3.0 were obtained with... 

Finally, Jessop and coworkers describe an organometallic approach to prepare in situ rhodium nanoparticles [78]. The stabilizing agent is the surfactant tetrabutylammonium hydrogen sulfate. The hydrogenation of anisole, phenol, p-xylene and ethylbenzoate is performed under biphasic aqueous/supercritical ethane medium at 36 °C and 10 bar H2. The catalytic system is poorly characterized. The authors report the influence of the solubility of the substrates on the catalytic activity, p-xylene was selectively converted to czs-l,4-dimethylcyclohexane (53% versus 26% trans) and 100 TTO are obtained in 62 h for the complete hydrogenation of phenol, which is very soluble in water. 

An organometallic approach to kinetically stabilized transition metal main-group element compounds of the half sandwich type involves formal substitution of the C5H5- against a CsMcs-unit at the metal promising both a better steric shielding of the coordination sphere and an increased electron donor capacity of the metal fragment [10],... 

Even the most stable and least reactive molecules such as H2O, CO2, and N2 can be photocleaved by intramolecular electron transfer between suitable redoxactive metal centers and these molecules or simple derivatives such as carbonate instead of carbon dioxide. The photoactivation is achieved by CT excitation which corresponds to a shift of electron density. The formation of stable products is determined by the availability of appropriate oxidation states at the metal centers. While these photoreactions are very interesting in their own right, applications are conceivable in particular for solar energy utilization. Especially, organometallic approaches to water splitting have been anticipated to be rather promising in the future (76). 

This organometallic approach to the preparation of mixed metals has also been used to prepare mixed metal species containing tin.201-206 As shown in Fig. 13.21 the reaction of tetraalkyl tin with silica or alumina supported platinum. 

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