Organomagnesium transition-metal

In this section, structures of compounds are described that contain both an organomagnesium moiety and a transition-metal-containing part. These moieties are linked via either... 

Low reactive aryl chlorides are converted to the respective organomagnesium species in excellent yields through transition metal catalysis using 2 mol% FeCU (4-6, equation 3). Alternatively, a safe and reproducible method for activation of aryl chlorides or bromides 7 uses microwave irradiation (equation 4). In a synthesis of a novel HIV-1 protease inhibitor, microwave irradiation was essential to generate the starting arylmagnesium halide as well as to promote the subsequent Kumada coupling reaction. ... 

The transition metal catalyzed carbon-carbon bond formation between organomagnesium reagents and aryl (vinyl) halides has been one of the pioneering entries into cross-coupling chemistry. The reaction has been widely utilized since than in azine chemistry,22 with the limitation that the functional group tolerance of Grignard reagents is only moderate. Here only some of the more recent developments will be mentioned. 

The mechanisms are complex, particularly in the organomagnesium (Grignard) reactions. Several reactive species are present, and the product metal alkoxide can complex with unreacted organometallic. Furthermore, trace transition metal impurities in the magnesium used to prepare Grignard reagents appear to facilitate electron transfer and may cause the reaction to proceed at least partly by a radical pathway. See (a) J. Laemmle, E. C. Ashby, and H. M. Neumann, J. Amer. Chem. Soc., 93, 5120 (1971) (b) E. C. Ashby, J. Laemmle, and H. M. Neumann, Accts. Chem. Res., 7,272 (1974). 

Hence, there are strong grounds to state that compounds with the tetra-nuclear cluster magnesium core formed in Mg-RH films have catalytic properties similar to those of transition metal complexes. The catalytic activity of organomagnesium clusters can be very high. 

Transition metal-catalyzed cross-coupling reaction has become the most important method for the formation of aryl-aryl bonds. The reaction of aryl halides and sulfonates with organomagnesium compounds is usually referred... 

Organomagnesium compounds sometimes behave as single-electron donors. Such behaviour is most likely to be observed in highly polar, cation-solvating media and/or when steric hindrance inhibits alternative pathways. Ate complexes of magnesium can also act as single electron donors (see p. 97). It should be noted that the presence of traces of transition metals may lead to electron transfer processes [2]. 

Some examples to illustrate these categories are listed in Table 8.1. Many examples of reactions between organomagnesium compounds and organic halides are tabulated in General Ref. [A] it should be noted, however, that for many of the earlier experiments, transition metals may have been present as impurities in the magnesium used. While under some circumstances transition metal catalysis may be beneficial, under the conditions commonly used in the early experiments it usually led to a proliferation of products. For reactions with primary alkyl and allylic halides, catalysis with copper(i) salts is often beneficial examples of such reactions are also shown in Table 8.1 and a procedure follows. 

It appears that the reduction of the transition metal oxidation state is a more frequent cause of activity loss of the centre than is usually acknowledged. Such reduction can occur as a consequence of a bimolecular reaction according to eqn. (92) but it may also be caused by the organoaluminium (organomagnesium) catalyst component, by H2 intentionally added in order to reduce the molecular mass of the product, and perhaps also by other, so far unknown, processes. 

It is noteworthy that reaction of a transition-metal organomagnesium reagent with Me3SiCl may be employed to produce a Si—Fe bond... 

Aluminum alkyls, although inconvenient to handle because of their spontaneous inflammability, are powerful reducing agents and are applied to the synthesis of transition-metal hydrides . They, and the organomagnesium halide reagents to be discussed in 1.10.8.4, function by initial alkylation of the transition metal (e.g., for EtjAl) ... 

Reaction of a transition metal halide with organolithium, organomagnesium, or organoaluminum reagent. 

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