Grignard reactions complexes

Mechanism of the Grignard reaction. Complexation of the carbonyl oxygen with the Lewis acid Mg " " and subsequent nucleophilic addition of a carbanion to an aldehyde or ketone is followed by protonation of the alkoxide intermediate to yield an alcohol. 

In Grignard reactions, Mg(0) metal reacts with organic halides of. sp carbons (alkyl halides) more easily than halides of sp carbons (aryl and alkenyl halides). On the other hand. Pd(0) complexes react more easily with halides of carbons. In other words, alkenyl and aryl halides undergo facile oxidative additions to Pd(0) to form complexes 1 which have a Pd—C tr-bond as an initial step. Then mainly two transformations of these intermediate complexes are possible insertion and transmetallation. Unsaturated compounds such as alkenes. conjugated dienes, alkynes, and CO insert into the Pd—C bond. The final step of the reactions is reductive elimination or elimination of /J-hydro-gen. At the same time, the Pd(0) catalytic species is regenerated to start a new catalytic cycle. The transmetallation takes place with organometallic compounds of Li, Mg, Zn, B, Al, Sn, Si, Hg, etc., and the reaction terminates by reductive elimination. 

A Grignard reaction begins with an acid-base complexation of Vfg2+ to the carbonyl oxygen atom of the aldehyde or ketone, thereby making the carbonyl group a better electrophile. Nucleophilic addition of R then produces a tetrahedral magnesium alkoxide intermediate, and protonation by addition of water... 

However, there is evidence that reactions of aluminium hydride produced in situ involve single-electron-transfer (SET) processesThe reactions described by Trost and Ghadiri have most likely not been studied in sufficient detail to permit an adequate description of the reaction mechanism to be given at this stage. It is, however, quite likely that the Grignard reactions catalyzed by copper(II) and nickel(II) complexes , as developed by julia - and by Masaki , do involve SET processes, although, if this is so, the preservation of stereochemistry in some of the examples described by these workers is quite remarkable. (In this context, the reader s attention is drawn to Reference 196, end of this section.)... 

Germanium Porphyrins. The electrosynthesis of a-bonded mono-alkyl or mono-aryl germanium porphyrins involves the conversion of (P)Ge(R)2 to (P)Ge(R)X where X is an anionic ligand. A standard Grignard reaction between (P)Ge(Cl)2 and RMgX leads to the o-bonded bis-alkyl or bis-aryl complexes, (P)Gp(R)2. These complexes were initially synthesized as lH IWR shift reagents(28,29). However, almost no reactivity of these species was reported until several years later when it was shown that the... 

The Grignard reaction is one general method for the access to fiuori-nated phosphines [118]. Fluor mated phosphines are also related to catalysis applications in fluorous reaction media [119-121]. As a first example, trans-Co2(CO)6[P(z-Pr)2 3,5-fozs(CF3)-C6H3 ] (5b) was synthesized in two steps by the reaction of 3,5-fozs(CF3)-C6H3MgBr with (z-Pr)2PCl to 5a with a yield of 90%, followed by the complexation of Co2(CO)8 in refluxing benzene (Fig. 10). 

Biguanides are accessible by a Grignard reaction of guanidinomagnesium halide with a substituted cyanamide, and hydrolysis of the resulting complex [59—61). The method is of considerable theoretical interest but so far only of limited practical importance. 

The kinetics of the Grignard reaction was recognized to be much more complex than originally considered because of several factors the Schlenk equilibrium produces many reacting species, a metal impurity induces an ET process, and the primary product, ROMgX, contributes to the reaction . 

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). 

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