With Magnesium Alkyl

In conclusion, as far as this section is concerned, it should be mentioned that highly active catalyst systems, prepared by ball-milling solid supports e.g., MgO, Si02, y-AljOj, etc.) in the presence of ethereal solutions of Grignard reagents followed by milling in the presence of TiCU, were reported in 1974 by 

Duck et al. These systems are of particular interest in that they combine the use of a hydroxyl-containing support with a magnesium alkyl halide compound. Activities of some two orders of magnitude (on the basis of Ti) greater than that of a traditional TiCU catalyst were reported although no actual active centre determinations were carried out Also in this case much higher ratios of AI Ti were employed (typically 240 1) than for traditional catalysts although the actual alkyl aluminium concentration employed was about the same in both cases. 

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On catalytic hydrogenation it furnishes mainly dihydroquinidine. Benzoyl chloride converts it into the enol benzoate, m.p. 114-5°, and on interaction with magnesium alkyl halides it forms alkylquinidines, e.g., methylquinidine, 2H2O, m.p, 105-12°, [a] ° -(- 168° (EtOH),... 

Grignard Synthesis.— The most important method for synthesizing aromatic alcohols is by the Grignard reaction, with magnesium alkyl or aryl halides (p. 77). The one given as an example of a tertiary aromatic alcohol may be prepared by the action of magnesium phenyl bromide, CeHs—Mg—Br, upon acetone. 

SCHEME 23.16 Postulated formation of the propagating enolate species in MMA polymerization with magnesium alkyl initiators. 

At ordinary temperatures, zinc forms an addition compound with an alkyl halide (cf magnesium) ... 

Alkyl and aryl iodides usually react with magnesium more rapidly than the corresponding bromides, and the bromides very much more rapidly than the chlorides. Aryl (as distinct from alkyl) chlorides have usually only a slow reaction with magnesium and are therefore very rarely used. With alkyl and aryl iodides in particular, however, a side reaction often occurs with the formation of a hydrocarbon and magnesium iodide ... 

Uses. Magnesium alkyls are used as polymerization catalysts for alpha-alkenes and dienes, such as the polymerization of ethylene (qv), and in combination with aluminum alkyls and the transition-metal haUdes (16—18). Magnesium alkyls have been used in conjunction with other compounds in the polymerization of alkene oxides, alkene sulfides, acrylonitrile (qv), and polar vinyl monomers (19—22). Magnesium alkyls can be used as a Hquid detergents (23). Also, magnesium alkyls have been used as fuel additives and for the suppression of soot in combustion of residual furnace oil (24). 

The reaction is carried out by first reacting the alkyl or aryl halide with magnesium shavings in an ether suspension and then treating with silicon tetrachloride (prepared by passing chlorine over heated silicon). With methyl chloride the following sequence of reactions occur ... 

Grignard reagent (Section 14.4) An organomagnesium compound of the type RMgX formed by the reaction of magnesium with an alkyl or aryl halide. 

Oddo reported that the organomagnesium derivatives of p3Trole, indole, skatole, and carbazole could be prepared in a single operation by mixing the parent heterocyclic compound with an alkyl halide and magnesium in anhydrous ether.The product formed was reported to be the same as that obtained by the more conventional procedure. However, this approach to the synthesis of the indole Grignard reagents does not seem to have been exploited in subsequent work. 

Grignard reagents are a very important class of organometallic compounds. For their preparation an alkyl halide or aryl halide 5 is reacted with magnesium metal. The formation of the organometallic species takes place at the metal surface by transfer of an electron from magnesium to a halide molecule, an alkyl or aryl radical species 6 respectively is formed. Whether the intermediate radical species stays adsorbed at the metal surface (the A-modelf, or desorbs into solution (the D-model), still is in debate ... 

Schemes 3-7 describe the synthesis of cyanobromide 6, the A-D sector of vitamin Bi2. The synthesis commences with an alkylation of the magnesium salt of methoxydimethylindole 28 to give intermediate 29 (see Scheme 3a). The stereocenter created in this step plays a central role in directing the stereochemical course of the next reaction. Thus, exposure of 29 to methanol in the presence of BF3 and HgO results in the formation of tricyclic ketone 22 presumably through the intermediacy of the derived methyl enol ether 30. It is instructive to point out that the five-membered nitrogen-containing ring in 22, with its two adjacent methyl-bearing stereocenters, is destined to become ring A of vitamin Bi2. A classical resolution of racemic 22 with a-phenylethylisocyanate (31) furnishes tricyclic ketone 22 in enantiomerically pure form via diaster-eomer 32.

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