Grignard reaction, effective reagent

These compounds are sources of the nucleophilic anion RC=C and their reaction with primary alkyl halides provides an effective synthesis of alkynes (Section 9 6) The nucleophilicity of acetylide anions is also evident m their reactions with aldehydes and ketones which are entirely analogous to those of Grignard and organolithium reagents... 

Cesium forms simple alkyl and aryl compounds that are similar to those of the other alkah metals (6). They are colorless, sohd, amorphous, nonvolatile, and insoluble, except by decomposition, in most solvents except diethylzinc. As a result of exceptional reactivity, cesium aryls should be effective in alkylations wherever other alkaline alkyls or Grignard reagents have failed (see Grignard reactions). Cesium reacts with hydrocarbons in which the activity of a C—H link is increased by attachment to the carbon atom of doubly linked or aromatic radicals. A brown, sohd addition product is formed when cesium reacts with ethylene, and a very reactive dark red powder, triphenylmethylcesium [76-83-5] (C H )2CCs, is formed by the reaction of cesium amalgam and a solution of triphenylmethyl chloride in anhydrous ether. 

Modified MPV-type reductions carried out with chiral magnesium alkoxides and with chiral Grignard reagents have been discussed in detail (1). These reagents differ from the aluminum alkoxides since the Grignard reaction is essentially irreversible. Chiral alkali metal alkoxides have also been used to effect asymmetric reductions (1). 

Today one of the most common chemical applications of ultrasound is the initiation of a reluctant Grignard reaction. The quantitative effects of ultrasound on the induction times for the formation of a Grignard reagent in various grades of ether is given in Tab. 3.2 [88]. 

The regioselectivity in the Grignard reactions of tetrasubstituted pyrylium salts is the consequence of simultaneous effects of substitution patterns in both the substrate and the reagent. Thus the reaction of MeMgl with 2,3,4,6-or 2,3,5,6-tetramethylpyrylium ions provide mixtures of isomeric 2//-pyrans 187 and 188 in the ratio 91 9 or of 2//-pyran 189 and 4i/-pyran 190 in the ratio 48 52, respectively.237... 

The sonication method of magnesium activation has been employed to initiate sluggish Grignard reactions in ether. Sonication is the preferred technique for effecting the Barbier variation, in which the electrophilic species and organohalide are introduced concomitantly, and the Grignard reagent is then intercepted as fast as it is formed (Fig. 15) [36]. 

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