Metal agents

Two examples of pyrimidines 57, 58 were included in a study of direct magnesiation of heterocycles. Both of these metallated selectively at C-4 using inverse addition i.e. substrate added to the metallating agent <06AG(I)2958>. 

The analogous dithio- and diseleno-imidodiphosph(in)ates are also accessible as their alkali-metal salts via reaction of the imino-fe(phosphine chalco-genide) 41 with raBuLi, NaH, NaOMe or KO/Bu. However, the most commonly used of these metallating agents is KOtBu since it gives near quantitative yields and the resultant potassium salt is a good starting material for metathesis reactions (see Section 5.3.8). 

Ethyl pyridine-2-acetate and ethyl 6-methylpyridine-2-acetate have previously been prepared by carboxylation of the lithio derivatives of a-picoline and lutidine, respectively. Use of ethyl carbonate to acylate the organometallic derivative avoids the intermediacy of the (unstable) carboxylic acid, and the yields are better. In the present procedure potassium amide is used as the metalating agent the submitters report that the same esters may be formed by metalation with sodium amide (43% yield) or with w-butyllithium (39% yield). The latter conditions also yield an appreciable amount of the acid (which decarboxylates). 

AUtylmagnesium halides and dialkyhnagnesium compounds are efficient metallating agents toward alkyl mesityl ketones such as Kohler s ketone (2,2-diphenylethyl 2,4,6-trimethylphenyl ketone) and hindered carboxylic esters such as f-butyl acetate. 

The alkali metal hydrides, LiH, NaH, and KH, when prepared in a special way, are very rapid metallation agents Klusencr Brandsma Vcrkruijsse Schleyer Friedl Pi Angew. Chem. Int. Ed. Engl. 1986, 25, 465 Angew. Chem. 98, 4581. 

Systematic studies on 3-fluoropyridine, the first mono halopyridine to be shown to undergo the DoM process [72CR(C)(275)1535], showed that metalation regioselectivity was dependent on reaction conditions (solvent, temperature, time, metalating agent). 

Heterosubstitution at nonactivated positions is also possible by using highly reactive metalating agents such as pentylsodium. When used in the presence of tm-BuOK, it readily and selectively forms vinylsodium derivatives that can subsequently be converted to organosilicium compounds.243 Norbomadiene and bicyclo-[3.2.0]hepta-2,6-diene are also readily silylated via the corresponding sodium compounds formed in the reaction with BuLi-rm-BuONa.244 Similar transforma-tions of strained polycyclic cyclopropanes were also achieved [Eq. (10.45) ] ... 

With the extremely efficient metalating agent B uLi-ferf-B uOK-TMEDA even direct metallation of ethylene and the transformation of the intermediate to hetero-substituted product were accomplished 246... 

Alkali metal hydrides can be used as metallating agents in the synthesis of base-free cyclopentadienylmetal derivatives.27 28... 

The use of phosphorus compounds has two advantages the reactions are performed in homogeneous mixtures and secondly, phosphorus derivatives are milder dehalogenation agents and, therefore, react with a better regioselectivity compared with most metallic agents, e.g. zinc.112... 

Treating l-chloro-l-(trichlorovinyl)cyclopropanes with three equivalents of BuLi afforded the the dilithiated acetylene This, upon reaction with excess trimethylsilyl chloride, yielded the bis(trimethylsilyl)ethynylcyclopropane (equation 163). The same results can be achieved using magnesium metal in THF as the metallating agent, instead of the preferable BuLi236 238. 

Carboxylic acid derivatives or ketones with an alkylthio group at C-3 can be meta-lated and alkylated at the a position without elimination of thiolate [446] (Scheme 5.56). Amides of 3-(alkylthio)-2-alkylpropanoic acids can, however, also be alkylated at the /3 position if strong metalating agents are used as base [53, 460]. 

On the other hand, catalyst efficiency was defined as the ratio of the calculated Mn to the experimental Mn (Eq. (34b)]. The calculated M was determined from the ratio of grams of monomer per mole of metalating agent the experimental Mn was determined by measuring the Mn of the styrene block after oxidative degradation of the polybutadiene backbone. 

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