Grignard reagents halides

The effect of the leaving group was briefly examined, but cinnamyl bromide gave a substantially lower ee (38%). Cinnamyl dimethyl phosphonate, or acetate, gave very poor results. The cyclohexyl-substituted allylic acetate 21, on the other hand, afforded a completely y-selective reaction, but the product turned out to be racemic. Changing the Grignard reagent halide from bromide to either chloride or iodide resulted in very low ees. 

It is noteworthy that c/s-epoxides lead to the uMt/ -allenic product as the major compound, but the nature of the Grignard reagent halide can reverse the selectivity when bromide is used. For rruns-epoxides, the syn-product is always the major compound, whatever the nature of the counterion. In the steroid series [33], the cross-coupling of a... 

Since Grignard reagents can easily be obtained from aryl halides, they are of special value in the s nthesis of many aromatic compounds, particularly as, for reasons already stated (pp. 270, 276), aromatic compounds cannot generally be prepared by means of ethyl acetoacetate and ethyl malonate. 

Anilides and a-Naphthalides. The Grignard reagents prepared from alkyl halides react with phenyl isocyanate (CgHjN=C=0) or with a-uaphthy l isocyanate (C,oH, N=C=0) to yield addition products that are converted by hydrolysis into anihdes and a-naphthalides respectively RX + Mg —> RMgX... 

Alkyl mercuric halides. Grignard reagents, prepared from alkyl halides, react with a mercuric halide that contains the same halogen as the reagent to form alkyl mercuric halides ... 

Strictly speaking the alkyl halides are esters of the halogen acids, but since they enter into many reactions (t.g., formation of Grignard reagents, reaction with potassium cyanide to yield nitriles, etc.) which cannot be brought about by the other eaters, the alkyl halides are usually distinguished from the esters of the other inorganic acids. The preparation of a number of these is described below. 

Many organic halides do not react satisfactorily with lithium to form RLi ecMnpounds or with metallic magnesium to form Grignard reagents. The desired organolithium compound can often be prepared by a halogen-metal interconversion reaction ... 

Grignard reagents are prepared from organic halides by reaction with magnesium a Group II metal... 

The order of halide reactivity is I > Br > Cl > F and alkyl halides are more reac tive than aryl and vinyl halides Indeed aryl and vinyl chlorides do not form Grignard reagents m diethyl ether When more vigorous reaction conditions are required tetrahy drofuran (THF) is used as the solvent... 

Acetylenic Grignard reagents of the type RC CMgBr are prepared not from an acetylenic halide but by an acid-base reaction in which a Grignard reagent abstracts a proton from a terminal aUcyne... 

Grignard reagents react with ethylene oxide to yield primary alcohols containing two more carbon atoms than the alkyl halide from which the organometallic compound was prepared... 

Overall the carboxylation of Grignard reagents transforms an alkyl or aryl halide to a carboxylic acid in which the carbon skeleton has been extended by one carbon atom... 

The major limitation to this procedure is that the alkyl or aryl halide must not bear substituents that are incompatible with Grignard reagents such as OH NH SH or... 

Formation of aryl Grignard reagents (Section 14 4) Aryl halides react with magnesium to form the corresponding arylmagnesium halide Aryl iodides are the most reac tive aryl fluorides the least A similar reaction occurs with lithium to give aryllithium reagents (Section 14 3)... 

The reaction proceeds by formation of the Grignard reagent from o bromofluorobenzene Because the order of reactivity of magnesium with aryl halides is Arl > ArBr > ArCl > ArF the Gngnard reagent has the structure shown and forms benzyne by loss of the salt FMgBr... 

Grignard reagent (Section 14 4) An organomagnesium com pound of the type RMgX formed by the reaction of magne Slum with an alkyl or aryl halide... 

Alkyl and arylboranes are obtained (35) from BF using the appropriate Grignard reagent, alkylalurninum halide, or zinc alkyl, using diethyl ether as the solvent (see also Organometallics) ... 

Allyl Complexes. Allyl complexes of uranium are known and are usually stabilized by cyclopentadienyl ligands. AEyl complexes can be accessed via the interaction of a uranium halide and an allyl grignard reagent. This synthetic method was utilized to obtain a rare example of a "naked" homoleptic allyl complex, U(T -C2H )4 [12701 -96-17, which decomposes at 0°C. Other examples, which are more stable than the homoleptic allyl complex have been synthesized, ie, U(allyl)2(OR)2 (R = alkyl), U(allyl)2X (X = halide), and U(allyl)(bipy)2. 

Pyrroles do not react with alkyl halides in a simple fashion polyalkylated products are obtained from reaction with methyl iodide at elevated temperatures and also from the more reactive allyl and benzyl halides under milder conditions in the presence of weak bases. Alkylation of pyrrole Grignard reagents gives mainly 2-alkylated pyrroles whereas N-alkylated pyrroles are obtained by alkylation of pyrrole alkali-metal salts in ionizing solvents. 

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