Elimination Grignard reagent from

Reactions of lithium derivatives and Grignard reagents from polyfluoroalkenes or polyfluoroaromatic compounds are often complicated by eliminations but are generally much more useful in synthesis than polyfluoroalkane derivatives. Generation of arynes is discussed later (see Chapter 9). 

Typically, attempts to synthesize 1,2-di-Grignard reagents from 1,2-dihaloalkanes fail, because metal halide elimination after insertion of the first magnesium atom is faster than formation of a second magnesium-carbon bond. The only l,2-di(halomagnesio)alkane [16] synthesized in the classic fashion is a l,2-di(bromomagnesio)cyclopropane 22 [Eq. 

Bakuzis has explored the use of an interesting (B-acrylate anion equivalent in his synthesis of 9 (Scheme 4.5) ° The Grignard reagent from 3-bromopropyl phenyl sulfide was condensed with acetaldehyde and the resulting alkoxide was acylated to procure 24 (70%). Oxidation of the sulfide to aldehyde 25 proceeded in 74% yield. Reaction of 25 with ethyl 3-nitropropionate followed by elimination of HNO2 from the intermediate afforded 26. The latter step, equivalent to the addition of the p-carbanion of ethyl acrylate, occurred in 38% yield. Oxidation of alcohol 26, protection of the resulting ketone, and saponification gave 17. This intermediate was then dimerized as previously described to a mixture of 9 and its meso diastereomer. 

Finally, it may be noted that the treatment of dialkyl 2-chlorophenyl phosphates with metallic sodium and the formation of the Grignard reagent from dialkyl 2-bromophenyl phosphates both yield dialkyl (2-hydroxyphenyl)phosphonates after work-up in the case of dialkyl 2,4-dibromophenyl phosphate, the elimination of bromine is restricted to the 2-position . ... 

The mechanism of the Heck reaction with Pd(OAc)2 probably involves the initial in situ reduction of the Pd " ion to Pd(0), which then inserts in the C-X bond (recall the formation of the Grignard reagent from Mg and halides). The species R-PdX that is formed adds to the double bond in the sense of R and PdX, which in an intramolecular reaction will lead to a cyclic product. The adduct eliminates H-PdX to regenerate a double bond. This proposed mechanism is illustrated with the synthesis of the benzofuran 4.36 in Scheme 4.39. 

Cooling is routinely appHed, either with ambient process water if THF is the solvent or with chilled brine if diethyl ether is used. Since Grignard reagents are particularly reactive with water, Hquid hydrocarbon coolants may be preferred, to eliminate the risk that could arise from a cooling-system leak. 

Trialkylsilyl groups have a modest stabilizing effect on adjacent carbanions (see Part A, Section 3.4.2). Reaction of the carbanions with carbonyl compounds gives (3-hydroxyalkylsilanes. (3-Hydroxyalkylsilanes are converted to alkenes by either acid or base.270 These eliminations provide the basis for a synthesis of alkenes. The reaction is sometimes called the Peterson reaction.211 For example, the Grignard reagent derived from chloromethyltrimethylsilane adds to an aldehyde or ketone and the intermediate can be converted to a terminal alkene by acid or base.272... 

The 2,2-dialkylchromenes can easily be obtained from the reaction of coumarin 27 with a Grignard reagent.48 This method has been known for a long time and has not been modified much. The parent chromene 28 has been prepared by reduction and dehydration of 4-chromanone.63 Elimin-... 

Metal alkyls can be prepared in a simple manner from the main group halides (X = Cl, Br, I) and the appropriate alkyl Grignard reagent (RMgl) or the alkyllithium salt (RLi), as shown for the cadmium alkyls (Equation (2)).13 The elimination of impurities from the precursor source is of great importance, as any remaining impurities are invariably carried over into the growing semiconductor layers. Incorporation of impurities, even at levels as low as 1015 free carriers per cubic centimeter (one part in ca. 107), can drastically affect the electronic properties of the... 

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