Grignard reagents addition-elimination

The overall outcome of this reaction looks somewhat puzzling, but the reaction course, which includes successive alkylation with the Grignard reagent and elimination of magnesium thiolate, was proposed as in Scheme 3 [228]. The first oxidative addition of Ni° species to the Csp -S bond was promoted through coordination by an additional intramolecular oxygen or sulfur functional group. 

The reaction of propargylic ethers proceeds through an addition-elimination pathway, which docs not involve the coppcr(III) intermediate. The stereochemical outcome varies with the nature of the halogen component of the Grignard reagent of RMgX6Sc. If the halogen is chloride, overall syn selectivity is obtained however, anti substitution results in the case of iodide. 

The MgX of Grignard reagents can migrate to terminal positions in the presence of small amounts of TiCU." The proposed mechanism consists of metal exchange (12-33), elimination-addition, and metal exchange ... 

The reaction has been applied to nonheterocyclic aromatic compounds Benzene, naphthalene, and phenanthrene have been alkylated with alkyllithium reagents, though the usual reaction with these reagents is 12-20, and Grignard reagents have been used to alkylate naphthalene. The addition-elimination mechanism apparently applies in these cases too. A protected form of benzaldehyde (protected as the benzyl imine) has been similarly alkylated at the ortho position with butyl-lithium. ... 

Aldehydes can be obtained by reaction of Grignard reagents with triethyl orthoformate. The addition step is preceded by elimination of one of the alkoxy groups to generate an electrophilic oxonium ion. The elimination is promoted by the magnesium ion acting as a Lewis acid.93 The acetals formed by the addition are stable to the reaction conditions, but are hydrolyzed to aldehydes by aqueous acid. 

Attack of Grignard reagents on esters, e.g. (161), follows the general pathway indicated above, so that the initial product of addition/ elimination (eOR as leaving group) is a ketone (162) ... 

The morpholin-4-yl substituent in 7-position behaves similarly to the ethoxy group. Compound 481 is easily prepared by double addition of benzotriazole to acrolein followed by elimination of one of the benzotriazolyl moieties induced by treatment with NaH. Lithiation of derivative 481 followed by addition to a Schiff base results in formation of diarylpyrrole 476. Lithiated product 481 is alkylated exclusively at the carbon a, in relation to the benzotriazolyl substituent, giving intermediate 482. Subsequent treatment with a Grignard reagent leads to enamine 483 (Scheme 79) <1995TL343>. 

Finally, Cristau and coworkers have reported on a quite efficient preparation of triphenylphosphine oxide (Figure 2.13) by a similar addition-elimination reaction of red phosphorus with iodobenzene in the presence of a Lewis acid catalyst followed by oxidation of an intermediate tetraarylphosphonium salt.42 This approach holds the potential for the preparation of a variety of triarylphosphine oxides without proceeding through the normally used Grignard reagent. Of course, a variety of approaches is available for the efficient reduction of phosphine oxides and quaternary phosphonium salts to the parent phosphine, including the use of lithium aluminum hydride,43 meth-ylpolysiloxane,44 trichlorosilane,45 and hexachlorodisilane.46... 

Subsequent addition of aryl Grignard reagents then provides the secondary alcohols 161, which easily undergo elimination under the conditions shown in the scheme to provide the 1,3-disubstituted allenes 162 in excellent yields (-90%). 

The reaction sequence to the latter hydrocarbons is the most flexible one and starts from the allenic alcohols 212, which are first converted to the l,3-hexadien-5-ynes 213 by an elimination reaction the allene group is then generated by a pro-pargylic rearrangement initiated by the addition of a Grignard reagent. 

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