Grignard Reagents and Organic Halides

Propylene sulfide can be resolved more efficiently by using a mixture of diethylzinc and (S)-binaphthol (179). The polymerization affords, at 67% conversion, unreacted monomer, [a]D25 +51.8°, in 92% ee, together with optically active polymer, [a]D25 —103°. 

Ring-opening polymerization of racemic A-carboxy amino acid anhydrides can be achieved with a chirally modified aluminum alkoxide (180). 

Racemic 1-phenylethyl methacrylate is resolved efficiently by a cy-clohexylmagnesium chloride-(—)-sparteine complex to give, at 70% conversion, optically active polymer and the unreacted monomer in greater than 90% ee (181). Similarly, reaction of racemic phenyl-2-pyridyl-o-tolylmethyl methacrylate in the presence of 4-fluorenyllithium and ( + )- or ( — )-2,3-dimethoxy-l,4-bis(dimethylamino)butane proceeds with a high degree of kinetic resolution (182). 

SCHEME 76. Mechanism of Ni-catalyzed Grignard coupling reaction. 

SCHEME 79. Asymmetric cross-coupling using allyl esters and ethers. 

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Metal-halogen exchange between Grignard reagents and organic halides... 

Similar to nickel catalysts, palladium complexes that have a chiral phosphine ligand enable asymmetric-coupling reactions of secondary-alkyl Grignard reagents and organic halides [246,256,257]. A study giving excellent e.e.s was selected [Eq. (173) 408] from relevant works [359,363,369]. 

Metal-halogen exchange between Grignard reagents and organic halides is an alternative useful method for the preparation of organomagnesium compounds (Scheme 3.30). We will discuss this method in detail in Section 3.4. 

Selective cross-coupling reactions between C(sp ) and C(5p ) centers had been one of the most difficult tasks in carbon-carbon bond synthesis until the early 1970s, when it was first reported that iron, nickel, palladium and copper - catalysts are extremely effective for cross-coupling of Grignard reagents with organic halides. Now, nearly 20 years later, transition metal catalyz cross-coupling has become the reaction of first choice for this purpose. 

Similar reactions between Grignard reagents and organotin halides give tetraorgano-tin compounds with mixed organic groups. This is illustrated by the preparation of (methyl)(ethyl)(isopropyl)(cyclohexyl)tin 12... 

Table 15.1 Synthesis of Grignard Reagents from Organic Halides and Magnesium Metal...

Two chapters primarily concern Grignard reagent formation. Garsi and Ihtgvaty consider in detail mechanisms lor the formation of Grignard reagents from organic halides Rasion describes use of the Mg-... 

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