Halogen-magnesium exchanges, lithium

Preparation of some azulenylmagnesium species was achieved by the halogen-magnesium exchange reactions of iodoazulenes with lithium tributylmagnesate at low temperatures (equations 29-33) . The reactions offer access to a variety of functionalized azulenes including azulenylphosphine, -stannane and -boronic ester. 

Convenient alternatives to direct deprotonation of ethers are tin-lithium exchange [199, 258-261], halogen-magnesium exchange [262], or reductive cleavage of 0,Se-acetals [263, 264], Another synthetic equivalent of a-metalated ethers are (alkoxymethyl)phosphonium salts [265]. 

Acrylic acid derivatives with a heteroatom (N, O, S, Hal) at C-3 can be cleanly deprotonated at this position with BuLi or LDA at low temperatures [329, 333-338] (Scheme 5.37). Some of these anions rearrange to the a-metalated acrylates on warming [329], but can also decompose (see Section5.4.7). Non-heteroatom-substi-tuted lithium /3-lithioacrylate [329] or /3-magnesioacrylic acid derivatives [339, 340] have been prepared by bromine-lithium or halogen-magnesium exchange. 

One alternative to the halogen-lithium exchange that has been employed in a Parham cyclization is halogen-magnesium exchange, as exemplified by the synthesis of 5-substituted 2,3-dihydrobenzofurans 05)- The use of n-butyllithium is less successful and leads to butylated by products. Lithium tributylmagnesate has also been employed, e.g., 16- 17. ... 

Halogen-Magnesium Exchange Reaction of Alkenyl Iodides. The reaction of alkenyl iodides with isopropylmagnesium chloride-lithium chloride is one of the best ways to prepare functionalized alkenylmagnesium reagents. This reaction is... 

Scheme 2-21. Halogen-magnesium exchange reactions using lithium trialkyl...

The functionalized arylzinc reagents are best prepared either starting from an aryllithium obtained by halogen-lithium exchange followed by a low-temperature (-80°C) transmetalation with ZnBrj or by performing an iodine-magnesium exchange reaction. The latter reaction tolerates temperatures up to -30°C and is more convenient for industrial applications. ... 

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