Lithium-halogen exchange conditions

In an analogous case (7.6.) the 9-borabicyclo[3.3.1]nonane (BBN) unit was introduced onto the quinoline core through lithium-halogen exchange and the quenching of the intermediate with 9-BBN-OMe. The quinolylborane coupled with 3-bromopyridine under the earlier mentioned conditions (THF, , TBAB) to give the pyridylquinoline in acceptable yield.8... 

The neurotoxic quaterpyridine natural product nemertelline was successfully synthesized by S. Rault et al. using a Suzuki cross-coupiing as the key step. The boronic acid coupling partner, required for the Suzuki reaction, was prepared by first subjecting 3-amino-2-chloropyridine to the conditions of the Sandmeyer reaction followed by a lithium-halogen exchange and trapping the lithiopyridine derivative with triisopropylborate. 

Most lithium halocarbenoids decompose at temperatures above — 80 to — 90 °C. An essential condition for a successful synthesis via a (preformed) lithium halocarbenoid is that its generation proceeds at a high rate at temperatures below this range. For this reason, the more polar THF is preferred to Et20 as a main solvent for the deprotonation or lithium-halogen exchange reactions by which the carbenoids are formed. 

Likewise, we have synthesized another key core segment based on a p-polyphenylene conducting unit which fits the general electronic architectural requirements. Conversion of 2-aminobiphenyl to the corresponding iodide under Sandmeyer conditions followed by lithium halogen exchange and quenching with fluorenone afforded the alcohol 11. Acid treatment to close the spiro system followed by reaction with bromine... 

Even if not impaired by side reactions, the condensation of lithium, sodium, or potassium acetylides with alkyl halides is only moderately fast in ethereal media unless alkyl iodides or sulfonates are applied. Good results can be obtained in liquid ammonia. To speed up the reaction with primary alkyl bromides in THF, they may be performed in the presence of 10% of sodium iodide, in other words, under halogen/halogen exchange conditions. Polar solvents such as dimethylformamide (DMF) are mandatory with alkyl bromides and, a fortiori, chlorides. ... 

A synthetic method for the generation of trifluoropro-penyl lithium via lithium-halogen exchange reaction of 2-BrTFP has been first developed by Kobayashi et al. Later, Ichikawa et al. reinvestigated the reaction conditions in detail (Scheme 26.46). Thus, treatment of 2-BrTFP with... 

Aryl triflates have been used to generate arynes via other routes than metal-halogen exchange. For example, fluoride ion displacement of the trimethylsilyl group in 26 provides a convenient route to benzyne under mild conditions. The required benzyne precursor 26 was prepared from o-trimethylsilylphenoxytrimethylsilane by sequential treatment with BuLi and (Tf)20. Analogous aryne precursors have also been prepared less directly, as in the case of amide benzyne 31. Examples of 32 include various methylfurans, diphenylsobenzofuran and tetraphenylcyclopentadienone. Examples of nucleophiles include methanol, phenol, lithium thiophenoxide and others. 

Treatment of dibromoalkene with a lithium base (BuLi or LDA) generates a bromoalkyne intermediate via dehydrohalogenation, which undergoes metal-halogen exchange under the reaction conditions and yields the terminal alkyne upon workup. 

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