7 Magnesium Halogen

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Myers Review: Magnesium-Halogen Exchange Chem 215 Knochel, P.; Dohle, W.; Gommermann, N.; Kneisel, F. F.; Kopp, F.; Korn, T.; Sapountzis, I.; Ahn Vu, V. Angew. Chem., Int. Ed. Engl. 2003, 42, 4302. General References on the Preparation and Reactions of Grignard Reagents: ã Unlike many lithium-halogen exchange protocols, only one equivalent of i-PrMgX is used in typical experimental procedures. ã THF is the most common solvent. Ethyl ether has been employed as a solvent for selective exchange
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  Myers  ã Analogous to lithium-halogen exchange. The position of the equilibrium varies with thestabilities of the carbanion intermediates involved (sp >> sp 2  >> sp 3 ).ã Although the first example was reported in 1931 (above), the preparation of Grignardreagents via metal-halogen exchange has not been widely used until recently. Knochel andcoworkers have demonstrated the functional-group tolerance of magnesium-halogenexchange, which is now the method of choice for the preparation of highly functionalizedorganomagnesium reagents. Magnesium-Halogen Exchange Chem 215  Jason Brubaker Review: Knochel, P.; Dohle, W.; Gommermann, N.; Kneisel, F. F.; Kopp, F.; Korn, T.; Sapountzis, I.;Ahn Vu, V. Angew. Chem., Int. Ed. Engl. 2003 , 42  , 4302. General References on the Preparation and Reactions of Grignard Reagents: Main Group Metals in Organic Synthesis  , Yamamoto, H., Oshima, K., Eds.; John Wiley andSons: New York, 2004 . Handbook of Grignard Reagents, Silverman, G. S., Rakita, P. E., Eds.; Marcel Dekker:New York, 1996 . Organomagnesium Methods in Organic Synthesis, Wakefield, B. J.; Academic Press:San Diego, 1995 . Development and General Aspects: RX + R'MgX' RMgX' + R'XBr+EtMgBrEt 2 O 20 °C, 12 h MgBr+EtBr Prévost, C. Bull. Soc. Chim. Fr. 1931 , 49  , 1372.ã i-  PrMgCl or i-  PrMgBr are the most common reagents. In most cases, these reagents can beused interchangeably. i-  PrMgBr is made by the Grignard reaction of isopropyl bromideand magnesium turnings. It is less soluble than the chloride (solutions are ~0.8 M), and thetitre must be checked more often. i-  PrMgCl is commercially available as a 2.0 M solutionin THF or diethyl ether.ã The reactivity of Grignard reagents is highly temperature dependant. Only highly reactiveelectrophiles such as aldehydes and ketones react at significant rates below 0 °C. Thisallows for the preparation of organomagnesium reagents containing cyano, nitro, ester,and imine functional groups, provided that the rate of the exchange reaction is fast enoughto allow for exchange at temperatures below 0 °C.ã Unlike many lithium-halogen exchange protocols, only one equivalent of i-  PrMgX is usedin typical experimental procedures. IOCH 3 i  -PrMgBrTHF, 25 °C, 1 hMgBrOCH 3 ICO 2 CH 3 i  -PrMgBrTHF, –20 °C, 30 minMgBrCO 2 CH 3 Jensen, A. E.; Dohle, W.; Sapountzis, I.; Lindsay, D. M.; Ahn Vu, V.; Knochel, P. Synthesis    2002 , 565.Cali, P.; Begtrup, M. Synthesis  2002 , 63.ã Solutions of i-  PrMgX are titrated by the method of Paquette (Lin, H.-S.; Paquette, L. A. Synth.Commun.   1994 , 24  , 2503.) According to this procedure, a flame-dried flask is charged withmenthol (a non-hygroscopic solid), 1,10-phenanthroline (indicator) and THF. The Grignardreagent is then added until a distinct violet or burgundy color persists.ã The rate of magnesium-halogen exchange is accelerated by electron-withdrawing groupson the aromatic ring, and is slowed by electron-donating groups:ã THF is the most common solvent. Ethyl ether has been employed as a solvent forselective exchange of geminal dihalides to generate magnesium carbenoids. INO 2 NO 2 MgClNO 2 NO 2 PhMgClTHF, –40 °C, 30 s Sapountzis, I.; Knochel, P. Angew. Chem., Int. Ed. Engl.   2002 , 41 , 1610.
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