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Preparation of functionalized aryl iron(II) compounds and a nickel-catalyzed cross-coupling with alkyl halides.

Iron-catalyzed cross-coupling reactions of organometallic reagents have found numerous applications in organic synthesis. Although this method has found wide acceptance, the mechanism of the cross-coupling is still not completely elucidated. This goal may be achieved by preparing stoichiometric amounts of organometallic Fe compounds and studying their stability and reactivity towards electrophiles. Although aryl Fe derivatives can be prepared by transmetalation or by direct ferration using a tetramethylethylenediamine(tmeda)-stabilized sodium ferrate base, no iron base has been reported that allows for the general preparation of organometallic aryl Fe compounds. Building on our previous reports on LiCl-solubilized Mg, Zn, Al, and Mn tmp bases (tmp = 2,2,6,6-tetramethylpiperidyl), we now report a new convenient iron(II) base tmp2Fe·2 MgCl2·4 LiCl (1), which allows room-temperature ferration of a broad range of functionalized arenes 2 to diaryl Fe derivatives 3. A subsequent efficient cross-coupling with alkyl iodides (or bromides) and benzylic chlorides promoted by 4-fluorostyrene (4) resulted in the formation of polyfunctional arenes 5 in 60–88 % yield (Scheme 1). The importance of metallic impurities in FeCl2·2 LiCl was examined, and such impurities were found to catalyze this crosscoupling reaction. Thus, the reaction of tmpMgCl·LiCl (2 equiv) with FeCl2·2 LiCl [12] (1 equiv) at 0 8C and further stirring at 25 8C for 3 h produces quantitatively tmp2Fe·2MgCl2·4 LiCl (1). This base has excellent solubility in THF (0.5m) and can be stored in solution without decomposition for at least eight weeks at 25 8C. The reaction of 1 with ethyl-3-fluorobenzoate (2a ; 25 8C, 3 h) leads to the corresponding diaryl Fe species 3a, which reacted smoothly with 1-iodooctane (1.2 equiv) to provide the 1,2,3-trisubstituted benzoate 5a in 86% yield. The reaction time was 14 h, but by adding 4-fluorostyrene (4 ; 10 mol%), the cross-coupling was complete within 7 h at 25 8C (88 % yield; Table 1, entry 1). 4-Fluorostyrene is known to promote Ni-catalyzed cross-coupling reactions. It is assumed that it accelerates the reductive elimination step through a coordination of the electron-poor olefin to the metal center.