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Alkali-metal-mediated manganation(ii) of functionalized arenes and applications of ortho-manganated products in pd-catalyzed cross-coupling reactions with iodobenzene

Blair, V.L. and Clegg, W. and Conway, B. and Hevia, E. and Kennedy, A. and Klett, J. and Mulvey, R.E. and Russo, L. (2008) Alkali-metal-mediated manganation(ii) of functionalized arenes and applications of ortho-manganated products in pd-catalyzed cross-coupling reactions with iodobenzene. Chemistry - A European Journal, 14 (1). pp. 65-72. ISSN 0947-6539

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Abstract

Extending the recently introduced concept of alkali-metal-mediated manganation to functionalised arenes, the heteroleptic sodium manganate reagent [(tmeda)Na(tmp)(R)Mn(tmp)] (1; TMEDA=N,N,N,N-tetramethylethylenediamine, TMP=2,2,6,6-tetramethylpiperidide, R=CH2SiMe3) has been treated with anisole or N,N-diisopropylbenzamide in a 1:1 stoichiometry in hexane. These reactions afforded the crystalline products [(tmeda)Na(tmp)(o-C6H4OMe)Mn(tmp)] (2) and [(tmeda)Na(tmp){o-{C(O)N(iPr)2C6H4}Mn(CH2SiMe3)] (3), respectively, as determined from X-ray crystallographic studies. On the basis of these products, it can be surmised that reagent 1 has acted, at least partially and ultimately, as an alkyl base in the first reaction liberating the silane Me4Si, but as an amido base in the second reaction liberating the amine TMPH. Both of these paramagnetic products 2 and 3 have contacted ion-pair structures, the key features of which are six-atom, five-element (NaNMnCCO) and seven-atom, five-element (NaNMnCCCO) rings, respectively. Manganates 2 and 3 were successfully cross-coupled with iodobenzene under [PdCl2(dppf)] (dppf=1,1-bis(diphenylphosphino)ferrocene) catalysis to generate unsymmetrical biaryl compounds in yields of 98.0 and 66.2 %, respectively. Emphasizing the importance of alkali-metal mediation in these manganation reactions, the bisalkyl Mn reagent on its own fails to metalate the said benzamide, but instead produces the monomeric, donor-acceptor complex [Mn(R)2{(iPr)2NC(Ph)(O)}2] (5), which has also been crystallographically characterised. During one attempt to repeat the synthesis of 2, the butoxide-contaminated complex [{(tmeda)Na(R)(OBu)(o-C6H4OMe)Mn}2] (6) was obtained. In contrast to 2 and 3, due to reduced steric constraints, this complex adopts a dimeric arrangement in the crystal, the centrepiece of which is a twelve atom (NaOCCMnC)2 ring.