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''Meta''-selective C-H functionalization refers to the regioselective reaction of a substituted aromatic ring on the C-H bond ''meta'' to the substituent. Substituted aromatic ring is an important type of substructure in pharmaceuticals and industrial compounds. Thus, synthetic methods towards substituted aromatic rings are always of great interest to chemists. Traditionally, regioselectivity on the aromatic ring is achieved by the electronic effect of substituents. Taking the well-known Friedel-Craft electrophilic aromatic substitution as example, electron donating groups direct the electrophile to ''ortho-/para-''position while electron withdrawing groups direct the electrophile to ''meta''-position. However, with complicated systems, electronic difference between different C-H bonds can be subtle and electronic directing effect alone could become less synthetically useful. The fast development of C-H activation in the past few decades provides synthetic chemists with the powerful tools to synthesize functionalized aromatic compounds with high selectivity. The widely used approach to achieve ''ortho''-selectivity involves metal-chelating directing groups, which forms a relatively stable 6- or 7-membered cyclic pre-transition state to bring the metal catalyst to the proximity of the ''ortho-''hydrogen.〔Hartung, C. G.; Snieckus, V. in Modern Arene Chemistry (ed. Astruc, D.) 330–367 (Wiley-VCH, 2004).〕〔Colby, D. A., Bergman, R. G.&Ellman, J. A. Chem. Rev. 2010, 110, 624–655.〕〔Lyons, T. W. & Sanford, M. S. Chem. Rev. 2010, 110, 1147–1169.〕〔Jun, C. H.; Moon, C. W.; Lee, D-Y. Chem. Eur. J. 2002, 8 2422 – 2428.〕〔Engle, K. M.; Mei, T-S.; Wasa, M.; Yu, J-Q. Acc. Chem. Res., 2012, 45, 788–802.〕 However, applying the same strategy to ''meta''- or ''para''- C-H functionalization does not work because the corresponding cyclophane-like cyclic pre-transition state is highly strained.〔Leow, D.; Li, G.; Mei, T-S.; Yu, J-Q. Nature 2012, 486, 518.〕 Thus, while ''ortho''-selectivity has been achieved by numerous catalytic systems, ''meta''- and ''para''-selectivity remains a challenge. In recent years, new strategies that override the electronic and steric bias have been developed to address ''meta''-C-H functionalization. However, before these discoveries, synthesis of ''meta''-substituted aromatic compounds could be either limited or cumbersome. For example, before the development of the C-H activation-involving one-pot synthetic route to ''meta''-substituted phenol derivatives by Maleczka and co-workers, the traditional synthesis requires 10 steps from TNT.〔Boele, M. D. K.; van Strijdonck, G. P. F.; de Vries, A. H. M.; Kamer, P. C. J.; de Vries, J. G.; van Leeuwen, P. W. N. M. J. Am. Chem. Soc. 2002, 124, 1586.〕〔Hodgson, H. H.; Wignall, J. S. J. Chem. Soc. 1926, 129, 2077.〕〔Prakash Kumar Shee, Meta-Selective C-H Bond Activation. Department of Chemistry, Michigan State University, November 27, 2013, https://www2.chemistry.msu.edu/Courses/CEM958/FS13_SS14/Prakash_Kumar_Shee.pdf retrieved Nov 27th, 2014〕 Some early attempts utilize steric and electronic effects to achieve ''meta''-selectivity.〔Cho, J.-Y.; Tse, M. K.; Holmes, D.; Maleczka, R. E., Jr.; Smith, M. R., III Science 2002, 295, 305.〕〔Murphy, J. M.; Liao, X.; Hartwig, J. F. J. Am. Chem. Soc. 2007, 129, 15434.〕〔Zhang, Y-H.; Shi, B-F.; Yu, J-Q. J. Am. Chem. Soc. 2009, 131, 5072 - 5074.〕 However, they are either limited to certain structure of substrates or are not highly selective. In recent years, several highly selective ''meta''-C-H functionalization strategies have been reported which can override the intrinsic electronic and steric properties of the substrates and can apply to a wide range of substrate derivatives. The development of the modern ''meta''-C-H functionalization strategies “open doors for numerous possibilities” for synthesis and catalyst development.〔Zhou, Y.; Zhao, J.; Liu, L. Angew. Chem. Int. Ed. 2009, 48, 7126 - 7128.〕 ==Recently-developed ''meta''-selective C-H activation strategies== 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Meta-selective C-H fuctionalization」の詳細全文を読む スポンサード リンク
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