With my graduate school circus hopefully about to fold up tent, I will leave you with the abstract for this recent interesting Science paper which challenges a classic sophomore organic chemistry notion; that electron donating groups on benzene direct para and ortho substitution in electrophilic aromatic substitution reactions. By using a clever copper catalyst the authors manage to coax an aryl group to neatly substitute meta to an amido substituent, thus effecting a valuable C-H bond arylation.
"For over a century, chemical transformations of benzene derivatives have been guided by the high selectivity for electrophilic attack at the ortho/para positions in electron-rich substrates and at the meta position in electron-deficient molecules. We have developed a copper-catalyzed arylation reaction that, in contrast, selectively substitutes phenyl electrophiles at the aromatic carbon–hydrogen sites meta to an amido substituent. This previously elusive class of transformation is applicable to a broad range of aromatic compounds."I also want to state that I remember many in my sophomore organic class misunderstanding the facts about the effects of e-withdrawing and donating substituents. For some reason they used to think that electron donating groups activate ortho and para positions and electron withdrawing groups activate meta positions. But that's completely incorrect. The correct statement is one which I still remember from a then classic organic chemistry textbook (which sadly went out of print).
Electron donating groups on benzene activate all positions; it's just that they activate ortho and para positions more than meta. Similarly, electron withdrawing groups on benzene deactivate all positions; it's just that they deactivate para and ortho more than meta. Thus the effect of any group, whether electron donating or electron withdrawing, is greatest at the ortho and para positionsReference:
Phipps, R., & Gaunt, M. (2009). A Meta-Selective Copper-Catalyzed C-H Bond Arylation Science, 323 (5921), 1593-1597 DOI: 10.1126/science.1169975