Steric effects Group 1 elements

Hellwinkel (22) ingeniously removed the problem of para bromination by starting with tris(4-methylphenyl) amine, which exclusively bromi-nates in the required three ortho positions. Unfortunately, although ortho dimetallation occurs when diphenylamine is treated with alkyl-lithium reagents, triphenylamine is lithiated in the meta positions (23) presumably due to the steric effects the triphenyls of other Group V elements cleave when reacted with LiR derivatives. 

A substituent affects reactivity in two general ways (a) by its electronic effect, that is, by its effect on the availability of electrons at the reaction center and (b) by its steric effect, that is, by its effect on crowding in the molecule. Since hydrogen is the element most commonly attached to carbon, it is used as the standard of reference. We consider a substituent G, which may be an atom or a group of atoms, to be attached to a carbon in place of a hydrogen, and we ask the question How does G C— compare with H—C-- ... 

In contrast to alkali metals or transition metals, complexation of [R2P(E)NP(E )PR 2] with main group elements is less well documented. Syntheses and structures of a number of tellurium complexes Te[R2P(E)N-P(E )PR 2]2 (E = S, Se) have been reported. It has been argued that electronic rather than steric effects control the geometry around the central tellurium atom. As examples four-coordination in (79) and two-coordination in (80). ... 

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