Electronic effects, stereochemical consequences

Wolfe, S. (1972). Gauche effect. Stereochemical consequences of adjacent electron pairs and polar bonds. Accounts of Chemical Research, 5(3), 102-111. 

As to the first route, we started in 1969 (1) in investigating unconventional transition metal complexes of the 5 and 4f block elements of periodic table, e.g., actinides and lanthanides as catalysts for the polymerization of dienes (butadiene and isoprene) with an extremely high cis content. Even a small increase of cistacticity in the vicinity of 100% has an important effect on crystallization and consequently on elastomer processability and properties (2). The f-block elements have unique electronic and stereochemical characteristics and give the possibility of a participation of the f-electrons in the metal ligand bond. 

In bridged metal-metal bonded dimeric complexes, the relative importance of metal-metal and bridging ligand effects are more difficult to unravel. Dahl and his co-workers have elegantly exploited systematic crystallographic analyses to detail the stereochemical consequences of valence-electron addition or removal in dimeric metal complexes (46, 47, 65, 230) and clusters (66, 88, 204, 205, 213, 216, 222). Their experimental work has been neatly underpinned by nonparameterized approximate Hartree-Fock molecular orbital calculations (217) on the phosphido-bridged dimers [Cr2(CO)80ti-PR2)2]n"2 and [Mn2(CO)g(/i.-PR2)2]n (rt = 0, + 1, or +2) ... 

The magnesium porphyrin radical, Mg(tetraphenylporphyrin)C104 (136), has been used as a model for the structural and stereochemical consequences of loss of an electron in photosynthetic chromophores.521 The primary photosynthetic reaction in plants and bacteria consists of a transfer of an electron, in the picosecond time domain, from the chlorophyll phototrap to nearby acceptors yielding chlorophyll -cation radicals. The structure of (136) shows a five-coordinated Mg2+ cation which is not quite symmetrically sited in the porphyrin ring but has metal-ligand distances similar to those found in the previous structures. The perchlorate anion is tightly bound (Mg—O = 2.01 A) in a monodentate mode. It was concluded that the porphyrin can act as an electron sink and that no major effects are found in the bond lengths, or on the stereochemistry, of the macrocycle. 

Stereochemical consequences of the JT theorem are straightforward the higher symmetric polyhedra in electronic degenerate states should distort in order that their electronic ground state may become nondegenerate this is the static JT effect. Some possible distortions are exemplified by ... 

S. Wolfe, Acc. Chem. Res., 5, 102 (1972). The Gauche Effect. Some Stereochemical Consequences of Adjacent Electron Pairs and Polar Bonds. 

In this paper, we focus exclusive attention on the stereochemical consequences of structural modifications which effectively destroy a symmetrical electron distribution in the ligand fragment and thus cause U H rebonding of core and ligands. 

Note that we can write a similar resonance picture for esters, and we shall actually need to invoke this when we discuss enolate anions (see Section 10.7). However, electron donation from oxygen is not as effective as from the less electronegative nitrogen. We shall also see that this resonance effect in amides has other consequences, such as increased acidity of the amide hydrogens (see Section 10.7) and stereochemical aspects of peptides and proteins (see Section 13.3). In addition, the amide derivatives have... 

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