Stereoelectronic effects definition

Triorganosilyl groups can control stereochemistry in organic reactions through a steric, an electronic or a stereoelectronic effect. More than one of these effects may exist simultaneously in some chemical processes. The trend listed in Section II could provide chemists with a clear guideline, yet definitive order, to choose a silyl group with appropriate size in control of reactions based on the steric effect. 

The previously described experiment of Eschenmoser p.164) shows definitely that the 6-Endo-Tet process (61) is indeed not favored. These are numerous experiments in the literature which show that all the Exo-Tet processes are allowed and the relative ease with which these processes take place is the following 55 >56 < 57 > 58 > 69. In other words, 55 is easier than 56, and jj7 is easier than 56 and 58 while 58 is better than 59. However, Stork has clearly demonstrated that when Y is part of an oxirane ring, this order is changed to 55 >56 >57 <58. Thus, there is a change between 56, 57 and 58, and the fundamental reason is due to stereoelectronic effects. 

There is one common misunderstanding that needs to be addressed early stereoelectronic is not the same as steric+electronic By definition, stereoelectronic effects aie always stabilizing, reflecting increased delocalization at favorable conformations. Repulsive steric interactions also depend on the arrangement of orbitals in space but, historically, are not included under the umbrella of stereoelectronic effects. 

Orbital interactions are complex and depend on the combination of stabilizing delocalizations, sterics, and electrostatics. Again, in the spirit of the definition of stereoelectronic effects in the introduction, we will concentrate on stabilizing effects, such as hyperconjugation and conjugation. 

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