Stereoelectronic effects, donor-acceptor

Stereoelectronic effect (Section 5.16) An electronic effect that depends on the spatial arrangement between the orbitals of the electron donor and acceptor. 

Whichever of the above types of interaction occurs, a strong dependence of the magnitude of the interaction upon the relative orientation of the donor and acceptor fragments is observed. The results of these interactions are often referred to as stereoelectronic effects, a description that reflects the electronic basis of these effects, and recognizes their dependence upon stereochemistry. 

An NMR study of stereoelectronic effects in dithiolane sulfoxides such as 35 has appeared <07CEJ4273> and the new donor-acceptor system 36 has been examined for possible use in solar cells. A variety of new crystallographic studies of l,3-dithiole-2-thione structures have... 

We have already seen in Section 2.2.3.3 how conformation can be affected by anomeric interactions, which can lead electronegative substituents to be axial at the 2-position in tetrahydropyranyl rings, and sometimes cause a chain of atoms to adopt a seemingly more hindered gauche conformation 2.81-2.83, 2.85 and 2.86 rather than the more usual zigzag arrangement. Similar hyperconjugative interactions in neutral molecules between two a bonds, one a a donor and the other a a acceptor, can lead them to adopt conformations in which the stereoelectronic effect overrides the purely steric effect. 

The effects of directionality and symmetry are intertwined in stereoelectronic effects. A particularly simple, yet surprisingly powerful generalization - often referred to as the main stereoelectronic rule - is the pronounced preference for antiperiplanarity of the vicinal donor and acceptor orbitals (Figure 5.3). The rule can be expressed in the following way There is a stereoelectronic preference for conformations in which the best donor lone pair or bond is antiperiplanar to the best acceptor bond. ... 

This chapter outlined several fundamental factors that control the magnitude and importance of stereoelectronic effects. In the following chapters, we will discuss what the consequences of donor-acceptor orbital interactions are. We will focus on the two questions of general importance for organic chemists stability and reactivity. In the very next chapter, we will provide examples of conformational effects controlled by vicinal stereoelectronic interactions. We will illustrate that such effects often provide the key electronic stabilization that is responsible for the preferred conformational profiles and explains the shapes of many key organic functional groups. 

Stereoelectronic Effects with Donor and Acceptor Separated by a Single Bond Bridge The Broad Spectrum of Orbital Contributions to Conformational Analysis... 

This chapter outlines the variety of common stereoelectronic effects where donor and acceptor orbitals are separated by a single bond. With a few exceptions, the sigma bridge will not be directly considered as a contributor to the hyperconjugative interactions. It will be treated as simply a means for keeping the interacting vicinal orbitals in the parallel sidewise arrangement. 

Heteroatoms lone pairs and polar bonds The stereoelectronic component of conformational effects in the previous section was largely based on the relatively subtle differences between the donor and acceptor abilities of C-H and C-C bonds. Heteroatoms introduce a new array of stereoelectronic effects associated with the presence of polar bonds and lone pairs. These effects can be significantly stronger than the above C-H and C-C interactions, especially when lone pairs (good donors) and (good acceptors) can directly interact with each other. 

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