Donor stereoelectronic effects

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

The strong stereoelectronic effect of vicinal nN donors is also evident in the product guanidinium cation,... 

It has however been suggested by Cieplak (9) that the stereochemistry of nucleophilic addition to cyclohexanone is determined by a combination of steric and stereoelectronic effects. According to this interesting model, steric hindrance favors the equatorial approach while electron donation favors the axial approach. The stereoelectronic effect favors the axial approach because the axial C —H bonds next to the carbonyl group (C — Ha and Cn-Ha) are better electron donors than the Cn-C-j and Cc-Cfi a bonds (cf 7A 7 and 7A-8). J... 

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. 

The stereoselectivity of the second and key Michael-type conjugate addition reaction can be rationalized as follows. The conformation of 63 will be restricted to 63-A due to A(l 3) strain between the N-methoxycarbonyl and w-propyl groups in 63-B. Attack of the vinyl anion from the stereoelectronically favored a-axial direction provides the adduct 64 exclusively. It is noteworthy that the stereochemical course of the above reaction is controlled by the stereoelectronic effect in spite of severe 1,3-diaxial steric repulsion between the axial ethyl group at the 5-position and the incoming vinyl anion. This remarkable stereoselectivity can be also explained by Cieplak s hypothesis[31]. On the preferred conformation 63-A, the developing a of the transition state is stabilized by the antiperiplanar donor Gc-h at the C-4 position. 

In the explanation favored today, the reason for this stereoelectronic effect is as follows The electronically preferred direction of attack of a hydride donor on the 0=0 double bond of cyclohexanone is the direction in which two of the C—H bonds at the neighboring a positions are exactly opposite the trajectory of the approaching nucleophile. Only the axial C—H bonds in the a positions can be in such an antiperiplanar position while the equatorial C—H bonds cannot. Moreover, these axial C—H bonds are antiperiplanar with regard to the trajectory of the H nucleophile only if the nucleophile attacks via a transition state B, that is, axially (what was to be shown). The antiperiplanarity of the two axial C—H bonds in the a positions is reminiscent of the antiperiplanarity of the electron-withdrawing group in the a position relative to the nucleophile in the Felkin-Anh transition state (formula C in Fig. 8.8 cf. Fig. 8.11, middle row). 

In the attack by sterically undemanding reducing agents, this stereoelectronic effect is fully effective (for a completely different but perfectly diastereoselective reduction of 4-fert-butylcyclohexanone to the equatorial alcohol, see Figure 14.46). However, in the attack of such a bulky hydride donor as L-Selectride the stereoelectronic effect is overcompensated by the opposing steric effect discussed above. 

For the E(O) enolate, the stereoselection is best understood in terms of stereoelectronic effect. The most stable conformation of the 2,3-allylic bond of an acyclic enolate corresponds to the smallest substituent eclipsing the double bond whatever the geometry of the enolate (A1,3 strain)459. Then, in the transition state, the maximum overlap of the better ally lie a-donor and the n -orbital of the enolate directs the antiperiplanar attack of the electrophile (Scheme 95). 

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. 

In 12a, the empty orbital is antiperiplanar to crC2 ri and empty orbital is antiperiplanar to crC2 ca and crC5 ce bonds. Since the CTc h bond is a better electron-donor than the crc c bond, the principle of stereoelectronic effect dictates that 12a is of lower energy than 12b. The electron-rich nucleophile (a Lewis base) is expected to be electrostatistically drawn toward the empty orbital (a Lewis acid) on the axial face in 12a, leading to the formation of the equatorial... 

The fluorescence quenching dynamics of excited state electron donors by various pyrimidine and 5,6-dihydropyrimidine substrates have been examined and found to obey the Rehm-Weller relationship." In addition, an unexpected difference was observed between the reduction potentials for the trans-syn and cis-syn diastereoisomers of dimethylthymine cyclobutane dimers, and this has been ascribed to a stereoelectronic effect in the cis-syn dimer anion radical resulting from an unfavourable charge-dipole interaction between the added electron and the O carbonyl group of the pyrimidine ring... 

---