Orbital interactions stereoelectronic effect

The carbonyl n orbital is also assumed to be unsymmetrized arising from the out-of-phase interaction of the orbital attached to the more electron-donating aryl group (9 and 10). These unsymmetrizations of the carbonyl k orbital correspond well to syn addition (9) and anti addition (10), respectively. Thus, the electron-donation of the p-a orbitals controls the facial selectivities. The cyclopentane system was more sensitive to stereoelectronic effects, showing larger induced biases, than the adamantanone system [63]. 

FIGURE 7.13 Molecular model of the pyrrolo [ 1,2-<2] i ndol e showing the site of nucleophile attack that provides a favorable stereoelectronic effect. The inset shows expected orbital interactions. 

The assistance of some electron-rich groups [94-96], heteroatoms [90, 97, 98] or filled cr-orbitals [99,100] in the oxidation of organic sulfides was studied in a series of papers (Scheme 24). Since the 3p orbitals of sulfur are quite compact compared to those of Se, the mentioned electronic and stereoelectronic effects are rather remarkable. These interactions (discussed in terms of a trans-annular... 

This model is rationalized by a combination of steric and stereoelectronic effects. From a purely steric standpoint, an approach from the direction of the smallest substituent, involving minimal steric interaction with the groups L and M, is favorable. The stereoelectronic effect involves the interaction between the approaching hydride ion and the LUMO of the carbonyl group. This orbital, which accepts the electrons of the mcommg... 

Dioxin prefers the half-chair conformer 44 the corresponding boat and planar conformers proved to be less stable by 10.6 and 12.3 kcalmoP, respectively. If 4//-1,3-dioxin is alkyl-substituted at position 2, the same level of theory indicated the equatorial conformer 45-eq to be more stable than its axial counterpart, 45-ax (Me, 2.95 kcal moP Et, 2.89 kcal mol iso-Vi, 2.97 kcal moP neo-Vc,n, 2.16kcal moP and SiMc3, 4.45 kcal moP ) since the dipole moments of the two conformers are nearly equivalent, the position of the conformational equilibria of 2-alkyl-substituted 4//-1,3-dioxins is influenced by both steric effects (synclinal and H,H-, H,0-nonbonding interactions due to short C-O bonds) and a number of stereoelectronic orbital interactions. 

In contrast, the mannosyl radical 8 does not undergo such a conformational change, and the observed a-attack results from the shielding effect of the axial C-2 substituent in the chair conformation and the stereoelectronic effects mentioned earlier. In radicals 7b and 8 the C-O bonds adjacent to the radical center are coplanar with the singly occupied orbital. This reminds us of the anomeric effect in which an interaction between the nonbonding electron pair of the ring oxygen and the LUMO of the C-O bond stabilizes the conformation. 

The necessity for reacting groups of substrates to collide with an orientation that allows productive interaction of electronic orbitals is often called a stereoelectronic effect. An example is the addition reaction of Eq. 9-74. Tire orbital of an unshared pair of electrons on the HO- ion must be perpendicular to the plane of the double bond. Furthermore, if the proton becomes attached to the adjacent carbon in a synchronous or concerted manner it must enter from the opposite side, as it does in Eq. 9-74. 

The 1,6-addition to a,s, y, a-dienones is also subject to stereoelectronic effects. Addition on the bottom face of dienone 137 leads to a chair-like intermediate while that on the top face leads to a boat-like intermediate 140 in order to maintain maximum orbital overlap. Also, in 140 the R group encounters an eclipsed 1,2-R/H interaction and more importantly, a 1.4-CH3/R steric interaction which resembles the bowsprit flagpole arrangement of a twist-boat form of cyclohexane. This analysis of Marshall and Roebke (48) predicts that the trans product 139 should prevail over the cis product 141. 

Generally, the reactions are carried out in refluxing benzene solution, since the yield in benzene is better than that in other solvents. Probably, the radicals formed may be somewhat stabilized by the weak orbital-orbital interaction between the radicals and benzene. However, from the environmental point of view, toluene or dioxane is recently used. As substrates, alkyl bromides or alkyl iodides are used, and the reactivity increases in the order prim-alkyl < seoalkyl < te/t-alkyl. Sugar anomeric bromide (3) is generally not so stable, so the reaction is carried out under irradiation conditions with a mercury lamp at room temperature (eqs. 4.2 and 4.3). There are two types of anomeric glycosyl radicals as shown in Figure 4.1. One is the axial radical [I], and the other is the equatorial radical [I ]. The axial radical is more nucleophilic than the equatorial radical due to the stereoelectronic effect, where this effect comes... 

As you will probably realize, it s not only in six-membered rings that stereoelectronic interactions between filled and unfilled orbitals stabilize some conformations more than others. Stereoelectronic effects control the conformations of many types of molecules. We shall look at three common compounds that.are stabilized by stereoelectronic effects in two cases, the stabilization is specific to one conformation, and we can use stereoele ronics to explain what would otherwise be an unexpected result. 

Geometric constraints imposed by through-space interactions between two orbitals on the same molecule (stereoelectronic effects). 

Denmark argued that the synclinal transition state 19 may be favored due to stabilization by stereoelectronic effects such as secondary orbital overlap or minimization of charge separation. The allylstannane HOMO and the aldehyde LUMO could participate in. secondary orbital overlap in transition state 19, with specific-interactions between the allylstannane a-carbon and the aldehyde oxygen [50, 55]. Alternatively, the preference for the synclinal transition state 19 can also be attributed to minimization of charge separation in the transition state, compared to the situation in the antiperiplanar transition state 20 [50, 56],... 

The description and understanding of the nature of stereoelectronic effects is an appropriate held for the application of oiganic quantum chemistry. Molecular orbital (MO) methods " can describe the electron distribution in molecules, and the changes in internal rotation. In principle, they give the total potential energy of individual conformers completely, without the necessity to correct for various effects. Quantum chemical calculations offer a deeper insight into the orbital interactions in the molecule, and reveal the factors responsible for the stabilization of any conformation. 

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