Stereoelectronic effect

Take the simplest CH2X–CH3 system as an example; the donor orbital is σ(C–H) orbital and the acceptor is σ*(C–X). When moving from fluorine to chlorine, then to bromine, the electronegativity of the halogen and the energy level of the σ*(C–X) orbitals decreases. Consequently, the general trend of acceptors can be summarized as: π*(C=O)>σ*(C–Hal)>σ*(C–O)>σ*(C–N)>σ*(C–C), σ*(C–H). For donating orbitals, the nonbonding orbitals, or the lone pairs, are generally more effective than bonding orbitals due to the high energy levels. Also, different from acceptors, donor orbitals require less polarized bonds. Thus, the general trends for donor orbitals would be: n(N)>n(O)>σ(C–C), σ(C–H)>σ(C–N)>σ(C–O)>σ(C–S)>σ(C–Hal). Stereoelectronic effect can be directional in specific cases. The radius of sulfur is much larger than the radius of carbon and oxygen. Thus the differences in C–S bond distances generate a much-amplified difference in the two stereoelectronic effects in 1,3-dithiane (σ(C–H) → σ*(C–S)) than in 1,3-dioxane(σ(C–H) → σ*(C–O)). The differences between C–C and C–S bonds shown below causes a significant difference in the distances between C–S and two C–H bonds. The shorter the difference is, the better the interaction and the stronger the stereoelectronic effect. ==Influence on stability==

If there is an electropositive substituent (e.g. –SiR3, –SnR3, –HgR, etc.) at the β-position of carbocation, the positive charge could be stabilized which is also due largely to the stereoelectronic effect (illustrated below using –SiR3 as an example). The orientation of the two interacting orbitals can have a significant effect on the stabilization effect (σ(C–Si) → empty p orbital), where antiperiplanar (180°) > perpendicular (90°) > syn (0°). ==Influence on conformation==

Gauche effect One structural consequence of acyclic systems due to the stereoelectronic effect is the gauche effect. In 1,2-difluoroethane, despite the steric clash, the preferred conformation is the gauche one because σ(C–H) is a good donor and σ*(C–F) is a good acceptor and the stereoelectronic effect (σ(C–H) → σ*(C–F)) requires the energy minimum to be gauche instead of anti. This gauche effect and its impact on conformation are important in biochemistry. For example, in HIF-α subunit fragments containing (2S,4R)-4-hydroxyproline, the gauche interaction favors the conformer that can bind to the active site of pVHL. pVHL mediates the proteasomal degradation of HIF1A and with that the physiological response to hypoxia. Special effects of fluorine substituent Stereoelectronic effects can have a significant influence in pharmaceutical research. Generally, the substitution of hydrogen by fluorine could be regarded as a way to tune both the hydrophobicity and the metabolic stability of a drug candidate. Moreover, it can have a profound influence on conformations, often due to stereoelectronic effects, in addition to normal steric effects resulting from the larger size of the fluorine atom. For instance, the ground state geometries of anisole (methoxybenzene) and (trifluoromethoxy)benzene differ dramatically. In anisole, the methyl group prefers to be coplanar with the phenyl group, while (trifluoromethoxy)benzene favors a geometry in which the [C(aryl)–C(aryl)–O–C(F3)] dihedral angle is around 90°. In other words, the O–CF3 bond is perpendicular to the plane of the phenyl group. Further studies illustrate that even for only one or two hydrogen atoms in a methyl group being replaced by a fluorine atom, the distortion in the structure can also be significant, with the [C(aryl)–C(aryl)–O–C(H2F)] dihedral angle in the energy minimized structure being around 24° and the [C(aryl)–C(aryl)–O–C(HF2)] dihedral angle 33°. ==Influence on reaction selectivity==

Reductive cyclizations Although the energy difference between coplanar anisole and its isomer is quite large, the rotation between the O–CH3 bond becomes favorable when the electronic properties of methoxy group on aromatic rings need to be altered to stabilize an unusual intermediate or a transition state. In the following reaction, the regioselectivity could be rationalized as the out-of-plane rotation of the O–C bond which changes the methoxy group from an in-plane donor group to an out-of-plane acceptor group. The intermediate of the above reaction is the di-anion and the stereoelectronic effect that stabilizes this intermediate over the other one is the fact that the anionic charge at the para position could delocalize to the oxygen atom via orbital interaction: π(benzene) → σ*(O–CH3). The relevant stereoelectronic interaction in the starting material is the nO → σ*(Ccarbonyl–Caryl) interaction. The electron-withdrawing substituent on the benzene ring depletes the electron density on the aromatic ring and thus makes the σ*(Ccarbonyl–Caryl(nitro)) orbital a better acceptor than σ*(Ccarbonyl–Caryl(methoxy)). These two stereoelectronic interactions use different lone pairs on the oxygen atom (the one antiperiplanar to the σ* in question for each), leading to lone pairs with different electron densities. In particular, the enhanced depletion of electron density from the lone pair antiperiplanar to the 4-nitrophenyl group leads to weakened ability for that lone pair to coordinate to boron. This in turn results in the lone pair antiperiplanar to the 4-methoxyphenyl binding preferentially to the catalyst, leading to well-defined facial selectivity. Under optimized conditions, the product is formed with excellent levels of enantioselectivity (95% ee). ==Influence on thermodynamics==

Influence on equilibrium The stereoelectronic effect influences the thermodynamics of equilibrium. For example, the following equilibrium could be achieved via a cascade of pericyclic reactions. Despite very similar structures, one of the two isomers is strongly favored over the other because of a stereoelectronic effect. Since the σ*C-C orbital adjacent to the electron-withdrawing carbonyl group is lower in energy and is therefore a better acceptor than the σ*C-C orbital adjacent to the methoxy, the isomer in which the nO(σ) lone pair is able to donate into this lower-energy antibonding orbital will be stabilized (orbital interaction illustrated). Another example of the preference in the equilibrium within the area of pericyclic reaction is shown below. The stereoelectronic effect that affect the equilibrium is the interaction between the delocalized “banana bonds” and the empty p orbital on the boron atom. Influence on resonance structures In another case, the stereoelectronic effect can result in an increased contribution of one resonance structure over another, which leads to further consequences in reactivity. For 1,4-benzoquinone monoxime, there are significant differences in the physical properties and reactivities between C2-C3 double bond and C5-C6 double bond. For instance, in the 1H NMR, 3J23 higher than 3J56. The C2-C3 double bond also selectively undergoes Diels–Alder reaction with cyclopentadiene, despite the increased steric hindrance on that side of the molecule. These data illustrate an increased contribution of resonance structure B over structure A. The authors argue that the donation from nN to σ*C4-C3 orbital lengthens the C4–C3 bond (C4 is the carbon bearing the nitrogen substituent), which reduces the p-p overlap between these two atoms. This in turn decreasing the relative importance of structure A which has a double bond between C4 and C3. ==Application in asymmetric Diels–Alder reactions==

In the asymmetric Diels–Alder reactions, instead of using chiral ligands or chiral auxiliaries to differentiate the side selectivity of the dienolphiles, the differentiation of face selectivity of the dienes (especially for cyclopentadiene derivatives) using stereoelectronic effects have been reported by Woodward since 1955. A systematic research of facial selectivity using substituted cyclopentadiene or permethylcyclopentadiene derivatives have been conducted and the results can be listed as below. The stereoelectronic effect affecting the outcome of the facial selectivity of the diene in the Diels–Alder reaction is the interaction between the σ(C(sp2)–CH3) (when σ(C(sp2)–X) is a better acceptor than a donor) or σ(C(sp2)–X) (when σ(C(sp2)–X) is a better donor than an acceptor) and the σ* orbital of the forming bond between the diene and the dienophile. The ring opening of cyclobutene under heating conditions can have two products: inward and outward rotation. The inward rotation transition state of the structure shown below is relatively favored for acceptor R substituents (e.g. NO2) but is especially disfavored by donor R substituents (e.g. NMe2). ==Stereoelectronic effect versus steric clash==

Sometimes, stereoelectronic effects can win over extreme steric clash. In a similar cyclobutene ring-opening reaction, the trimethylsilyl group, which is very bulky, still favors the inward rotation. The stereoelectronic effect, which is the interaction shown above when the acceptor orbital is the σ*(Si–CH3), appears to be a more predominant factor in determining the reaction selectivity against the steric hindrance and even wins over the penalty of the disrupted conjugation system of the product due to steric clash. Furthermore, the acceptor orbitals are not limited to the antibonding orbitals of carbon-heteroatom bonds or the empty orbitals; in the following case, the acceptor orbital is the σ*(B–O) orbital. In the six-membered ring transition state, the stereoelectronic interaction is σ(C–X) → σ*(B–O). == Stereoelectronic effects in biomolecules ==

Molecular recognition events mediated through orbital interactions are critical in a number of biological processes such as enzyme catalysis. Stabilizing interactions between proteins and carbohydrates in glycosylated proteins also exemplify the role of stereoelectronic effects in biomolecules. ==References==