Cyclic Orbital Interaction Research Articles

Orbital theory for diastereoselectivity in electrophilic addition

Electrophilic attack to a double bond is often observed anti to the most electron-donating σ-bond at the α-position (hereafter, we refer to this as the extended anomeric effect). This preference is believed to result from the antiperiplanar effect between the bond that is formed between the double bond and the electrophilic reagent, and the donating vicinal σ-bond which is located on the substituent at the α-position. From an orbital viewpoint, however, it is still unclear why the approach of the electrophile anti to the substituent results in stabilization or why the frontier molecular orbital (FMO) deforms, expanding toward the reagent with this antiperiplanar interaction. We demonstrate here that cyclic orbital interaction including geminal bond participation plays an important role in the diastereoselectivity in electrophilic addition. We examined our idea using the electrophilic addition of chlorine to 3-substituted propenes as a model reaction. Our bond model approach should contribute to a better understanding of orbital mixing in FMO.

ChemInform Abstract: Orbital Phase Design of Diradicals

AbstractReview: 164 refs.

Thermal [3,3]-rearrangement of 1,1-disubstituted allyl carboxylates: lone pair participation and the geminal bond participation

The diastereoselectivity of the [3,3]-rearrangement of 1,1-disubsstituted allyl carboxylates was studied. In this heteroatom-containing system, the transition state has a boat-like transition structure (TS) because of the participation of the lone pairs and the secondary orbital interaction. Although the TS for the [1,3]-rearrangement has a far higher barrier, it does not proceed in the usual antarafacial manner due to the cyclic orbital interaction among two lone pairs of the carboxylate and the allylic lumo. In conjunction with the geminal bond participation, delocalization to the σ-bond at the Z-position shows a bonding character in the transition state of the [3,3]-rearrangement. Therefore, we predicted that a more electron-withdrawing σ-bond prefers the Z-position in the product. We designed the 1,1-disubstituted substrates with trimethylsilyl and pentyl groups, and found that the trimethylsilyl group prefers the Z-position despite its steric bulkiness. We confirmed our prediction by experimentation. This Z-selectivity was improved when a trimethylgermyl group was used.

The Origin of Cis Effect in 1,2-Dihaloethenes: The Quantitative Comparison of Electron Delocalizations and Steric Exchange Repulsions

Abstract The “cis effect” is a phenomenon in which the cis isomer is more stable than the corresponding trans isomer or almost the same stability in some molecules with double bonds. In order to clarify the predominant factor of this cis effect in the 1,2-dihaloethenes (XHC=CHX; 1: X=F, 2: Cl, and 3: Br), the energetic amount of electron delocalizations and steric exchange repulsions were theoretically estimated using the natural bond orbital (NBO) theory at MP2/6-311++G(3df,3pd) level. Two delocalization mechanisms, periplanar hyperconjugations (synperiplanar and antiperiplanar effects) and halogen lone pair delocalizations into the C=C bond antibonding orbitals (LP effect), were found as the cis stabilizing forces, in which the total amount of LP effect was greater than those of periplanar effects, the dominant factor of the cis effect. Moreover, the origin of the cis preference of the LP effect was clearly elucidated with the application of orbital phase theory, i.e., the cyclic orbital interaction was continuous only in the cis isomers of 1–3. The total steric exchange repulsion between two isomers were all trans stabilizing and their energetic gains were 1.26, 16.48, and 23.22 kJ mol−1 for 1, 2, and 3, respectively. These steric forces obviously counteract against cis preferable delocalization mechanisms, especially in compounds with larger halogen atoms, but their amounts are apparently less than those of electron delocalizations (29.82, 40.00, and 34.46 kJ mol−1 for 1, 2, and 3, respectively). Therefore, electron delocalization, not exchange repulsion, has the largest responsibility for the relative energies of 1,2-dihaloethene systems. The importance of this work is the quantitative elucidation of the dominance of delocalization mechanisms over steric effects on the electronic and energetic view of a simple molecular structure.

Geminal bond participation and torquoselectivity in cheletropic reactions

We applied the geminal bond participation theory to cheletropic reactions. The interaction between σ-orbital of the inner bond and the π*-orbital of the NN bond was predicted to be bonding, while that involving the outer bond should be antibonding. The difference in the bonding/antibonding character of these interactions is due to the cyclic orbital interaction between the π*-orbital of the NN bond and the geminal bonds at the reaction center, which is influenced by the phase continuity between the π*–σ–σ orbitals. This prediction was confirmed by the bond model analysis of model compound 1a . These results suggest that reactivity could be enhanced by substituting the inwardly rotating σ-bond by a more electron-donating one, i.e. inward rotation of a substituent with a more electropositive atom. Theoretical calculations were performed for systematically substituted substrates. The prediction was confirmed except in the case of 1g , which was affected by the inductive effect of oxygen. Silyl/methyl-substituted 1e was subjected to the bond model analysis to confirm that the preferred inward rotation of the silyl group of 1e results from the geminal bond participation and little affected by steric effects.

Relaxation of ring strain by introduction of a double bond

Ring strains are relaxed by the cyclic orbital interaction between a π-bond and the σ-bonds on the saturated atoms in the rings. An effective relaxation by the π–σ* interaction occurs in the three-, four-, and five-membered silicon rings. Cyclotrisilene 2b , cyclotetrasilene 4b , and cyclopentasilene 6b have lower strains than those of the parent saturated analogs. However, the stabilization is less significant for the hydrocarbons.

Cyclic Orbital Interaction and Orbital Phase in Acyclic Conjugation

Abstract The orbital phase continuity-discontinuity property is shown to control electron delocalization and polarization even in open chain conjugated molecules. Its chemical consequences are discussed.