Orbital unsymmetrization

Keywords Facial selection. Orbital phase, Secondary orbital interaction. Orbital unsymmetrization. Ketones, Olefins, Diels-Alder dienophiles, Diels-Alder dienes, Michael acceptor. Amine nitrogen atom... 

R2=C02CH3) exhibit little difference in face selectivity, i.e., syn selectivity when subject to NaBH symanti = 65 35 in 18d 62 38 in 18e) and DIBAL-H syn.anti = 66 34 in 18d 61 39 in 18e) reduction. The behavior of 18d and 18e is also consistent with orbital unsymmetrization, as in 19. On the other hand, Mehta et al. suggested the presence of significant electrostatic contributions from exo-electron-withdrawing groups, rationalizing the syn face selectivity in 18b [75]. 

Cycloaddition of bicyclo[2.2.1]hept-2-ene-2,3-dicarboxylic anhydride 81 with cyclopentadiene was also studied by Bartlett et al., who found exclusive top addition, the top-endo/top-exo ratio being 3 2 [147]. The endolexo ratio is significantly different from that of 80 (60-70 1). The observed top selectivity in norbornadiene (80) and norbomene (81) derivatives is consistent with the inherent top reactivity of norbomanone 25 and norbomene 57. Orbital unsymmetrization of the dienophile... 

Diels-Alder cycloadditions involving norbomene 57 [34], benzonorbomene (83), 7-isopropylidenenorbomadiene and 7-isopropylidenebenzonorbomadiene (84) as dienophiles are characterized as inverse-electron-demand Diels-Alder reactions [161,162], These compounds react with electron-deficient dienes, such as tropone. In the inverse-electron-demand Diels-Alder reaction, orbital interaction between the HOMO of the dienophile and the LUMO of the diene is important. Thus, orbital unsymmetrization of the olefin it orbital of norbomene (57) is assumed to be involved in these top selectivities in the Diels-Alder cycloaddition. 

The substituent effect of the aromatic nitro group can be accounted for in terms of n orbital unsymmetrization. The LUMO of the dibenzobicyclic lactone (106) can be analyzed as an in-phase combination of three vacant ir orbitals, i.e., those of benzene, nitrobenzene and the 2(5//)-furanone moiety. The energetically... 

In the Spiro systems 30, the aromatic orbitals unsymmetrize the carbonyl orbital. Simultaneously, the carbonyl group can perturb the orthogonal aromatic ring. Nitration of the fluorene derivatives (30) bearing a spiro substituent was studied (Fig. 17) [96, 97]. 

Distortion arises from the repulsion of ligands by the electrons occupying eg orbitals (unsymmetrical eg orbitals in both the cases). Thus d9 configuration in both the fields show strong distortion. 

Fig. 10.3. Orbital coefficients for HOMO and next highest n orbital for some substituted benzenes. (From CNDO/2 ealculations. Ortho and meta eoefficients have been averaged in the case of the unsymmetrical methoxy and formyl substituents. Orbital energies are given in atomic units.)...

When both the 1,3-dipoIe and the dipolarophile are unsymmetrical, there are two possible orientations for addition. Both steric and electronic factors play a role in determining the regioselectivity of the addition. The most generally satisfactory interpretation of the regiochemistry of dipolar cycloadditions is based on frontier orbital concepts. As with the Diels-Alder reaction, the most favorable orientation is that which involves complementary interaction between the frontier orbitals of the 1,3-dipole and the dipolarophile. Although most dipolar cycloadditions are of the type in which the LUMO of the dipolarophile interacts with the HOMO of the 1,3-dipole, there are a significant number of systems in which the relationship is reversed. There are also some in which the two possible HOMO-LUMO interactions are of comparable magnitude. 

The geometries and relative energies of the different conformations of model chalcogen diimides E(NR)2 (E = S, Se R = H, Me, Bu and SiMe3) have been investigated by using ab initio and DET molecular orbital methods.The cis,trans conformation is predicted to be most stable with the exception of the parent molecules E(NH)2 and the unsymmetrical systems RNSNH, for which the cis,cis conformation is slightly more stable than the cis, trans isomer. 

Nitrous oxide is a moderately unreactive gas comprised of linear unsymmetrical molecules, as expected for a 16-electron triatomic species (p. 433). The symmetrical structure N-O-N is precluded on the basis of orbital energetics. Some physical properties are in Table 11.8 it will be seen that the N-N and N-O distances are... 

The asymmetry of sp3 orbitals arises because, as noted previously, the two lobes of a p orbital have different algebraic signs, + and -. Thus, when a p orbital hybridizes with an s orbital, the positive p lobe adds to the s orbital but the negative p lobe subtracts from the s orbital. The resultant hybrid orbital is therefore unsymmetrical about the nucleus and is strongly oriented in one direction. 

In addition to its effect on stability, delocalization of the unpaired electron in the allyl radical has other chemical consequences. Because the unpaired electron is delocalized over both ends of the nr orbital system, reaction with Br2 can occur at either end. As a result, allylic bromination of an unsymmetrical alkene often leads to a mixture of products. For example, bromination of 1-octene gives a mixture of 3-bromo-l-octene and l-bromo-2-octene. The two products are not formed in equal amounts, however, because the intermediate allylic radical is... 

Various ab initio and scmi-cmpirical molecular orbital calculations have been carried out on the reaction of radicals with simple alkenes with the aim of defining the nature of the transition state (Section 1.2.7).2I>,j , 6 These calculations all predict an unsymmetrical transition state for radical addition (i.e. Figure 1.1) though they differ in other aspects. Most calculations also indicate a degree of charge development in the transition state. 

The orbital interaction depicted in Scheme 1.15 shows that the two cr-bonds form at the same time but do not develop to the same extent. The Diels-Alder cycloaddition of unsymmetrical starting materials is therefore concerted but asynchronous. A highly unsymmetrical diene and/or dienophile give rise to a highly unsymmetrical transition state and a stepwise pathway can be followed. 

The n orbital amplitudes of ethene are identical on both carbons. Unsymmetrical substitutions polarize the n orbital. Electron acceptors or electrophiles attack the carbon with the larger r amplitude. The polarization of frontier orbitals is important for regioselectivities of reactions. Here, mechanism of the n orbital polarization of ethene by methyl substitution [4] is described (Scheme 5). 

Steric repulsions come from two orbital-four electron interactions between two occupied orbitals. Facially selective reactions do occur in sterically unbiased systems, and these facial selectivities can be interpreted in terms of unsymmetrical K faces. Particular emphasis has been placed on the dissymmetrization of the orbital extension, i.e., orbital distortions [1, 2]. The orbital distortions are described in (Chapter Orbital Mixing Rules by Inagaki in this volume). Here, we review the effects of unsymmetrization of the orbitals due to phase environment in the vicinity of the reaction centers [3]. 

Deformation of symmetrical orbital extension of carbonyl or olefin compounds was proposed to be the origin of the facial selectivities. We illustrate the unsymmetrical orbital phase environment of % orbitals of carbonyl and olefin groups and facial selectivities in Fig. 1 [3, 4]. There are in-phase and out-of-phase combinations of... 

Fig. 1 Unsymmetrical orbital phase environment and preferential attack of reagents...

The SOI concept is akin to the unsymmetrization of orbitals. The only difference is in the sites of the subsidiary interactions, which occur between the non-reacting centers (positions 3 and 4 in Fig. 3a) in SOI and between the reacting and non-reacting centers (sites 2 and 3 in Fig. 3b) for the unsymmetrization of orbitals (Fig. 1). The orbital phase environment around the reaction centers is a general idea... 

In this review we will focus on the unsymmetrization of the orbital phase environment in the vicinity of reacting n systems, and its effect on facial selectivities. This idea can be applied to many kinds of recently observed facial selectivities, such as those involving ketones [10-21], olefins [22-31], dienes [32-46] and others [47-49]. 

Fig. 3 Orbital interactions of interest in secondary orbital interaction and the unsymmetrization...

---