Orbital interaction between

There are probably several factors which contribute to determining the endo exo ratio in any specific case. These include steric effects, dipole-dipole interactions, and London dispersion forces. MO interpretations emphasize secondary orbital interactions between the It orbitals on the dienophile substituent(s) and the developing 7t bond between C-2 and C-3 of the diene. There are quite a few exceptions to the Alder rule, and in most cases the preference for the endo isomer is relatively modest. For example, whereas cyclopentadiene reacts with methyl acrylate in decalin solution to give mainly the endo adduct (75%), the ratio is solvent-sensitive and ranges up to 90% endo in methanol. When a methyl substituent is added to the dienophile (methyl methacrylate), the exo product predominates. ... 

Fig. 12.5. Frontier orbital interactions between different combinations of substituted radicals and aikenes.

The T—3 CC bond-orbital interactions between opposite CC bonds in a cyclobutane ring are an interesting exception. In this system there is significant mixing between the acc (and a c) orbitals on opposite bonds (Fig. 29). The two acc molecular orbitals are both occupied. The reader will recognize these orbitals, and the... 

The unsymmetric n face of carbonyl groups is postulated to be attributable to orbital interactions between a o-fragment and a tt-fragment. Interactions between two 7t fragments in a carbonyl molecule can also lead to an unsymmetrical orbital phase environment [3]. 

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. 

Gleiter and Ginsburg found that 4-substituted-l,2,4-triazoline-3,5-dione reacted with the propellanes 36 and 37 at the syn face of the cyclohexadiene with respect to the hetero-ring. They ascribed the selectivity to the secondary orbital interaction between the orbitels (LUMO) of 36 and 37 with antisymmetrical combination of lone pair orbitals (HOMO ) of the triazolinediones (Scheme 24) [29]. 

Mataka and coworkers reported the studies of the Diels-Alder reactions of [3.3] orthoanthracenophanes 96 and 97, of which anthraceno unit, the potential diene, has two nonequivalent faces, inside and outside. The reactions of 96 with dien-ophiles gave the mixtures of inside and outside adducts with the ratios between 1 1 and 1 1.5. However, the ratio changes drastically, in favor of the inside adducts, when 97 reacts with dienophiles such as maleic anhydride, maleimide and naphto-quinone [55] (Scheme 46). Mataka suggested that the Jt-facial selectivity is controlled by an orbital interaction between the electron-poor dienophiles and the Jt-orbital of the facing aromatics, which would lead to a stabilization of the transition state, while Nishio suggested that the selectivity is due to the attractive k/k or CH/jt interaction [53]. 

In the case of the reverse-electron-demand Diels-Alder reactions, the secondary orbital interaction between the Jt-HOMO of dienophile and the LUMO of 114 or the effect of the orbital phase enviromnents (Chapter Orbital Phase Enviromnents and Stereoselectivities by Ohwada in this volume) cannot be ruled out as the factor controlling the selectivity (Scheme 55). 

The rationale behind this choice of bond integrals is that the radical stabilizing alpha effect of such radicals are explained not by the usual "resonance form" arguments, but by invoking frontier orbital interactions between the singly occupied molecular orbital of the localized carbon radical and the highest occupied molecular orbital (the non-bonding electrons atomic orbital) of the heteroatom (6). For free radicals the result of the SOMO-HOMO interaction Ts a net "one-half" pi bond (a pi bond plus a one-half... 

In Entry 2 a similar triene that lacks the activating carbonyl group undergoes reaction but a much higher temperature is required. In this case the ring junction is trans, which corresponds to an exo TS and may reflect the absence of secondary orbital interaction between the diene and dienophile. 

The /5-effect of silyl groups on -systems depends on the geometry of the molecule, because the orbital interaction between the C-Si a orbital and the n orbital reaches its maximum when they are in the same plane. For example, the... 

Variations to the cis addition have been found in the transtition state in some cases and a mixture of products has been reported. Two possible stereochemical variations have been reported because of endo and exo addition. Thus in the dimerisation of butadiene Hoffmann and Woodward have shown that besides the primary orbital interactions between C, and C4 of the diene and Cj and C2 of the dienophile, there are also secondary interactions (shown by dotted lines and also called endo addition) between C-2 of the dieno and C-3 of the dienophile. Such orientations are only possible in endo orientation and this will stabilize the transition state. 

Figure 5. Frontier orbital interaction between a transition metal peroxo group and an olefin.

The frontier orbital interaction between the olefin HOMO 7t(C-C) and the orbitals with c (0-0) character in the LUMO group of the metal peroxo moiety controls the activation of 0-0 bond. Electron donating alkyl substituents at the olefin double bond raise the energy of the HOMO, with the epoxidation barrier dropping concomitantly. On the other hand, a base coordinated at the metal center pushes the a (0-0) LUMO to higher energies and thus entails a higher barrier for epoxidation. 

These opposite signs can be explained by considering a twofold orbital interaction between the two parts of an arenediazonium ion, namely between the jr-HOMO of the diazonio group and the cr-LUMO of the aryl residue, and between the jr-HOMO of the aryl residue and the jr-LUMO of the diazonio group. These two overlaps stabilize the C—N bond and reduce the rate of dediazoniation into a phenyl cation and a nitrogen molecule. The two opposing HOMO-LUMO interactions are shown in Figure 1. Thus... 

FIGURE 3. Primary and secondary orbital interactions between diene and dienophile... 

Cycloaddition reactions using tropone or another cyclic triene as the 6ji partner have been abundantly described in the literature. It has been found that virtually all metal-free [6 + 4] cycloadditions of cyclic trienes afford predominantly exo adducts. This has been rationalized by consideration of the HOMO-LUMO interactions between the diene and triene partners. An unfavorable repulsive secondary orbital interaction between the remaining lobes of the diene HOMO and those of the triene LUMO develops during an endo approach. The exo transition state is devoid of this interaction (Figure 9). 

Fig. 10.7. Orbital interactions between RzCu and substrates in (a) an early stage of interaction of the cuprate with methyl bromide, and (b) n-complexation to acetylene or olefin.

Calculations on the isoelectronic series Me(Ph)B , Me(Ph)C , and [Me(Ph)N ]+ show that the singlet-state geometries are different, reflecting differences in the orbital interactions between the hypovalent atom and the 7r-system. The high calculated barrier (21.5 kcal moP ) for [1,2]-H shift in the nitrenium ion is the result of migration using the orbital which is conjugated with the tt-system. 

The delocalization of the HOMO and LUMO arises from pseudo-ir orbital interactions between adjacent 3px and 3py atomic orbitals of Si atoms, respectively. Since the 3p orbital is on the Si-Si-Si plane, the degree of the interaction depends on the dihedral angle between adjacent Si-Si-Si planes. Fluctuation of the dihedral angles along the polymer skeleton then causes the so-called Anderson localization of the HOMO and LUMO [41]. 

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