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Orbitals in Chemistry by Satoshi Inagaki (auth.), Satoshi Inagaki (eds.)

By Satoshi Inagaki (auth.), Satoshi Inagaki (eds.)

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3 [4+2] Cycloadditions The mechanistic spectrum suggests that [4+2] cycloadditions should shift from concerted reactions in the delocalization band to stepwise reactions through intermediates in the pseudoexcitation band. 5 in Chapter “Elements of a Chemical Orbital Theory” by Inagaki in this volume). The reactions occur in a concerted manner. In the pseudoexcitation band the HOMO–HOMO–LUMO–LUMO interaction is important. The HOMO– HOMO and LUMO–LUMO interactions are, however, forbidden by the orbital symmetry at the six-membered ring transition states for the Diels–Alder reactions.

The frontier orbital interaction is the n–πC=C interaction (Scheme 30b), which is impossible at the four-membered ring transition states. This is not good for the retention of the configuration. R R O R R hυ + + O R O R O O SOMO' ( p *CO) LUMO (p *CC) HOMO (pCC) a SOMO (nO) b Scheme 30a,b [2 + 2] Cycloaddition reactions of excited carbonyl compounds with the alkenes substituted by electron-accepting (a) and -donating (b) groups 4 Interactions of More Than Two Orbitals The theory of two-orbital interactions has been described in the preceding sections.

15. 16. 17. 18. 19. 20. 21. 22. 23. 1007/128_2008_27 1 A Mechanistic Spectrum of Chemical Reactions Satoshi Inagaki Abstract The mechanism of chemical reactions between electron donors and acceptors continuously changes with the power of the donors and the acceptors. The interaction between the HOMO (d) of the donors and the LUMO (a*) of the acceptors or delocalization of electrons is important for the reactions. The electron d-to-a* transferred configuration mixes to a significant extent. As the donors and/or the acceptors are strong, their excited configurations appreciably mixes together with the transferred configuration.

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