Acceptor orbitals reactivity effects

This chapter outlined several fundamental factors that control the magnitude and importance of stereoelectronic effects. In the following chapters, we will discuss what the consequences of donor-acceptor orbital interactions are. We will focus on the two questions of general importance for organic chemists stability and reactivity. In the very next chapter, we will provide examples of conformational effects controlled by vicinal stereoelectronic interactions. We will illustrate that such effects often provide the key electronic stabilization that is responsible for the preferred conformational profiles and explains the shapes of many key organic functional groups. 

In summary, this chapter has discussed stereoelectronic effects on reactivity and stability which benefit from the rigid preorganization and closer spatial orbital proximity imposed by the presence of a double bond between the donor and acceptor orbitals. 

Phosphorus.—Oxoanions and Related Species. Ab initio and semi-empirical molecular orbital calculations on the metaphosphate anion [PO3]- attribute the electrophilic reactivity of this species to a low-lying acceptor orbital of a symmetry, nearly degenerate with a n molecular orbital. This species, metaphosphate, is suggested as intermediate in one of the two pathways involved in phosphorylation by phosphorocreatine. Kinetic isotope effect studies indicate that metaphosphate is also generated in the hydrolysis of 2,4-dinitrophenylphosphate ... 

Both the reactivity data in Tables 11.3 and 11.4 and the regiochemical relationships in Scheme 11.3 ean be understood on the basis of frontier orbital theory. In reactions of types A and B illustrated in Seheme 11.3, the frontier orbitals will be the diene HOMO and the dienophile LUMO. This is illustrated in Fig. 11.12. This will be the strongest interaction because the donor substituent on the diene will raise the diene orbitals in energy whereas the acceptor substituent will lower the dienophile orbitals. The strongest interaction will be between j/2 and jc. In reactions of types C and D, the pairing of diene LUMO and dienophile HOMO will be expected to be the strongest interaction because of the substituent effects, as illustrated in Fig. 11.12. 

Wanzlick showed that the stability of carbenes is increased by a special substitution pattern of the disubstituted carbon atom [12-16]. Substituents in the vicinal position, which provide n-donor/a-acceptor character (Scheme 2, X), stabilize the lone pair by filling the p-orbital of the carbene carbon. The negative inductive effect reduces the electrophilicity and therefore also the reactivity of the singlet carbene. 

Electronic excitation of molecules lead to a drastic change of their reactivities. One effect of the excitation is the powerfiil change of the redox properties, a phenomenon which may lead to photoinduced electron transfer (PET) [1-4] The electron-donating as well as the electron-accepting behavior of the excited species are approximately enhanced by excitation energy. This can be explained by means of a simple orbital scheme. By excitation of either the electron donor (D) or the acceptor (A) of a given pair of molecules, the former thermodynamically unfavorable electron transfer process becomes exergonic (A et) (Scheme 1). 

During the 1960s, organic chemists were surprised to find that the reactivity of a di-polarophile is always increased by substitution, whether the substituent is a donor or an acceptor.28 This result is difficult to explain by classical effects. Can you do better using frontier orbitals ... 

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