Bonds as Electron Acceptors

The spectroscopic and chemical properties of l,4-diazabicyclo[2.2.2]octane (DAB-CO) are consistent with a strong interaction of the in-phase combination of the nonbonded electron pairs of the nitrogen atoms with the symmetric combination of the C—C a bonds, one consequence of which is that the in-phase combination is the HOMO. The two lowest IPs are 7.6 and 9.7 eV [89]. Compare these to the IPs of trimethyl amine (8.44 eV) and ammonia (10.5 eV) [90]. The relative importance of intramolecular orbital interactions through space and through bonds has been reviewed by Hoffmann [6]. 

The acceptor ability may be improved in two ways, by lowering the energy of the a orbital and by polarizing the orbital toward one end. The first improves the interaction between it and a potential electron donor orbital by reducing the energy difference, ea — Eb, the second by increasing the possibility of overlap and therefore increasing the value of the intrinsic interaction matrix element, h,)/ . 

---

An especially clean example of electron transfer chemistry involving carbon-carbon multiple bonds as electron acceptors, i.e., proceeding via anion radicals is the cyclobutanation of phenyl vinyl sulfone induced by reduction at a mercury pool cathode (Scheme 76) [122]. 

All other results are consistent with the picture of d orbitals as electron acceptors that strengthen the S—O bond. S—O BOPs are larger, and C—S ones are smaller, in the calculations with d orbitals C—C BOPs show a decrease as the number of oxygen atoms... 

The ability to ionically polymerize apparently correlates in many cases with the capacity of the substituents to act as electron acceptors (anionic polymerizability) or as electron donors (cationic polymerizability) on the rt-bond of the vinyl group. These relationships should be visible in carefully chosen quantum chemical parameters. 

Bending of unsaturated bonds reduces the overlap between the p-orbitals and weakens the interaction. The n orbital hes high in energy and the Jt orbital lies low. Bent unsaturated bonds are electron acceptors as well as donors. The energy gap between Jt and Jt is small. Bent unsaturated bonds are readily pseudoexcited to undergo [2+2] cycloaddition reactions. 

The ability of compounds with double bonds to act both as electron donors and as electron acceptors in charge transfer complex formation is well known (81,82). Hammond (83) has studied the correlations of association constants and of the energy of the charge transfer absorption of 2-substituted-l,4-benzoquinones complexed with hexamethylbenzene with the Hammett equation. Charton (84) has studied the correlation with eq. (2) of association constants of 1-substituted propenes with Ag. ... 

B The molecules from this group incorporate electronegative (i.e., basic) heteroatoms (e.g., O, N, S, etc.) and 71-electrons, and, hence, they are able to participate in the hydrogen bonds as proton acceptors. Ketones and ethers are among the most representative classes of analytes belonging to this particular group. 

Diels-Alder cycloaddition reactions of electron-poor dienophiles to electron-rich dienes, which are generally carried out thermally, afford widespread applications for C—C bond formation. On the basis of their electronic properties, numerous dienes can be characterized as electron donors and dienophiles as electron acceptors. Despite the early suggestions by Woodward,206 the donor/ acceptor association and electron-transfer paradigm are usually not considered as the simplest mechanistic formulation for the Diels-Alder reaction. However, the examples of cycloaddition reactions described below will show that photoirradiation of various D/A pairs leads to efficient cycloaddition reactions via electron-transfer activation. 

Cationic polymerisations of vinyl derivatives proceeds by a polar or heterolytic cleavage of the double bond by the action of initiators which act as electron acceptors. Initiation is carried out by proton donation by acids or co-catalysts... 

Cao, X. P. and Hamers, R. J. Silicon surfaces as electron acceptors Dative bonding of amines with Si(001) and Si(lll) surfaces. Journal of the American Chemical Society 123, 10988... 

Studies of the model compounds III and IV 33) and others 39) have shown that the NH-groups take part in hydrogen bonding both in the crystalline and amorphous states not only as electron donors but also as electron acceptors. Table 4 presents thermodynamic parameters for a series of model complexes. 

The next step is another oxidative decarboxylation, in which a-ketoglutarate is converted to succinyl-CoA and C02 by the action of the a-ketoglutarate dehydrogenase complex NAD+ serves as electron acceptor and CoA as the carrier of the succinyl group. The energy of oxidation of a-ketoglutarate is conserved in the formation of the thioester bond of succinyl-CoA ... 

The interaction between selective metal oxides and molecules to be oxidized is, of course, based on electron-accepting and electron-donating properties, respectively. In this way, Mo6+, Vs+, etc. act as electron acceptors and molecules with 7r-bonds as donors. Ai et al. [5—12] have drawn attention to the fact that this can also be described by acid—base properties. An electron donor molecule like butene is a basic entity interacting with acidic sites on the catalyst. Hence it follows that activity and selectivity depend on the relative acidity and basicity. Mo03, for example, is an acidic oxide, while Bi203 is a basic oxide. Different compositions Bi Mo have different acidities. The rate of oxidation depends on the number of acid sites (=acidity) and the acid strength, viz. 

It can also catalyze the reverse reaction when ferricyanide or cytochrome c is used as electron acceptor (374, 375). The phosphosulfate bond of... 

Inclusion complexing partners are classified as hosts and guests [46]. There are two types of hosts that were successfully employed in the chromatographic separation of enantiomers hosts that have a hydrophobic interior and hosts with a hydrophilic interior. The hydrophilic interior means that the cavity contains heteroatoms such as oxygen, where lone-pair electrons are able to participate in bonding to electron acceptors such as an organic cation (e.g., chiral crown ethers). In contrast, a host with a hydrophobic interior cavity is able to include hydrocarbon-rich parts of a molecule [47]. This type of host is found in the cyclodextrins. 

---