Acceptor Orbital Interactions

Reactions can be understood by considering interactions between filled orbitals and empty orbitals. We emphasized in several places in this book that it is always favorable to mix a filled orbital with an empty orbital. The definitions of a Lewis acid and Lewis base imply an empty and a filled orbital, respectively, as do the definitions of an electrophile and a nucleophile. Most of the time this is obvious, such as in the reaction of water with a carbe-nium ion (Eq. 10.3). The water has the filled lone pair orbital (it is the nucleophile or Lewis base) that donates its electron density to the empty p orbital on the carbenium ion (the electrophile or Lewis acid). 

---

C) Synergistic H2 coordination. When the metal has both donor and acceptor orbitals, interaction with the cthh and ohh orbitals of H2 can act synergistically (cooperatively) in the Dewar-Chatt-Duncanson mode. Such self-reinforcing charge flow enables a significantly stronger interaction than does simple dative coordination of H2. 

A prominent feature of this mechanism is that the growing polymer chain alternately swings between two r/.v-disposed coordination sites during each monomer insertion. General mechanistic outlines of this reaction have been extensively examined by large-scale computations and confirmed by experimental means.59 Our present goal is to clarify the localized donor-acceptor-orbital interactions that underlie (4.106), particularly the nature of the alkyl-alkene complex II. 

To produce an interaction diagram we need the frontier orbitals of Cr and CO. The former are 3d, 4s and 4p whereas the latter are 5a and 2ir and Cr possesses six valence electrons whereas each CO has two in the HOMO. For simplicity we will first consider only the CO donor orbitals and then add the acceptor orbital interactions later. For the first problem then we have nine metal functions and six ligand functions plus 18 electrons total. Thus, we must generate 15 MOs of which 9 will be filled. Is this a simplification The full problem requires nine metal functions and 6 x (4 + 4) ligand functions for a total of 57 MOs Of course, in a real calculation, the fragment analysis is done by the computer on the MO solutions of the complete... 

Ethylene provides a different situation because it has only one it acceptor orbital. The ethylene s preferred orientation has this acceptor orbital interacting with the [CpMn(C0)2] a orbital. There... 

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. 

There are at least three different definitions and vantage points that chemists use to expand upon the electrostatic paradigm. They are nucleophile-electrophile combinations, Lewis acid-base reactions, and donor-acceptor orbital interactions. We have used these terms repeatedly throughout this book because they are presented in introductory organic chemistry classes, but here we give them strict definitions. Each of these definitions is a subtle variation on the other, and often it is essentially a case of semantics to decide which best describes a particular reaction. 

Now, examine the orbital on cyclohexanone lithium enolate most able to donate electrons. This is the highest-occupied molecular orbital (HOMO). Identify where the best HOMO-electrophile overlap can occur. Is this also the most electron-rich site An electrophile will choose the best HOMO overlap site if it is not strongly affected by electrostatic effects, and if it contains a good electron-acceptor orbital (this is the lowest-unoccupied molecular orbital or LUMO). Examine the LUMO of methyl iodide and trimethylsilyl chloride. Is backside overlap likely to be successful for each The LUMO energies of methyl iodide and trimethylsilyl chloride are 0.11 and 0.21 au, respectively. Assuming that the lower the LUMO energy the more effective the interaction, which reaction, methylation or silylation, appears to be guided by favorable orbital interactions Explain. 

Alkyl substituents accelerate electrophilic addition reactions of alkenes and retard nucleophilic additions to carbonyl compounds. The bonding orbital of the alkyl groups interacts with the n bonding orbital, i.e., the HOMO of alkenes and raises the energy (Scheme 22). The reactivity increases toward electron acceptors. The orbital interacts with jt (LUMO) of carbonyl compounds and raises the energy (Scheme 23). The reactivity decreases toward electron donors. 

Keywords Cycloadditions, Chemical orbital theory. Donor-acceptor interaction. Electron delocalization band. Electron transfer band, Erontier orbital. Mechanistic spectrum, NAD(P)H reactions. Orbital amplitude. Orbital interaction. Orbital phase. Pseudoexcitation band. Quasi-intermediate, Reactivity, Selectivity, Singlet oxygen. Surface reactions... 

The pseudoexcitation in donors occurs through the d-a -d interaction. The a-d-a orbital interaction causes the pseudoexcitation in the acceptors. The simultaneous pseudoexcitations in the donors and acceptors are caused by the a-d-a -d interaction (Scheme 2). 

Scheme 2 Change of the frontier orbital interactions with the power of donors and acceptors...

With the power of the donors and acceptors, changes occur in the important frontier orbital interactions (Scheme 2) and in the mechanism of chemical reactions. The continuous change forms a mechanistic spectrum composed of the delocalization band to pseudoexcitation band to the electron transfer band. 

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

The cyclic orbital interaction of p and q with o or with o can significantly occur at the transition state of the ring closure of 1,3-diradicals. The continuous orbital phase for the cyclic orbital interaction with o implies effective stabilization of the transition states when the o bonds are electron acceptors. 

The introduction of heteroatoms into the hydrocarbon diradicals is a frequently applied strategy to tune the spin preference and relative stabilities of diradicals. The heteroatoms may change the energies of donor or acceptor orbitals, and consequently affect the donor-acceptor interaction involved in the cyclic orbital interaction. Take 2-oxopropane-l,3-diyl, or so-called oxyallyl (OXA, 18) as an example [29]. It is a hetero analog of TMM, as shown in Fig. 14. The replacement of CH with oxygen in the central fl unit leads to a decrease in energies of Jt and k orbitals. This may enhance the orbital interaction through one path (denoted by bold lines) and weaken that via the other (denoted by wavy lines) relative to the continuous cyclic orbital interaction in the parent species 1 (Fig. 14). As a result, the p-Jt -q... 

In more detail, the interaction energy between donor and acceptor is determined by the ionisation potential of the donor and the electron affinity of the acceptor. The interaction energy increases with lowering of the former and raising of the latter. In the Mulliken picture (Scheme 2) it refers to a raising of the HOMO (highest occupied molecular orbital) and lowering of the LUMO (lowest unoccupied molecular orbital). Alternatively to this picture donor-acceptor formation can be viewed in a Born-Haber cycle, within two different steps (Scheme 3). 

Fig. 3. Top n and

Figure 1.3 The two-electron stabilizing interaction between a filled donor orbital (pi<(]) and an unfilled acceptor orbital corresponding to perturbation...

---