SOMO-HOMO orbital interactions

SOMO-HOMO orbital interaction with electron-rich olefins (Mode B)... 

Addition reactions with Mode B are not popular, but are occasionally useful. Eq. 4.7 indicates the reaction of ethyl bromoacetate and sugar vinyl ether with Bu3SnH initiated by AIBN. The ethyl acetate radical is electrophilic and it reacts with electron-rich sugar vinyl ether through SOMO-HOMO orbital interaction to form a ribosyl anomeric radical, as shown below. Then, the formed ribosyl anomeric radical abstracts... 

The alkene substituted with the electron accepting group has the LUMO (it ) lowered by the interaction with the vacant orbital of the substituent. The high-lying SOMO interacts with the LUMO of the alkene more effectively than with the HOMO. The interaction is the symmetry-allowed it - n interaction (Scheme 30a). The configuration of the alkene is retained. 

The alkene with the electron donating group has the HOMO (n) raised by the interaction with the occupied orbital of the substiment. The low-lying SOMO (n ) interacts with the HOMO of the alkene more effectively. The frontier orbital interaction is the interaction (Scheme 30b), which is impossible at the four-membered... 

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... 

Figure 13 Schematic o orbital interaction diagrams for PC—CP (5a), CP—PC (5c), NC—CN (4a), and CN—NC (4c). MO and FMO energies in electron volts. Left panels FMOs central panels primary or first-order interaction (i.e., no °somo homo mixing) tight panels final situation including all interactions.

Considering the very large number of interactions to be taken into account, the FO approach loses much of its charm. Even when a radical can be described by a single configuration, scheme (a) above shows that it may be necessary to consider four orbital interactions including the three-electron SOMO-HOMO interaction, which may pose a problem as it can be either attractive or repulsive (p. 12). 

This interpretation is consistent with the nucleophilic properties which are generally associated with the methyl radical. In passing, note that Canadell s rule states that any radical, nucleophilic or electrophilic, reacts with an alkene at the site having the largest HOMO coefficient.62 Canadell and co-workers argue that the three-electron SOMO-HOMO interaction is stabilizing, due to the energetic proximity of these orbitals. See, however, p. 12. 

A word of warning. In this chapter, we have not tried any quantitative correlations. No doubt they could be made, but they will be complicated. The reason is that the interactions we have been looking at, especially the SOMO/ HOMO interactions, are often between orbitals quite close in energy. Such interactions lead to first-order perturbations and the third term of equation 2-7 is not appropriate. 

Similarly, orbital interaction diagrams can also explain the stabilization of a radical center by a pi donor (Fig. 11.3fc). The pi donor has an accessible full orbital, HOMO, close in energy to the orbital bearing the single electron, SOMO. The interaction of the SOMO with a full HOMO destabilizes one electron and stabilizes two for a net stabilization of one electron overall. We have created a pi bond with one electron in the antibonding orbital the pi bond order is thus half (our resonance structures were unable to indicate this partial pi bond with lines and dots). 

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