SOMO-LUMO interactions

In recent years, direct, time-resolved methods have been extensively employed to obtain absolute kinetic data for a wide variety of alkyl radical reactions in the liquid phase, and there is presently a considerable body of data available for alkene addition reactions of a wide variety of radical types [104]. For example, rates of alkene addition reactions of the nucleophilic ferf-butyl radical (with its high-lying SOMO) have been found to correlate with alkene electron affinities (EAs), which provide a measure of the alkene s LUMO energies [105,106]. The data indicate that the reactivity of such nucleophilic radicals is best understood as deriving from a dominant SOMO-LUMO interaction, leading to charge transfer interactions which stabilize the early transition state and lower both the enthalpic and entropic barriers to reaction, with consequent rate increase. A similar recent study of the methyl radical indicated that it also had nucleophilic character, but its nucleophilic behavior is weaker than that expressed by other alkyl radicals [107]. 

Generally, as the potential energy level of SOMO increases (becomes a more reactive radical), free radicals have nucleophilic character, while as the potential energy level of SOMO decreases (becomes a stable radical), free radicals have electrophilic character. Thus, when effective radical reactions are required, small energy difference in SOMO-HOMO or SOMO-LUMO interactions is necessary. For example, the relative reactivities of radical addition reactions of a nucleophilic cyclohexyl radical to alkenes,... 

The addition of alkyl radicals to alkenes is important for C-C bond formation. A tert-butyl radical, a typical nucleophilic radical, reacts with acrylonitrile taking a rate constant of 2.4 X 106 M-1 s-1 (27 °C), through a SOMO-LUMO interaction. However, it reacts with 1-methylcyclohexene, an electron-rich alkene, taking a rate constant of 7.4 X 102M-1 s-1 (21 °C). On the other hand, the diethyl malonyl radical, a typical electrophilic radical, shows the opposite reactivity [66-71]. Similarly, the rate constant for the reaction of nucleophilic C2H5 and cyclohexene is2X 102 M 1 s 1, while that of electrophilic C3F7 with cyclohexene is 6.2 X 105 M-1 s 1. 

The rate constants of these cyclizations via 5-exo-trig manner are 2.4xl05s-1, 7.0 X 105 s 1, 7.5 X 105 s 1, respectively, and they are close to that of the parent 5-hexen-l-yl radical (2.4 X 105 s-1). Moreover, the introduction of dimethyl groups at the 2,2-, 3,3-, and 4,4-positions of the 5-hexen-l-yl radical, increases the rate constants for cyclization about 10 times and they become approximately 106 s 1. These cyclization reactions proceed via the SOMO-LUMO interaction. Therefore, the introduction of three fluorine atoms to the olefinic group in the 5-hexen-l-yl radical accelerates the cyclization rate, 2 times. Probably, the substitution of hydrogen atoms by fluorine atoms onto the olefinic group induces the decrease of LUMO energy [5-12]. 

Acyl radical has a nucleophilic character and cyclizes via SOMO-LUMO interaction. Eq. 3.22 shows the cyclization of acyl radicals formed from the reaction of selenol esters (67) with Bu3SnH/AIBN or Bu3SnH/Et3B, to give the cyclic ketones (68) via 5-exo-trig or 6-exo-trig manner through the transition state [II] [84-90]. 

Sml2 can be used for SET reagent to carbonyl groups. Thus, eq. 4.13 shows the initial SET from Sml2 to a carbonyl compound (26) to generate a ketyl radical, a nucleophilic radical, which then reacts with electron-deficient ethyl acrylate through SOMO-LUMO interaction to form y-lactone (28) [34-36]. 

In the previous sections, the reactions of nucleophilic alkyl and acyl radicals with electron-deficient aromatics via SOMO-LUMO interaction have been described. At this point, we introduce the reactions of electrophilic alkyl radicals and electron-rich aromatics via SOMO-HOMO interaction, though the study is quite limited. Treatment of ethyl iodoacetate with triethylborane in the presence of electron-rich aromatics (36) such as pyrrole, thiophene, furan, etc. produces the corresponding ethyl arylacetates (37) [50-54]. 

Generally, treatment with electron-deficient olefins such as nitroethylene or vinyl sulfone is effective for radical addition reactions, since alkyl radicals derived from O-acyl esters (2) are nucleophilic and take SOMO-LUMO interaction. However, treatment of O-acyl esters ) derived from perfluoroalkyl carboxylic acids (RfC02H) generates electrophilic radicals, Rf, which react preferably with electron-rich olefins such as vinyl ether, as shown in eq. 8.16 [52]. 

With anisole, the SOMO/HOMO interaction (B) is strong, and with nitrobenzene the SOMO/LUMO interaction (A) is strong, but with benzene neither is stronger than the other. Product development control can also explain this, since the radicals produced by attack on nitrobenzene and anisole will be more stabilised than that produced by attack on benzene. However, this cannot be the explanation for another trend which can be seen in Table 7.1, namely that a p-nitrophenyl radical reacts faster with anisole and benzene than it does with nitrobenzene. This is readily explained if the SOMO of the p-nitrophenyl radical is lower in energy than that of the phenyl radical, making the SOMO/HOMO interactions (C and D) strong with the former pair. 

Carbon-centered radicals are nucleophilic or electrophilic species, depending upon the substituents at the radical center. Electron-donating substituents like alkyl or alkoxy groups increase the nucleophilicity - of the radicals whereas electron-withdrawing groups (EWG) like ester or nitrile groups increase their electrophilic nature . In nucleophilic radical reactions SOMO LUMO interaction dominates whereas electrophilic radical reactions are controlled by SOMO-HOMO interactions. 

LUMO of the O — O bond i.e. the energy of the low lying CTg 0 orbital (Scheme-6). The energy effect associated with the SOMO-LUMO interaction becomes more favorable as the energy gap decreases. So tertiary radicals having a higher SOMO than secondary and primary radicals are more reactive. If the reaction were controlled by an orbital interaction between the SOMO and an occupied MO of the peracid group, i.e. the a0 0 MO, the reverse reactivity should have been observed the primary radical should have been more reactive than the tertiary one. 

For addition of nucleophilic radicals to an alkene, the dominant interaction is between the SOMO of the radical and the LUMO of the olefin. Complexation of a Lewis acid to the alkene lowers its LUMO and magnifies the SOMO-LUMO interaction [3a]. Thus, one expects that the rate of addition of carbon radicals to alkenes should be dependent on the presence of Lewis acid if that alkene is capable of complexation to Lewis acid. The important processes for such a reaction are shown in Eqs. (7)-(9). 

These substituent effects are consistent with an FMO interpretation with a dominant SOMO-LUMO interaction. As shown in Figure 11.6, ERG substituents will raise the energy of the radical SOMO and increase the strength of interaction with the relatively low-lying LUMO of alkenes having electron-withdrawing groups. When... 

Frontier Molecular Orbital (FMO) theory may also be applied to provide qualitative understanding. The frontier orbital of the radical is that bearing the free spin (the SOMO) and during radical addition this will interact with both the jt antibonding orbital (the LUMO) and the n-orbital (the HOMO) of the olefin. Both the SOMO-HOMO and the SOMO-LUMO interactions lead to a net drop in energy i.e. E respectively - Figure 1.4J. The dominant interaction and... 

A SOMO, defined above and in Section ll.lO.B, can react with either a HOMO or a LUMO of another molecule, as shown in Figure 13.4.53 The SOMO-LUMO interaction (as in Figure 13.4) occurs with nucleophilic radicals (high energy SOMO) that react best with molecules possessing a low energy LUMO... 

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