Radicals polarity

Zheludev, A., Baron, V., Bonnet, M. et al. (1994) Spin density in a nitronyl nitroxide free radical. Polarized neutron diffraction investigation and ab initio calculation, J. Am. Chem. Soc., 116, 2019-2027. 

When a radical or atom attacks a polar O—H or N—H bond, the reactant Y forms a hydrogen bond of the type O—H Y or N—H Y in polar solvents. The hydrogen bond shields the reactant and slows down the reaction regardless of of the type of radical (polar or nonpolar) attacking it (see Chapters 12 and 13). 

Ketone and the formed a-ketoperoxyl radical are polar molecules. Hence the polar effect influences the reactivity of the ketones and the peroxyl radicals. Polar solvents also influence the reactions of peroxyl radicals with ketones as well as other free radical reactions. 

Scheme 16. Application of radical-polar crossover reaction in the total synthesis of ( )-aspidospermidine...

Radicals add to unsaturated bonds to form new radicals, which then undergo addition to other unsaturated bonds to generate further radicals. This reaction sequence, when it occurs iteratively, ultimately leads to the production of polymers. Yet the typical radical polymerization sequence also features the essence of radical-induced multicomponent assembling reactions, assuming, of course, that the individual steps occur in a controlled manner with respect to the sequence and the number of components. The key question then becomes how does one control radical addition reactions such that they can be useful multicomponent reactions Among the possibilities are kinetics, radical polar effects, quenching of the radicals by a one-electron transfer and an efficient radical chain system based on the judicious choice of a radical mediator. This chapter presents a variety of different answers to the question. Each example supports the view that a multicomponent coupling reaction is preferable to uncontrolled radical polymerization reactions, which can decrease the overall efficiency of the process. 

In all the following examples, the targeted double bonds were activated by suitable substituents to increase the efficacy of the desired cyclization mode. For the total synthesis of acutumine (26), an activated a,p-unsaturated ketone 27 was chosen as precursor (Scheme 10) [74, 75], Aryl radical additions to this type of alkenes are known to proceed about ten times faster than to comparable allylic alcohols. In a radical-polar crossover reaction, the spirocyclic product 28 was obtained in the presence of triethylaluminum as promoter and an oxaziridine as hydroxylating agent. The fact that even the efficient hydrogen donor tetrahydrofuran could be used as solvent nicely demonstrates the high efficacy of the cyclization step. 

IarUum catalyzed radical-polar carbosilylation of styrenes and 1,3-dienes... 

Narasaka and coworkers reported radical-polar crossover addition/cyclization reactions of phenacyl bromides 204 and electron-rich alkenes such as (silyl) enol ethers 205, catalyzed by the rhenium(I) complex 206 (Fig. 57) [302], The active catalyst 206A formed after thermal nitrogen elimination from 206 reduced 204 either directly or by oxidative addition/homolysis via rhenium enolate 204A to... 

Fig. 57 Rhenium(I)-catalyzed radical-polar crossover addition/cyclization reactions...

Fig. 15 Nickel(0)-catalyzed radical-polar cyclization/addition or substitution reactions...

Fig. 17 Total synthesis of methyl curcurbate 76 and methyl epijasmonate 77 via a radical-polar 5-exo cyclization/alkylation sequence...

Fig. 106 Radical-polar cyclization/addition reactions catalyzed by a Mn(II)-Cu(i) catalyst system...

Scheme 6. Comparison of Smij- and TTF-induced radical/polar sequential reaetions.

Scheme 9. Application of the radical/polar crossover reaction to the total synthesis of aspidospermidine.

During a microsecond time frame, which is the typical paramagnetic relaxation time of free radicals, polarized free radicals can participate in addition to multiple bonds, to dioxygen, in hydrogen (electron) transfer, in addition to polyradicals, etc. Products of these reactions are polarized in most cases, and they demonstrate TR ESR signals. 

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