Planar radicals

Figure 15.39 Structures of (a) the planar radical cation S3N2, (b) its dimer SeN4 + and (c) the corresponding planar diamagnetic dication S3N2 +.

A great deal of information on the electronic structure and geometry of radicals in solution can be extracted from their ESR spectra, as it is well established that the values of hyperfine coupling constants (hfcc), arising from the spin density of the s-orbitals, markedly increase with increasing of the SOMO s-character. The pyramidalization of the radicals is manifested in higher values of their hfccs (o-radicals), whereas smaller values of the hfccs are indicative of the more planar radicals (tt-radicals). 

Dietz and Peover examined the electrochemical reduction of cis and trans stilbene (114) in DMF containing carbon dioxide, 9>. The first electron transfer to trans-114 affords a planar radical anion (115) which then undergoes rapid reaction with carbon dioxide to produce, ultimately, 2,3-diphenylsuc-cinic acid (116) in... 

Abstraction of a hydrogen atom from C2 produces a trigonal planar radical that is achiral. This radical is achiral then reacts with chlorine at either face [by path (a) or path (b)]. Because the radical is achiral the probability of reaction by either path is the same therefore, the two enantiomers are produced in equal amounts, and a racemic form of 2-chloropentane results. 

The main difference between e -transfer and the last mentioned a-addition is in the intermediates HF1 is a blue (Xmax = 580nm) and planar radical, while RX —... 

As most of the work with this type of salts was essentially motivated by the results obtained with the salts based on decamethylmetalocenium cations and polynitrile planar radical anions, the use of different metallocenium derivatives was limited to a small number of compounds. Among these only [Fe (C5Me4SCMe3)2][M(mnt)2], M = Ni and Pt, present crystal structures based on mixed linear chain motives. 

A trivalent n radical is planar, and a trivalent a radical is pyramidyl. Both types of trivalent radicals have staggered and eclipsed conformations. For a planar radical, staggered and eclipsed refer to the substituents at the radical center and not to the p orbital. A divalent n radical is linear, and a divalent a radical is bent. In the case of heteroatom radicals, such as alkoxyl and aminyl radicals, two low-energy electronic states exist, and the odd electron can be in a p orbital with the lone pair in a hybrid orbital (ti radical) or vice versa (a radical). 

Interconversions of acychc carbon-centered radicals between n and a types are low-energy processes. The methyl radical is planar, but increasing alkyl substitution at the radical center results in an increasing preference for pyramidalization. The ferf-butyl radical is pyramidalized with the methyl groups 10° from planarity (the deviation from planarity for a tetrahedral atom is 19°) and a barrier to inversion of 0.5 kcal/mol. When a radical center is in a carbocycle, a planar radical is favored for all cases except the cyclopropyl radical, and the barrier for inversion in cyclopropyl is only 3 kcal/mol. ... 

For the vinyl radical, the hyperfine coupling for the a-carbon is 107.6 G, which would suggest 10% s character in the hybrid orbital. The vinyl radical clearly has the odd electron in a c-type orbital because the (3 protons of the vinyl radical have distinct hyperfine couplings with a = 37 G for the c/i-H, and a = 65 G for the trans-M. The cyclopropyl radical also is a c radical with an a- C a value of 98 G, whereas the cyclohexyl radical, which has a nearly planar radical center, has an a- C a value of 41 G. 

Pairs of radical ions of like charge also react by electron transfer (i.e., they disproportionate). One classic example involves reduction of tetraphenylethylene and subsequent ET between two tetraphenylethylene anions. A more recent interesting example is that of cyclooctatetrene radical anion 148 . Alkali metals readily reduce the nonplanar cyclooctatetraene, generating a persistent planar radical anion... 

In the course of the reaction, the nitrite ion leaves the primary anion radical. This produces the cyclohexyl radical in a pyramidal configuration. The vicinal methyl group steri-cally hinders the conversion of the pyramidal radical into the planar one. With a high concentration of the nucleophile, the rate of addition exceeds the rate of conversion i.e. radd > rconv Then the entering PhS group occupies the axial position. With a low concentration of the nucleophile, the conversion occurs earlier than the addition (radd rconv) and the planar radical center is attacked from both the axial and equatorial sides. This results in the formation of the isomer mixture. 

It was at this stage that Whiffen and co-workers succeeded in unravelling the very involved spectrum obtained from X-irradiated single crystals of glycine in terms of the planar radical... 

Racemization of brinzolamide to the S isomer occurs under heat and light (pH independent) conditions (Fig. 113). This can occur via a radical mechanism from radical formation at the chiral center to form a resonance stabilized planar radical with hydrogen atom addition occurring on both sides of the planar carbon centered-radical to racemize the stereocenter (163). 

In an ESR study of 1,1,3,3-difluoroallyl radicals, Krusic and coworkers were able to demonstrate that the barrier to rotation of such apparently planar radicals is substantially reduced [18]. Although allyl itself has a rotational barrier of 15 kcal/mol [19, 20], 1,1,3,3-tetrafluoroallyl, 1, had a barrier of but 7.2 kcal/mol. The observed 19F hfs constants (42.6 and 39.7 G) were consistent with 1 being a planar system. It is likely that the lowering of the rotational barrier of 1 derives from a destabilizing interaction between the fluorine lone pairs and the doubly-occupied allyl tt-MO which diminishes the net allylic resonance energy, as well as from stabilization of the transition state due to pyramidalization. 

Likewise, Pittman has examined the a,a-difluorobenzylradical (2) by ESR [21]. This radical also exhibited small 19Fahfs (51.4 G) which is consistent with a planar or near planar radical. Because of the symmetry of the system, he could obtain no information on the radical s rotational barrier. Yoshida et al. have recently calculated a 20° distortion from planarity for this radical and have rationalized its relatively high reactivity on the basis of its non-planar nature [22],... 

We have analyzed the H H and A—H interactions for three geometries of each AH" radical (Eq. [33]) (1) AH3-planar, the optimized planar structure (2) AH3-pyramidal, in which dAH is kept fixed to its value in the planar radical, whereas the H—A—H angle P is bent to its value in the optimized pyramidal structure (3) AH -pyramidal, the optimized pyramidal structure in which dAH is allowed to elongate to its equilibrium value. Note that for both CH3-pyramidal and CH3-pyramidal the optimum H-A-H angle p of... 

A first possible mechanism proceeds through a planar radical that can rotate freely about the C-C bond. Both diastereomers would thus pass through the same radical intermediate, which can be used to explain the ( -selectivity. 

Side Note 1.3. Pyramidalized Bridgehead Radicals—and/or Less Substituted Planar Radicals as Intermediates of Radical Chlorinations... 

The mechanism of this defunctionalization was discussed in connection with Figure 1.14. It took place via approximately planar radical intermediates. This is why in the reduction of alkyl mercury(II) acetates, the C—Hg bond converts to a C—H bond without stereocontrol. The stereochemical integrity of the mercury-bearing stereocenter is thus lost. When the mer-curated alcohol in Figure 3.48 is reduced with NaBD4 rather than NaBH4, the deuterated cyclohexanol is therefore produced as a mixture of diastereomers. 

Most common for the synthetic practitioner are inhibition studies or probe reactions making use of typical competitive radical reactions with known rate constants. Stereochemical probe studies pointing to the involvement of planar radical intermediates are also valuable. An approximate lifetime of radicals can in principal be estimated from such reactions. 

This brings up an important point. The DFT calculations discussed here were performed on isolated molecules, whereas the experimental results reported involve free radical formation in the solid-state, mainly in single crystals. Therefore the theoretical calculations are ignoring the electrostatic environment of the radicals discussed, in particular the intricate hydrogen bonding structure that the free radicals are imbedded in. This often leads to non-planar radicals which may or may not represent what is believed to be observed experimentally. 

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