Radicals preferred geometries

What is the preferred geometry about the radical center in free radicals Carbocation centers are characterized by a vacant orbital and are known to be planar, while carbanion centers incorporate a nonbonded electron pair and are typically pyramidal (see Chapter 1, Problem 9). 

As expected, fluorine substitution has some consequences on structure and stability of the radicals, which are different from the hydrocarbon counterparts. a-F radicals prefer the pyramidal structure because of minimizing 1 repulsion. The trifluoromethyl radical F3C is essentially tetrahedral and has a significant barrier to inversion of about 25 kcal mol - .39 In contrast, the methyl radical H3C itself is planar. Fluorine /J to the radical site is of minor structural consequence. Thus, the pcrfluoro-/er/-butyl radical exhibits a more planar geometry. 

Removal of an electron from a hydrazine unit changes the lone pair orbital occupancy from four to three, which has a large effect on the preferred geometry with respect to the nitrogens. The formation of a three-electron bond has also been demonstrated in the cation radical of octamethyl-l,2,4,5-tetra-aza-3,6-disilacyclohexane. In this cation radical, the spin density is distributed between only two of the four nitrogen atoms. There is a pronounced interaction of the unpaired electron with protons of the methyl groups joined to these two nitrogen atoms. 

Dussault and coworkers described the preparation of allylstannanes (116, 117) as part of their synthetic studies (equation 93)731. It is interesting to note the preferred geometries of the products which appear to be dependent on the nature of the stannane employed. In this last example, Yu and Oberdorfer reported the use of free-radical hydrostannylation in their preparation of (tributylstannyl)vinyl-substituted 2-deoxyuridine derivatives (e.g. 118) for use in halogenation and radiohalogenation reactions (equation 94)733. 

Bonding and Preferred Geometries in Carbon Radicals, Carbenium Ions and Carbanions... 

We will discuss the preferred geometries and the MO descriptions of carbon radicals and the corresponding carbenium ions or carbanions in two parts. In the first part, we will examine carbon radicals, carbenium ions, and carbanions with three substituents on the carbon atom. The second part treats the analogous species with a divalent central C atom. Things like alkynyl radicals and cations are not really important players in organic chemistry and won t be discussed. Alkynyl anions, however, are extremely important, but will be covered later. 

The diastereoselectivity of the reduction depicted in Figure 17.53 is determined when the hydroxylated radical A is reduced to the hydroxylated organosodium compound B. For steric reasons, the OH group assumes a pseudoequatorial position in the trivalent and moderately pyramidalized C atom of the radical center of the cyclohexyl ring of intermediate A. Consequently, the unpaired electron at that C atom occupies a pseudoaxially oriented AO. This preferred geometry is fixed with the second electron transfer. It gives rise to the organosodium compound B. B isomerizes immediately to afford the equatorial sodium alkoxide. 

Which geometries are preferred at the valence-unsaturated C atom of C radicals, and how do they differ from those of carbenium ions or carbanions And what types of bonding are found at the valence-unsaturated C atoms of these three species It is simplest to clarify the preferred geometries first (Section 1.1.1). As soon as these geometries are known, molecular orbital (MO) theory will be used to provide a description of the bonding (Section 1.1.2). 

The rate-determining step in the Na/NH3 reduction of alkynes is the protonation of the radical anion A. The next step, the reaction of the alkenyl radical C to the alkenyl-sodium intermediate B, determines the stereochemistry. The formation of B occurs such that the substituents of the C=C double bond are in trims positions. This trans-selectivity can be explained by product-development control in the formation of B or perhaps also by the preferred geometry of radical C provided it is nonlinear at the radical carbon. The alkenylsodium compound B is protonated with retention of configuration, since alkenylsodium compounds are configurationally stable (cf. Section 1.1.1). The Na/NH3 reduction of alkynes therefore represents a synthesis of fnms-alkencs. 

The interaction with a cationic center, as in the cyclopropylcarbinyl cation, has a similar character. The main difference is that whereas a proton has no stereochemical requirements with respect to its bond to carbon, the cationic center may take different orientations with respect to the cyclopropane ring. Experiment and theory both agree that the preferred geometry is that known as bisected and that rotation of the cationic center by 90° will raise the energy by about 14 kcal mol The preferred conformation is that which allows the p orbital at the cationic center to interact with the in-plane carbon orbitals in the highest occupied MO. This type of interaction also may be seen in the energies of radical cation states of cyclopropane derivatives as determined by photoelectron spectroscopy . 

As shown in Figure 6.22 in the example of the formyl radical, the o acyl radical prefers a bent geometry with the unpaired electron In an approxi-... 

Chiral auxiliary-mediated diastereoselective allylations of a-bromoglycine derivatives 65 have also been established. 8-Phenylmenthol has been successfully employed as a chiral auxiliary in glycine allylations (Eq. (13.19)) [29]. The captoda-tive radical intermediate generated in this reaction benefits from the observation that a-amino acid radicals prefer an s-cis geometry about the single bond, presum-... 

As described above for a-carbon-centered radicals, where it is not feasible to access a side-chain amino acid radical by direct manipulation of a functional group, radical translocation may be employed instead, as illustrated in Scheme 10 [60]. Regiocontrol may be achieved by exploiting product radical stability and the preferred geometry of intramolecular hydrogen atom transfer. 

The qualitative energy levels of appropriate to P, T and G N2H would be radically different had we assumed that nonbonded overlap of the hydrogen Is AO s Is zero. In such an event, we would have obtained the symmetry orbitals shown In Figure 3b. Note that now excitation or deexcitation can no longer be defined simply because all four MO s are degenerate. We will see that assumption of one or the other MO patterns makes a "night-day" difference Insofar as the final prediction of the preferred geometry of N2H Is concerned. 

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