Carbanions orbitals

Ostoja Starzewski and Bock83 reported the photoelectron spectra of an extensive series of phosphonium ylides, focusing on the substituent effects. Their lowest ionization potential (IP) is associated with the carbanion orbital. Their main results are given in Table 9. A number of important trends can be identified and interpreted. Replacement of the methyl groups on P with phenyl groups reduces the IP of the carbanion. The phenyl group is able to stabilize the P+ charge, which reduces the ability of the phosphonium to stabilize the... 

The sp2 hybridized carbanion 496 can also be viewed as an sp3 hybridized anion and can therefore look like 497 or 498. In 487, the electron pair is antiperiplanar to the two C —0 bonds of the dioxane ring, so that the carbanion orbital can be delocalized by an overlap with the antibonding orbitals of the two C —0 sigma bonds (n-o interaction). On that basis, carbanion 496 would be closer to 497 than 498, and the equatorial approach of the electrophile is thus readily understood. Banks has however given a different explanation based on the work of Klein (152, 153). 

However, there are small differences due to carbanion orbital orientations and the degree of charge delocalization 13a). Both 10 and 12 have essentially the largest symmetry possible (C2v) although all the carbons in 11 lie in the same plane, the lithiums are displaced to opposite sides. Symmetry C2 is also found in 13, but both rings are twisted. 

An important point should be made here. The HOMO of the contact radical anion complex contains very little lithium s orbital character and correctly predicts a relatively small lithium hyperfine contribution. This does not necessarily mean that there is little or no metal-organic group covalent bonding as some workers have implied. The comparatively large lithium 2p coefficients in the HOMO (Table III) suggests in fact that overlap of the lithium 2P orbital with the ir-carbanion-orbital may indeed be significant. 

Anions Many other examples exist for the importance of stereoelectronics in n-it interactions. Even under conditions of very large electronic demand such as formation of carbanions, the directly connected x-system can only assist when it aligns properly with the forming carbanionic orbital. For example, deprotonation at the bridged positions is remarkably difficult (this is, of course, one more manifestation of Bredt s rule. Figure 6.126). 

The antiperiplanar arrangement of the carbanionic orbital and the o-acceptors in aromatic systems can be achieved by introducing endocyclic heteroatoms. The known hydride affinities for N-containing aromatics generally reflect the stabilizing effect of endocyclic o C-N orbitals (Figure ll ijfP- However, the situation is far from simple and cannot be readily explained by the combination of electrostatic and hyperconjugative interactions alone. A computational dissection of the complex interplay of multiple orbital interactions in these systems has not been undertaken so far but it is likely that the observed non-additivity of such effects reflects cooperativity and anticooperativity of stereoelectronic interactions. 

More recent developments are based on the finding, that the d-orbitals of silicon, sulfur, phosphorus and certain transition metals may also stabilize a negative charge on a carbon atom. This is probably caused by a partial transfer of electron density from the carbanion into empty low-energy d-orbitals of the hetero atom ( backbonding ) or by the formation of ylides , in which a positively charged onium centre is adjacent to the carbanion and stabilization occurs by ylene formation. 

Thus with dihalocarbenes we have the interesting case of a species that resem bles both a carbanion (unshared pair of electrons on carbon) and a carbocation (empty p orbital) Which structural feature controls its reactivity s Does its empty p orbital cause It to react as an electrophile s Does its unshared pair make it nucleophilic s By compar mg the rate of reaction of CBi2 toward a series of alkenes with that of typical electrophiles toward the same alkenes (Table 14 4) we see that the reactivity of CBi2... 

The pA of 1,3-dithiane is 36.5 (Cs" ion pair in THF). The value for 2-phenyl-1,3-dithiane is 30.5. There are several factors which can contribute to the anion-stabilizing effect of sulfur substituents. Bond dipole effects contribute but carmot be the dominant factor because oxygen substituents do not have a comparable stabilizing effect. Polarizability of sulfur can also stabilize the carbanion. Delocalization can be described as involving 3d orbitals on sulfur or hyperconjugation with the a orbital of the C—S bond. MO calculations favor the latter interpretation. An experimental study of the rates of deprotonation of phenylthionitromethane indicates that sulfur polarizability is a major factor. Whatever the structural basis is, there is no question that thio substituents enhance... 

It provides electrostatic stabilization of the carbanion formed upon removal of the C-2 proton. (The sf hybridization and the availability of vacant d orbitals on the adjacent sulfur probably also facilitate proton removal at C-2.)... 

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). 

FIGURE 2.8 Energy levels for the benzyl cation, free radical, and carbanion. Since a is the energy of a p orbital (p. 35) the nonbonding orbital has no bonding energy. 

Stabilization by Sulfur or phosphorus. Attachment to the carbanionic carbon of a sulfur or phosphorus atom causes an increase in carbanion stability, though the reasons for this are in dispute. One theory is that there is overlap of the unshared pair with an empty d orbital" (pn-dn bonding, see p. 45). For example, a carbanion containing the SO2R group would be written... 

However, there is evidence against d-orbital overlap and the stabilizing effects have been attributed to other causes. In the case of a PhS substituent, carbanion stabilization is thought to be due to a combination of the inductive and polarizability effects of the group, and d-pn resonance and... 

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