Hyperconjugation negative

1 Negative Hyperconjugation with a Cation. If instead the carbon is bonded to an electronegative element like fluorine, the interaction diagram corresponding to Fig. 2.13 changes. The orbitals of the X—C bond, taken from 

This simple difference alone seems to account for why anti arrangements, both in anomeric effects and in / -eliminations (to be discussed in Chapter 4) are so common, but it is not the whole story, because there are systems where this factor is not present, and yet there is still a preference for anti anomeric effects [and anti eliminations, see (Section 5.1.2.1) page 156]. 

A tempting way to explain the inherent preference for anti over syn arrangements is to picture the antibonding hybridised orbitals with the large lobes outside the bond instead of between the atoms. Thus we might redraw the (T cx orbital in 2.52 as 2.54. This seems to make sense—the atomic orbitals of opposite sign will repel each other. Many organic chemists succumb to this temptation, for we 

Bond distances and bond angles in fluorinated molecules often provide useful informations about the bonding state of the C—F bond related to the negative hyperconjugation. Calculations indicate that the C—C bond of 10 is shortened by 0.13 A and the antiperplanar 

This is dependent upon the geometry, as seen in the structure of c/.v-1,1 -dichlorodioxan 2.85.126 The length of the equatorial C—Cl bond is the same as that in methyl chloride, because it is oriented at an angle giving little conjugation with the lone pairs on the neighbouring 0-1. In contrast, the axial CCl bond is lined up for an anomeric effect with the axial lone pair on 0-4, and it is longer. At the same time, the bond between 0-4 and C-3 is shortened, whereas the bond between 0-1 and C-2 is close to that for the C—O bond in a normal ether. 

In symmetrical systems, anomeric effects are acting in both directions, but it is clear that bond-shortening from the anomeric effect in the one direction is stronger than the bond-lengthening in the other, in line with the overall stabilisation provided by the anomeric effect. Thus, with dimethoxymethane 2.86,127 the central pair of C—O bonds are equal in length and both are shorter than normal because of the anomeric effects, while the other pair of C—O bonds, the O—Me groups, have normal C—O bond lengths. 

Similarly, the fluoromethanes have F—C bonds that shorten128 as the number of fluorines increases from one in 2.87 to four in 2.90, and the number of generalised anomeric effects accumulates. The bond-strengthening represented by these bond-shortenings contributes to the reduced reactivity towards nucleophilic substitution seen in polyhalogenated alkanes. 

4 Syn-coplanar and Anti-periplanar Overlap. In the discussion about the anomeric effect, the lone pair has been oriented, without comment, anti to the C—X bond. The lone pair and the C X bond are able to overlap in this orientation 2.91 since they are coplanar, but at first sight they could equally easily have overlapped had they been syn 2.92. Undoubtedly, coplanarity is the single most important constraint for good overlap, but what about the choice between syn and anti One answer, immediately apparent even in these simplified drawings, is that the syn arrangement 2.92 carries with it at least one eclipsing 

Unfortunately it is illegitimate. When we mix two atomic orbitals, the bonding orbital with an attendant drop in energy is paired with an antibonding orbital with its corresponding rise in energy, and a mathematical formulation determines the sizes of the lobes in each. One cannot arbitrarily move the lobes in and out, 

This interaction leads to a stabilisation of the anionic centre, but also to a weakening of the C-X-bond and partial double bond character in the CC-bond. If these changes are extrapolated further (Fig. 3.23), they lead to elimination of X to give an olefin. 

The HOMO and the LUMO+2 of the a-fluoroethyl anion demonstrate negative hyperconjugation. 

A further impressive example of negative hyperconjugation, this time in a neutral compound, is NF3O, whose highest occupied and lowest unoccupied MOs show strong mixing between lone pairs and a -orbitals. This leads to unusual stability and bond lengths. 

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P Fluonnation always strongly stabilizes carbamons both by induction and by negative (anionic) hyperconjugaQon, 7 The latter "no-bond resonance has been controversial, but its importance is now well established both theoretically [133, 134] and expenmentally [67] The X-ray crystal structures of salts 8 [fi5] and 9 [136] provide cogent evidence for negative hyperconjugation... 

In addition to the nucleophilic displacement of the halogen atom, subsequent substitution of the heteroarylium moiety has been observed in numerous cases. Tliese substitution reactions beneht from the increasing importance of the negative hyperconjugation (Scheme 13). Such a pathway has been intensively exploited to synthesize (fused) hve-, six-, and seven-membered heterocycles. 

This can happen only if the two orbitals are in the positions shown. The situation can also be represented by this type of hyperconjugation (called negative hyperconjugation) ... 

If an electron pair is present on metal, its direction should be such that a negative hyperconjugation of this lone pair with the a (Si-X) antibonding... 

On a thermodynamic level, the presence of fluorine atoms in the P position strongly stabilizes the anions (planar or not) either by inductive effect or by negative hyperconjugation (Figure 1.8). 

Figure 4.2 (a) The zwitterionic model and (b) negative hyperconjugation for cyclo-... 

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