A pyramidal carbocation is a type of carbocation with a specific configuration. This ion exists as a third class, besides the classical and non-classical ions. In these ions, a single carbon atom hovers over a four- or five-sided polygon, in effect forming a pyramid. The four-sided pyramidal ion will carry a charge of 1+, and the five-sided pyramid will carry 2+. In the images (at upper right), the black spot on the vertical line represents the hovering carbon atom.
The apparent coordination number of five, or even six, associated with the carbon atom at the top of the pyramid is a rarity as compared to the usual maximum of four.
Studying these cations was sparked, at the time, by amazing results in computational chemistry. While calculating the optimal geometry of the mono-cation which arises from the extraction of chloride from 3-chlorotricyclo[2.1.0.0<sup>2,5</sup>]pentane, the three bridges were expected to orient in space with angles of roughly 120°. The calculations however showed the four-sided pyramid to be the most stable configuration. At the top of this pyramid, there resides a carbon atom, still connected to a hydrogen. The original expected structure turned out to be not even close to an energy minimum: it represented a maximum.[1]
Figure 1: Several possibilities for (CH)5 cation.
1a starting situation in the calculations: the chloride ion just left.
1b the expected structure. Charge has been delocalized over three carbon atoms
1c representation of the pyramidal ion.
Depending on the method used, the ion 1c in figure 1 is an absolute or just a relative minimum.
A complete theoretical discussion will use all orbitals of all contributing atoms. A first approximation might use a LCAO of the molecular orbitals in the polygon forming the base of the pyramid and the orbitals on the apical atom, as the carbon atom at the top of the pyramid. This approximation will provide insight into the intrinsic stability of the structures.
The apical carbon atom is connected to only one other substituent, so an sp-hybridisation is to be expected. The substituent will be oriented upward. Towards the basic polygon, three orbitals are available:
| Figure 2: Orbitals of the apical carbon atom (above) and the MOs of the base (below) | |
| center|300px | |
| Figure 3: Interaction between the apical and basal orbitals. The "A" on top is apical carbon, "P" indicates the pyramidal structure, "B" is for the basal part of the pyramid. | |
| ring size | energy level | |
|---|---|---|
| 3 | (α + β) | |
| 4 | α | |
| 5 | (α - 0.618β) | |
| 6 | (α - β) |
To obtain bonding interactions between atoms or parts of molecules, two conditions should be met:
The orbitals at the apical carbon and the basic polygon are able to combine with respect to their symmetries. The result will be a more stable configuration for the pyramids. In figure 2, the symmetry aspects are depicted.
Filling the atomic and molecular orbitals in pyramidal structures of different base size leads to the next table. Only bonding orbitals are accounted for.
| n=3 (trigonal) | n=4 (square) | n=5 (pentagonal) | n=6 (hexagonal) | ||||||
|---|---|---|---|---|---|---|---|---|---|
| orbitals | charge | orbitals | charge | orbitals | charge | orbitals | charge | ||
| 1s orbitals on carbon | 4 | −8 | 5 | −10 | 6 | −12 | 7 | −14 | |
| σ bond between hydrogen and the apical carbon | 1 | −2 | 1 | −2 | 1 | −2 | 1 | −2 | |
| σ bond between hydrogen and the basic carbon | 3 | –6 | 4 | –8 | 5 | 10 | 6 | –12 | |
| σ bond in between basic carbons | 3 | –6 | 4 | –8 | 5 | –10 | 6 | –12 | |
| bonding MO between apical and lowest basic orbital | 1 | –2 | 1 | –2 | 1 | –2 | 1 | –2 | |
| bonding MO between apical and second-lowest basic orbitals | 2 | –4 | 2 | –4 | 2 | –4 | 2 | –4 | |
| total number of electrons | –28 | –34 | –40 | –46 | |||||
| total nuclear charge: (n+1)*(C+H)=(n+1)*(6+1) | +28 | +35 | +42 | +49 | |||||
| Net charge of structure | 0 | 1+ | 2+ | 3+ | |||||
In the case of the three-sided pyramid, clearly no ion results; a known neutral species arises: tetrahedrane. To this molecule this way of description is an alternative quantum mechanical description.
The other pyramidal structures will be charged in relation with their base size.
Figure 4: A number of derivatives of triclo[3,1,0,0<sup>2,4</sup>]pentane (TCP) leading to the same pyramidal cation. The carbon atom carrying the leaving group becomes basic, while carbon at the anti position becomes apical.
The group "R" is either 1H or 2H (D):
In 1972 Masamune describes the results of dissolving a number of precursors to 4d (figure 4) at in superacid (a mixture of SO2ClF and FSO3H). Based on both the 13C as well as the 1H-NMR-spectrum the evidence is clear: in each case the same intermediary is formed. Also, when the super acidic medium is destroyed, with either methanol or benzoic acid, the same product is formed. (see: Reaction... below).[3]
| group/atom( | ) | 13C | 1H | ||
|---|---|---|---|---|---|
| 1 | 93.56 | - | 200px | ||
| 2 / 4 | 73.00 | 4.62 | |||
| 3 (if R= 1H) | 60.97 | 4.68 | |||
| 5 | -23.04 | - | |||
| Methyl at 1 | 7.45 | 2.15 | |||
| Methyl at 5 | -1.03 | 1.84 | |||
| ( | )In this table carbon atoms are called, in 1H-NMR the signal of the hydrogen carried by the called carbons are depicted | ||||
Figure 5: De reaction products of the dimethyl pyramidal cation with methanol and benzoic acid.
The "R"-group is either1H or 2H (D):As described above, independent from its synthetic route, pyramidal ion 5a reacts with methanol or benzoate giving rise to products governed by reagent and the reaction medium as is clear by the substitution patterns. In 1972 Masamune is unable to explain the different behavior of the intermediate. In terms of the HSAB-theory an explanation might be given.
In 1975 Masamune calculated[7] in the non-substituted ion most of the charge at the hydrogen atoms. Replacing hydrogen for carbon, the central atom of the methyl group, a more electronegative substituent (2.5 versus 2.1 on the Pauling scale) will concentrate charge on the skeletal carbon. This charge concentration has several effects:
In chemistry, the prefix "homo-" denotes a homolog, a likewise compound containing one, or as in this case two, extra CH2-groups. The common aspect of the bishomo ions is the possession of a 1,4-cyclohexadiene ring instead of a cyclobutadiene one.The stability of this ion at first may seem strange, as enlargement of the ring in general will diminish the bonding overlap between the orbitals at the center of the pyramidal structure. Here the sp2 hybridization, and consequently the planarity of the atoms of and those directly bonded to the sp2 centers, forces the tops of the p-orbitals of the basal carbons towards each other, thus creating a solid base for the apical carbon to sit on. Stiffening the configuration by a bridge between the homo-atoms, converting the base of the pyramid, to a norbornadiene, creates an even more stable structure.
According to the results presented in Table 1, a five-sided pyramidal carbocation will be divalent. This is confirmed by theoretical[8] and practical work by Hogeveen.[9] [10] In contrast to the monocation, which is described with several patterns of substitution, the dication is mainly studied by its hexamethyl derivative. The synthesis starts at hexamethyl Dewar benzene (compound I in table 4) reacting with Cl2 into 5,6-dichloro-1,2,3,4,5,6-hexamethylbicyclo[2.1.1]hex-2-ene (compound II in table 4). Dissolution of this compound in fluorosulfonic acid gives rise to the dication (structure III in table 4).
| 200px | 200px | ||
| I: Me6-Dewar benzene | II: Product of reaction of Me6 Dewar benzene with chlorine | III: the pyramidal dication |
The presence of a pyramidal ion in the solution of fluorosulfonic acid is evidenced by the 1H- and 13C-NMR-spectrum (Table 5).
| 1H ! | 13CSingulet | 13CQuartet | ||
|---|---|---|---|---|
| 1 | 1.96(s) | 22,5 | - 2.0 | |
| 5 | 2,65(s) | 126,3 | 10,6 |
The assignment of the signals is based on their intensities and multiplicities. The assignment of the pyramidal structure is based on the observed simplicity of the spectra: five equal C-CH3 groups combined with one outstanding C-CH3 group. The only way to construct a molecular entity from this data is a five-sided pyramid. Rapid equilibriums between degenerated classical or non-classical carbocations are discarded as the position of the signals does not match the expected values for those kind of structures.
The crystal structure of [C<sub>6</sub>(CH<sub>3</sub>)<sub>6</sub>]2+ (SbF6−)2 • HSO3F was obtained in 2017. Although the apical carbon atom is hexacoordinated, the rule of the tetravalency of carbon is still fulfilled. While the C-CH3 bond length of 1.479(3) Å is typical for a C-C single bond, the other five very long C-C distances of 1.694(2)-1.715(3) Å indicate a bond order of <1.[11]
The product of the reaction of the dication with triethylamine offers a pathway to other substitution patterns then hexamethyl.[12] One or both double bonds are oxidized to a keton. The keton then is reacted with an organometallic compound producing an alkylated hydroxide. The compounds formed in this way possess one or two other alkyl groups, depending on the number of oxidized double bonds. When the alcohols are dissolved in fluorosulfonic acid, they again give rise to new pyramidal dications. Both non-methyl groups occupy basal positions. Each other position at the pyramidal skeleton still carries a methyl group. Table 6 summarizes these findings.
Up to this point the substitution pattern of the divalent pyramidal ion is of minor importance to its behavior. A clear difference arises when the thermal stability if the ions of type V (Table 6) is studied: the apical ethyl substituted ion is stable for 48 hours, whereas no trace of the apical iso-propyl ion is detectable anymore.
At the time of the literature survey (end of 1978), there were no reports on tervalent or higher pyramidal cations.