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2-Norbornyl cation
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2-Norbornyl cation
In organic chemistry, the term 2-norbornyl cation (or 2-bicyclo[2.2.1]heptyl cation) describes a carbonium ionic derivative of norbornane. A salt of the 2-norbornyl cation was crystallized and characterized by X-ray crystallography to confirm the non-classical structure.
Advocates of the non-classical nature of the stable 2-norbornyl cation typically depict the species using either resonance structures or a single structure with partial bonds (see Figure 2). This hypovalent interaction can be imagined as the net effect of i) a partial sigma bond between carbons 1 and 6, ii) a partial sigma bond between carbons 2 and 6, and iii) a partial pi bond between carbons 1 and 2. Each partial bond is represented as a full bond in one of the three resonance structures or as a dashed partial bond if the cation is depicted through a single structure.
There has been some debate over how much the pi-bonded resonance structure actually contributes to the delocalized electronic structure. Through 1H and 13C NMR spectroscopy, it has been confirmed that significant positive charge lies on methylene carbon 6. This is surprising as primary carbocations are much less stable than secondary carbocations. However, the 2-norbornyl cation can be formed from derivatives of β-(Δ3-cyclopentenyl)-ethane, indicating that the pi-bonded resonance structure is significant.
The 2-norbornyl cation was one of the first examples of a non-classical ion. Non-classical ions can be defined as organic cations in which electron density of a filled bonding orbital is shared over three or more centers and contains some sigma-bond character. The 2-norbornyl cation is seen as the prototype for non-classical ions. Other simple cations such as protonated acetylene (ethynium, C
2H+
3), protonated ethylene (ethenium, C
2H+
5), and protonated ethane (ethanium, C
2H+
7) have been shown to be best described as non-classical through infrared spectroscopy.
The most frequently proposed molecular orbital depiction of the 2-norbornyl cation is shown in Figure 3. Two p-type orbitals, one on each of carbons 1 and 2, interact with a sp3-hybridized orbital on carbon 6 to form the hypovalent bond. Extended Hückel Theory calculations for the 2-norbornyl cation suggest that the orbital on carbon 6 could instead be sp2-hybridized, though this only affects the geometry of the geminal hydrogens.
According to proponents of a classical double-well potential, the 2-norbornyl cation exists in dynamic equilibrium between two enantiomeric asymmetric structures. The delocalized species central to the non-classical picture is merely a transition state between the two structures. Wagner-Meerwein rearrangements are invoked as the mechanism that converts between the two enantiomers (see Figure 4).
Efforts to isolate the asymmetric species spectroscopically are typically unsuccessful. The major reason for this failure is reported to be extremely rapid forward and reverse reaction rates, which indicate a very low potential barrier for interconversion between the two enantiomers.
Some chemists have also considered the 2-norbornyl cation to be best represented by the nortricyclonium ion, a C3-symmetric protonated nortricyclene. This depiction was first invoked to partially explain results of a 14C isotope scrambling experiment. The molecular orbital representation of this structure involves an in-phase interaction between sp2-hybridized orbitals from carbons 1, 2 and 6 and the 1s atomic orbital on a shared hydrogen atom (see Figure 5).
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2-Norbornyl cation
In organic chemistry, the term 2-norbornyl cation (or 2-bicyclo[2.2.1]heptyl cation) describes a carbonium ionic derivative of norbornane. A salt of the 2-norbornyl cation was crystallized and characterized by X-ray crystallography to confirm the non-classical structure.
Advocates of the non-classical nature of the stable 2-norbornyl cation typically depict the species using either resonance structures or a single structure with partial bonds (see Figure 2). This hypovalent interaction can be imagined as the net effect of i) a partial sigma bond between carbons 1 and 6, ii) a partial sigma bond between carbons 2 and 6, and iii) a partial pi bond between carbons 1 and 2. Each partial bond is represented as a full bond in one of the three resonance structures or as a dashed partial bond if the cation is depicted through a single structure.
There has been some debate over how much the pi-bonded resonance structure actually contributes to the delocalized electronic structure. Through 1H and 13C NMR spectroscopy, it has been confirmed that significant positive charge lies on methylene carbon 6. This is surprising as primary carbocations are much less stable than secondary carbocations. However, the 2-norbornyl cation can be formed from derivatives of β-(Δ3-cyclopentenyl)-ethane, indicating that the pi-bonded resonance structure is significant.
The 2-norbornyl cation was one of the first examples of a non-classical ion. Non-classical ions can be defined as organic cations in which electron density of a filled bonding orbital is shared over three or more centers and contains some sigma-bond character. The 2-norbornyl cation is seen as the prototype for non-classical ions. Other simple cations such as protonated acetylene (ethynium, C
2H+
3), protonated ethylene (ethenium, C
2H+
5), and protonated ethane (ethanium, C
2H+
7) have been shown to be best described as non-classical through infrared spectroscopy.
The most frequently proposed molecular orbital depiction of the 2-norbornyl cation is shown in Figure 3. Two p-type orbitals, one on each of carbons 1 and 2, interact with a sp3-hybridized orbital on carbon 6 to form the hypovalent bond. Extended Hückel Theory calculations for the 2-norbornyl cation suggest that the orbital on carbon 6 could instead be sp2-hybridized, though this only affects the geometry of the geminal hydrogens.
According to proponents of a classical double-well potential, the 2-norbornyl cation exists in dynamic equilibrium between two enantiomeric asymmetric structures. The delocalized species central to the non-classical picture is merely a transition state between the two structures. Wagner-Meerwein rearrangements are invoked as the mechanism that converts between the two enantiomers (see Figure 4).
Efforts to isolate the asymmetric species spectroscopically are typically unsuccessful. The major reason for this failure is reported to be extremely rapid forward and reverse reaction rates, which indicate a very low potential barrier for interconversion between the two enantiomers.
Some chemists have also considered the 2-norbornyl cation to be best represented by the nortricyclonium ion, a C3-symmetric protonated nortricyclene. This depiction was first invoked to partially explain results of a 14C isotope scrambling experiment. The molecular orbital representation of this structure involves an in-phase interaction between sp2-hybridized orbitals from carbons 1, 2 and 6 and the 1s atomic orbital on a shared hydrogen atom (see Figure 5).
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