Reactions were done at 120 °C and were studied by 1H-NMR spectroscopy of the reaction mixture. The forward rate k was found to decrease by a factor of 4.5 going from cyclohexane to methanol. The reverse rate k was found to increase by a factor of 53 going from cyclohexane to methanol, while the equilibrium constant K decreased by a factor of 140. It is suggested that there is a change of the polarity during the activation process as evidenced by relationships between the equilibrium and kinetic data. The authors say that the reaction appears to be influenced by the polarity of the solvent, and this can be shown by the change in the dipole moments when going from reactant to transition state to product. The authors also state that the cheletropic reaction doesn’t seem to be influenced by either solvent acidity or basicity.
The results of this study lead the authors to expect the following behaviors:
1. The change in the solvent polarity will influence the rate less than the equilibrium.
2. The rate constants will be characterized by opposite effect on the polarity: k will slightly decrease with the increase of E(30), and k will increase under the same conditions.
3. The effect on k will be larger than on k.
One of the most synthetically important cheletropic reactions is the addition of a singlet carbene to an alkene to make a cyclopropane (see figure at left). A carbene is a neutral molecule containing a divalent carbon with six electrons in its valence shell. Because of this, carbenes are highly reactive electrophiles and generated as reaction intermediates. A singlet carbene contains an empty "p" orbital and a roughly "sp" hybrid orbital that has two electrons. Singlet carbenes add stereospecifically to alkenes, and alkene stereochemistry is retained in the cyclopropane product. The mechanism for addition of a carbene to an alkene is a concerted [2+1] cycloaddition (see figure). Carbenes derived from chloroform or bromoform can be used to add CX to an alkene to give a dihalocyclopropane, while the Simmons-Smith reagent adds CH.
Interaction of the filled carbene orbital with the alkene π system creates a four-electron system and favors a non-linear approach. It is also favorable to mix the carbene empty "p" orbital with the filled alkene π orbital. Favorable mixing occurs through a non-linear approach (see figure 2 at right). However, while theory clearly favors a non-linear approach, there are no obvious experimental implications for a linear vs. non-linear approach.
Sigmatropic reaction
A sigmatropic reaction in organic chemistry is a pericyclic reaction. A sigmatropic reaction does not use a catalyst and involves a single molecule (an uncatalyzed intramolecular process). It changes one σ-bond into a different σ-bond. The name "sigmatropic" is the result of a compounding of the long-established "sigma" name for single carbon-carbon bonds and the Greek word "tropos", meaning turn. This is a rearrangement reaction which means that the bonds in a molecule shift between atoms without any atoms leaving or new atoms added to the molecule. In a sigmatropic reaction, a substituent moves from one part of a π-bonded system to another part in an intramolecular reaction with simultaneous rearrangement of the π system. True sigmatropic reactions usually do not need a catalyst. Some sigmatropic reactions are catalyzed by a Lewis acid. Sigmatropic reactions often have transition-metal catalysts that form intermediates in analogous reactions. The most well-known of the sigmatropic rearrangements are the [3,3] Cope rearrangement, Claisen rearrangement, Carroll rearrangement and the Fischer indole synthesis.
A special notation is used to describe sigmatropic shifts. Each of the carbon atoms on the backbone of the molecule are assigned a position number. Sigmatropic rearrangements are described by an order term [i,j]. This means the migration of a σ-bond adjacent to one or more π systems to a new position (i-1) and (j-1) atoms removed from the original location of the σ-bond. When the sum of i and j is an even number, this is an indication of the involvement of a neutral, all C atom chain. An odd number suggests that there is a charged C atom or of a heteroatom lone pair replacing a carbon-carbon double bond. Thus, [1,5] and [3,3] shifts become [1,4] and [2,3] shifts with heteroatoms, while preserving symmetry considerations. Hydrogens are omitted in the third example for clarity.
Here is a way to find the order of a given sigmatropic rearrangement. The first step is to give numbers to each atom starting with the atoms of the bond being broken as atom 1. Chemists count the atoms in each direction from the broken bond to the atoms that form the new σ-bond in the product. The numbers that correspond to the atoms forming the new bond are then separated by a comma and placed within brackets. This creates the sigmatropic reaction order descriptor.
Chemists also count atoms when naming a sigmatropic shift where a hydrogen atom moves. The carbon chain does not break in a hydrogen atom migration. So, chemists count across all atoms involved in the reaction rather than only across the closest atoms. For example, the following hydrogen atom migration is of order [1,5], attained by counting counterclockwise through the π system, rather than the [1,3] order through the ring CH group that would mistakenly result if counted clockwise.
Chemists have studied sigmatropic reactions where the migrating group has a sterocenter. In principle, all sigmatropic shifts can occur with either the same (retention) or the opposite (inversion) geometry of the migrating group. This depends on whether the original bonding lobe of the migrating atom or its other lobe is used to form the new bond.
In cases of stereochemical retention, the migrating group translates without rotation into the bonding position. In the case of stereochemical inversion the migrating group both rotates and translates to reach its bonded conformation.
There is another way that a sigmatropic reaction can produce products with different sterochemistry. The migrating group can stay on the original face of the π system after rebonding. Or, it can go to the opposite face of the π system. If the migrating group remains on the same face of the π system, the shift is known as suprafacial. If the migrating group transfers to the opposite face is called an antarafacial shift. Transformations that occur within small- or medium-sized rings cannot make antarafacial shifts.
In a thermal [1,3] hydride shift, a hydride moves three atoms. The Woodward-Hoffmann rules dictate that it would proceed in an antarafacial shift. Although such a shift is symmetry allowed, the Mobius topology required in the transition state prohibits such a shift. It is geometrically impossible. This is why enols do not isomerize without an acid or base catalyst.
Thermal alkyl [1,3] shifts, similar to [1,3] hydride shifts, must proceed antarafacially. The geometry of the transition state is prohibitive. But an alkyl group, due to the nature of its orbitals, can invert its geometry and form a new bond with the back lobe of its sp orbital. This reaction will result a suprafacial shift. These reactions are still not common in open chain systems because of the highly ordered nature of the transition state. So, the reactions work better in cyclic molecules.
Photochemical [1,3] shifts should be suprafacial shifts; however, most are non-concerted because they proceed through a triplet state (that is, they have a diradical mechanism, to which the Woodward-Hoffmann rules do not apply).
A [1,5] shift involves the shift of 1 substituent (-H, -R or -Ar) down 5 atoms of a π system. Hydrogen has been shown to shift in both cyclic and open chain systems at temperatures at or above 200 ˚C. These reactions are predicted to proceed suprafacially, by a Huckel-topology transition state.