Cheletropic reaction

A **cheletropic reaction** is a type of pericyclic reaction where one atom on one of the reagents gets two new bonds. A pericyclic reaction is one that involves a transition state with a cyclic array of atoms and an associated cyclic array of interacting orbitals. A reorganization of σ and π bonds occurs in this cyclic array.

Cheletropic reactions are a subclass of cycloadditions. What sets cheletropic reactions apart is that on one of the reagents, both new bonds are being made to the same atom.  A few examples are shown to the right in Figure 1. In the first case, the single atom is the carbon atom in the carbonyl group. That carbon atom ends up in carbon monoxide. The end result is making two new bonds to one atom. The first two examples are known as "cheletropic extrusions" because a small stable molecule is given off in the reaction. The driving force for these reactions is often the entropic benefit of releasing a gas (e.g. CO or N2).

### Kinetics
One example of this is the cheletropic reactions of 1,3-dienes with sulfur dioxide. Chemists have carefully looked at the kinetics of this reaction. In 1976, Isaacs and Laila measured the kenetic factors for the addition of sulfur dioxide to butadiene derivatives. Rates of addition were monitored in benzene at 30 °C with an initial twentyfold excess of diene. They used a spectrophotometer to study light at 320&nbsp;nm to measure the disappearance of SO2. The reaction showed "pseudo first-order kinetics". They found that electron-withdrawing groups on the diene decreased the rate of reaction. The reaction rate was affected considerably by steric effects of 2-substituents, with more bulky groups increasing the rate of reaction. (In other words, the larger the group of atoms hanging from the second carbon atom, the quicker the reaction ran.) They attribute this to the tendency of bulky groups to favor the cisoid conformation of the diene which is essential to the reaction (see table below). In addition, the rates at four temperatures were measured for seven of the dienes. They used the Arrhenius equation to calculate the enthalpy of activation (ΔH‡) and entropy of activation (ΔS‡) for each reaction. This was one of the first important efforts to study the kinetics of a cheletropic reaction. 

Monnat, Vogel, and Sordo, in 2002, measured the kinetics of addition of sulfur dioxide to 1,2-dimethylidenecycloalkanes. They wrote that the reaction of 1,2-dimethylidenecyclohexane with sulfur dioxide can give two different products depending on reaction conditions. The reaction makes the corresponding sultine through a hetero-Diels-Alder reaction under kinetic control (≤ -60 °C), but, under thermodynamic control (≥ -40 °C), the reaction makes the corresponding sulfolene through a cheletropic reaction. The activation enthalpy for the hetero-Diels-Alder reaction is about 2 kcal/mol smaller than that for the corresponding cheletropic reaction.  The sulfolene is about 10 kcal/mol more stable than the isometric sultine in CH2Cl2/SO2 solution.

 or a Sulfolene Through a Cheletropic Reaction Under Thermodynamic Control]]

The authors were able to experimentally work out a rate law at 261.2 K for the reaction of 1,2-dimethylidenecyclohexane with sulfur dioxide to give the corresponding sulfolene.  The reaction was first order in 1,2-dimethylidenecyclohexane but second order in sulfur dioxide (see below).  This confirmed a prediction of theoretical chemists based on high-level *ab initio* quantum calculations.  Using computational methods, the authors proposed a transition structure for the cheletropic reaction of 1,2-dimethylidenecyclohexane with sulfur dioxide (see figure at right). Similar results were found in an 1995 study by Suarez, Sordo, and Sordo which used *ab initio* calculations to study the kinetic and thermodynamic control of the reaction of sulfur dioxide with 1,3-dienes.

## Carbene Additions to Alkenes
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 singlet carbene contains an empty *p* orbital and a roughly *sp**2* hybrid orbital that has two electrons. Singlet carbenes add stereospecifically to alkenes, and alkene stereochemistry is retained in the cyclopropane product.

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.

## References