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Cheletropic reaction
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Cheletropic reaction
In organic chemistry, cheletropic reactions, also known as chelotropic reactions, are a type of pericyclic reaction (a chemical reaction that involves a transition state with a cyclic array of atoms and an associated cyclic array of interacting orbitals). Specifically, cheletropic reactions are a subclass of cycloadditions. The key distinguishing feature of cheletropic reactions is that on one of the reagents, both new bonds are being made to the same atom.
In the pericyclic transition state, a small molecule donates two electrons to the ring. The reaction process can occur in two different geometries: the small molecule can approach in a linear or non-linear fashion. In the linear approach, the electrons in the orbital of the small molecule are pointed directly at the π-system. In the non-linear approach, the orbital approaches at a skew angle.
The π-system's ability to rotate as the small molecule approaches is crucial in forming new bonds. The direction of rotation will be different depending on how many π-electrons are in the system. The rotation will be disrotatory if the small molecule approaches linearly and conrotatory if the molecule approaches non-linearly. Disrotatory and conrotatory are sophisticated terms expressing how the bonds in the π-system are rotating. Disrotatory means opposite directions while conrotatory means the same direction.
Using Hückel's rule, one can tell if the π-system is aromatic or antiaromatic. If aromatic, linear approaches use disrotatory motion while non-linear approaches use conrotatory motion. The opposite goes with an anti-aromatic system. Linear approaches will have conrotatory motion while non-linear approaches will have disrotatory motion.
In 1995, Suarez and Sordo showed that sulfur dioxide when reacted with butadiene and isoprene gives two different products depending on the mechanism. This was shown experimentally and using ab initio calculations. A kinetic and thermodynamic product are both possible, but the thermodynamic product is more favorable. The kinetic product arises from a Diels–Alder reaction, while a cheletropic reaction gives rise to a more thermodynamically stable product. The cheletropic pathway is favored because it gives rise to a more stable five-membered ring adduct.
The cheletropic reactions of 1,3-dienes with sulfur dioxide have been extensively investigated in terms of kinetics (see above for general reaction).
In the first quantitative measurement of kinetic parameters for this reaction, a 1976 study by Isaacs and Laila measured the rates of addition of sulfur dioxide to butadiene derivatives. Rates of addition were monitored in benzene at 30 °C with an initial twentyfold excess of sulfur dioxide, allowing for a pseudo first-order approximation. The disappearance of SO2 was followed spectrophotometrically at 320 nm. The reaction showed pseudo first-order kinetics. Some interesting results were that electron-withdrawing groups on the diene decreased the rate of reaction. Also, the reaction rate was affected considerably by steric effects of 2-substituents, with more bulky groups increasing the rate of reaction. The authors 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 permitting calculations of the enthalpy of activation (ΔH‡) and entropy of activation (ΔS‡) for these reactions through the Arrhenius equation.
More recently, a 2002 study by Monnat, Vogel, and Sordo measured the kinetics of addition of sulfur dioxide to 1,2-dimethylidenecycloalkanes. An interesting point presented in this paper is that the reaction of 1,2‑dimethylidenecyclohexane (1) with sulfur dioxide can give two different products depending on reaction conditions. The reaction produces the corresponding sulfine (2) through a hetero-Diels–Alder reaction under kinetic control (≤ -60 °C), but, under thermodynamic control (≥ -40 °C), the reaction produces the corresponding sulfolene (3) through a cheletropic reaction:
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Cheletropic reaction
In organic chemistry, cheletropic reactions, also known as chelotropic reactions, are a type of pericyclic reaction (a chemical reaction that involves a transition state with a cyclic array of atoms and an associated cyclic array of interacting orbitals). Specifically, cheletropic reactions are a subclass of cycloadditions. The key distinguishing feature of cheletropic reactions is that on one of the reagents, both new bonds are being made to the same atom.
In the pericyclic transition state, a small molecule donates two electrons to the ring. The reaction process can occur in two different geometries: the small molecule can approach in a linear or non-linear fashion. In the linear approach, the electrons in the orbital of the small molecule are pointed directly at the π-system. In the non-linear approach, the orbital approaches at a skew angle.
The π-system's ability to rotate as the small molecule approaches is crucial in forming new bonds. The direction of rotation will be different depending on how many π-electrons are in the system. The rotation will be disrotatory if the small molecule approaches linearly and conrotatory if the molecule approaches non-linearly. Disrotatory and conrotatory are sophisticated terms expressing how the bonds in the π-system are rotating. Disrotatory means opposite directions while conrotatory means the same direction.
Using Hückel's rule, one can tell if the π-system is aromatic or antiaromatic. If aromatic, linear approaches use disrotatory motion while non-linear approaches use conrotatory motion. The opposite goes with an anti-aromatic system. Linear approaches will have conrotatory motion while non-linear approaches will have disrotatory motion.
In 1995, Suarez and Sordo showed that sulfur dioxide when reacted with butadiene and isoprene gives two different products depending on the mechanism. This was shown experimentally and using ab initio calculations. A kinetic and thermodynamic product are both possible, but the thermodynamic product is more favorable. The kinetic product arises from a Diels–Alder reaction, while a cheletropic reaction gives rise to a more thermodynamically stable product. The cheletropic pathway is favored because it gives rise to a more stable five-membered ring adduct.
The cheletropic reactions of 1,3-dienes with sulfur dioxide have been extensively investigated in terms of kinetics (see above for general reaction).
In the first quantitative measurement of kinetic parameters for this reaction, a 1976 study by Isaacs and Laila measured the rates of addition of sulfur dioxide to butadiene derivatives. Rates of addition were monitored in benzene at 30 °C with an initial twentyfold excess of sulfur dioxide, allowing for a pseudo first-order approximation. The disappearance of SO2 was followed spectrophotometrically at 320 nm. The reaction showed pseudo first-order kinetics. Some interesting results were that electron-withdrawing groups on the diene decreased the rate of reaction. Also, the reaction rate was affected considerably by steric effects of 2-substituents, with more bulky groups increasing the rate of reaction. The authors 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 permitting calculations of the enthalpy of activation (ΔH‡) and entropy of activation (ΔS‡) for these reactions through the Arrhenius equation.
More recently, a 2002 study by Monnat, Vogel, and Sordo measured the kinetics of addition of sulfur dioxide to 1,2-dimethylidenecycloalkanes. An interesting point presented in this paper is that the reaction of 1,2‑dimethylidenecyclohexane (1) with sulfur dioxide can give two different products depending on reaction conditions. The reaction produces the corresponding sulfine (2) through a hetero-Diels–Alder reaction under kinetic control (≤ -60 °C), but, under thermodynamic control (≥ -40 °C), the reaction produces the corresponding sulfolene (3) through a cheletropic reaction:
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