Question

What products would you expect from the Diels-Alder addition of tetracyanoethylene to cis, trans \(-2,4\) -hexadiene and cis, cis \(-2,4\) -hexadiene? Explain.

Short answer

The Diels-Alder reaction of tetracyanoethylene with cis,trans-2,4-hexadiene and cis,cis-2,4-hexadiene will form tetracyanocyclohexene products with preserved stereochemistry on the 6-membered ring. This is due to both reactions following endo-selectivity.

Step-by-step solution

Identify the diene and dienophile

Here, our dienophile is tetracyanoethylene, which has the general formula \(N=C-C=N\), and the dienes are cis,trans-2,4-hexadiene and cis,cis-2,4-hexadiene. Note: The diene should have the double bonds in conjugation (alternating single and double bonds), and the dienophile typically has an electron-deficient bond that interacts with the diene's electron-rich bond.

Understand the stereochemistry of both the dienes

In the case of cis,trans-2,4-hexadiene, there's a cis conformation between C1-C2 and a trans conformation between C3-C4. For cis,cis-2,4-hexadiene, both the double bonds have a cis conformation between C1-C2 and C3-C4.

Perform the Diels-Alder reaction for both scenarios

We will now perform the Diels-Alder reaction for both diene and dienophile pairs. Remember that the interaction takes place between the double bond in the dienophile and the diene's outer double bonds, leading to the formation of a six-membered ring containing a new double bond. Scenario 1: cis,trans-2,4-hexadiene and tetracyanoethylene Here, the Diels-Alder reaction occurs as follows: \[ \:\text{cis,trans}\-2,4\text{-hexadiene}\: + \: \text{tetracyanoethylene} \to \:\text{cyclohexene}\: \text{product} \] Scenario 2: cis,cis-2,4-hexadiene and tetracyanoethylene Here, the Diels-Alder reaction occurs as follows: \[ \:\text{cis,cis}\-2,4\text{-hexadiene}\: + \: \text{tetracyanoethylene} \to \:\text{cyclohexene}\: \text{product} \]

Predict the products formed

Based on the Diels-Alder reaction performed, we anticipate the following products: Scenario 1: With cis,trans-2,4-hexadiene The product formed would be a 6-membered ring with tetracyanocyclohexene, ensuring the original stereochemistry on the 6-membered ring (endo selectivity). Scenario 2: With cis,cis-2,4-hexadiene Similarly, the product formed would be a 6-membered ring with tetracyanocyclohexene, ensuring the original stereochemistry on the 6-membered ring (endo selectivity). Note: The endo-selectivity is a result of a secondary orbital interaction, which increases the stability of the transition state. In conclusion, both cis,trans-2,4-hexadiene and cis,cis-2,4-hexadiene will form tetracyanocyclohexene products with preserved stereochemistry on the 6-membered ring, and both reactions will follow endo-selectivity.

Key concepts

Tetracyanoethylene

Tetracyanoethylene is a fascinating molecule used as a dienophile in Diels-Alder reactions. A dienophile is a partner molecule that pairs with a diene to form larger cyclic structures in these reactions. Tetracyanoethylene is characterized by its strong ability to attract electrons, making it highly reactive. This electron deficiency is due to the presence of four cyano groups (C≡N) bonded to a central ethylene unit (C=C).

• The formula for tetracyanoethylene is C₂(CN)₄.
• Each cyano group contributes to pulling electrons away from the central double bond.

In a Diels-Alder reaction, the electron-poor tetracyanoethylene interacts with an electron-rich diene. This interaction forms a six-membered ring, with the cyano groups potentially influencing the stereochemistry of the resulting product. The reactivity and functionality make tetracyanoethylene an important tool in creating complex organic molecules.

Stereochemistry

Stereochemistry plays a crucial role in determining the three-dimensional arrangement of atoms in a molecule, which in turn affects its physical and chemical properties. In a Diels-Alder reaction involving tetracyanoethylene, the stereochemistry of both the diene and dienophile is an essential factor.

• The cis or trans configuration of the diene's double bonds significantly influences the final product.
• With tetracyanoethylene, the product will form with configurations influenced by these starting positions.

For example, different arrangements of a diene such as cis,trans-2,4-hexadiene and cis,cis-2,4-hexadiene will lead to different spatial arrangements in the resulting cyclohexene product. Each of these arrangements maintains the ye original stereochemistry due to the conformation of the reactant molecules and drives the selectivity of the reaction.

Endo-Selectivity

Endo-selectivity is a preference observed in certain chemical reactions for the formation of one product over another. Specifically, in Diels-Alder reactions, it refers to the tendency to form products where substituents are oriented towards the core of the newly formed cyclic ring. This positioning often results from secondary orbital interactions, which stabilize the transition state.

• The 'endo' product is typically favored because the interaction of orbitals in the molecule is more efficient.
• These additional interactions lower the energy of the transition state, making this pathway more favorable.

In the case of reactions involving tetracyanoethylene, endo-selectivity ensures that the cyano groups of the dienophile are oriented towards the interior of the cyclic structure. This orientation maximizes attractive interactions and leads to a more stable product, confirming that endo-selectivity is an important characteristic in predicting the outcome of these reactions.