Question

The following triene undergoes an intramolecular Diels-Alder reaction to give a bicyclic product. Propose a structural formula for the product. Account for the observation that the Diels-Alder reaction given in this problem takes place under milder conditions (at lower temperature) than the analogous Diels- Alder reaction shown in Problem 20.34.

Short answer

Short Answer: The intramolecular Diels-Alder reaction is a cycloaddition process that involves a diene and dienophile within the same molecule. To identify the product, we must locate the diene and dienophile, apply the reaction rules, and propose the structural formula for the bicyclic product. The intramolecular Diels-Alder reaction proceeds under milder conditions than the analogous intermolecular Diels-Alder reaction due to the reacting fragments being in closer proximity and a favorable entropic factor.

Step-by-step solution

Understanding the intramolecular Diels-Alder reaction

Intramolecular Diels-Alder reaction involves a cycloaddition process where the diene and dienophile are part of the same molecule. The reaction yields a bicyclic product with two new adjacent single and double bonds. To solve this problem, we need to identify the diene and dienophile fragments in the starting triene and apply the intramolecular Diels-Alder reaction rules.

Identifying the diene and dienophile in the given triene

When looking at the given triene molecule, we must identify the diene and dienophile fragments that could undergo the intramolecular Diels-Alder reaction. The diene should consist of a conjugated system, while the dienophile should have an electron-withdrawing group and a double bond.

Applying intramolecular Diels-Alder reaction rules

In the intramolecular Diels-Alder reaction, the diene approaches the dienophile in a perpendicular orientation, and the double bonds are shared between the diene and dienophile fragments. The new double bond will form between the dienophile's electron-withdrawing group and the last double bond of diene. The two single bonds are shared between the first double bond of the diene and the dienophile.

Proposing the structural formula for the product

Based on the intramolecular Diels-Alder reaction rules, we can now propose the structural formula for the bicyclic product. Connect the fragments according to the new bond formation stated in step 3 and visualize the formation of a new six-membered ring.

Comparing the reaction with an analogous Diels-Alder reaction

In order to account for the observation that the intramolecular Diels-Alder reaction proceeds under milder conditions (lower temperature), we should compare this reaction to an analogous intermolecular Diels-Alder reaction. Intramolecular reactions usually take place under milder conditions because the reacting fragments are within closer proximity, and this increases the reaction rate. Moreover, the entropic factor is favourable due to the decrease in the number of particles in the system.

Key concepts

Cycloaddition Process

The cycloaddition process is a cornerstone in the realm of organic reactions, specifically exemplified by the Diels-Alder reaction. This [4+2] cycloaddition involves a diene, with a conjugated system of two double bonds, reacting with a dienophile, which is typically a double or triple bond carrying an electron-withdrawing group. The cycloaddition results in the formation of a six-membered ring with the creation of two sigma (σ) bonds and the loss of two pi (π) bonds.

Engaging in this orchestrated dance of electrons, the diene and dienophile must achieve a specific orientation to facilitate the reaction—the diene assumes an s-cis conformation, enabling the p orbitals to overlap effectively with those of the dienophile. It's a pericyclic reaction where the transition state involves a cyclic array of orbitals, the symmetry of which determines the course of the reaction following the Woodward-Hoffmann rules. With its concerted mechanism, the cycloaddition process is a stereospecific reaction leading to high regio- and stereochemical control.

Bicyclic Product Formation

In an intramolecular Diels-Alder reaction, the diene and dienophile reside within the same molecule, which allows for the formation of a bicyclic product. This maneuver takes the molecule from simple acyclic architecture to a more complex bicyclic structure in one fell swoop. The new double bond forms between the last double bond of the diene and the electron-withdrawing group of the dienophile, and the new single bonds form at the ends of the diene that was not involved in the new double bond formation.

What's fascinating about this intramolecular variant is that it can generate polycyclic compounds that often resemble natural products or crucial synthetic intermediates. Indeed, the bicyclic structure's inherent rigidity confers robustness and can be a key element in the behavior and properties of the synthesized compound.

Reaction Mechanism

Exploring the intricacies of the reaction mechanism, the intramolecular Diels-Alder reaction is no different in essence from its intermolecular counterpart. Both are concerted processes, meaning that bond-making and bond-breaking occur simultaneously. But it’s the details of this inner mechanism that draw the distinction.

In the case we’re examining, the diene and dienophile engage in a dance of electrons within the same molecular framework. This allows for a unique approach angle and a transition state that's fundamentally different from those in intermolecular scenarios. During the transition state, the electrons from the double bonds in the diene and the double bond in the dienophile redistribute to form one new double bond and two new single bonds, leading to a closed ring system—the bicyclic product.

Structural Formula Prediction

The ability to predict the structural formula of the product resulting from an intramolecular Diels-Alder reaction leans heavily on understanding the mechanistic dance previously mentioned. This predictive power is bestowed by recognizing the diene and dienophile within the molecular structure and envisioning how they might come together. Due to the concerted nature of the reaction, the stereochemistry of the product can also be anticipated.

To undertake this, one must scrutinize the triene and locate the diene and dienophile based on electronic considerations and spatial proximity. Once these are identified, applying the rules of Diels-Alder reactions can guide you towards the new bonds. With this understanding, the drawing of the two fused rings emerges with an elegance that belies the complexity of the reaction.

Comparison of Reaction Conditions

Diving deeper into why an intramolecular Diels-Alder reaction might proceed under more lenient conditions compared to an intermolecular version is revelatory. Proximity plays a huge role; the diene and dienophile are part of the same molecule, thus circumventing the need for diffusion and encounter in solution. Additionally, the entropic cost—a hurdle in reactions—is significantly reduced since the intramolecular process does not increase the number of molecules in the system.

The proximity of the reactive sites accelerates the reaction rate, and the intramolecular nature favors the transition state formation. This efficiency can often be achieved at lower temperatures and in a more controllable fashion, reducing the energy needed to overcome the activation barrier and allowing the reaction to proceed briskly under milder conditions.