On heating at 225^∘ C, 5-allylcyclohexa-1,3-diene, 17-A,...

On heating at $225^{\circ} \mathrm{C}$, 5-allylcyclohexa-1,3-diene, 17-A, undergoes intramolecular cycloaddition to give the tricyclononene $\mathbf{1 7}-\mathbf{B}$. The same product is predicted for both $[2+2]$ and $[2+4]$ cycloaddition. The mechanism of the reaction has been probed by using the deuterium-labeled derivative, as shown. Indicate the position of the deuterium labeling in the product if the reaction proceeds by (a) a [2+2] cycloaddition or (b) a [2+4] cycloaddition.

Key Concepts

Cycloaddition Reactions

Cycloaddition reactions involve the simultaneous formation of multiple bonds to construct a cyclic structure by combining unsaturated molecules. They are classified by the number of electrons or atoms contributed from each reactant (e.g., [2+2], [4+2], etc.), which helps in understanding the reaction mechanism, stereochemistry, and regioselectivity.

[2+2] Cycloaddition

A [2+2] cycloaddition involves the union of two reaction centers, each contributing two ?-electrons, to form a four-membered ring. This type of cycloaddition is generally symmetry-forbidden under thermal conditions due to orbital interactions, but can proceed under high energy conditions or photochemically. In mechanistic studies using isotopic labels such as deuterium, the position of the label can help ascertain whether the bonds formed follow the [2+2] pathway.

[2+4] Cycloaddition

A [2+4] cycloaddition, more commonly referred to as a Diels–Alder reaction, involves a diene (contributing four ?-electrons) combining with a dienophile (contributing two ?-electrons) to form a six-membered ring. This reaction typically proceeds in a concerted manner with predictable regioselectivity and stereochemistry; deuterium labeling is used to probe which atoms participate in the bond formation.

Intramolecular Cycloaddition

Intramolecular cycloaddition occurs when the reacting partners are contained within the same molecule, leading to the formation of cyclic or polycyclic structures. This process can offer advantages in terms of reaction rates and selectivity, and it is particularly useful for synthesizing complex frameworks such as tricyclic systems.

Isotopic Labeling in Mechanistic Studies

Isotopic labeling, such as replacing hydrogen with deuterium, is a powerful tool in mechanistic studies. The subtle physicochemical differences between isotopes allow chemists to track the fate of particular atoms during a reaction, distinguish between competing pathways, and thus determine the detailed mechanism by which the product is formed.

Transcript

00:03 This is the answer to chapter 27, problem number 20 from the smith organic chemistry textbook.

00:10 And in this problem, we are asked to predict the product from a 1 -7 -sigmatobic rearrangement of a deuterium atom in a triene.

00:23 And then we're asked to discuss whether it would be superficial or antarificial under the two different sets of conditions that we talk about in this chapter.

00:37 And so the way to begin this problem to make it easier to visualize is to redraw the molecule.

00:47 And so, okay, so we'll redraw the molecule like this.

01:07 Okay.

01:08 And so there's a double bond.

01:13 There's a double bond.

01:15 And there's a double bond.

01:17 And so our three deuteriums are going to be coming off of this carbon.

01:24 So deuterium, deuterium, deuterium.

01:28 And then it is a one -seven -sigmatropic rearrangement.

01:34 So we can use the proper notation with the brackets.

01:48 So it's going to be a one -seven.

01:51 And then we can draw at least the carbon.