Hydrogenation of alkene A with D2 in the presence of Pd-C affords a single product B. Keeping th

Hydrogenation of alkene A with D2 in the presence of Pd-C...

Key Concepts

Halohydrin Formation and Intramolecular Cyclization

When an alkene is treated with Br? in the presence of water, a bromonium ion intermediate is formed, which is attacked by water to give a bromohydrin via anti addition. Subsequent treatment with base removes a proton and triggers intramolecular nucleophilic displacement of the bromide, leading to an epoxide. Although the initial bromohydrin formation is anti, the ring closure step is stereospecific, ultimately giving the same epoxide as that formed by the peracid oxidation when the alkene is symmetric.

Epoxidation with Peracids

Oxidation of alkenes using peracids such as mCPBA typically proceeds via a concerted syn-addition pathway to form an epoxide. In this reaction, an oxygen atom is transferred to the alkene from the peracid, and the stereochemistry of the starting alkene is preserved in the product. When applied to a symmetric alkene, this results in a single epoxide isomer.

Catalytic Hydrogenation

Catalytic hydrogenation (or deuteration, in this case) of an alkene over Pd-C is a syn addition process in which two hydrogen (or deuterium) atoms add simultaneously to the same side of the double bond. This reaction is stereospecific, and when the starting alkene is symmetric, it gives a single stereoisomer, which provides insight into the stereochemical course of other addition reactions on that alkene.

Syn Addition Mechanism

Many reactions of alkenes, such as catalytic hydrogenation with D? on Pd-C and epoxidation with peracids, proceed via syn addition, meaning that the two groups add to the same face of the double bond. This stereospecificity is key in determining the stereochemistry of the final product and can lead to a single isomer when the starting alkene is symmetric.

Transcript

00:02 This is the answer to chapter 12, problem number 45 from the smith organic chemistry textbook.

00:11 And this problem starts by giving us this molecule and telling us that when it's treated with deuterium gas and palladium on carbon, the deuteriums both add from the top and only from the top.

00:29 And so this is important because it tells us that the bottom face of this molecule is blocked.

00:37 It's hindered.

00:38 And so anything that adds is going to add from the top.

00:42 So when we're then asked to predict products for the other two reactions or series of reactions that we're given, we can do so because we know that the bottom face of this molecule is blocked.

00:55 So, for example, for a, the mcpba is only going to be able to add from that same top face that the deuterium added from.

01:05 And so, the product here is going to look like this.

01:12 So here's our molecule.

01:16 And the epoxide here is going to be up.

01:22 So the epoxyide is going to look like this, right? because the mcpba can only add from that top face.

01:30 So it's going to add from that top face there.

01:34 So now, in part b of this problem, we are asked to do a series of reactions that are also going to have us end with an epoxide.

01:43 But it's actually going to be the opposite epoxide to part a here.

01:47 And so the reason for that looks like this.

01:50 So we need to think about this mechanistically in order to see that's ugly.

01:56 We need to think about this mechanistically in order to see why it makes sense that we're going to get the opposite product from this series of reactions.

02:05 So after the bromination here, we're going to have this bridged intermediate with the bromine in it.

02:15 And that's also only going to be able to have added from the top.

02:19 So there's our bromine.

02:21 It's going to have a positive charge on it.

02:24 So there's our bromine.

02:26 So now when we next treat this with water, or i guess we used water at the same time as the bromine, regardless, there's water in this solution.

02:44 So the water, the oxygen is going to attack from below.

02:49 So it can attack either of these carbons, but it's going to come from below...

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