Benzene-sensitized photolysis of methyl 3-cyclohexene-1-carboxylate in acetic acid leads to addi

Benzene-sensitized photolysis of methyl...

Key Concepts

Photochemical Sensitization

This concept involves the absorption of light energy by a sensitizer, which in turn transfers its excitation energy to a substrate to initiate a reaction. In benzene?sensitized photolysis, benzene absorbs UV light and passes the energy to the alkene, allowing formation of reactive intermediates under mild conditions. This process often enables reaction pathways, such as radical formation, that are not readily accessible in thermal reactions.

Radical Reaction Mechanism

Many photochemical reactions proceed through radical intermediates, where the alkene first forms a radical or diradical species after energy transfer. Because these intermediates are typically planar, the addition of reagents like acetic acid can occur from the less hindered or more electronically favorable side. The nature of the transient radicals is crucial in determining both the regioselectivity and the stereoselectivity of the outcome.

Anti Addition and Stereoselectivity

Stereoselectivity in this context refers to the preferential formation of a single stereoisomer, in this case the trans adduct. The exclusive formation of trans isomers suggests that the addition occurs in an anti fashion, meaning that the reagent attacks the intermediate from the face opposite to that of the existing substituent. Steric hindrance and conformational constraints in the intermediate favor an approach that minimizes repulsions, leading to the observed stereoselectivity.

Regioselectivity Principles

Regioselectivity dictates which of two possible isomeric products predominates. In reactions involving radical intermediates, the stability of these intermediates generally guides the formation of the major product. Factors including hyperconjugative stabilization, electronic effects, and the steric environment influence which site on the alkene is more susceptible to attack. The most stabilized radical intermediate will tend to lead to the major regioisomer in the product mixture.

Transcript

00:03 Similar to how we previously explored the reactions or the possible reactions of acyl halides, now we will be exploring the reactions of the next strongest derivative, or the second strongest carboxylac acid derivative, which is the inhydride.

00:20 And a good rule of thumb is acetic inhydrides and acyl halides do the same reactions.

00:28 They can do basically the same things as one of the other, except acyl halides are just a bit stronger than acetic in hydrates, or just in hydrodes in general.

00:39 This, for example, is acetic inhydrate.

00:41 So if you're ever stuck on what a reagent could be, just think about if this was, say, acyl halide, what would the reaction produce? and that would pretty much be your answer.

00:57 The only exception is there is no way to go from a weaker carboxylic acid to a stronger carboxylic acid.

01:07 This does not happen.

01:09 And that's a good note to have.

01:13 So let's take a first reaction.

01:15 Let's take nh3 in excess.

01:23 Well, if we had this reaction with an acyl halide, for example, acetyl chloride, this would make anaseyl chloride.

01:32 Amide.

01:33 And well, it does the same thing here.

01:39 Produces an amine.

01:47 Let's try another example.

01:54 What would happen if we were to react acetic and hydride with water? well, if this was acyl halide reacting with water, acyl halide and acetic and hydride are very similar in their strengths, except acetyl halide is a tiny bit stronger.

02:16 But this is enough to make.

02:18 Acetic acid or just carbacylacic acid in general.

02:27 Let's take another example...

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