20
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Theoretical basis behind orbital correlation diagram for pericyclic reactions
Introduction
Instead of the usual cycloadditions (which has some extra complications in terms of the reacting orbitals), let's consider the concerted reaction $\ce{H2 + D2 -> 2HD}$. We'll come ...
- 70.2k
14
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Reaction of substituted furan with methyl vinyl ketone with subsequent rearomatisation
Your Diels–Alder adduct is pretty much correct. Without any further evidence we cannot definitely say whether the exo or the endo adduct is formed (you have drawn the exo adduct, which generally ...
- 70.2k
14
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Singlet/triplet oxygen cycloadditions
Normal dioxygen ($\ce{O2}$) exists as a ground state, triplet biradical. This is an example of a molecule that, thermodynamically should be quite reactive, yet is kinetically unreactive - once again, ...
- 83.7k
11
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Why can anthracene, but not phenanthrene, take part in Diels–Alder reactions?
Have a look at the reaction mechanism of the Diels-Alder Reaction, e.g. at Wikipedia.
We have talked about this reaction before on this site, and concluded that a sufficient explanation is only ...
- 43.5k
11
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Why is it the middle ring of anthracene which reacts in a Diels–Alder?
To provide a reason for the observed regioselectivity, it is helpful to draw anthracene's aromatic π-electron system in alternance of single and double bonds. In this instance, it is more beneficial ...
- 26.7k
10
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Orbital correlation diagram for the cleavage of cyclopropylmethylene carbene
What is the type of the ring opening; is it a concerted or radical
based C-C bond cleavage?
This is a chelotropic reaction, a subset of cycloaddition\fragmentation reactions. As to whether it ...
- 83.7k
9
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Why is the endo product the major product in a Diels-Alder reaction?
What is the endo-selectivity of Diels-Alder reactions?
If the diene used in the Diels-Alder reaction has asymmetric substituents at the end carbons, and if the dienophile is unsymmetric, then two ...
- 5,567
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Retro Diels–Alder reaction in synthesis of corallopyronin A
Here is my assessment of the mechanism. You are correct about the Dess-Martin periodinane (DMP) oxidation of 1 to 2. The resulting ketone 2 can also exist as the enols (E)-3 and (Z)-4. Any one of the ...
- 15.4k
8
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Why can't we use orbital correlation diagrams for sigmatropic rearrangements?
The reason that comes up again and again is the one below, taken from Orbital Interaction Theory of Organic Chemistry.
Orbital correlation diagrams are useful for cycloadditions and electrocyclic ...
- 17k
8
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Treatment of 2-vinylbicyclo[2.2.2]oct-5-en-2-ol with potassium hydride
The reaction in question is one of the early examples of an oxy-Cope rearrangement [1] in which a 3-hydroxy-hexadiene undergoes a [3,3]-sigmatropic rearrangement to afford a ketone.
The general ...
- 17k
8
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Claisen rearrangement in substituted ring
Another great example of how it's important to look at actual evidence. There have been various studies on Claisen rearrangement selectivity, here's a combined experimental/theoretical one on meta-...
- 959
7
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Identification of endo and exo products in this Diels-Alder reaction
The product you have drawn looks like an exo-product. Conformation 1 leads to an exo-transition state that produces exo 2 as the enantiomer shown. With an achiral catalyst ent-exo 2 would also be ...
- 15.4k
6
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Explaining stereochemistry in a sigmatropic 1,3-alkyl migration
Problem #1 is that if you interchange the methyl and hydrogen groups, you get diastereomers, not enantiomers. But this is a relatively minor point. I assume your question is about the ...
- 70.2k
6
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Stereoelectronic effects in torquoselectivity of 4π electrocyclic opening
The torquoselectivity of the above reactions is dictated by stereoelectronic effects, which (in this case) are significant enough to take precedence over steric effects. In general, electron-donating ...
- 70.2k
6
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Identification of endo and exo products in this Diels-Alder reaction
For me, the most foolproof way to identify the endo and exo products is to look at the stereochemistry in the product. Consider first a standard intermolecular Diels–Alder reaction:
I labelled the ...
- 70.2k
5
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Can vinyl alcohol react as a dienophile in Diels-Alder reactions?
Firstly, your Diels-Alder products are missing the double bond in the ring (formed between what was formerly C-2 and C-3 of the diene). You could always simply hydrogenate it down to the alkane, but I ...
- 70.2k
5
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Do eta-4 benzene ligands participate in Diels Alder or other cycloadditions?
Coordinated benzene as a dienophile
It would be logical to suggest that an η4-benzene ligand could behave as a dienophile. The reason why benzene is characteristically unreactive, both as a ...
- 70.2k
5
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Claisen rearrangement in substituted ring
First, consider if this reaction reversible? If so, we'll just form the thermodynamically favorable product. But the temporarily breaking of aromaticity and tautomerization to reform aromaticity ...
- 17.2k
5
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Does the reaction of 2‐hydroxy‐6‐methyl‐5‐nitrocyclohepta‐2,4,6‐trien‐1‐one with NaOH involve ring contraction?
Drawing on the suggestion of @user55119 I propose the mechanism shown below.
Whether the first intermediate bicyclic [4.1.0] heptanone arises from an intramolecular electrocyclic process or an ...
- 20.6k
4
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Pericyclic Reaction Mechanism
Although you probably know the answer by now, here is the cyclic transition state. It is an oxy-ene reaction. The Lewis acid lowers the activation energy of the reaction.
- 15.4k
4
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Kinetic isotope experiment used to distinguish radical and concerted mechanisms for the Diels Alder reaction
What Fleming is likely referring to is work by Edward Thornton (J. Am. Chem. Soc. 1972, 94, 1168) in which he attempted to make use of secondary kinetic isotope effects (KIE) to prove whether the ...
- 17k
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Reaction between 1,2-dicyanoethylene and 2-phenylbuta-1,3-diene in toluene and heat
The presence of an electron-donating group on the diene, or an electron-withdrawing group on the dienophile, is not an absolute requirement for the Diels-Alder reaction to occur. There are many Diels-...
- 70.2k
4
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Why is a racemic mixture formed in the Diels-Alder cycloaddition?
The concerted Diels-Alder reaction has in principle eight different possible transition states. Depending on the substrates some of them will lead to the same products.
Below you can see an overview ...
- 43.5k
4
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Can the Smirnov-Zamkow reaction be described as a pericyclic process?
I was going to write a comment, but I think this is good enough for an answer.
The issue that symmetry rules forbid a [2+2] thermal cycloaddition if the addition is suprafacial-suprafacial.
However, ...
- 17.2k
3
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Application of the Woodward-Hoffman rules to a [14+2] cycloaddition
General rules for cycloadditions
Before getting into the actual Woodward Hoffman 'analysis', one can quickly check any proposed cycloaddition reaction against the following table of general results.
...
- 17k
3
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Direction of arrows in electrocyclic reactions
The answer is rather bland and probably not what you want to hear. Electrocyclic reactions are called electrocyclic because they are cyclic. (Please don’t see cyclic circular reasoning in there.) They ...
- 66.4k
3
votes
How to classify the hydride shift in the following diazacyclopenta[cd]pentalene system?
It took me a while to see what was actually happening. Initially, I was focused on a [1,5] suprafacial sigmatropic proton shift followed by a diaza-electrocyclic ring opening reaction. However, after ...
- 66.4k
3
votes
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Torquoselectivity: Disrotatory ring opening
No, there is no general orbital symmetry rule for predicting which of the two possible disrotatory motions will be favored. However, by examining steric effects in the two possible transition states ...
- 83.7k
3
votes
Relative rates of two acyclic dienes in the Diels-Alder reaction
You are correct that the having the diene in an s-cis conformation is extremely important. Only from this conformation can the Diels-Alder occur. If there is a high concentration of the s-cis ...
- 83.7k
3
votes
How fast is the interconversion triplet → singlet in organic molecules?
It depends on the organic molecule.
When spin-orbit coupling facilitates intersystem crossing, it can be fast: 100 femtoseconds for nitronapthalene (viz. Chem. Eur. J. 2018, 24, 5379 – 5387).
The ...
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